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Integrated Project






1. Summary of the project objectives and organization.

The general long-term goal of the SCALA Integrated Project is the realisation of a scalable quantum computer, by using individually controlled atoms, ions and photons in order to encode, store, process and transmit qubits. This general goal naturally divides into two more specific objectives, which appear as necessary intermediary steps :

  • A) Realize interconnected quantum gates and quantum wiring elements, which are required as building blocks of a general purpose quantum computer.
  • B) Realize first approaches of "operational" quantum computing, which include

(i) small-scale quantum algorithms, such as quantum error correction
(ii) special-purpose quantum processors, such as quantum simulators
(iii) entanglement-assisted metrology.

The project is designed to foster integration between experimental and theoretical approaches, which include both modelizing experiments and improving quantum hardware, and attacking long-term issues such as quantum simulations and quantum information theory, which will be crucial on the path between the present experimental state of the art, and a scalable general purpose quantum computer.

Both objectives A and B appear to be very important realisable steps, going towards the general long-term goal of the project. The tasks of the SCALA partners within objective A will be to combine and integrate all experimental and theoretical activities necessary to realise the basic elements of a general purpose scalable quantum computer. Objective B aims at accomplishing non-trivial "quantum calculations", and though this is quite ambitious, it can reasonably be expected to be successfully achieved within the four years of the duration of SCALA. 

In order to attack in a coordinated way its various goals, SCALA  is using all available experimental and theoretical tools based on trapped atoms, ions and photons. The experimental research area of the IP, as well as its internal dynamics, are thus based on a set of common tools and methods, defined by the "atomic, molecular and optical" (AMO) physics that we shall use to reach the planned objectives. The theoretical research area of the IP obviously also includes AMO physics, but is actually much broader in scope, in order to attack long-term issues which are important on the path between the present experimental state of the art, and a scalable general purpose quantum computer.

One can also note that the objectives A and B given above are associated with two possible routes for achieving scalability. The first consists of developing the elementary information registers, gates and processors, and then interconnecting and networking them. Alternatively, one may use the opposite approach and work from the very beginning with large, strongly interconnected, distributed systems, such as atoms in optical lattices. These kinds of systems are natural candidates for quantum simulators, and the issue is to control them well enough to perform quantum information processing and communication tasks. Obviously succeeding in combining both approaches - "bottom-up" and "top-down" - would be a major achievement for the future of quantum computers.

The general objectives of SCALA allow a very natural division of the RTD activities into four technical sub-projects, denoted below as "SP", summarized in the nearby graph. From the progression of the project, it is expected that the activities in SP1 and SP2 will produce a continuous flow of new techniques, to be used in SP3 and SP4. On the other hand, the theoretical work carried out in SP3 and SP4 may require the realization of new types of quantum gates or wiring, which will then be implemented in SP1 and SP2. The resulting flow of information gives the structure of the whole project, as it is summarized by the figure on the right. It is thus crucial goal to achieve the best possible integration of all these activities, and to optimize the relations between the different subprojects by a careful organization of the internal communications. This is  addressed in the subproject SP0 entitled "Management, Communications and Training".


2. Contractors involved and  coordination activities.

In its second year SCALA has carried out successfully an internal procedure for integrating 11 new groups, corresponding to 8 new legal partners, which started contributing to the work on November 1st, 2007, at the beginning of Year 3. As it will be seen from the many results obtained this year by the new partners, this allowed us to keep in pace with the rapid emergence of new ideas in all domains which are of fundamental importance for SCALA, and especially quantum simulations.

Presenty the project comprises 26 active legal partners, located in Palaiseau (CNRS.IO1 and CNRS.IO2), Paris (CNRS.ENS), Garching (MPQ.EX+TH), London (ICSTM.EX+TH), Innsbruck (IBK.EX+TH), Barcelona (ICFO.EX+TH and UAB), Darmstadt (TUD), Düsseldorf (UDUS), Mainz (UMAINZ), Bonn (UBONN), Oxford (UOXF), Cambridge (CAMBRIDGE), Aarhus (UAARHUS), Camerino (UNICAM), Rehovot (WEIZMANN), Gdansk (GUT and UG), Teddington (NPL), Vienna (TUW), Braunschweig (TUBS), Jerusalem (HUJI), Leeds (UNIVLEEDS), Ulm (UULM.EX+TH), Sussex (UoS), Zurich (ETH) and Florence (LENS). Overall there is a fair balance between experimental groups (13 “old” + 7 “new” = 20) and theory groups (13 “old” + 4 “new” = 17), and the partners know each other well, having already collaborated in many instances.

The management office in Palaiseau is in charge of internal and external communication in the project, and the ICFO partner in Barcelona is in charge of Training and Meetings. In 2008 SCALA has supported several conferences directly related to its activities, where SCALA speakers were invited, and many SCALA team members were present. Overall the interactions between the project members are excellent, resulting in many joint publications, mostly between the theory teams, but also as collaborations between experimental and theoretical teams.


3. Objectives and major achievements of each subproject.

This section presents a compilation of the objectives and main achievements of the Integrated Project, on a SubProject and WorkPackage basis. A more compact summary is given in Section 5.


SP1 - SP2 - SP3 - SP4

SP1 : Physics of quantum gates and decoherence control :

This SP is related mainly to objective A, and aims at the development and control of elementary quantum registers, gates and processors.  Major issues are the improvement of the writing, processing and reading of qubits, and the development of new schemes to overcome decoherence. The experimental tools use atom chips, arrays of optically trapped neutral atoms, cavity QED techniques, and trapped ions. The theoretical tasks concern new methods for optimizing quantum gates and fighting decoherence, such as optimal control theory, and physical aspects of information processing, such as multi-particle entanglement. Major achievements in the four WP are listed below.

WP1.1 Atom chips for Quantum Information Processing

Y1 : During Year 1, all the milestones in this atom chips WP have been achieved. This includes in particular new design of atom chips (ICSTM.EX, UHEI), high finesse micro optics resonators for qubit detection (CNRS.ENS, CNRS.IO, ICSTM.EX), trapping of atoms in a cryogenic superconducting atom chip (CNRS.ENS), and the theoretical study of optimized optical quantum gates with static potentials on atom chips (Trento, Aarhus). Special highlights are a new and versatile method for atom micro manipulation using RF fields, developed by UHEI and not foreseen at the time of SCALA creation, and  the first clear detection of single atoms on a chip by CNRS.ENS.

Double fiber Fabry-Perot resonators for single-atom detection on an atom chip.
(CNRS.ENS Paris, WP1.1)
Mirrors are realized on the fiber tips. Two fibers mounted face-to-face form a resonator in the gap between the fiber tips, visible in the zoom on the right.


Y2 : In the second year, six out of seven planned milestones have been reached. In addition, two Y3 milestones were achieved in advance, and a new one was added, on the study of a BEC on a superconducting atom chip (CNRS.ENS). Several groups have devised new ways to reduce the noise causing fragmentation on atom chips (ICSTM.EX, CNRS.IO2, TUW). The team in TUW has obtained very nice results about arrays of qubit sites with sizable couplings between them (published in “Nature”), and about the detection of single atoms “on the chip” using tapered fibers (without cavity). Results have also been obtained by ICSTM and ENS with micro-cavities, including cavity QED effects by dragging a BEC inside a cavity “on the chip” (CNRS.ENS, published in “Nature”).

Direct observation of the phase dynamics through images of interference patterns in the atomic density distribution for various hold times, in the case of isolated 1D systems (TUW, WP1.1).
Blue areas indicate zero density, while red areas indicate high atomic density, and white lines show bright nodes.


Y3 : Four out of eight milestones have been reached, and the other four ones have made good progress. These results include two highlights papers : one by TUW on “Probing quantum and thermal noise in an interacting many-body system”, and one on a new quantum computing scheme using trapped polar molecules, named “Holographic quantum computing”, by UAARHUS. In addition, nice results on atom detection and photon production in a microcavity were obtained by the ICSTM.EX team, as well as new studies to control the surface roughness in atom chips (CNRS.IO2), and Bose-Einstein condensation on a superconducting atom chip (CNRS.ENS).


Experimental set-up for probing noise in an interacting many-body system (TUW, WP 1.1)  :

Two independent 1D Bose gases are created by first splitting a single highly elongated magnetic trap on an atom chip holding a thermal ensemble of atoms into a double well using RF-induced potentials. In a second step the separate parts are evaporatively cooled to degeneracy, producing two individual 1D condensates The two systems are then simultaneously released from the trapping potential and the resulting interference pattern is recorded with standard absorption imaging.


Y4 : Seven out of nine milestones have been reached or partly reached, one has been cancelled, and one is making progress. These results include very nice achievements on controlling single atoms on atom chips, including embedded optical fibers, as demonstrated in TUW (without a cavity), and at ENS (with a fiber-based cavity). There is one highlight paper by ICSTM on this subject, “An integrated atom-photon junction”, illustrated by the figure below. In addition, various new designs were successfully implemented, such as silicon pyramid structures (ICSTM), microtraps using super-conducting structures in the critical state (CNRS.ENS), and monolithic optical cavities (CNRS.IO).

 Schematic diagram of the integrated-waveguide atom chip (ICSTM.EX).
A silicon substrate supports a layer of silica cladding, within which 4 µm-square doped silica waveguide cores are embedded. There are 12 parallel waveguides (only 4 are shown), with output connected to optical  fibres. A 22 µm-deep trench cuts across the waveguides so that 65% of the light from the trench is collected by the waveguides. An atom in the trench affects the phase and the intensity of the transmitted light, and is also affected by this light, so each waveguide provides a microscopic atom-photon junction. The top layer of the chip is coated with gold to reflect laser light used for cooling the atoms. Current-carrying wires below the chip provide magnetic fields to trap and move the atoms


WP1.2 Addressable arrays of optically trapped neutral atoms

Y1 : With neutral atoms in optical traps, the expected milestones of demonstrating  atom transport with optical micro traps (CNRS-IO1, TUD), and selective transport of individual atoms using an optical conveyor belt and spin state control (MPQ.EX, UBONN) have both been achieved. A special highlight are exceptionally long lifetimes (of up to a minute most recently) of single atoms within a high-Q optical cavity, observed by the MPQ.EX team, and associated with full three-dimensional cooling of the trapped atoms.

Y2 : Two out of three milestones have been reached by several groups simultaneously. The team CNRS.IO1 was able to initialize and measure qubits encoded on individual atoms. By using a photon echo technique, the measured qubit coherence time (40 ms) is up to 106 time longer than the single qubit gate time (40 ns), and is robust when moving the qubit in an optical tweezer. Experiments about the coherence and the displacements (up to 55 µm) of small atomic clouds were also carried out by TUD. The UBONN team is now successfully operating a new optical dipole trap at the ‘magic wavelength’ required for spin-dependent transport, including a high-resolution optical system that allows direct continuous monitoring of individual atoms. First experiments have shown good indications of spin dependent transport.

Y3 : Here 5 out of 6 milestones have been reached. In particular, cooling of single atoms was achieved in an optical tweezer by an adiabatic method (CNRS-IO) and by a novel sideband cooling technique (UBONN). For controlling atomic qubits, new results were a Raman technique causing single qubit rotations (CNRS-IO), preliminary results on a spatial single atom interferometer (UBONN), and reduction of dephasing by compensating additional light fields (TUD). Quantum jumps of a coupled atom-cavity system have been observed for single atoms inserted into a high finesse cavity (UBONN). On the theory side, optimized control sequences were found for optical potentials allowing faster transport faster and fidelities than adiabatic transport. This result, obtained from a collaboration between UULM.TH and the group of Bill Phillips at NIST (USA), is the highlight of this WP for Y3.

Y4 : Here 5 out of 7 milestones have been reached. In particular, the UBONN team has generated coherent superpositions of quantum product states involving atomic hyperfine states and motional oscillator states. A novel result on quantum walks using single trapped neutral atoms is a Highlight of the project (see figure below). The TUD team used focused pairs of laser beams to coherently and site-selectively manipulate internal quantum states of trapped samples of cold atoms. The CNRS-IO team demonstrated a new method to measure the temperature of a single atom in an optical tweezer, and high-fidelity single-qubit rotation by driving a Raman transition. Finally, the ULM-TH team has built theoretical models for single-photon nonlinearities using arrays of cold polar molecules. The nonlinearities result from dipole-dipole interactions of cold polar molecules and implement a two-qubit controlled phase operation between two single photons.


Controlling a quantum walk by time-reversal  (UBONN) :

 (A) Time-reversal sequence for refocusing the delocalized state of a six-step quantum walk. After six steps, the total application of the coin and shift operator is reversed. (B) The resulting probability distribution shows a pronounced peak at the center, to where, ideally, the amplitude should be fully refocused. We observe a refocused amplitude of 30%, surrounded by a Gaussian background (fitted curve).



WP1.3 Novel schemes for deterministic quantum gates with trapped ions

Y1 : For trapped ions, the four expected milestones for Y1 have been completed : Proposal of fast gate operation for ion string (UAARHUS, UOXF), measurement of entanglement witnesses (IBK-EX), design and construction of linear array Penning trap (ICSTM-EX), design of quantum gates between ions in arrays of Penning traps through Coulomb interaction (UNICAM). In addition, two milestones expected for year 2 have made significant progress : Implementing magnetic-field-independent hyperfine qubits (UOXF), and qubit rotations and gates in decoherence-free subspaces (IBK-EX).

Y2 : Six milestones have been reached, two of them in advance on Y3. Main experimental results this year are the first observations of ions in microfabricated traps, observation and cooling of small Coulomb crystals in Penning traps, and 2-qubit gate fidelity above 99%. On the theoretical side, methods for implementing qubits in Penning traps, for moving information along strings of electrons in Paul trap arrays, and new gate methods based on STIRAP and geometric phase have been explored theoretically. The latter avoid high sensitivity to forces, that limited most previous gate proposals.

Y3 : Three out of five milestones have been reached. A major result is the realization by the IBK-EX group of single- and 2-qubit logic gates between logical qubits stored in ion pairs, which create “decoherence free subspaces”. In parallel, the ICSTM and UOXF groups have developed new types of ion trap array. The ICSTM group have developed a unique multiple-Penning-trap configuration that operated for the first time this year. They have been able to observe a very weak magnetic mixing effect through the quantum-jump method, and also controlled movement of ion clouds between trap centers. The Oxford group implemented a Paul trap array with an unusual design, which will permit a test of a concept for rapidly splitting and combining ion strings. The Penning trap serves as the context for the theoretical research on quantum communication by spin chains (UNICAM).

Y4 : Four out of ten milestones have been reached, four are in progress, and two have been cancelled. A major result is the implementation by the IBK.EX group of a universal set of operations on up to 8 ions at high fidelity. Using a combination of joint and single-qubit pulses, they report the generation of n-particle entangled “cat” states from n = 3 at 98.7% to n = 8 at 82% fidelity, and also the first deterministic creation of a bound entangled state. This work is not published yet, but the related “Realization of Universal Ion Trap Quantum Computation with Decoherence Free Qubits” is a highlight of the project. The groups in Oxford, Imperial College, Innsbruck and Ulm have developed different types of multiple-electrode trap structure. The effort worldwide to fabricate small (10 -100 µm) ion traps in arrays is continuing despite difficulties, with many traps never functioning or experiencing electrical breakdown after a short lifetime. Within the SCALA consortium, the Oxford group this year reports a new very stable surface-electrode Paul trap, and the Innsbruck group have implemented operations to shift and swap ions in a trap array. ICSTM continues to study single ions in Penning traps, and have begun a collaboration with UULM to develop a unique multiple-Penning trap structure.

WP1.4 Theoretical aspects of decoherence control

Y1 : Theoretical studies in this SP had many milestones, and most of them have been completed : Multilevel equations for storage and gate operations in presence of decoherence-suppressing modulation (WEIZMANN), schemes for the controlled generation of entanglement in atom-cavity systems (ICSTM-TH, UNICAM), derivation of criteria for coherence of time evolutions in open quantum systems (ICSTM-TH), atom-ion interactions for qubit cooling and quantum gate operation (TRENTO), evaluation of effects of decoherence under composite pulses and pulse-shaped driving of quantum gates (AARHUS), quantum interference and adiabatic passage structures with atomic waveguides and cavity QED setups (ICFO-TH). In addition, the milestone about providing parametrization of noise level in quantum processors has been postponed, and replaced by an unexpected result about macroscopic qubits (UG,GUT).

Y2 In this theoretical WP the 5 planned milestones have been reached, one of them being redefined. This include schemes for the controlled generation of entanglement in trapped atom and ion systems (ICSTM-TH, UNICAM), and dynamical protection of decoherence (UNICAM, WEIZMANN). The teams at UG and GUT carried out an analysis of topological codes, where a properly designed self-Hamiltonian is protecting the quantum information, while TRENTO and AARHUS designed schemes using quantum optimal control theory to perform quantum gate operations with robustness against decoherence. The ICFO postponed the planned work on EIT, but redefined and reached a new milestone on the realization of Mott insulators using ultracold atoms in a standing wave optical cavity.

Y3 : In this WP five milestones have been reached, one related to error correction within a microwave cavity is still in progress, and another one related to interfacing molecular qubits has been redirected One particularly important result is the determination of optimal gates in decoherence-free subspaces (UAARHUS). It refers to a theoretical proposal on a new scheme for a robust two-qubit gate for Rydberg atomic qubits. Other results are the study of Surface-induced heating of cold polar molecules by the ICSTM group, as well as results on topological quantum memory for finite temperature (UG) and the emission of entangled light pulses from a single-atom emitter (UNICAM with ICFO and UAB).

Y4 : In this WP seven out of nine milestones have been reached, and two are in progress. On the other hand two new milestones have been defined and reached, of quite general interest for quantum information. The first one is related to the derivation of the capability of optimal control theory to reach the ultimate quantum speed limit, and it is the highlight of this WP : “Optimal Control at the Quantum Speed Limit” by the UULM.TH group. The second one concerns the possibility to observe or not sudden death of entanglement when one has only restricted experimental access to a system.


SP2 : Quantum networking and quantum communications :

The goal of this SP is to employ the elementary registers, processors and gates developed in SP1, and to interconnect them into information processing networks. Major issues are the development of new networking protocols, in particular with "hybrid systems" (using for instance atom-photon, or atom-phonon links), as well as entanglement sharing and teleportation between remote nodes in a quantum network. The experimental tools again use cavity QED techniques, and trapped neutral atoms or ions. The theoretical tasks concern new schemes for quantum repeaters, investigations of communication complexity, and development of protocols for distributed quantum computing.


WP2.1 Quantum networking protocols with neutral atoms and photons

Y1 The three milestones for this WP have all been completed : single photon sources using either trapped single atoms or neutral atoms at rest strongly coupled to an optical cavity (CNRS.IO, MPQ.EX), development of  high finesse optical microcavities (ICSTM.EX, UHEI), and the design of new protocols for the implementation of 2-qubit gate operations, entanglement generation and teleportation between distant single photon sources (ICSTM.TH).

Single Photon Generation.
(MPQ.EX Garching, WP2.1)
A single atom is strongly coupled to an optical cavity (a), and laser pulses drive Raman transitions in the atom. Excitation by a vertically polarized laser produces photons with alternate circular polarizations, as shown on (b) and (c).

Y2 : In this WP all seven Y2 milestones have been reached. This includes the demonstration of the Hong-Ou-Mandel effect on the light emitted by the two atoms (CNRS.IO1), the implementation of a cavity-QED scheme for realizing a single-photon source, and for spin-polarization entanglement of single atoms and photons (MPQ.EX, published in « Science »). The progress realized by ICSTM.EX and VIENNA to integrate optical microcavities in atom chips is also described here, as well as results obtained by TUW for entangling two atomic ensembles, one major step in the direction of quantum repeater protocols.

Y3: Many results were obtained in this WP, where four out of eight milestones were reached, while four are still in progress. In particular, two highlights were published in Nature Physics : one (from MPQ.EX) is non-linear spectroscopy of photons bound to one atom in cavity QED, and the other one (from CNRS.IO1) is the observation of Rydberg blockade and collective excitation for two individual atoms in tweezer traps. Other achievements include the loading of atoms from a magneto-optical surface trap into a dipole trap, a series of papers obtained by J. Schmiedmayer in collaboration with the group of J.-W. Pan, which culminated in the demonstration of a quantum repeater node. There are many significant theoretical results obtained by UAB1, in collaboration with other nodes in the network (ICFO and ULM.TH).

Excitation of one atom versus collective excitation of two atoms separated by 3.6 µm, in presence of Rydberg dipole blockade (CNRS.IO1, WP 2.1).

The Rabi frequency for exciting one atom is increased by √2 by the presence of the other atom (blue curve) with respect to one atom only (red curve), though only atom can be excited. This is associated with the creation of an entangled state of the pair of atoms.

Y4 : Many results were obtained in this WP, going even beyond the planned milestones. Formally six out of twelve milestones were reached, while six are partly reached or in progress, but many papers are associated both with reached and non-reached milestones. The experiments in Oxford and Barcelona have made good progress, and the two main highlights are the demonstration of phase shaping of single-photon wave packets by MPQ.EX, and the deterministic entanglement of two distant grounds state atoms using the Rydberg interaction (see figure). By a detailed characterization of the trapped pair of atoms, it was measured that 61% of the initial pairs are still present after the entangling sequence, and that for these remaining pairs, the fidelity with respect to the target Bell state is 75%.


Experimental setup for entangling two atoms using Rydberg blockade (WP2.1, CNRS.IO1).

(a) Two atoms are held at a distance of 4 µm in two optical tweezers formed by focused laser beams at 810 nm. The atomic qubits can be driven by Raman beams, and the fluorescence of each atom is directed onto separate avalanche photodiodes (APDs).
(b) Atomic level structure and lasers used for exciting the atoms towards the Rydberg state, where the blockade creates a state (|up, r> + |r, up>)/√2, and bringing them back in the ground state, to store the entanglement in a Bell state (|up, down> + |down, up>)/√2.


WP2.2 Atom-photon interface with trapped ions

Y1 : In this WP the two experimental milestones : Generation of single photons from a coupled ion-cavity system (IBK.EX), and operation of two-trap system & narrow-band twin photon source (ICFO.EX), have made good progress, while the theoretical one : Gaussian state analysis of continuous variable quantum state storage in a trapped ion string (AARHUS) has been completed.

Y2 : In this WP, the generation of single photons from a coupled ion-cavity system has been achieved, and the demonstration of entanglement between atomic and photonic states is in progress (IBK.EX). The second milestone for achieving distant atomic entanglement also made good progress at ICFO.EX, while the two groups worked together for studying photon correlation vs interference of single atom fluorescence in a half cavity (joint article in Phys. Rev. Lett.). On the theory side, the third milestone about the theory of non-Gaussian storage and manipulation in large ion ensembles was reached by the AARHUS team, who also published a relevant work on engineering quantum light states using quantum feedback control.

Single Photon Generation with trapped ion (IBK.EX Garching, WP2.2).

 (A) 40Ca+ partial level scheme showing the relevant levels and transitions.
(B) Correlation between photon arrival times for the single photon source with a repetition rate of 55 kHz.

Y3 : Out of four planned milestones, two have been reached, and one is a highlight : it deals with quantum interference of photons emitted by two continuously laser-excited single ions, independently trapped in distinct vacuum vessels. High contrast two-photon interference is observed in two experiments with different ion species, Ca+ and Ba+. The experimental findings are quantitatively reproduced by Bloch equation calculations, and the coherence of the individual resonance fluorescence light fields is experimentally determined from the observed interferences.


Sketch of the experimental setup for interfering two photons from two trapped ions (WP2.2, ICFO).

Fluorescence photons are coupled into the two inputs of a single mode fiber beam splitter. Second order correlations are computed by recording arrival times at the detectors (PMT) using single-photon counting electronics. The inset shows the relevant electronic levels of a 40Ca+ ion.

Y4 : The work on this work package has been active and successful, although the focus of some of the experimental work changed during the reporting period. This was partially due to bigger-than-expected technical challenges, partially because preliminary or sideline objectives were explored in more detail than planned. IBK.EX report further advances towards controlling ion-cavity interactions, including controlled single-photon creation from the ion-cavity system. UULM.EX improved their microcavity to a significantly higher finesse, and they demonstrated several tools such as shuttling and cooling in the microtrap, which is being combined with the cavity. Highlights are the works from AARHUS about “Few qubit atom-light interfaces with collective encoding” and from Barcelona about “Bandwidth-Tunable Single-Photon Source in an Ion-Trap Quantum Network”.

WP2.3 Quantum channels, entanglement distillation and communication complexity
Y1 : A lot of theoretical results have been obtained in this WP, including the completion of the three expected milesones : Evaluation/estimation of capacities for simple distributed channels and channels for indistinguishable particles (ICFO.TH, UDUS, UG, GUT), characterizing the robustness of informational properties of low-partite quantum states (UG, GUT), comparative study of the efficiency of bipartite versus multipartite purification protocols (UIBK.TH1), as well as advanced work on a Year 2 milestone : Evaluation/estimation of bounds for distillation / key rate  (UG, GUT, CAMBRIDGE)

Y2 : The four Y2 milestones were reached, as well as one Y3 milestone. The first milestone had to deal with capacities of noisy multiparty channels, was solved as a joint work between ICFO.TH and UG. In other milestones, UIBK.TH1 investigated the influence of memory errors in quantum repeater schemes, and UG and GUT gave two new proofs of quantum Shannon theorem stating that the coherent information is an achievable rate for the transmission of quantum information through a noisy quantum channel. Several new results have also been obtained by UG, GUT, UCAM and HHUD about bounds for distillation.

Y3 : Five out the six planned milestones have been reached, as well as three Y4 milestones which have been reached in advance. There are two highlight results, one is abour “Epsilon-measures of entanglement », which are smooth entanglement measures with an operational meaning (IBK.T1), and the other one about the possible use of non-maximally entangled states for multiple linear optical teleportation (UG, GUT). Othert results include secure key distillation (UG, GUT), and multipartite entangement and secret key distillation (CAMBRIDGE, HHUD, UG, GUT)..

Y4 : All seven planned milestones have been reached, with a large amount of novel theoretical results on quantum communications and quantum networks, by ICFO.TH, CAMBRIDGE, HHUD, UG, GUT. The highlight result is actually a review paper by UIBK.TH, which presents the state of the art on measurement-based quantum computation, which was initially introduced by this SCALA group, and is still developing very actively (see figure below).


Measurement-based quantum computation (IBK.TH) 
Symbolic representation of a 3D cluster for a CNOT gate between two encoded qubits. The gate is implemented by a monodromy between world lines of holes in the code surface. The holes evolve in ‘simulated time’ (the third cluster dimension). Also shown is the string corresponding to an encoded Pauli operator X on the control qubit and its evolution from the initial to the final code surface.


SP3 : Elementary quantum algorithms :

This SP addresses both objectives A and B. The tasks concern the implementation, realisation, and demonstration of elementary algorithms and information processing protocols, such as quantum error correction for a logical qubit encoded over several physical qubits, by using tools developed in SP1 and SP2. The theory parts of SP3 involve development of new algorithms and new error-correction schemes. This SP is expected to undergo significant further evolution during the two last years of the project, as new algorithms become accessible due to experimental and theoretical advances.

WP3.1 Implementation and experimental realisation of elementary quantum algorithms

Y1 : The two milestones of this WP : Demonstration of entanglement swapping with 4 ions (IBK.EX), and realization and assessment of performances of a two-cavity set-up (CNRS.ENS), have been completed. In advance on Y2, the IBK.EX and IBK.TH groups were able to reconstruct the full density matrix for a 8-qubit e,ntangled state (the density matrix consists of 256x256 = 65536 complex numbers, that required more than 650.000 individual measurements). A highlight result is a super-long (> 100 ms) storage time for a open-structure microwave cavity, allowing repeated QND measurements of a single photon, which can interact with hundreds of atoms before its final decay (CNRS.ENS).

Quantum Byte.
(IBK.EX, Innsbruck, WP3.1)
Reconstructed density matrix for an entangled state with eight qubits implemented with 8 trapped ions. This is the first time that an experimentally determined density matrix of that size has become available for theoretical analysis. For a quantum system consisting of eight qubits the density matrix consists of 256x256 = 65536 complex numbers, that required more than 650.000 individual measurements. The theory group at IBK.T1 analysed the density matrices with various methods and found that all states carry genuine multipartite entanglement.


Y2 : Several milestones made good progress (deterministic entanglement swapping with 4 ions, GHZ states with 4 and 5 ions (IBK.EX), implementation of “AND” gate between two trapped ions, by UOXF.DU). A major result (published in “Nature”), obtained by the team CNRS.ENS, consists in repeated Quantum Non Demolition (QND) measurements of discrete numbers of photons stored in a very high Q cavity (lifetime > 0.1 s). Besides providing a striking illustration of the most fundamental properties of quantum measurements, this experiments opens the way to studies of many-qubits decoherence effect, and to explicit demonstrations of quantum non-locality with mesoscopic fields.

Progressive collapse of field into photon number state (CNRS.ENS, WP3.1)

The figure shows photon number probabilities plotted versus photon and atom numbers n and N. The histograms evolve, as N increases from 0 to 110, from a flat distribution into a well defined peak at n = 5.

Y3 : In this WP three out of five milestones were reached. One of them on the demonstration of GHZ- states with at least 5 ions was actually cancelled, and redirected to the realization of the quantum Toffoli gate with trapped ions, which was achieved and became a highlight (IBK.EX). The other highlight (CNRS.ENS1) reports the complete reconstruction and pictorial representation of a variety of radiation states trapped in a cavity, in which several photons survive long enough to be repeatedly measured. This allows to follow the evolution of decoherence by reconstructing snapshots of Schrödinger cat states at successive times. This reconstruction procedure is a useful tool for further decoherence and quantum feedback studies of fields trapped in one or two cavities.



Elementary quantum algorithms (WP 3.1) : 

Characterization of a Quantum Toffoli gate with 3 trapped ions (IBK.EX, left), and measurements of a quantum superposition of photonic states (Schrödinger cat state) in a high Q cavity (CNRS.ENS1, right).

Y4 : In this WP  three out of six milestones were reached, including two highlights, one is in progress and two were cancelled. The UIBK.EX group concentrated on the implementation of quantum error correction, and a protocol based on three qubits could be demonstrated. In addition, this combination of unitary operations was used for carrying out QND measurements of spin correlations, which were then employed in a state-independent test of quantum contextuality (highlight). The UOXF.DU group worked on other primitives for quantum error correction, e.g. dynamic decoupling techniques, and  a multi-qubit readout with 99.99% single-shot fidelity was achieved. Based on previous achievements the CNRS.ENS group proposed  a protocol for producing Fock states on demand, by introducing a real-time  feedback loop that modifies the cavity field based on the available measurement results (highlight, see figure below).


Preparing arbitray Fock states in a cavity by quantum feeback (CNRS.ENS1): 

Theoretical calculation of the feedback convergence. Histogram of convergence times with Fconv = 0.95. The solid line gives the probability of the feedback convergence after a given time. The inset shows the average density matrix of the converged states.


WP3.2 Developing and optimising novel algorithms and protocols for trapped ions and atoms

Y1 : Two milestones have been completed, about theoretical proposals for neural network with cold ion chains or dipolar atomic lattice gases (ICFO.TH), and for collective manipulation of quantum states of a larger number of particles (ICFO.TH, ICSTM, AARHUS). A third milestone on the investigation of the rate of collective, simultaneous cooling of many atoms cooled in an optical cavity is in progress (ICSTM).

Y2 : The three Y2 milestones of WP3.2 have all been reached, and are about (i) studies of generation and control of entanglement in model neural network systems (such as spin glasses, or disordered cold gases) by ICFO.TH, (ii) investigations of quantum walks as a new algorithmic tool in quantum information processing by ICSTM.TH, and (iii) protocols and algorithms with built-in robustness against experimental noise and unknown parameters by UAARHUS.

Y3 : In this WP five milestones were reached out of six, and one of them became a highlight, about the optomechanical coupling between a Bose-Einstein condensate considered as a mechanical oscillator and the electromagnetic field in the cavity (ETH). Other results concern  matter-wave gap solitons (TUD, UAB), quantum gates and multiparticle entanglement by Rydberg excitation blockade (UAARHUS), and double species Bose-Einstein condensate with tunable interspecies interactions.

Y4 : In this WP four milestones were reached, and six other ones, which were “new milestones” added last year, made good progress though they are not fully completed yet. The highlight concerns the area law for the entropy of low energy states of spatially-extended quantum systems. This issue confronts the observation that the entropy of a large sub-region of a ground state quantum system scales with the surface area, while for almost all general states of the system Hilbert space, the entropy scales with the volume. Based on an analysis of how fluctuations of the energy within a region is determined by a superficial shell and the exterior of the region, it is shown that under well specified conditions, the area law prevails for low energy states. Other results concern matter-wave gap solitons (TUD, UAB), quantum gates and multiparticle entanglement by Rydberg excitation blockade (UAARHUS), and double species Bose-Einstein condensate with tunable interspecies interactions.

WP3.3 Theory of quantum information processing and communication protocols

Y1 : All the planned milestones have been completed : Design of error correction codes in d dimension (UNICAM), new tight multi-particle Bell inequalities for many local settings and their links with various quantum information processing tasks (UG), bounds on quantum cloning under restrictions including super-selection rules (UDUS, ICFO.TH), bounds for information transmission for most typical/important families of multipartite states/channels (ICFO.TH, GUT, UG, UDUS), and novel parameters describing entanglement (UG, GUT). In addition, advanced work have been carried out in view of Y2 milestones : Design of error correction codes in the infinite dimensional case, and improvement of error correction codes by the  application of noise-assisted strategies (UNICAM). Finally, a new milestone has been set up :  Novel tools and methods for characterizing/detecting entanglement of d-level and  continuous variables systems (UDUS, ICFO.TH, UG, GUT).

Y2 : In this theoretical WP most milestones have been reached, about advanced error correcting schemes, tools for studying multipartite entanglement, novel application of quantum resources to quantum information tasks, realistic multiphoton scenarios for certain quantum communication tasks and unified approach to dynamical protection of entanglement. During the reported period several collaborations between UG and GUT, and also between ICFO, HHUD and UG took place. Another collaboration between ICFO.TH and MPQ.TH, on “Entanglement percolation in quantum networks”, resulted in a publication illustrated on the cover page of “Nature Physics” in april 2007.

Y3 : Here many milestones were reached (11 out of 15), three are in progress, and one has been redirected. The two highlights deal with “Thermodynamic control by frequent quantum measurements”, and with “Nonadditivity effects in classical capacities of quantum multiple-access channel”. These results show on the one hand the non-standard  behaviour of qubits coupled to heat bath under fast measurements (WEIZMANN, Nature) and the nonadditivity of rate regions for transmission of classical information with multi-access channel (GUT, PRL). Finally let us stress that collaboration between ICFO and UG, ICFO and IBK.T1, and also GUT and UG took place in Y3 research..

Y4 : Here most milestones were reached (13 out of 15), and two are in progress. There are two highlights, dealing with “Multipartite entanglement detection via structure factors” (HHUD), and Master Equation and Control of an Open Quantum System with Leakage” (WEIZMANN). Other results by ICFO.TH, UG, GUT concern various issues about QIPC in distributed quantum systems.


SP4 : Medium and large scale quantum state control and applications :

This SP also addresses both objectives A and B, i. e. quantum state control and applications in quantum statistical systems, such as ion chains, or ultracold atoms in optical lattices. The possibilities for the implementation of large size atom registers in arrays of microtraps, on microchips, or inside high Q cavities are also studied theoretically and experimentally. Important issues are the development and implementation of quantum simulators, studies of quantum statistical systems of special significance (such as for instance various types of Hubbard models), quantum phase transitions, quantum state control in distributed systems, and generation of massive entanglement. Like SP3, this SP evolved significantly due to the addition of new groups.

WP4.1 Quantum state engineering and control in distributed systems

Y1 : All Y1 milestones were completed : Analysis of robustness of state transfer and other components of ‘always-on’ QIP (CAMBRIDGE), schemes for cooling atoms moving in an optical lattice (IBK.TH), internal-translational entanglement schemes of cold-atom interferometry (WEIZMANN). In addition, CAMBRIDGE have made progress on the Y2 milestone : New primitives for quantum computing based on spin systems with always-on interaction. Similarly WEIZMAN has partly realized its Year 2 milestone as well. Finally, UDUS, has already achieved a Year 2 milestone : Determination of maximal nearest neighbour entanglement in XXZ spin chains

Theoretical design of Spin Lattice Hamiltonians with Polar Molecules in Optical Lattices.
(IBK.T1Innsbruck, WP4.1)
This is illustrated from two models:(a) gapped model with error tolerant 2-fold degenerate ground states, and (b) Kitaev"s model with anyonic excitations and ground states allowing for topologically protected quantum memory.


Y2: The five Y2 milestones have all been reached, about fault-tolerant quantum computation based on 'always-on-interaction', quantum phase transitions in optical lattices (CAMBRIDGE), studies of the Bose-Hubbard model with dissipation via phonon cooling (IBK.T2), entangled state interferometry in ultracold atomic systems (WEIZMANN). Overall, many theoretical problems have been studied, ranging from efficient initialization of quantum registers in atomic lattices, and the quantum simulation abilities of doped coupled cavity arrays, up to implementation issues such as the preparation of cluster states, reproduction of spin models in coupled cavities, and detection of multi-particle entanglement.

Y3 : In this WP ten out of fourteen milestones were reached, including one highlight on « Quantum States and Phases in Driven Open Quantum Systems with Cold Atoms », which deals with schemes to exploit dissipative coupling to phonon baths as a means of preparing many-body states of atoms in optical lattices. A variety of other theoretical problems was studied, ranging from efficient initialization of quantum registers in atomic lattices to the characterization of entanglement in spin chains, as well as experimental issues, including preparation of cluster states and detection of multi-particle entanglement.

Y4 : In this WP nine out of twelve milestones were reached, including two highlights : the proof of the quantum analogue of the Local Lovasz Lemma, by which significant improvements about the threshold for the satisfaction of random quantum SAT instances were derived, and the experimental investigation of the spin dynamics of one and two neutral atoms strongly coupled to a high finesse optical cavity, with quantum jumps between hyperfine ground states of a single atom. Other results address theoretical problems ranging from efficient initialization of quantum registers in atomic lattices, to the characterization of entanglement in spin chains, as well as experimental issues, including preparation of cluster states and detection of multi-particle entanglement.

WP4.2 Cold atoms and ions as quantum simulators

Y1 : There was two theoretical milestones in Y1, which have both been completed, about quantum optical systems to implement quantum simulators (MPQ.TH, UDUS), and classical algorithms for quantum simulation in 1D (MPQ.TH). Experimental milestones are expected for the next periods.

Y2 : Five out of six Y2 milestones have been reached, and the central achievements are about simulating complex many-body quantum systems using atoms and photons, and analyzing algorithms to describe those systems, specially in two spatial dimensions. This concerns cold-atom quantum simulators of strongly interacting particles, and optical detection of the quantum phases (HHUD, TUD, MPQ.TH), quantum simulations with trapped ions (MPQ.TH), and the possibility to realise and detect excitons using cold atoms in optical lattices (IBK.TH). A new unexpected result was the demonstration of entanglement percolation in quantum networks, done in collaboration between MPQ.TH and ICFO.TH. On the experimental side, ICSTM.EX has been working to integrate microscopic optical devices into atom chips in order to detect small numbers of atoms, in order to eventually manipulate large atom arrays as quantum simulators.


Left : cover of Nature Physics,  April 2007, Volume 3, No 4, about “Entanglement Percolation in Quantum Networks” (MPQ.TH and ICFO.TH, WP 4.2)

Right : Quantum noise correlation of a charge density wave (CDW) type ordering prepared using an optical superlattice. The CDW is revealed by observing the distance of the noise correlation peaks in the horizontal being half as large as the distance along the vertical direction (U. Mainz, WP4.3)

Y3 This WP is split in a theoretical and an experimental part, with the central goals of simulating complex many-body quantum systems using atoms and ions, as well as analysing of new theoretical methods to describe those systems. all devoted to quantum simulations of complex quantum systems. Six out of twelve milestones were reached, and most of the other ones made good progress. The WP includes three highlights, on “Simulation of Quantum Many-Body Systems with Strings of Operators and Monte Carlo Tensor Contractions” (MPQ.TH), ] “Simulating the quantum magnet with trapped ions” (MPQ.EX2), and “A Mott insulator of fermionic atoms in an optical lattice” (ETH).

Y4 : This WP, in which 13 out of 15 milestones have been reached,  is split in a theoretical and an experimental part, with the central goals of simulating complex many-body quantum systems using atoms and ions, as well as analysing new theoretical methods to describe those systems. Three major highlights of this workpackage for the current year concern Quantum computation and quantum-state engineering driven by dissipation (MPQ.TH), “Quantitative Determination of Temperature in the Approach to Magnetic Order of Ultracold Fermions in an Optical Lattice” (ETH), and “Symmetry breaking in quantum systems : the case study of vortex nucleation”. This work constitutes a paradigm example of symmetry breaking/change of the order parameter of quantum many-body systems in the course of adiabatic evolution (see figure below).

 Vortex nucleation in quantum systems (WP4.2)

Density of the ground state and phase maps of  the first (y1) and second (y2) most populated states, for two different values of the rotation frequency W for N=6 particles, upper : W = Wc, lower : W = 1.03 Wc. The first column is the contour plot of the total density, and the second and third columns show the local phase maps of y1 and y2, respectively. Vortices are localized at the singularities of the phase maps, surrounded by diffuse change of the phase.
This figure shows that the nucleation of the first centered vortex in a rotating condensate by a slow frequency sweep does not occur through a smooth entrance of the vortex. The system passes through a correlated, non-mean-field state where two single-particle states have equal weight

WP4.4 : The UIBK.EX group has performed precision spectroscopy of the S1/2-D5/2 line of 40Ca+ and 43Ca+, for determining the isotope shift, which are of paramount importance for employing this ion as a qubit. The UAARHUS group has started to develop the theoretical treatment of large collection of atoms which can become spin squeezed. At NPL a segmented linear trap had been designed for the planned experiments with 88Sr+, but due to fabrication problems a conventional linear trap has been realised. The CNRS-ENS plans to develop cavity QED experiments based on mesoscopic atomic samples were delayed because the cavity QED set-up  was used for the unexpected milestones reported in WP3.1.


WP4.3 Disordered cold atom and ion systems

Y1  : This very rich WP achieved a large number of milestones, both theoretical :  Identification and proposals for realisations of various interesting disordered systems of various kinds (ICFO.TH), characterization of entanglement properties of weighted graph states (IBK.T1), measurement scheme for q-overlap in disordered Bose-Hubbard model (IBK.T2), illustration of measurement procedure for disordered 1D BH model (IBK.T2), and experimental : Two-atomic species loading of atoms in optical lattice potentials (UMAINZ), creation of short ranged disorder via a second atomic species in the lattice potential (UMAINZ). The last milestone : Characterization and arbitrary manipulation of 1d disorder potentials (UHEI), though not fully completed, has also made very good progress.

Y2 Seven out of eight milestones have been reached, including the investigation of several Hamiltonians which can be simulated experimentally, and the interplay of entanglement and frustration in disordered quantum systems (ICFO.TH). The IBK.T1 team has continued to investigate the thermodynamical and quantum mechanical properties of disordered systems, using weighted graph states. The CNR Trento team has obtained unexpected results, on the entanglement in quantum disordered systems with an Ising-type interaction, and their potential for quantum information processing. On the experimental side, the UMAINZ team has carried out detailed studies of Bose-Fermi mixtures in 3D optical lattices, and specifically investigated the influence of the fermionic species on the coherence properties of the bosonic superfluid component. Furthermore, the team has realized for the first time superexchange spin-spin interactions between atoms in superlattices. The TUW team has performed in-depth studies on the spectrum and height dependence of disorder potentials in atom chips. Especially, the scaling of corrugation amplitude and frequency with atom-wire separation is now well understood.

Y3 : : In this WP, three milestones were fully reached, two are in progress, and one is postponed. In addition, two other milestones were partly reached, and give rise to two highlights : « Metallic and Insulating Phases of Repulsively Interacting Fermions in a 3D Optical Lattice » (UMAINZ), and  “Anderson localization of a non-interacting Bose-Einstein condensate”(LENS). Other topics are the detection of correlations in disordered quantum systems, the effect of dipolar interactions for the generation of novel complex quantum phases and also the effect of fermionic quantum gases in an optical resonator (ICFO). IBK.T1 has focussed the work on thermodynamical and quantum mechanical properties of disordered systems, and IBK.T2 on weighted graph states, which are good candidates for the numerical simulation of quantum systems. Furthermore, general entanglement properties of condensed matter systems and the algorithmic complexity of solving classical spin models were addressed in the work of Year 3.



Cloud sizes of repulsively interacting spin mixture versus compression (UMAINZ, WP4.3).

The cloud size is measured as a function of the external trapping potential for various interactions. Dots denote single experimental shots, lines denote the theoretical expectations. The insets (A to E) show the- quasi-momentum distribution of the noninteracting clouds (averaged over several shots).
The inset (F) shows the resulting cloud size for different lattice ramp times, for a noninteracting and an interacting Fermi gas. The arrow marks the actial ramp time of 50 ms. These experimental results demonstrate the potential to model interacting condensed-matter systems using ultracold fermionic atoms.

WP4.4 Entanglement and quantum correlations for precise metrology

Y1 : In this WP two theoretical milestones have been completed : Determination of the interferometric phase sensitivity with separate squeezing of two atomic ensembles (UDUS), and detailed studies of squeezing and its application in precision probing (UAARHUS). One of the experimental milestone has been completed : Absolute frequency measurement of Ca+ S-D transition with a single ion (IBK.EX), and actually a Y2 milestone on the same experiment wera also achieved, about the absolute electric quadrupole moment of Ca+ (IBK.EX). The two other experimental milestones, though not fully completed, have made very good progress : Observation of 2-ion entanglement (NPL), and preparation of mesoscopic atomic samples (CNRS.ENS).

Y2 : The UIBK.EX group has performed precision spectroscopy of the S1/2-D5/2 line of 40Ca+ and 43Ca+, for determining the isotope shift, which are of paramount importance for employing this ion as a qubit. The UAARHUS group has started to develop the theoretical treatment of large collection of atoms which can become spin squeezed. At NPL a segmented linear trap had been designed for the planned experiments with 88Sr+, but due to fabrication problems a conventional linear trap has been realised. The CNRS.ENS plans to develop cavity QED experiments with mesoscopic atomic samples were delayed, because the cavity QED set-up was used for the unexpected milestones reported in WP3.1.

Y3 : Two out of four milestones have been reached. In particular, the UAAHRUS group has developed a general theoretical description of the continuous probing of a squeezed atomic sample coupled to an optical cavity. Optimal probing strategies as well as the use of feedback for reaching a desired state were investigated. The UIBK.EX group, have performed precision spectroscopy of the S1/2-D5/2 line of 40Ca+. Together with measurements performed with 43Ca+, this study provides a precise understanding of the level spectrum and of systematic shifts of the considered quadrupole transition. This is of great importance for employing it as a qubit.

Y4 : In this WP most milestones were reached already in previous periods (IBK.EX), or have been redirected to other WP (CNRS.ENS). During year 4 one out of three milestones has been reached, and two others are still in progress at NPL, to achieve 2-ion entanglement within one segment of a linear trap, because two kind of traps were investigated and have both presented difficulties.


4. Using and Disseminating Knowledge.

The main channel for using and disseminating knowledge is through scientific publications, and communication to conferences. The scientific production of SCALA along the four years of the project is shown in the table below : it should be noticed that SCALA results have been disseminated by more than 500 invited talks and 1000 articles, including 200 “high impact” publications. Most of these results have been already exploited within the Project, as a way to achieve its objectives.


Year 1

Year 2

Year 3

Year 4


Letters in high-impact journals (Nature, Science, PRL)






Articles in international refereed journals (PRA etc)






Articles in Books and Proceedings






Preprints (arXiv)






Total Articles






Invited Talks in International Conferences






Miscelleanous Communications (oral or posters)






Seminars, Colloquium






Total Communications






The list of publications acknowledging SCALA has been published on this website. Click here

The other instruments which are used for disseminating knowledge are :

    • Through the industrial and commercial involvement of the partners, which are used to disseminate and exploit the spinoffs of the project in the best possible way.
    • Through project clustering and collaboration within the FET QIPC cluster, in close contact with the QIPC Coordination Actions “QUROPE”, especially for joint conferences. 
    • Through the web site The webpage is linked to the existing webpages of the individual partners, and it has a private part which has been used a communication tool between the partners, especially for preparing the reports.


    5. General assessment of the results in SCALA.

    Beyond the very large number of results and publications listed in sections 3 and 4, it is interesting to assess how well the general objectives listed in section 1 have been achieved. Without quoting again the groups names, which are spelled out in section 4, here are some general statements.

    A) Realize interconnected quantum gates and quantum wiring elements, which are required as building blocks of a general purpose quantum computer.
    This was essentially the objective of SP1 and SP2, which have met many success, among which :
    * very significant progress in manipulating single trapped material particles : single atoms on chips, in free space, in cavities, single ions in many kinds of traps.  All aspects of the “manipulation” have been considerably improved, especially for neutral atoms : writing a single qubit, reading it out, driving it with high fidelity, shuttling it, all this both in free space and in cavities. With trapped ions, record values in the fidelity of single qubit manipulations have been reached, e.g. 99.9%.
    * very significant progress in producing “tailored”, indistinguishable, single photons with high count rate, from either trapped atoms in free space, or from cavity QED systems with atoms or ions. This includes also the production of entangled photon pairs, or of entangled atom-photon systems, as well as using parametric photons as atomic probes. Interconnecting quantum gates seems in view.
    * in addition to optical photons, long-range Rydberg interactions have been used in several contexts : with high finesse microwave cavities (see below), but also without cavities, as a way to deterministically entangle two neutral atoms, as proposed by the Innsbruck group in 2000. These new experimental developments have stimulated many new theoretical ideas by SCALA theory groups, which turn out to be very important for new post-SCALA projects.
    * at the theory level, many results relative to quantum networks have been obtained, both on the more abstract side of quantum information theory, and on a more experiment-oriented side. In addition, as recommended by the referees during the negociation of the project, several new groups have significantly added to SCALA expertise and productivity in (quantum) computer science.

    B) Realize first approaches of "operational" quantum computing, which include
     (i) small-scale quantum algorithms, such as quantum error correction
    (ii) special-purpose quantum processors, such as quantum simulators
    (iii) entanglement-assisted metrology.

    This was essentially the objectives of SP3 and SP4, which have also met many success.
    * For objective B(i), two implementations were considered. For trapped ions, many very spectacular results were obtained, including the preparation of a “quantum byte” in a W state, QND measurements and elementary quantum error correction (with 3 qubits), composite (two-ions) “decohence-free” qubits, and many others. A second implementation is microwave photons in superconducting cavity, which has reached during SCALA an unprecedented level of control, based on super-long-lived cavities (0.1 s). This allowed to carry out repeated measurements and tomography of quantum states up to 7 photons, to prepare Fock states and Schrödinger cat states, and to study their decoherence. Although these experiments are not “quantum computing” as such, they deal with questions which are just the ones which will arise in a computer : influence of losses, of thermal excitations, possibility to apply quantum feedback, etc. One can thus conclude that objective B(i) has indeed be achieved.
    * Objective B(ii) was not strongly emphasized at the beginning of the project, but it has been “boosted” by the addition of new SCALA groups. It is probably the one where achievements go further beyond expectations, and one can tell that now the quantum simulator concept is essentially validated. A confirmation is that this subject now attracts the attention of “top-notch” solid-state physicists, who are actively collaborating with SCALA groups (see e.g. highlights in Year 4). Again, it is fair to say that objective B(ii) has been reached, and has also stimulated many more present developments.
    * Finally, the main results in objective B(iii) was obtained during Y2 and Y3, about ultra-precise measurements assisted by entanglement. Though this is quite significant, the general outcome seems here to remain below expectations. This is partly because some groups redirected their effort towards other goals, and other groups met unexpected difficulties. Clearly the issue of quantum metrology remains quite important, and post-SCALA projects will have to deal again with it.

    Overall, it seems fair to say that SCALA has reached its objectives, and has also contributed to develop new promising fields, such as quantum simulators, and “hybrid systems”, which appeared during the project without being an explicit objective. In the future, very large projects like SCALA may be harder to organize and to fund, but smaller projects will also be able to progress further towards the realisation of a scalable quantum computer, both on theoretical and on the experimental side.





    Scala - March 2010

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