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Quantum Information Processing & Communications
Projects contracted in the 5th Framework Programme (1998-2002)
FET web site with links to all FP5 project fact sheets
Presentations of Success stories & Project Objectives and success stories in a nut shell per project
as given at the QIPC Cluster review 14-16 February 2005, Innsbrück (AT)
Project web sites
It is with a deep sadness and regret that we notify the loss of our colleague
who passed away on Saturday 9 February 2008. We have lost a highly valued colleague and friend and she will be greatly missed by all of us.
Presentations of Success stories & Project Objectives and success stories in a nut shell per project
as given at the QIPC Cluster review 14-16 February 2005, Innsbrück (AT)
- Presentation of success story part 1 ( PDF, 1862 KB )
- Presentation of success story part 2 ( PDF, 1841 KB )
- Project objectives and success story in a nut shell
CECQDM: FET project fact sheet
MAGQIP: FET project fact sheet
NSP-SI: FET project fact sheet
Q-ACTA: FET project fact sheet
QIPD-DF: FET project fact sheet
QIPDDF-ROSES: FET project fact sheet
REQC HARDWARE: http://www-atom.fysik.lth.se/photonEcho/Esquire/index.html
SQID: FET project fact sheet
The main objectives for the ACQP collaboration in the next year will focus on three topics, which will bring atom chip technologies together to establish the toolbox for QIPC. The main goal is directed towards the implementation of a quantum gate on the atom chip.
- Achieving fast and high fidelity detection of single qubit states on an atom chip. We will employ integrated micro optics and micro cavity detectors to state-selectively detect single atoms. Our current detection limit is in the 1-3 atom range, improvements currently being implemented should allow us to detect single atoms with high fidelity.
- Further develop techniques for qubit loading and state (qubit)-selective atom micro manipulation. These will include developing further our atom chip fabrication capabilities, and implementing new tools on the chip, like RF or MW manipulation of atoms.
- Developing atom chip devices which are capable of two qubit operations, and implementing entangling operations on the atom chip.
All there main objectives are central to the QIPC initiative. The Atom Chip is a micro fabricated, integrated devices in which electric, magnetic and optical fields confine, control and manipulate cold atoms as qubits. Through miniaturization, atom chips offer a versatile new technology for implementing the well established ideas in quantum optics and atomic physics in a robust way to develop tools for quantum measurement and quantum information processing.
Success story: Coherence in magnetic micro traps (paper in Phys. Rev. Lett. 92, 203005 (2004) by P. Treutlein, P. Hommelhoff, T. Steinmetz, T. W. Hänsch and J. Reichel)
Atom Chips are micro fabricated, integrated devices in which electric, magnetic and optical fields can confine, control and manipulate cold atoms. Through miniaturization, atom chips offer a versatile new technology for implementing modern ideas in quantum optics, quantum measurement and quantum information processing. One of the key promises of Atom Chips is, that all the advantages of atomic physics qubits remain valid, when controlled by micro fabricated and integrated devices (the Atom Chip). A central issue is, if the exceptional coherence properties of atomic qubits will still be valid if the atoms are trapped µm above the (room temperature) surface of the atom chip. This was demonstrated in a beautiful ACQP experiment carried out in Munich, which shows that long quantum coherence, similar to atomic clocks can be achieved between qubit states trapped on an atom chip. 87Rb atoms were used to encode the qubit states into the two hyperfine ground states |F=1, mF=-1> and |F=2, mF=1> which can be magnetically trapped on the chip. These states differ in energy by 6.8GHz, causing the phase of the superposition to oscillate at that frequency, representing a Rubidium atomic clock on a chip. The beat between the atomic oscillation and a stable reference clock was used to demonstrate the long coherence time (>2.8 s). In the experiment the long term stability of the qubit oscillation was not limited by decoherence, but by the quality of the reference oscillator.
This long coherence time became possible by the choice of the two trapped qubit states. The two chosen states have exactly the same coupling to magnetic fields, and the differential shifts (which cause the decoherence) cancel, similar to the 0-0 transitions used in atomic clocks. With the fabrication techniques developed by the ACQP collaboration, inhomogeneous broadening of such a clock transition in trapped 87Rb can be as low as 10-3 Hz, corresponding to dephasing times as long as 10 minutes.
The activity concerning the above described research lines fits well the ATESIT objectives whose main goal is to address the realization of new methods for an efficient generation and manipulation of various types of two- and multi-particle entangled states, to be used within some relevant quantum communication protocols. Some QI tasks can be realized by using these states. As an example, in the case of hyper-entangled states, the complete analysis of the four orthogonal Bell states with 100\% efficiency, a result otherwise impossible to achieve with standard linear optics, represents a fundamental tool for many QI objectives.
The current research within the ATESIT project is aimed also to demonstrate the scalability of fundamental protocols as quantum cloning, universal NOT gate and purification. Such experimental objectives will be reached within the end of the Project by increasing the complexity of the linear optics setup and the NL gain of the QI-OPA device.
The high brilliance parametric source realized in Rome represents one of the main points of interest of the ATESIT research activity. Because of its peculiar spatial characteristics and flexibility in terms of state generation it has been possible to generate by this source polarization entangled mixed states with tunable degree of mixedness (Werner states end maximally entangled mixed states). By this source it has been realized for the first time an entanglement witness method for entanglement detection. More recently, two photon states simultaneously entangled in polarization and linear momentum (hyper-entangled states), have been produced by the same system. The importance of these states in quantum information (QI) resides on the fact that they represent a way to overcome the intrinsic limit of SPDC where no more than one photon pair is created time by time within each microscopic annihilation-creation process.
Another point of interest is represented by the nonlinear Quantum Injected Optical Parametric Amplifier (QI-OPA) scheme, a unique nonlinear (NL) optics application for QI manipulations. In the present case a single photon stimulates the emission of pairs of photons by NL interaction with the pump beam. Such process has been adopted to realize the quantum analogues of two fundamental processes of classical information: the NOT gate and the cloning (copying) machine. A ''high gain'' version of this machine consists of a multi-particle, all-optical ''Schroedinger Cat'' structure. There, in virtue of the basic information-preserving character of the OPA amplification process, the quantum superposition character of a single photon qubit is transferred, in conditions of high intensity pump, onto a multiparticle quantum superposition, thus realizing a ''multi-particle qubit'' (M-qubit). A different approach which has been undertaken to manipulate QI is the combination of photonic states generated by different sources by exploiting basic linear optics components. Within the ATESIT project linear optics methods have been adopted to implement projection over the symmetric subspace (SSP) of polarization encoded qubits. By this technique the protocol of quantum cloning, tele-UNOT and purification have been carried out.
The current EDIQIP objectives includes the further tests and developments of PAREC method with applications to various quantum algorithms. Its possible implementations in quantum computers operating at the groups of R.Blatt (Innsbruck) and D.Cory (MIT) will be worked out. The effects of dissipative decoherence will be investigated on the basis of quantum trajectories method for the quantum saw-tooth map and the Grover algorithm with up to 10-12 qubits. The universal fidelity decay law induced by dissipative effects should be determined and compared with the effects of static imperfections. New quantum algorithms for models of quantum chaos, random walks in complex networks will be developed. We will study the quantum chaos border induced by static imperfections in a solid state quantum computer based on atoms implemented in a solid state matrix with magnetic field gradient and dipole-dipole couplings between qubits. The numerical codes will be developed for simulations of the dissipative decoherence, noise in quantum gates and static imperfections. These codes will be made available to the public access via the EDIQIP web site. The global universal laws for accuracy bounds in quantum computations will be determined for dissipative decoherence, noise random errors in quantum gates and static imperfections errors.
Quantum computers, relying on the weird logics of the quantum world, can be much faster than any classical machine, since they perform, so to speak, all the calculations at once. However, their construction is rather challenging, since they are prone to errors and decoherence. The EDIQIP project addresses an important question: what is the influence of static imperfections in computer hardware (parasitic coupling between qubits, imperfections in qubit energies). It determined a universal law, based on random matrix theory invented by Wigner for complex nuclei and atoms, for the computation fidelity decay induced by internal static imperfections. The theoretical predictions are confirmed by extensive numerical simulations of a polynomial quantum algorithm for quantum chaos in a dynamical map with up to 18 qubits. The decay law provides a transition between exponential and gaussian behaviours and is characterized by two time-scales analogous to the Thouless and Heisenberg times for probability decay in mesoscopic systems. These studies establish the universal accuracy bounds for quantum computation in presence of residual static couplings between qubits. They open a link between random matrix theory and realistic quantum computations. They are essential to estimate the individual gates fidelity required for a large scale quantum computation. In addition the project developed a generic Pauli random error correction method (PAREC) which allows to suppress significantly the effects of static errors. Without redundancy PAREC illuminates rapid gaussian decay and gives a parametric improvement of fidelity. In numerical tests with 10 qubits it gives the fidelity increase by two orders of magnitude with only five percents increase of the number of quantum gates.
The overall objective of the ESQUIRE project is to show that rare-earth-ion-doped inorganic crystals (RE crystals) are strong solid state candidates for quantum computing. The aim is to reach this objective by:
- Demonstrating single qubit operations and two-qubit quantum gates of good fidelity in RE crystals. This work includes the development of schemes and techniques for robust operations on the ensembles of ions in the qubit for qubit preparation and control.
Demonstrating that RE crystal quantum computing can be scalable by
a. Growing new crystal materials specially designed for quantum gate operations
b. Developing schemes and architectures for RE crystal quantum computers usingmaterial parameters that are consistent with, or that could be achieved based on, the experimental results in 1 and the properties of the crystals developed in 2a.
This is consistent with the FP5 QIPC Work Programme objective to "develop novel systems for information processing and transmission by exploiting the properties of quantum mechanical operations" and the medium term goal to "develop elementary but scalable quantum processors"
The four approaches A-D below are followed in order to reach the project objectives.
- A. A brute force method is used by taking existing rare-earth-ion-doped crystal materials and experimentally demonstrate simple quantum gates in these materials.
- B. Appropriate laser pulse excitation techniques for interacting with the ions in the crystal, using light pulses in a controlled way in order to minimise accumulated errors and improve the fidelity of the gate operations are developed.
- C. New RE crystal materials specially designed for supporting scalable quantum operations are grown.
- D. Novel schemes for QC in RE crystals which can be scalable are developed.
The ESQUIRE project aims at constructing quantum computer hardware from rare-earth-ion-doped inorganic crystals (slide 1). The rare earth ions are doped into the crystal sitting at random positions (slide 2). Different ions absorb light at different frequencies and a frequency tuneable laser beam is used for addressing different ions and for carrying out operations on the ions. Qubits consist of an ensemble of ions which all absorb light around some arbitrarily selected frequency. Quantum gate operations require that qubits can control each other. Slide 3 shows one qubit (CONTROL) controlling the absorption frequency of another qubit (TARGET). The upper part of slide 3 shows the target qubit when the ions in the control qubit are in their ground state. Here laser pulses can be applied for carrying out operations on the ion ensemble constituting the target qubit. In the lower part the control qubit is in the excited state and as can be seen the target qubit does not any longer absorb at its original frequency and pulses applied at the (normal) target absorption frequency should have no effect on the target qubit. As stated each qubit consists of an ensemble of ions. Still all ions in the ensemble must have the same wave function during the qubit operations. It turns out that pulses can be designed that are remarkably robust for differences in transition strength, Rabi frequency, detuning from the qubit centre frequency etc. Slide 4 illustrates that pulses yielding quantum operation fidelities of 0.9999 can be designed using optimal control theory. A remarkable finding is that the transition strengths between different levels in the crystal to some extent can be controlled by magnetic fields. Slide 5 shows how the orientation of a magnetic field can be used to change the transition strength. Finally some selected results are briefly summarised in slide 6.
The project ProSecCo aims at developing new distributed quantum applications for tasks which cannot be realized classically. Protocols for secure computations evaluate a function which depends on local inputs of the participants. The protocols ensure the correctness of the results and do not expose inputs which should remain secret. Starting with a quantum security model incorporating realistic threats and faults, we develop methods for building complex secure protocols from basic primitives. We aim to identify where security can be guaranteed by physics and, where it cannot, to identify reliable technological or computational security assumptions. We develop new primitives and new applications which involve entangled states and investigate their practicability in the presence of noise.
Everyday examples are authentication, voting schemes, or online auctions. The project will develop new distributed quantum applications and analyse their security against quantum faults/attacks. Such protocols will probably be among the first applications of quantum technology. A further scientific motivation is that new quantum protocols as well as new impossibility theorems will elucidate the borderline between tasks which are possible in a quantum world, but impossible classically and tasks which are completely impossible. A key question.
Secure message transfer is one of the main applications of quantum cryptography. Security means here that the message should remain secret and unaltered even in presence of an attacker.
However, security in the above sense is not necessarily preserved under composition and it is difficult to design secure protocols from secure primitives, as can be seen from the worlds first quantum bank transfer, which was broken within the project.
A classical notion of security, called universal composability, has been generalised to quantum protocols and quantum attackers. The composition theorem which allows for modular design of secure protocols has been proven for the quantum case and the important primitive of quantum key exchange could be shown to be universally composable in this sense.
A new security proof for a classical protocol for secure message transfer which makes use of an ideal key exchange has been given. And a theorem has been proven which states when quantum primitives can be used within classical protocols without lowering the security.
Putting all the results together we obtain a complete proof for a secure message transfer protocol which is itself securely composable.
This result was obtained by a collaboration between the projects PROESECCO, QUPRODIS, and RESQ.
We are working towards the demonstration of elementary quantum processors, using a range of technologies involving individually addressed ions or neutral atoms, and cavity QED methods. Special attention is being given to elementary scalability. An exciting new direction currently being explored by a number of our ion trapping groups is the development of miniature ion trapping structures that consist of interconnected traps. One of our top priorities is to study the coherent transport of ions in these structures. In the meantime we will continue to work with our existing ion traps performing novel coherent operations and testing their fidelity. Some of our groups working with neutral atoms have already demonstrated the ability to work with single atoms in their systems. Other groups are close to achieving this important milestone and it is their objective to reach this point in the coming year. The top priority over the coming year for the groups already manipulating single neutral atoms is to demonstrate entanglement in their experiments. In our Cavity QED experiments we intend to capitalize on our newly developed ultra-high-Q open microwave cavity to perform a range of experiments with entangled Rydberg atoms. We are also developing a superconducting atom chip for the production of individual Rydberg atoms on demand. The next priority for a number of other groups within the consortium is to use an optical cavity to mediate entanglement between neutral atoms or ions. Our project integrates theoretical work with experimental work and a very wide range of topics is currently under investigation. In particular we will extend our very successful work on the design of programmable quantum processors and we will also continue the development of methods of coherent control of single trapped atoms in cavity fields using open system dynamics and quantum-trajectories techniques.
A fundamental law of quantum mechanics states that it is impossible to ?clone? a quantum state i.e. starting, for instance, with a single atom in a particular unknown quantum state it is impossible to create other atoms in exactly the same state, leaving the original atom unaffected. It is therefore quite surprising that it does turn out to be possible to ?teleport? a quantum state provided the original atom?s quantum state is destroyed in the process. In this process the unknown quantum state of one atom can be ?copied? onto another atom by a series of quantum operations involving entanglement, with some classical communication mixed in to the recipe. In the language of information theory teleportation of a quantum state involves the complete transfer of information from one particle to another. In a genuine milestone experiment from the QGATES project the group led by Rainer Blatt at the University of Innsbruck have demonstrated deterministic quantum-state teleportation between a pair of trapped calcium ions. Following closely the original proposal by Bennett et al., they create a highly entangled pair of two ions and perform a complete Bell-state measurement involving one ion from this pair and a third source ion. State reconstruction conditioned on this measurement is then performed on the other half of the entangled pair. Clearly, to test their protocol the input state cannot actually be unknown so instead they prepare the first ion in one of a set of four nonorthogonal test states. To obtain directly the fidelity of the teleportation, they perform on the target ion the inverse of the unitary operation used to create the input state of the source ion. The obtained teleportation fidelities range from 73% to 76% for all test states, demonstrating unequivocally the quantum nature of the process.
The primary objective of the project has been to develop a suitable single qubit readout technique for a spin-based solid-state qubit implementation. Such a readout technology would greatly assist a variety of solid-state quantum information processor implementations, including endohedral fullerene as well as some quantum dot and doped crystal, implementations. It is known that ensemble quantum information processing (NMR quantum computing), is not scalable unless one can achieve very highly polarized initial states. Further, the synchronization and scalability issues related to ensemble solid-state quantum computing makes the path forward less clear than in the case of "so-called", single-issue quantum processors (where there is only one quantum processor with its associated qubit readout technology). Thus the development of a single qubit readout technology for any viable quantum processor technology is vital. This project has shown that an optical readout technique (Nitrogen-Vacancy defect in Diamond), has virtually unique features in its capabilities to store/manipulate/magnetically couple and optically reset and readout spin-based solid-state qubits. The further development of this type of readout will allow not only Buckyball based quantum processors, but a number of other types of solid-state processors to avail of this qubit.
A previous project (QIPDDF), showed that certain molecules called endohedral fullerenes (a Buckyball of 60 Carbon atoms containing an implanted atom within it), possessed especially good properties needed to store and potentially manipulate quantum information in the electronic and nuclear spins of the implanted atom. The ROSES project's goal was to develop a technique to readout the qubit information stored in these molecules. A variety of single-spin detection techniques were examined but the primary success story is the project's development of an optical readout technique based on coupling a Nitrogen-Vacancy defect centre in ultra-pure Diamond magnetically into the Buckyball. In Slide 1 we summarize the relevant characteristics of the N-V defect, including its atomic level structure and crystal lattice configuration. The defect has the very desirable properties of combining a 2-level atomic system with very long coherence times (>200microseconds), which can be manipulated with microwave radiation, coupled magnetically to other spin systems, and optically read out via optical excitation and subsequent fluorescence with single-shot readout capability at low-temperature but with reasonable repetitive measurement cycles even at room temperature. We also show a possible schematic for the resulting endohedral fullerene/ NV-defect quantum computer device. On Slide 2 we present the experimental results of a controlled two-qubit conditional quantum gate between the 2-level electronic spin system of the NV-defect and a nearby Carbon 13 nucleus. In Slide 3 & 4 we present further more controlled quantum manipulations of the 2-level system of a single NV-defect by performing quantum process tomography. This maps out how the 2-level system is effected by decoherence at room temperature. This is the first execution of quantum process tomography for an individual solid-state 2-level system that we are aware of. Finally on Slide 5 we present the other major highlights of the project, in the development of near perfect purification of the Buckyball endohedral material, the experimental self- assembly of an array of nanoscopic ordered Buckyball wires within a molecular matrix and the self-assembley of single individual Buckyball wires (imaged by a scanning tunneling microscope). These material developments will be crucial in fabricating the ultimate device envisaged on Slide 1.
The QUPRODIS is a Working Group within FP5 which aims to support, enhances, and complement the research on quantum information processing and communication in distributed quantum systems by promoting and facilitating appropriate links between the QUPRODIS partner, between the QUPROIDS partner and other projects of the QIPC cluster, and the QUPRODIS partner and researcher/research-groups working in other fields.
The QUPRODIS main objective is the theoretical foundations of quantum distributed information processing, and the role of entanglement in this field. On the more applied side, the QUPRODIS investigates schemes for the solid-state implementation of quantum computation, and schemes for multi party ommunication. Currently the main activities are in the development of novel architectures, like quantum cellular automata, the theoretical study of nano-mechanical devices, and the cocenptual interfacing of entanglement theory with the quantum phase transitions in standard lattice models.
Success story: ?Entanglement in Nano Electro-Mechanical Systems?
In a publication ?Towards Quantum Entanglement in Nanoelectromechanical Devices? [Physical Review Letters 93, 190402 (2004)] the authors Eisert, Plenio, Bose and Hartley (Potsdam-London Collaboration) study arrays of mechanical oscillators in the quantum domain and demonstrate how the motions of distant oscillators can be entangled without the need for control of individual oscillators and without a direct interaction between them. The spatial dimensions, Q-factors, temperatures and decoherence sources are discussed in some detail. The autthors find a distinct robustness of the entanglement and discuss various schemes for its detection.
RESQ is an interdisciplinary theoretical project with partners from physics, computer science, and mathematics and statistics backgrounds. In its proposal RESQ addressed essentially all of the objectives put out in the above document, although its contribution could of course only be theoretical. Some of the main objectives of the project were to develop new protocols and algorithms for processing information at the quantum level, particularly in networks and distributed systems and in the presence of noise, to develop specific applications for small scale quantum systems, to devise tests of quantum devices, to further characterize quantum entanglement, and to further characterize the resources required to carry out quantum information processing. Important progress towards all these objectives, without exception, has been made. The project produced a very large number of scientific publications ?more than 70 in 2004 alone- which cover all the above aspects of QIPC and many others as well. In addition to its scientific objectives, the project had a cultural objective which was to help "close the persistent cultural gap between computer scientists and physicists/engineers" as observed by the European Commission in the above document. Towards reaching this goal the project has realized, approximately every 6 months, interdisciplinary workshops involving all partners of the project and a few external experts. These workshops have been a big success. The key quantitative result which testifies to this is the large and increasing number of scientific articles which scientists from different partners, often from different disciplines, co-authored. In summary RESQ has without doubt more than exceeded all the objectives it set out to realize, and will continue to do so during its last year.
After the discovery of quantum key distribution by Bennett and Brassard in 1984, there was much hope that quantum communication would also help for other cryptographic tasks, and in particular for mistrustful cryptography. This hope was apparently destroyed by no-go theorems by Mayers, and Lo and Chau concerning the impossibility of quantum bit commitment. But in 2004 important progress was made on these questions in project RESQ. The key advance in the new work which circumvents the no-go theorems is to consider commitment or flipping of many bits, rather than the commitment or flipping of a single bit. Thus we are concerned with the scenario in which two parties who do not trust each other want to generate a string of random bits; and the scenario in which one party wants to commit a string of bits to the other party. The first result in this direction was obtained by Barrett and Massar have devised a protocol for flipping a string of N bits for which they could prove that, even if one party is dishonest, the entropy of the string is greater than H > N ? N-a. Based on this work an experimental demonstration of provably secure string flipping was realised. Using another approach Buhrman, Christandl, Hayden, Lo, Wehner and Winter devised a protocol for string commitment in which a cheater can only affect O(logN) of the bits. This protocol can be used for a variety of other tasks like string flipping (with even better security than the protocol of Massar and Barrett), or zero knowledge proofs. In summary: up to now quantum cryptography was essentially restricted to Quantum Key Distribution. The work reported here shows for the first time that quantum communication may also have a major impact on mistrustful cryptography.
Ultimately RAMBOQ seeks to develop the technologies supporting elementary scalable quantum processors and robust optical links between separated quantum processors. It aims to do this by developing on recent novel schemes for efficient optical quantum computation using conditional linear logic. We have developed working cascadable CNOT and C-phase gates within the first 2 years of this project. We have also developed individual components, notably single photon sources, entangled sources and detectors to further develop this technology.
However, these gates are probabilistic, they can only work at most 50% of the time and thus do not scale well. We are thus trying to develop a full understanding of the theoretical limits to conditional linear logic, particularly gate efficiency and complexity. Theoretical work is now also exploring gates exploiting linear optics coupled to weak non-linearity which show promise for 100% success probability. We are also looking now at cluster state methods and will present first experimental demonstrations of cluster state generation and computing in the final project year.
We are also like to developing novel communications schemes and quantum networks relying on linear optics. These include
- teleportation and entanglement swapping over distance
- higher dimensional coding schemes with application to for quantum networks.
- multi-party quantum protocols based on multi-party entanglement and on single photon techniques.
Clearly these objectives tie in with the goals of the QIPC cluster. We are contributing strongly to the toolbox for scalable quantum information processing and addressing small scale applications through our work on quantum networks.
For linear optics quantum computation and quantum networks we need:
- High efficiency sources of single photons, entangled photons and interference between them.
- A scalable 2-qubit CNOT gate.
- High efficiency and long-distance teleportation and entanglement swapping.
Within the project we have made major strides towards these goals:
Sources: In the first two years of the RAMBOQ project we have developed a high efficiency source of single photons based on InAs quantum dots in pillar microcavities. These sources show spontaneous emission enhancement of a factor of four and produce pure state single photons. This has been demonstrated by an interference experiment (of the Hong-Ou-Mandel type) showing indistinguishability between successive single photon emissions.
Scalability of CNOT gates requires sources of ancilla photons in the form of entangled photon pairs. Such sources need to be bright, narrowband and single spatial mode. We have been developing various single waveguide sources of pair photons and this year succeeded in developing a source of picosecond photon pairs from photonic crystal fibre. This source is promising because we expect to be able to couple it efficiently to standard single mode fibre using conventional fusion splicing techniques.
Gates: A cascadable 2-qubit CNOT gate has been built and a fidelity of 0.8 has been measured. The gate design is based on the Franson teleportation gate and which was foreseen as possibly the simplest gate circuit in the initial project proposal. We have also made quantum phase gates and full Bell state analysers.
Quantum Networks: We have performed experiments demonstrating long distance teleportation and long distance entanglement swapping. The teleportation experiment was set up to teleport qubits across the river Danube with communication via the sewage under the river. The entanglement swapping experiment was done using fibre optic links with receiving stations separated by 2.2km of fibre.
Applications: In the area of applications we find that our work is already producing spin-outs. We have demonstrated the first worldwide bank transfer protected by entanglement-based QC and free-space distribution of entangled photon pairs over 7.8 km in an intra-city link.
? 4-photon path-entanglement for quantum metrology
? Complete Bell-state analysis of entangled states
Publications: We have published over 50 papers in the project so far. This has included 9 PRL and 2 Nature publications. Two of this years papers were selected as 2004 physics highlights by the UK Institute of Physics. Our achievements can be viewed on our website http://www.ramboq.net
The main objective of the SAWPHOTON project was to develop a new type of single photon on demand source. By combining a single electron generator based an a surface acoustic wave propagating through a 1-dimensional ballistic channel and a pn diode to absorb and recombine these electrons with semiconductor holes the main constituencies of such a source exist. The combination of these parts of the total SAWPHOTON device is notoriously difficult and not a success yet. The final SAWPHOTON device will however have some extraordinary properties, which is certainly worth a considerable effort. The frequency of single photon generation is high (>3 GHz); the emission is fully compatible with a vertical cavity using a Bragg mirror and the surface of the semiconductor, possibly yielding very high efficiency. It is also interesting to notice that the single electron in the SAW trough may have a particular spin and actually constitute a flying qubit, which in the recombination process is converted to another flying qubit, a polarized photon. A further fascination lies in the fact that the troughs can bring 2 or more electrons forward to the recombination and thus generate two entangled photon. This could add new perspective to the quantum cryptography of the single photon on demand source. These and also several simultaneous photons on demand emissions belong to the future, but are of considerable basic interest. Finally the manipulation of the spin properties of the single electron in the troughs using magnetic semiconductors or variations in the g-factor in the heterostructures is other aspects, which our device may profit from. The development of the SAWPHOTON device will continue, because the possible final applications are numerous.
Ballistic channels made by split gates or by trench etching have been one of the major preconditions for this project. The insert of figure 1 shows one of the long trench-etched constrictions formed in a high mobility 2-dimensional electron gas of a high mobility field effect transistor (HEMT) III-V heterostructure. The insert to the lower right shows the gate-voltage dependence of the conductance (here on a top-gated sample), where the 2-dimensional electron gas is depleted for gate voltages below about ?1 Volt. The main curve shows a blow up of the insert with a large number of conductance plateaus corresponding at each conductance step of 2e2/h, of an extra 1-dimensional channel entering the ballistic channel. Based on the high technology thus developed we have subsequently studied the effect of a surface acoustic wave formed by a 100-fingered f=3 GHz transducer. The surface acoustic wave has both mechanical and potential amplitude and each trough therefore in the ballistic channel forms a 0-dimensional moving quantum dot carrying an integer number of electrons through the ballistic channel. Our devices allows up to exactly 20 electrons entering through the channel for each trough leading to a quantized current of 20*e*f. The integer number of electrons must be dumped into a lateral p-n diode. The pn-diodes have also been perfected during our project in several different ways. Figure 3 shows a lateral pn-diode based on a from p-type modulation doping and a back gate forming an 2-dimensionaæ electron layer (by etching the acceptor away in that region, as shown. The figure shows the measured light intensity emitted in the pn diode by recombination. This is the first lateral pn diode based on million mobility 2-dimensional electron and ½ million 2-dimensional holes.
The SQUBIT-2 workplan for 2004 is focussed on two major objectives: 1) Reliable operation of single qubits, and 2) fabrication, testing and operation of 2-qubit circuits with charge-phase and flux quibts.
The first objective has been achieved with great success with the quantronium charge-phase qubit of CEA-Saclay, demonstrating accurate NMR-style quantum logic operations around the "magic point" that may serve as Hadamard and phase gates. The conclusion is that the CEA quantronium charge-phase single qubit and operation protocol already represent a "mature" basic qubit design that may serve as a basis for multi-qubit circuits. The same is true for the TU Delft 3-junction flux qubit design. These schemes involve operation with microwave pulses at the "magic" point.
The second objective is of course much more difficult, and represents a major step towards a scalable solid-state quantum computer. Concerning fabrication of two coupled qubits, this part of the objective has been achieved by TU Delft, CEA-Saclay and Chalmers. The objective of testing and elementary operation has so far been achieved by TU Delft for (i) a flux qubit coupled to a quantum oscillator (SQUID) and very recently to some extent also with (ii) two coupled flux qubits, demonstrating conditional spectroscopy as well as conditional excitation and manipulation of 4-level systems. Logic 2-qubit gate operations have not yet been achieved but represent the goal for 2005.
Qubit circits for quantum computers need to meet conditions for scalability and for implementing a universal set of 1- and 2-qubit gates. To achieve this, different qubit-qubit coupling schemes exist. One of these is to employ a ?bus-type? harmonic oscillator to couple two (or more) qubits. This approach has some similarity to a method used in ion trap schemes where the ion-ion coupling is mediated by the lowest phonon mode of the trap (sloshing mode).
We have implemented a first variety of this scheme using a flux qubit coupled to a harmonic oscillator. The qubit is our standard three Josephson junction flux qubit, and the harmonic oscillator is a DC SQUID with attached capacitor. The potential for this scheme is very large, as it provides a very local system. We have demonstrated for the first time the quantum dynamics of a solid state two-component system. Entangling a phase-coupled 2-level flux qubit and a SQUID-based harmonic oscillator, we have shown controlled and conditional Rabi flopping, employing the lowest four levels of the composite system.
The current objectives are:
- Implementations of optimal connections with parametric Hamiltonians
- Experimental proposal for detection of topological phases
- Schemes for manipulations of abelian and non-abelian anyons
- Proposal of a quantum interferometric setup for detecting quantum phases and topologically generated entanglement
TOPQIP fits the FET QIPC program in that it provides a clear example of a focused research action devoted to an emerging field still not mature for fully experimental realizations, but of manifest long term interest. The Project successfully brings together people from different countries in a coherent and entangled effort aimed at realizing the Project goals. Even though some aspects TOPQIP can be termed as visionary its outcomes should encourage experimentalist to analyze systems where the control and use of quantum geometrical as well as topological quantum phases, is most likely. This last aspect is going to be one of the major concern of the Project coordination over the last year of the TOPQIP lifetime.
Success Story: Spin-1/2 geometric phase driven by decohering fields
In this work we continued the analysis of geometric phases in open quantum systems within the Quantum jump approach we developed in
. We have started an analysis of the behaviour of this phase under a realistic two-level system interacting with a quantised environment. An analysis of the behaviour of geometric phase in such a scenario is presented in
. In this work the geometric phase of a spin-1/2 system driven by one and two mode quantum fields subject to decoherence is considered. There are some works that investigate the behavior of geometric phases under some typical errors sources like random classical fluctuations to the driving fields, as well as generic reservoirs acting in spin ½ evolutions. All of them consider the driving field as a classical system. However, any driving field is also a quantized system and, in most of the typical experimental situations, this quantum behaviour is relevant, especially when decoherence affects those fields. In fact, decoherence in the driving field may become critical, particularly when geometric phases are used to implement quantum protocols, like communication and computational ones. In Ref , we have studied the behaviour of the geometric phase of a spin 1/2 particle interacting with a driving magnetic field when this field is not only quantized but also subjected to decoherence. We analyzed the effect of decoherence of the driving field on both adiabatic and non-adiabatic evolutions of the spin 1/2 particle. We calculate Berry's phases for different interactions of spin 1/2 systems and decohering fields both in the adiabatic and non-adiabatic scenarios. Finally we point out the differences between these two situations and how this noise source compares to previously analyzed ones. This result has implications both in fundamental issues as well as in geometric quantum computation.
Ref  França Santos, I. Fuentes-Guridi, V. Vedral, Geometric phase in Open System Phys. Rev. Lett. (2003).
Ref  A. Carollo, M. Santos, I. Fuentes-Guridi, V. Vedral Spin-1/2 geometric phase driven by decohering fields,,Phys. Rev. Lett.(2004).