ASSESSMENT OF CURRENT RESULTS AND OUTLOOK ON FUTURE EFFORTS
QUANTUM COMMUNICATION
QUANTUM INTERFACES AND MEMORY
An interface between quantum information carriers (quantum states of light) and quantum information storage and processors (atoms, ions, solid state) is an integral part of a full-scale quantum information system. In classical communication information is transferred encoded in pulses of light. The pulses are detected by photodetectors, transformed into electrical current pulses, amplified by electronics, and sent to computers, phones, etc. This transformation of light into electrical signals forms classical light-matter interface. In quantum information processing simple classical detection of light is inadequate for recording into memory, because it destroys the quantum state by adding extra noise to it. Hence a quantum interface has to be developed. Instead of direct transformation of light pulses into electrical pulses, as in classical communication, quantum state transfer of light qubits (or continuous variables) with atomic qubits (or continuous variables) has to be developed in QIPC. Certain kinds of quantum interfaces, based on cavity QED, are discussed in part 6.2 with an emphasis on computing tasks. Other kinds of quantum interfaces, such as quantum memory and long-distance quantum teleportation of long lived atomic states, are important for communication and quantum secret sharing tasks. It is obvious that long lived entanglement shared over a long distance requires transfer of entanglement from light (the long distance carrier) to atoms (the long lived objects). Such transfer can only be done via a special light-atoms quantum interface. Distant long lived entangled objects can serve as secure "quantum identification cards". These kinds of tasks can be address via such physical implementations as atomic ensembles, which are easier to implement and to scale.
Currently various aspects of light-atoms quantum interface and memory are investigated mainly by the European groups at Copenhagen University (E. Polzik); University of Aarhus, Denmark (K. Molmer and M. Drewsen); Max Planck Institute for Quantum Optics, Garching, Germany (I. Cirac and G. Rempe); Institute for Photonic Sciences, Barcelona, Spain (M. Mitchel); University of Kaiserslautern, Germany (M. Fleischhauer); University of Heidelberg, Germany (J. Schmiedmayer); and Lab Kastler Brossel, CNRS, Paris (M. Pinard and E. Giacobino). In the US this research is primarily carried out at Harvard University (M. Lukin); Caltech (J. Kimble), University of Michigan, Ann Arbor (Ch. Monroe); and Georgia Institute of Technology, Atlanta (A. Kuzmich).
Quantum memory for light and quantum repeaters
For coherent pulses used in classical communications, a classical approach via simple detection limits the fidelity of the memory to 50%. For non-classical states the fidelity of the classical memory is even lower. Classical communications where weak pulses of light of different colors are sent in parallel (frequency multiplexing) approach quantum limits exponentially with time (at today's pace it will be reached by 2020). Hence new - quantum - approaches to memory have to be considered for both quantum and classical communications.
State of the art: Proposals for quantum memory for light have been put forward during the past decade, in Europe and in the US. Recently the first quantum memory for a weak coherent pulse has been demonstrated in Europe [1]. A quantum memory which is to be used for storage, and not for quantum processing, is based on a simple physical system consisting of a small cell filled with atomic gas at room temperature - an atomic ensemble. Demonstrated quantum state storage time of up to 4 msec corresponds to propagation time over a distance of about 1000 km. The storage cell works close to the free space propagation wavelength.
Visions and perspectives: Quantum memory provides a stored version of quantum cryptography and quantum secret sharing (in the long run, counterfeit proof bank cards, etc). It also poses a potential threat to quantum cryptography via more efficient eavesdropping protocols, and hence has to be taken seriously in quantum communication security issues. Quantum memory for light provides a necessary ingredient for quantum networks, as discussed in the next section. Future work on quantum memory based on the atomic ensemble approach should be concentrated on
- Extending memory capabilities to single photon/qubit storage.
- Achieving efficient retrieval of the stored quantum state.
- Improving the fidelity of storage.
- Quantum error correction necessary for achieving extra long storage times.
- Memory micro-cell arrays for multi-channel storage including quantum image storage - quantum holograms.
- Exploring other types of atomic/solid state ensembles useful for storage applications; solid-state system such as those used for slow light experiments are potentially suitable for quantum memory and should be investigated.
- Developing probabilistic repeater schemes possibly integrated using atoms on chip technology.
European labs are presently ready to tackle each of the above strategic challenges.
Long distance atomic teleportation and repeaters
State of the art: Atomic teleportation over a distance of a fraction of a millimeter has been recently demonstrated by two groups, in Europe and in the US. Long distance teleportation of atomic states requires interface with light. A significant progress has been achieved on the way towards implementation of a repeater primarily by US groups [Monroe, Kimble and Kuzmich]. Entanglement of atomic ensembles at a distance of half a meter has been demonstrated in Europe [2]. The technology is simple and relies on glass cells filled with atomic gas at room temperature. At present the technology is limited to near infrared wavelength suitable for free space propagation.
Vision and perspectives: Long distance deterministic teleportation will allow realization of distributed quantum networks. Extension of entanglement of atomic ensembles to up to a kilometer is possible with specially designed optical setups. For yet longer distances quantum repeaters proposed in Europe present an option. Towards this goal a combination of a repeater with entangled trapped ions will be useful. Another possible way to realize an efficient repeater is to use atomic ensemble quantum memory [1] to store one photon of an entangled pair produced by downconversion. The repeater approach may allow teleportation of atomic states over many kilometers.
Challenges and directions of future work are similar to those listed for quantum memory, i.e.:
- Extending memory capabilities to single photon/qubit storage.
- Achieving efficient retrieval of the stored quantum state.
- Improving the fidelity of storage.
- Exploring other types of atomic/solid state ensembles useful for storage applications; solid-state system, atoms on a chip.
Key research groups in Europe have been shown in the beginning of the section.
Key references
[1] B. Julsgaard, J. Sherson, J.I. Cirac, J. Fiurášek, and E.S. Polzik, Experimental demonstration of quantum memory for light, Nature 432, 482 (2004).
[2] B. Julsgaard, A. Kozhekin, and E.S. Polzik, Experimental long-lived entanglement of two macroscopic objects, Nature 413, 400 (2001).
