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Below are some Frequently Asked Questions and answers in connection to the quantum memory results published on 25th November 2004 in nature 432, 482 (2004).

Q: Please briefly describe in simple terms what you have accomplished.

A: Communications are performed with light pulses. When a light pulse arrives to its destination is has to be received and stored. Standard (classical) receivers - detectors of light inevitably add noise that spoils the signals. This noise is quantum; it was predicted already by Bohr and Heisenberg 80 years ago. This quantum noise is small, and until recently has been of little importance. Nowadays with ever increasing speed of communications and with the new - quantum - type of communications on the horizon, the quantum noise of classical receivers and of classical memory becomes increasingly important. We have demonstrated that a new type of memory - quantum memory using atoms - can work with much less noise than any classical memory could ever achieve. From the fundamental physics perspective we have, for the first time, demonstrated that two complementary (immeasurable) variables of light (amplitude and phase) can be transferred onto matter with higher-than-classical fidelity.

Q: How does this work differ from recent light-atomic ensemble systems?

A: This is the first realization of memory for light which works: 1. Deterministically (not probabilistically as in previous work). 2. With fidelity higher than any classical memory can achieve. Hence, according to the claim in the paper, this is the first realization of quantum memory for light, i.e., the memory which works for an unknown state of light with 100% efficiency and fidelity higher than the best classical memory fidelity.

Q: What are the practical applications, and what remains to be done to achieve them?

A: First of all it is fundamental research. Applications may be found in quantum information processing, as memory for quantum computers, as interface in quantum networks, and as an eavesdropping technique for quantum cryptography. However, also in classical communications this memory, with the noise less than that of classical memory, can prove useful as a buffer for light. Next steps towards applications will be 1. Demonstrate that this memory can work for entangled and single photon states of light, 2. Demonstrate the inverse transfer of an atomic state onto light, 3. Increase the speed (presently maximum 1 kHz repetition rate, 4. Possibly reduce the dimensions of the memory device and find a solid state implementation.

Q: Considering only technological issues, how soon could practical applications be possible?

A: 5-15 years from now.

Q: What is the source of funding for this research?

A: The research is funded by the Danish National Research Foundation and by European grants within Quantum Information networks.