![]() ![]() The ground hyperfine levels a and b of 87Rb have angular momenta F a=1 and F b=2, and the upper and lower clock states are written as |+〉≡| b, m=0〉 and |−〉≡| a, m=0〉, respectively. ![]() We thereby take advantage of the magnetic insensitivity of the so-called clock transition, the energy of which depends only quadratically on the magnetic field strength. Here, we report achieving this goal by confining rubidium-87 atoms in an optical lattice of 25 μm period using the ground-state atomic hyperfine transition for storage. To demonstrate quantum memory lifetimes of many milliseconds, we must suppress atomic motion and use magnetically insensitive atomic coherence as the basis of the quantum memory. ![]() Millisecond storage of classical, coherent light has been reported in atomic gases 24, 25, 26, whereas coherence times in excess of 1 s have been achieved in the solid state 27. Ballistic expansion of the freely falling gas provides a longer memory time limitation, which can be estimated from the time τ= Λ/(2π v) ∼100 μs it takes an atomic spin grating to dephase by atomic motion (we use some representative parameters typical of the magneto-optical trap (MOT) environment: grating wavelength Λ=50 μm, atomic velocity for T=70 μK and rubidium mass M). In short, equally populated atomic states of opposite magnetization, ± m μ B, where m is the angular momentum projection and μ B is the Bohr magneton, respond asymmetrically to ambient fields 16, 17, 18, 19, 20, 21, 22, 23. The rubidium sample, prepared in a state of zero average magnetization, was allowed to freely fall during the protocol and the quantum memory time was limited by the effects of small uncompensated magnetic fields. The longest quantum memory time previously reported, 32 μs in a cold rubidium ensemble 6, is insufficient to carry out quantum repeater protocols over the distances where direct transmission fails. Such long-lived quantum memories could revolutionize deterministic single-photon sources 6 and lead to the generation of entangled states over extended systems 13.Įnhancing the matter coupling to a single spatial light mode is an advantage shared by cold optically thick atomic ensembles 14 and single atoms in high-finesse cavities 15. For L ∼1,000 km, required memory times vary from many seconds for a simple network topology 8, 9 to milliseconds for more complex (for example, multiplexed) topologies and architectures 10, 11, 12. The entanglement distribution rate of a network depends critically on the memory time of these storage elements. Entanglement distributed over these shorter links is then connected over length L according to a family of protocols generically known as the quantum repeater 8. The division circumvents attenuation in the fibre provided the intermediate memory nodes, which terminate the links, have a non-zero quantum memory time. To distribute entanglement over longer distances, the channel should be divided into links of length ≤ l. In practice, direct entanglement distribution over optical fibres is limited by absorption to distances l ∼100 km. For L=1,000 km, L/ c≈5 ms for an optical fibre. ![]() For the distribution of entanglement over a length L, the characteristic timescale for storage is the light travel time L/ c, where c is the speed of light in the medium. The generation of such remote entanglement must necessarily be done locally and distributed by light transmission over optical fibre links or through free space 7. Protocols for quantum communication are typically based on remote parties sharing and storing an entangled quantum state. ![]()
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