Quantum memory for light

Abstract

We propose an efficient method for mapping and storage of a quantum state of propagating light in atoms. The quantum state of the light pulse is stored in two sublevels of the ground state of a macroscopic atomic ensemble by activating a synchronized Raman coupling between the light and atoms. We discuss applications of the proposal in quantum information processing and in atomic clocks operating beyond quantum limits of accuracy. The possibility of transferring the atomic state back on light via teleportation is also discussed. 42.50.Lc, 42.50.Dv, 42.50.Ct, 06.30.Ft Light is an ideal carrier of quantum information, but photons are difficult to store for a long time. In order to implement a storage device for quantum information transmitted as a light signal, it is necessary to faithfully map the quantum state of the light pulse onto a medium with low dissipation, allowing for storage of this quantum state. Depending on the particular application of the memory, the next step may be either a (delayed) measurement projecting the state onto a certain basis, or further processing of the stored quantum state, e.g., after a read-out via the teleportation process. The delayed projection measurement is relevant for the security of various quantum cryptography and bit commitment schemes [1]. The teleportation read-out is relevant for full scale quantum computing.In this Letter we propose a method that enables quantum state transfer between propagating light and atoms with an efficiency up to 100% for certain classes of quantum states. The long term storage of these quantum states is achieved by utilizing atomic ground states. In the end of the paper we propose an atom-back-to-light teleportation scheme as a read-out method for our quantum memory.We consider the stimulated Raman absorption of propagating quantum light by a cloud of Λ atoms. As shown in the inset of Fig.1, the weak quantum field and the strong classical field are both detuned from the upper intermediate atomic state(s) by ∆ which is much greater than the strong field Rabi frequency Ω s , the width of an upper level γ i and the spectral width of the quantum light Γ q . The Raman interaction "maps" the non-classical features of the quantum field onto the coherence of the lower atomic doublet, distributed over the atomic cloud.In our analysis we eliminate the excited intermediate states, and we treat the atoms by an effective two-level approximation. We start with the quantum Maxwell-Bloch equations in the lowest order for the slowly varying operatorQ:Q =σ 31 e −i(ωq−ωs)t+i(kq −ks)z (it will be assumed, that (k q − k s )L ≪ 1, where L is the length of the atomic cloud, z is the propagation direction, and ω q,s and k q,s are frequencies and wavevectors of "quantum" and "strong" fields respectively) [2,3]Γ is the dephasing rate of the 1 ↔ 3 coherence which also includes the strong field power broadening Γ s ≃ ω 3h κ 2 1 |E s | 2 /(3c 3 ) due to spontaneous Raman scattering [2],F (z, t) is the associated quantum Langevin force with correlation function F * (z,...

Towards high-speed optical quantum memories

“…In a Raman memory, the bandwidth is generated dynamically by ancillary write/read pulses, which dress the narrow atomic resonances to produce a broad virtual state to which the signal field couples. The off-resonant nature of the scheme confers some appealing features 7,8,27 . The first, as already mentioned, is the ability to store broadband pulses.…”

Teleportation and spin squeezing utilizing multimode entanglement of light with atoms

“…The collective spin of the ensemble is then probed by an off resonant pulse which propagates along z and is linearly polarized along x. Thorough descriptions of this interaction and the final state of light and atoms after the scattering can be found in [14,15,16,17,18] and especially in [19,20,21,22] for the specific system we have in mind. We derive the final state here with a special focus on the effects of Larmor precession and light propagation in order to identify the light modes which are actually populated in the scattering process.…”

“…More detailed descriptions can be found in [14,15,16,17,18,19,20,21]. The Hamiltonian of the system is given by H = H at + H li + V where the atomic part H at = Ω i F (i)…”

Experimental demonstration of quantum memory for light

“…3. Other proposals for quantum memory for light with better-than-classical quality of recording have also been published recently [1][2][3][4] .…”