Efficient and long-lived quantum memory with cold atoms inside a ring cavity

Abstract

Quantum memories are regarded as one of the fundamental building blocks of linear-optical quantum computation [1] and long-distance quantum communication [2]. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived [3,4] or long-lived but inefficient quantum memories [5][6][7] have been demonstrated so far. Here we report a high-performance quantum memory in which long lifetime and high retrieval efficiency meet for the first time. By placing a ring cavity around an atomic ensemble, employing a pair of clock states, creating a longwavelength spin wave, and arranging the setup in the gravitational direction, we realize a quantum memory with an intrinsic spin wave to photon conversion efficiency of 73(2)% together with a storage lifetime of 3.2(1) ms. This realization provides an essential tool towards scalable linearoptical quantum information processing.A high-performance quantum memory is of crucial importance for large-scale linear-optical quantum computation[1], distributed quantum computing, and long-distance quantum communication [2]. The lifetime and the retrieval efficiency of a quantum memory are two important quantities that determine the scalability of realistic quantum information protocols. For a certain quantum information task, e.g. creating a large-scale cluster state [8] or distributing entanglement through the quantum repeater protocol [9-12], the time overhead T r is inversely proportional to a power law of the retrieval efficiency R, T r ∝ R −n , where n is determined by the scale of the quantum computation or the communication distance. In order to implement one of those tasks, the lifetime of the quantum memory must be larger than this time overhead. To satisfy this condition, one has to improve the lifetime of the quantum memory and reduce the time overhead by improving the retrieval efficiency. Besides, different protocols also set thresholds on the retrieval efficiency and lifetime. For example, in loss-tolerant linear-optical quantum computation the minimum retrieval efficiency required is 50% [13] and in long-distance quantum communication distributing en-tanglement over 1000 km requires a communication time of at least 3.3 ms.Quantum memories for light have been demonstrated with atomic ensembles [14][15][16], solid state systems [17,18], and single atoms [19]. With these quantum memories, the principle of some quantum information protocols have been demonstrated, e.g., functional quantum repeater nodes were realized with atomic ensembles [20,21]. However, due to the low retrieval efficiency and short lifetime, the implementation of further steps is extremely difficult. Therefore, in recent years, many efforts have been devoted towards improving the retrieval efficiency and the lifetime of the quantum memories and significant progress has been achieved. However, an efficient and long-lived quantum memory remai...

An efficient quantum light–matter interface with sub-second lifetime

“…For the quantum memory parameters (lifetime + efficiency), we consider three representative results in the single quantum regime with cold atomic ensembles, which includes the result of 0.2 s + 16% by Radnaev el al. in 2010 [12], the result of 3.2 ms + 73% by Bao et al in 2012 [16] and the current result of 0.22 s + 76% in this paper. We calculate the generation rate of a pair of two-photon entanglement over distance L for the three groups of memory parameters.…”

“…So far, storage lifetime has been improved to the sub-second regime for single excitations in an atomic ensemble [12,14] and to the sub-minute regime for classical light storage [15], nevertheless, storage efficiency in these experiments is typically very low (∼16%). If we define a threshold of 50% for the memory efficiency, the longest storage time is limited to 1.2 ms [16], which is far away from the second regime requirement [6] of a quantum repeater. Memory efficiency is essentially important as it intervenes in every entanglement swapping operation between adjacent quantum repeater nodes.…”

“…The cavity has a finesse of F = 52, and a linewidth of 9.2 MHz. In our previous experiments [16], the cavity was intermittently stabilized to avoid influence from the locking beam on to the memory. In order to obtain a high retrieval efficiency for long storage durations, we have developed the cavity stabilization technique with large detuning of atomic ransitions.…”

Pure Photons for Quantum Communications

“…This free-space retrieval efficiency, relevant for most applications, exceeds those observed in previous similar experiments where efficient polariton-tophoton conversions were undermined by outcoupling or filtering losses [18,20].…”

“…The measured characteristic time τ ≈ 900 ns is in very good agreement with the Doppler time for a Maxwell-Boltzmann distribution of atoms at T ¼ 50 μK (τ D ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi m=k B T p =2k, where k is the wave vector of 795 nm light and m is the atomic mass of 87 Rb). This Doppler decay could, in principle, be reduced by placing the atoms in a light shift compensated optical lattice [21] or by increasing the wavelength of the atomic spin wave using an appropriate beam configuration [20], which should not fundamentally impact the purity of the retrieved photon. Our model also accounts for explicit dependence on the read pulse profile and intensity.…”