A Cavity-Enhanced Room-Temperature Broadband Raman Memory

Abstract

Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali vapour Raman memories combine high-bandwidth storage, on-demand read-out, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and suffered from fourwave mixing noise. Here we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering anti-resonance for the anti-Stokes field, we also suppress the four-wave mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapour memory, (15 ± 2) × 10 −3 photons per pulse, with a total efficiency of (9.5 ± 0.5)%. Quantum information technologies such as quantum key distribution and random number generators are beginning to transition into the commercial sphere, where key requirements are the ability to function at high speed, and in non-laboratory settings. The high carrier frequency of optical signals enables photonic quantum devices to operate noise-free at room temperature whilst offering GHz-THz operational bandwidths. However, direct photon-photon interactions are prohibitively weak, which has held back the development of photonic quantum processors. One solution to this problem has been to use probabilistic measurement-induced non-linearities [1], but the probability of success decreases exponentially with system size, limiting the state of the art to < 10 photons [2]. Further scaling photonic devices requires a multiplexing strategy. Quantum memories capable of storing photons and releasing them on-demand provide the ability to temporally multiplex a repeat-until-success architecture to achieve a freely scalable photonic quantum information platform operable at room temperature.There are many types of quantum memory for light under development: electromagnetically induced transparency [3], the full atomic-frequency comb protocol [4,5], gradient-echo memories [6,7], and the far-offresonant Raman memory [8,9]. Each of these protocols have advantages and challenges, see Ref.[10] for a recent review. A helpful metric for temporal multiplexing is the time-bandwidth product B = τ δ, with δ the acceptance bandwidth and τ the memory lifetime. B is the maximum number of time-bins over which a memory can synchronise an input signal. Time-bandwidth products of B > 1000 enable a dramatic enhancement in the multiphoton rate from parametric photon sources [11], as required to utilise multiphoton interference for computational speed-up (e.g. boson sampling [12]). Large timebandwidth products have been achieved with cold atom memories [13,14], cryogenic rare earth ion memories [15] and room-temperature Raman memories [8,16]. In this paper we present an implementation of an alkali-vapour Raman memory, operating at 75• C, and show the lo...

Practical decoy-state BB84 quantum key distribution with quantum memory*

“…[7] Despite the technology of QM is not mature yet, the characteristics of QM have been greatly improved, such as long storage times, [8,9] high fidelities, [10][11][12] and high storage efficiencies. [13] More importantly, QM can work at room temperature, [14][15][16] which makes it more applicable than before. Recently, works on QKD with QM have been reported.…”