Coherent Spin Control at the Quantum Level in an Ensemble-Based Optical Memory

Abstract

Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.

Memory-assisted quantum key distribution resilient against multiple-excitation effects

“…Specifically, we suggest memory parameters that could be obtained with realistic experimental improvements to the memories of [34][35][36][37][38][39][40][41][42][43][44][45]. We attempt to be conservative with our suggested parameters, in particular with those of efficiency and coherence time, and acknowledge that there are fundamental limitations of some parameters, e.g.…”

“…Europium-doped Y 2 SiO 5 crystals are employed in the investigations of [41,42] while the well-studied Pr:Y 2 SiO 5 is featured in [44,45]. On-demand storage at the single photon level is shown in [41], in which dynamical decoupling techniques are also used to overcome dephasing due to spin inhomogeneous broadening. Reference [42] utilizes a low-finesse cavity to show (up to 50%) efficient and on-demand storage of strong pulses.…”

“…We consider the five atomic frequency comb experiments described in [41][42][43][44][45]. Europium-doped Y 2 SiO 5 crystals are employed in the investigations of [41,42] while the well-studied Pr:Y 2 SiO 5 is featured in [44,45].…”

“…The latter may differ in coherence time, efficiency, bandwidth and access time, reading and writing procedures, and operating wavelength for example. In particular, we consider a selection of state-of-the-art memories based on warm vapors at room temperature [34][35][36][37], cold ensembles of rubidium atoms [38][39][40], and cryogenically-cooled rare-earth-ion-doped crystals [41][42][43][44][45]. In the latter case, we utilize the possibility of spectral multi-mode storage [46] in such memories.…”

“…The corresponding quantum memory properties and key rates are shown in table 7 and figure 10, respectively. We employ the 151 Eu:Y 2 SiO 5 memory of [41], except that we assume perfect dynamical decoupling is in use to achieve a coherence time that is entirely limited by the ground-level homogeneous broadening (Eu+DD), and we employ the Pr:Y 2 SiO 5 crystal of [44,45] (Pr+DD) in a similar way. Even with perfect efficiency, we find that neither of the quantum memories overcome the bound, mainly due to their limited bandwidth in comparison to the Raman quantum memories.…”