Optimal control of an optical quantum memory based on noble-gas spins

Abstract

In Ref. [Katz et al., arXiv:2007.08770 (2020], we present a mechanism and optimal procedures for mapping the quantum state of photons onto an optically inaccessible macroscopic state of noble-gas spins, which functions as a quantum memory. Here we introduce and analyze a detailed model of the memory operation. We derive the equations of motion for storage and retrieval of non-classical light and design optimal control strategies. The detailed model accounts for quantum noise and for thermal atomic motion, including the effects of optical mode structure and imperfect anti-relaxation wall coating. We conclude with proposals of practical experimental configurations of the memory, with lifetimes ranging from seconds to hours.

Optical quantum memory with optically inaccessible noble-gas spins

“…In Ref. [22], we derive the equations of the atomic operators using the Bloch-Heisenberg-Langevin model. While the full equations are stochastic, the memory efficiency and bandwidth are governed by the deterministic part [23],…”

“…We search for the temporal shapes of Ω(t), δ(t) and δ s (t) that maximize the storage efficiency η = K † K /N , where K † K is the number of noble-gas excitations after storage. From here on, we focus on the relevant regime of strong atom-photon coupling γ p CT 1, where the optical dipole P follows Êin and Ŝ and can be adiabatically eliminated [12,22]. We can then safely take ∆ = 0 while maintaining P † P 1.…”

“…(2-4) with an input optical signal Ê(t) † Ê(t) = A 2/T e (t−T )/T for −2T ≤ t ≤ T , where A guarantees the pulse contains a single photon. We then iterate on Ω(t), δ(t), and δ s (t) using the gradient ascent method [22]. The optimization calculations for given J, γ s , and T are done with no relaxation of the noble-gas spins γ k = 0 (since γ k ≪ 1/T ) and with cooperativity C 1, yielding the ideal storage efficiency η ∞ .…”

Coupling light to a nuclear spin gas with a two-photon linewidth of five millihertz

“…A large time-bandwidth product usually requires temporal variations of external classical fields. Indeed, variations of the magnetic and control fields can be done during the reading and writing processes to increase the bandwidth and, during the memory time, to decouple the noble-gas spins from the alkali and consequently reduce its decoherence rate down to γ b [35,36].…”

“…Since alkali spins do couple to light, the pickup of the NMR signal can be done optically, and in this way narrow spectra and long-lived spin precession signals are routinely obtained [28][29][30][31]. Yet, various quantum optics applications require an efficient bi-directional coupling between light and noble-gas spins [32][33][34][35][36]. Such coupling, corresponding to a resonant optical excitation of the long-lived nuclear spins, has never been realized.…”