Abstract
We describe a multi-mode quantum memory for propagating microwave photons that combines a solid-state spin ensemble resonantly coupled to a frequency tunable single-mode microwave cavity. We first show that high efficiency mapping of the quantum state transported by a free photon to the spin ensemble is possible both for strong and weak coupling between the cavity mode and the spin ensemble. We also show that even in the weak coupling limit unit efficiency and faithful retrieval can be obtained through time reversal inhomogeneous dephasing based on spin echo techniques. This is possible provided that the cavity containing the spin ensemble and the transmission line are impedance matched. We finally discuss the prospects for an experimental implementation using a rare-earth doped crystal coupled to a superconducting resonator.
Highlights
A spin ensemble is placed in a single-ended cavity, where we assume that the center of the inhomogeneous spin line of width is in resonance with a cavity mode
We have presented a memory for propagating photons in the microwave regime that combines an inhomogeneously broadened spin ensemble, microwave π pulses and a superconducting lowloss cavity
We have shown that even in the weak coupling regime, the memory efficiency can reach unity if the transmission line impedance matches the spin ensemble embedded in the cavity
Summary
We first describe the steps allowing storage and retrieval before we analyze in detail the memory properties. An input microwave photon is absorbed by the cavity-spin ensemble system, resulting in a single collective spin excitation. The π1 pulse inverts the spin population and associated to this inversion is a source of spontaneous emission noise. It has been shown, in the optical regime, that the resulting echo at 2τ1 would be a lowfidelity copy of the input pulse [32, 33]. The application of a second rephasing pulse denoted π2, delayed by τ1 + τ2 with respect to π1, ideally reverts the population, while rephasing the collective spin coherence after a total storage time of τM = 2(τ1 + τ2). Note that the echo technique makes the protocol inherently multi-mode, i.e. several temporal modes can be absorbed during the time interval τ1 without the need to increase the absorption depth of the ensemble nor the finesse of the cavity
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