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arXiv:2512.17490 · Physical Review A

Probing an electron spin ensemble with squeezed microwave signals

P. Oehrl, F. Fesquet, K. E. Honasoge, M. Handschuh, A. Marx, R. Gross, K. G. Fedorov, H. Huebl

quant-phcond-mat.mes-hallSubmitted Dec 19, 2025• Announced Dec 22, 2025• 0 citations

Abstract

The efficient transfer of quantum states into a long-lived storage unit such as solid-state spin ensembles is widely recognized as a critical challenge with significant implications for quantum communication, sensing and computing applications. Here, we experimentally investigate the interaction of propagating squeezed microwaves with an electron spin resonance transition in order to evaluate the use of spin ensembles as quantum memories for GHz signals. We generate continuous variable microwave states with a squeezing of up to 5dB below the vacuum level and let this signal interrogate a spin ensemble, which is inductively coupled to a lumped element superconducting microwave resonator with a cooperativity of C=0.3. Analyzing this signal using Wigner tomography, we observe a transfer efficiency of around 61% between the squeezed microwaves and the spin excitation. We successfully model our experimental results with a dedicated steady-state model based on the quantum input-output formalism and provide guidance for design parameters required to enable spin-based quantum memories.

Comments: 11 pages, 7 figures

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We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of

{10}^{10}

nitrogen-vacancy centers in a diamond crystal coupled to a superconducting

LC

resonator. The energy of the signal, retrieved $100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$ later by spin-echo techniques, reaches

0.3%

of the energy absorbed by the spins. This

0.3%

storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits.