Probing an electron-spin ensemble with squeezed microwave signals

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 gigahertz signals. We generate continuous-variable microwave states with squeezing levels up to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>5</a:mn> <a:mspace width="4pt"/> <a:mi>dB</a:mi> </a:mrow> </a:math> 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:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>C</c:mi> <c:mo>=</c:mo> <c:mn>0.3</c:mn> </c:mrow> </c:math> . By analyzing this signal using Wigner tomography and modeling our experimental results with a dedicated steady-state model based on the quantum input-output formalism, we extract a transfer efficiency of around <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mn>61</d:mn> <d:mspace width="0.16em"/> <d:mo>%</d:mo> </d:mrow> </d:math> between the squeezed microwaves and the spin excitations, resulting in squeezing levels of the spin mode up to 0.5 dB below the vacuum level.