Observation of Einstein-Podolski-Rosen correlations between microwave and optical photons

Entanglement is 'the characteristic trait’ [1] that distinguishes quantum mechanics from previous theories of physics. Most remarkable, it enables correlations between subsystems that are stronger than what is classically allowed and it is the essential resource behind scaling advantages in emerging quantum technologies. Today, entanglement between similar systems such as photons, ions, atoms, electronic spins or superconducting circuits is routinely generated, detected and used for basic quantum information processing and quantum communication tasks. However, a full-fledged development of such technologies will make it necessary to share this important resource also across very dissimilar physical platforms. Quantum transducers between microwave and optical photons would offer such a capability by coherently interfacing superconducting quantum processors with optical photons for transmitting quantum information over large distances. However, despite significant experimental progress, the ubiquitous trade-off between low conversion efficiencies and added classical noise have so far prevented the observation of genuine non-classical correlations in such devices. Here we report on the deterministic preparation of entanglement between microwave and optical fields in the continuous variable domain. We achieve this in a millikelvin environment using a triply resonant electro-optical transducer that is operated in pulsed mode to minimize added noise [2]. The resulting entangled state is squeezed by 0.7 dB below the vacuum level and violates the EPR bound for classical correlations. This demonstrates the feasibility of transducing genuine quantum correlations across vastly different frequency scales, with wide-ranging implications for quantum technology applications.

[1] E. Schrödinger, Discussion of probability relations between separated systems, Mathematical Proceedings of the Cambridge Philosophical Society, 31, 4 (1935)

[2] Rishabh Sahu, et al., Quantum-enabled operation of a microwave-optical interface, Nature Commun. 13, 1276 (2022)