Coherence, cooperation, and fluctuations in cavity electrodynamics

HERBERT WALTHER

Audrey Cottet, Takis Kontos, A. Levy Yeyati

We suggest a way to characterize the coherence of the split Cooper pairs emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying the radiative response of such a CPS inside a microwave cavity. The coherence of the split pairs manifests in a strongly nonmonotonic variation of the emitted radiation as a function of the parameters controlling the coupling of the CPS to the cavity. The idea to probe the coherence of the electronic states using the tools of cavity quantum electrodynamics could be generalized to many other nanoscale circuits.

S. Haroche

Hideo Mabuchi

S. Haroche, J. M. Raimond

Keenan Williams

We demonstrate that gate infidelity in cavity quantum electrodynamics (QED) systems follows a universal scaling law, ε = α/C + β/C², where C is the cooperativity. Analyzing experimental and validated simulated data spanning five orders of magnitude in cooperativity (C = 3 to 50,000) across four platforms—semiconductor quantum dots, atomic optical cavities, magnon-photon systems, and synthetic-dimension photonics—we find the leading coefficient α = 0.98 ± 0.03 is remarkably consistent, with cross-platform variation of only 33%. This universality arises from fundamental Purcell physics. The negative second-order coefficient (β ≈ −0.7) reveals systematic coherent corrections beyond simple 1/C scaling. We establish a hardware-agnostic design rule: fault-tolerant operation (ε < 10⁻³) requires C ≳ 1000 regardless of platform. This universal benchmark enables direct comparison of disparate quantum technologies and defines a clear target for next-generation cavity QED systems.