Down-conversion of a single photon as a probe of many-body localization

TL;DR

This paper demonstrates that many-body localization (MBL) prevents a single photon from down-converting into multiple lower-energy photons in a superconducting cavity, revealing MBL's spectral signatures and offering a new platform for fundamental studies.

Contribution

The study experimentally shows how MBL inhibits photon down-conversion, highlighting spectral fine structures that deviate from Fermi's Golden Rule predictions in a superconducting cavity.

Findings

MBL causes a spectral fine structure of multi-photon resonances.

Photon down-conversion is suppressed by MBL, contrary to classical expectations.

The experiment provides a new platform to study MBL without complex many-body control.

Abstract

Decay of a particle into more particles is a ubiquitous phenomenon to interacting quantum systems, taking place in colliders, nuclear reactors, or solids. In a non-linear medium, even a single photon would decay by down-converting (splitting) into lower frequency photons with the same total energy, at a rate given by Fermi's Golden Rule. However, the energy conservation condition cannot be matched precisely if the medium is finite and only supports quantized modes. In this case, the photon's fate becomes the long-standing question of many-body localization (MBL), originally formulated as a gedanken experiment for the lifetime of a single Fermi-liquid quasiparticle confined to a quantum dot. Here we implement such an experiment using a superconducting multi-mode cavity, the non-linearity of which was tailored to strongly violate the photon number conservation. The resulting interaction attempts to convert a single photon excitation into a shower of low-energy photons, but fails due to the MBL mechanism, which manifests as a striking spectral fine structure of multi-particle resonances at the cavity's standing wave mode frequencies. Each resonance was identified as a many-body state of radiation composed of photons from a broad frequency range, and not obeying the Fermi's Golden Rule theory. Our result introduces a new platform to explore fundamentals of MBL without having to control many atoms or qubits.