Emission of photon multiplets by a dc-biased superconducting circuit

TL;DR

This paper demonstrates the controlled emission of photon multiplets from a superconducting circuit, showing that specific bias voltages produce exactly k photons per tunneling event, with detailed experimental and theoretical analysis.

Contribution

It introduces a superconducting circuit that emits photon bunches at specific voltages, with quantitative agreement between RWA theory and experiments for multiple photon emissions.

Findings

Photon bunches emitted at specific bias voltages $V_k$

Emission intensity matches RWA predictions for $k=1$ to 6

Photon bunching characterized by Fano factor $F_k=k$ at low $E_J$

Abstract

We observe the emission of bunches of $k \geqslant 1$ photons by a circuit made of a microwave resonator in series with a voltage-biased tunable Josephson junction. The bunches are emitted at specific values $V_k$ of the bias voltage, for which each Cooper pair tunneling across the junction creates exactly k photons in the resonator. The latter is a micro-fabricated spiral coil which resonates and leaks photons at 4.4~GHz in a measurement line. Its characteristic impedance of 1.97~k$\Omega$ is high enough to reach a strong junction-resonator coupling and a bright emission of the k-photon bunches. We show that a RWA treatment of the system accounts quantitatively for the observed radiation intensity, from $k=1$ to $6$, and over three orders of magnitude when varying the Josephson energy $E_J$. We also measure the second order correlation function of the radiated microwave to determine its Fano factor $F_k$, which in the low $E_J$ limit, confirms with $F_k=k$ the emission of $k$ photon bunches. At larger $E_J$, a more complex behavior is observed in quantitative agreement with numerical simulations.