Experimental Realization and Characterization of Stabilized Pair Coherent States

TL;DR

This paper reports the experimental creation and detailed characterization of stabilized pair coherent states in superconducting cavities, advancing quantum error correction and quantum optics with new reservoir engineering and measurement techniques.

Contribution

It demonstrates the first experimental realization of pair coherent states and introduces quantum subspace tomography for high-dimensional state measurement.

Findings

Successfully stabilized pair coherent states in superconducting cavities.

Developed quantum subspace tomography for direct state measurement.

Identified cross-Kerr interaction as a key dephasing channel.

Abstract

The pair coherent state (PCS) is a theoretical extension of the Glauber coherent state to two harmonic oscillators. It is an interesting class of non-Gaussian continuous-variable entangled state and is also at the heart of a promising quantum error correction code: the pair cat code. Here we report an experimental demonstration of the pair coherent state of microwave photons in two superconducting cavities. We implement a cross-cavity pair-photon driven dissipation process, which conserves the photon number difference between cavities and stabilizes the state to a specific complex amplitude. We further introduce a technique of quantum subspace tomography, which enables direct measurements of individual coherence elements of a high-dimensional quantum state without global tomographic reconstruction. We characterize our two-mode quantum state with up to 4 photons in each cavity using this subspace tomography together with direct measurements of the photon number difference and the joint Wigner function. We identify the spurious cross-Kerr interaction between the cavities and our dissipative reservoir mode as a prominent dephasing channel that limits the steady-state coherence in our current scheme. Our experiment provides a set of reservoir engineering and state characterization tools to study quantum optics and implement multi-mode bosonic codes in superconducting circuits.