# Multiparametric Amplification and Qubit Measurement with a Kerr-free Josephson Ring Modulator

## TL;DR

This paper introduces a novel Josephson Ring Modulator design that nulls higher order Hamiltonian terms, enabling a bi-directional, broadband, phase-sensitive amplifier for superconducting qubit readout.

## Contribution

A new linearly shunted Josephson Ring Modulator with minimized 4th-order terms and a combined parametric drive scheme for enhanced amplification and bandwidth.

## Key findings

- Achieved nearly null 4th-order Hamiltonian terms at a specific flux bias.
- Demonstrated a bi-directional, phase-sensitive amplifier with large, gain-independent bandwidth.
- Successfully used the amplifier for superconducting qubit readout with high quantum efficiency.

## Abstract

Josephson-junction based parametric amplifiers have become a ubiquitous component in superconducting quantum machines. Although parametric amplifiers regularly achieve near-quantum limited performance, they have many limitations, including low saturation powers, lack of directionality, and narrow bandwidth. The first is believed to stem from the higher order Hamiltonian terms endemic to Josephson junction circuits, and the latter two are direct consequences of the nature of the parametric interactions which power them. In this work, we attack both of these issues. First, we have designed a new, linearly shunted Josephson Ring Modulator (JRM) which nearly nulls all 4th-order terms at a single flux bias point. Next, we achieve gain through a pair of balanced parametric drives. When applied separately, these drives produce phase-preserving gain (G) and gainless photon conversion (C), when applied together, the resultant amplifier (which we term GC) is a bi-directional, phase-sensitive transmission-only amplifier with a large, gain-independent bandwidth. Finally, we have also demonstrated the practical utility of the GC amplifier, as well as its' quantum efficiency, by using it to read out a superconducting transmon qubit.