# Quantum Gates for Propagating Microwave Photons

## TL;DR

This paper presents a scheme for implementing transmission-type quantum gates for propagating microwave photons using superconducting components, enabling arbitrary single-qubit gates and potential photon interactions for microwave quantum computing.

## Contribution

It introduces a tunable phase shifter using SQUIDs on transmission lines and proposes replacing SQUIDs with qubits for nonlinear interactions, advancing microwave quantum information processing.

## Key findings

- Demonstrated a broad-range, tunable phase shifter with full transmission.
- Showed that replacing SQUIDs with qubits enables strong nonlinearity and photon interactions.
- Paves the way for an all-microwave quantum computer based on propagating photons.

## Abstract

We report a generic scheme to implement transmission-type quantum gates for propagating microwave photons, based on a sequence of lumped-element components on transmission lines. By choosing three equidistant superconducting quantum interference devices (SQUIDs) as the components on a single transmission line, we experimentally implement a magnetic-flux-tunable phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, these phase shifters can be utilized to implement arbitrary single-qubit gates. Furthermore, we theoretically show that replacing the SQUIDs by superconducting qubits, the phase shifter can be made strongly nonlinear, thus introducing deterministic photon--photon interactions. These results critically complement the previous demonstrations of on-demand single-photon sources and detectors, and hence pave the way for an all-microwave quantum computer based on propagating photons.