Demonstration of measurement-free universal fault-tolerant quantum computation

TL;DR

This paper demonstrates a universal set of fault-tolerant quantum operations on a trapped-ion processor without mid-circuit measurements, enabling practical, measurement-free quantum algorithms like Grover's search.

Contribution

It introduces and experimentally validates a measurement-free fault-tolerant quantum computation method using logical state teleportation and coherent gate operations.

Findings

Fault-tolerant logical state teleportation demonstrated without measurements.

Universal gate set realized on an eight-qubit error-detecting code.

Grover's algorithm implemented fault-tolerantly on three logical qubits.

Abstract

The ability to perform quantum error correction (QEC) and robust gate operations on encoded qubits opens the door to demonstrations of quantum algorithms. Contemporary QEC schemes typically require mid-circuit measurements with feed-forward control, which are challenging for qubit control, often slow, and susceptible to relatively high error rates. In this work, we propose and experimentally demonstrate a universal toolbox of fault-tolerant logical operations without mid-circuit measurements on a trapped-ion quantum processor. We present modular logical state teleportation between two four-qubit error-detecting codes without measurements during algorithm execution. Moreover, we realize a fault-tolerant universal gate set on an eight-qubit error-detecting code hosting three logical qubits, based on state injection, which can be executed by coherent gate operations only. We apply this toolbox to experimentally realize Grover's quantum search algorithm fault-tolerantly on three logical qubits encoded in eight physical qubits, with the implementation displaying clear identification of the desired solution states. Our work demonstrates the practical feasibility and provides first steps into the largely unexplored direction of measurement-free quantum computation.