Realization of Constant-Depth Fan-Out with Real-Time Feedforward on a Superconducting Quantum Processor

TL;DR

This paper demonstrates a superconducting quantum fan-out gate with real-time feedforward on up to four qubits, showing potential for scaling to larger systems and improving quantum algorithm efficiency.

Contribution

It introduces a constant-depth quantum fan-out gate with real-time feedforward on superconducting qubits, enabling scalable quantum circuit depth reduction.

Findings

Achieved quantum fan-out with real-time feedforward on four qubits.

Extrapolated potential scaling advantage beyond 25 qubits with feedforward.

Benchmarking via quantum state tomography confirms gate performance.

Abstract

When using unitary gate sequences, the growth in depth of many quantum circuits with output size poses significant obstacles to practical quantum computation. The quantum fan-out operation, which reduces the circuit depth of quantum algorithms such as the quantum Fourier transform and Shor's algorithm, is an example that can be realized in constant depth independent of the output size. Here, we demonstrate a quantum fan-out gate with real-time feedforward on up to four output qubits using a superconducting quantum processor. By performing quantum state tomography on the output states, we benchmark our gate with input states spanning the entire Bloch sphere. We decompose the output-state error into a set of independently characterized error contributions. We extrapolate our constant-depth circuit to offer a scaling advantage compared to the unitary fan-out sequence beyond 25 output qubits with feedforward control, or beyond 17 output qubits if the classical feedforward latency is negligible. Our work highlights the potential of mid-circuit measurements combined with real-time conditional operations to improve the efficiency of complex quantum algorithms.