High-Rate Four Photon Subtraction from Squeezed Vacuum: Preparing Cat State for Optical Quantum Computation

TL;DR

This paper demonstrates high-rate four-photon subtraction from a squeezed vacuum to generate large-amplitude Schrödinger cat states, overcoming previous limitations and advancing optical quantum computing capabilities.

Contribution

It introduces a method for high-speed, high-resolution four-photon subtraction from a broadband squeezed vacuum, enabling scalable generation of cat states for quantum computing.

Findings

Achieved four-photon subtraction at high rates with picosecond wavepackets.

Generated cat states exhibit Wigner negativity without loss correction.

Verified quantum coherence through off-diagonal density matrix elements.

Abstract

Generating logical qubits, essential for error detection and correction in quantum computation, remains a critical challenge in continuous-variable (CV) optical quantum information processing. The Gottesman-Kitaev-Preskill (GKP) code is a leading candidate for logical qubits, and its generation requires large-amplitude coherent state superpositions -- Schr\"{o}dinger cat states. However, experimentally producing these resource states has been hindered in the optical domain by technical challenges. The photon subtraction method, a standard approach for generating cat states using a squeezed vacuum and a photon number-resolving detector, has proven difficult to scale to multi-photon operations. While the amplitude of the generated cat states increases with the number of subtracted photons, limitations in the generation rate have restricted the maximum photon subtraction to $n=3$ for over a decade. In this work, we demonstrate high-rate photon subtraction of up to four photons from a squeezed vacuum with picosecond wavepackets generated by a broadband optical parametric amplifier. Using a Ti-Au superconducting-transition-edge sensor, we achieve high-speed, high-resolution photon number discrimination. The resulting states exhibit Wigner function negativity without loss correction, and their quantum coherence is verified through off-diagonal density matrix elements in CV representation. These results overcome long-standing limitations in multi-photon operations, providing a critical foundation for generating quantum resources essential for fault-tolerant quantum computing and advancing ultrafast optical quantum processors.