Protecting the Quantum Interference of Cat States by Phase-Space Compression

Abstract

Cat states, with their unique phase-space interference properties, are ideal candidates for understanding fundamental principles of quantum mechanics and performing key quantum information processing tasks. However, they are highly susceptible to photon loss, which inevitably diminishes their quantum non-Gaussian features. Here, we protect these non-Gaussian features against photon loss by compressing the phase-space distribution of a cat state. We achieve this compression with a deterministic technique based on the echoed conditional displacement operation in a circuit QED device. We present a versatile technique for creating robust non-Gaussian continuous-variable resource states in a highly linear bosonic mode and manipulating their phase-space distribution to achieve enhanced resilience against photon loss. Such compressed cat states offer an attractive avenue for obtaining new insights into quantum foundations and quantum metrology, as well as for developing inherently more protected bosonic code words for quantum error correction.

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Plain Language Summary

Schrödinger’s cat states—superpositions of macroscopically distinct states—are great candidates for exploring the fundamental ideas of quantum decoherence and for developing continuous-variable (CV) quantum sensing and quantum computing. However, the unique characteristics of cat states are highly fragile, as they arise from inherently quantum mechanical interference effects that can be easily destroyed by decoherence. Here, we experimentally demonstrate the protection of these quantum interference features in cat states by manipulating their phase-space distribution.

We realize bosonic cat states in a superconducting cavity, which is designed to be a nearly ideal quantum harmonic oscillator. The dominant error channel is photon loss, which corresponds to a loss of high-frequency elements in the characteristic function, a convenient spectral representation of quantum states. These elements also encode valuable quantum interference information. To evade this loss, we compress the cat state using a deterministic gate-based protocol to lower the frequencies of these interference features. Our experiment shows that the quantum interference of compressed cat states is significantly more robust against photon loss compared to normal cat states.

Our results bring invaluable insights into investigating intrinsic dynamics of quantum interference in CV quantum states. Our technique also offers a versatile tool for enhancing the noise resilience of CV quantum states and opens new possibilities for quantum metrology and fault-tolerant quantum computing with bosonic codes.