Quantum process tomography of continuous-variable gates using coherent states

TL;DR

This paper demonstrates the use of coherent-state quantum process tomography (csQPT) to fully characterize quantum gates in a bosonic mode, enabling detailed error analysis beyond the logical subspace.

Contribution

It introduces and applies csQPT to bosonic modes, allowing complete process characterization and improved understanding of error mechanisms in continuous-variable quantum gates.

Findings

Successfully reconstructed Kraus operators for a bosonic mode

Achieved detailed error mechanism analysis

Enhanced gate fidelity assessment

Abstract

Encoding quantum information into superpositions of multiple Fock states of a harmonic oscillator can provide protection against errors, but it comes with the cost of requiring more complex quantum gates that need to address multiple Fock states simultaneously. Therefore, characterizing the quantum process fidelity of these gates also becomes more challenging. Here, we demonstrate the use of coherent-state quantum process tomography (csQPT) for a bosonic-mode superconducting circuit. CsQPT uses coherent states as input probes for the quantum process in order to completely characterize the quantum operation for an arbitrary input state. We show results for this method by characterizing a logical quantum gate constructed using displacement and SNAP operations on an encoded qubit. With csQPT, we are able to reconstruct the Kraus operators for the larger Hilbert space rather than being limited to the logical subspace. This allows for a more accurate determination of the different error mechanisms that lead to the gate infidelity.