# Efficient representation of Gaussian states for multi-mode non-Gaussian quantum state engineering via subtraction of arbitrary number of photons

## TL;DR

This paper introduces a comprehensive formalism for representing multi-mode Gaussian states and their non-Gaussian derivatives via photon subtraction, enabling analytical characterization of state fidelity, heralding probability, and applications in quantum state engineering.

## Contribution

It develops a novel $K$ function formalism for multi-mode Gaussian states, allowing analytical analysis of non-Gaussian states generated by photon subtraction and heralding.

## Key findings

- Heralding probability expressed as a Hafnian, linking to Gaussian boson sampling.
- Analytical fidelity and success probability for non-Gaussian state preparation.
- Proposed method for near-perfect two-mode coherent-cat Bell state generation.

## Abstract

We introduce a complete description of a multi-mode bosonic quantum state in the coherent-state basis (which in this work is denoted as "$K$" function ), which---up to a phase---is the square root of the well-known Husimi "$Q$" representation. We express the $K$ function of any $N$-mode Gaussian state as a function of its covariance matrix and displacement vector, and also that of a general continuous-variable cluster state in terms of the modal squeezing and graph topology of the cluster. This formalism lets us characterize the non Gaussian state left over when one measures a subset of modes of a Gaussian state using photon number resolving detection, the fidelity of the obtained non-Gaussian state with any target state, and the associated heralding probability, all analytically. We show that this probability can be expressed as a Hafnian, re-interpreting the output state of a circuit claimed to demonstrate quantum supremacy termed Gaussian boson sampling. As an example-application of our formalism, we propose a method to prepare a two-mode coherent-cat-basis Bell state with fidelity close to unity and success probability that is fundamentally higher than that of a well-known scheme that splits an approximate single-mode cat state---obtained by photon number subtraction on a squeezed vacuum mode---on a balanced beam splitter. This formalism could enable exploration of efficient generation of cat-basis entangled states, which are known to be useful for quantum error correction against photon loss.