Finite-dimensional approximations of generalized squeezing

TL;DR

This paper investigates the effects of finite-dimensional truncations in simulating generalized squeezing, revealing parity-dependent behaviors due to non-self-adjoint Hamiltonians, and proposes Kerr interactions to achieve consistent, physically meaningful results.

Contribution

It demonstrates how adding Kerr interactions regularizes the Hamiltonian, ensuring self-adjointness and consistent simulation outcomes regardless of truncation parity.

Findings

Parity of truncation affects simulation results due to non-self-adjoint Hamiltonians.

Adding Kerr interactions makes the Hamiltonian self-adjoint, leading to consistent simulations.

Spectral analysis clarifies differences between even and odd truncation schemes.

Abstract

We show unexpected behaviour in simulations of generalized squeezing performed with finite-dimensional truncations of the Fock space: even for extremely large dimension of the state space, the results depend on whether the truncation dimension is even or odd. This situation raises the question whether the simulation results are physically meaningful. We demonstrate that, in fact, the two truncation schemes correspond to two well-defined, distinct unitary evolutions whose generators are defined on different subsets of the infinite-dimensional Fock space. This is a consequence of the fact that the generalized squeezing Hamiltonian is not self-adjoint on states with finite excitations, but possesses multiple self-adjoint extensions. Furthermore, we present results on the spectrum of the squeezing Hamiltonians corresponding to even and odd truncation size that elucidate the properties of the two different self-adjoint extensions corresponding to the even and odd truncation scheme. To make the squeezing operator applicable to a physical system, we must regularize it by other terms that depend on the specifics of the experimental implementation. We show that the addition of a Kerr interaction term in the Hamiltonian leads to uniquely converging simulations, with no dependence on the parity of the truncation size, and demonstrate that the Kerr term indeed renders the Hamiltonian self-adjoint and thus physically interpretable.