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arXiv:2609.16435

Exact Work Characteristic Functions for Time-Dependent Bosonic Quadratic Hamiltonians

F. Nicacio, R. N. P. Maia

quant-phSubmitted Sep 14, 2026• Announced Sep 16, 2026• 0 citations
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Abstract

We derive an exact analytical expression for the characteristic function of work in closed many-body bosonic systems governed by general time-dependent quadratic Hamiltonians. This is made possible by adopting a unified prescription in which all operators appearing in the characteristic function are treated as elements of the inhomogeneous Metaplectic group. The esulting expression involves a single symplectic matrix, a single phase-space displacement, and a single accumulated phase. It holds for any number of degrees of freedom, arbitrary time dependence of the quadratic Hamiltonian, and any initial thermal equilibrium state of the system. The formalism trades the explicit time-ordering problem associated with the unitary evolution for the determination of a symplectic path in phase space, from which the corresponding driving Hamiltonian can be reconstructed. We also discuss the generalization to arbitrary nonequilibrium initial states within the two-projective-measurement scheme for quantum work. The same construction yields Loschmidt echoes and mixed-state total phases. Using the derived characteristic function, we obtain formulas for the Helmholtz free energy, the Jarzynski equality, the mean work, the work variance, and the nonequilibrium lag. Undisplaced Hamiltonians, sudden quenches, constant-Hessian protocols, and the adiabatic limit follow as special cases of the general result. As an illustration, we analyze a one-dimensional system constructed from an unconventional symplectic path and exhibiting physically relevant limiting cases. We then show how affine and purely symplectic contributions affect work statistics and irreversibility.

Comments
25 pages, 11 figures

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We derive an exact analytical expression for the characteristic function of work in closed many-body bosonic systems governed by general time-dependent quadratic Hamiltonians. This is made possible by adopting a unified prescription in which all operators appearing in the characteristic function are treated as elements of the inhomogeneous Metaplectic group. The esulting expression involves a single symplectic matrix, a single phase-space displacement, and a single accumulated phase. It holds for any number of degrees of freedom, arbitrary time dependence of the quadratic Hamiltonian, and any initial thermal equilibrium state of the system. The formalism trades the explicit time-ordering problem associated with the unitary evolution for the determination of a symplectic path in phase space, from which the corresponding driving Hamiltonian can be reconstructed. We also discuss the generalization to arbitrary nonequilibrium initial states within the two-projective-measurement scheme for quantum work. The same construction yields Loschmidt echoes and mixed-state total phases. Using the derived characteristic function, we obtain formulas for the Helmholtz free energy, the Jarzynski equality, the mean work, the work variance, and the nonequilibrium lag. Undisplaced Hamiltonians, sudden quenches, constant-Hessian protocols, and the adiabatic limit follow as special cases of the general result. As an illustration, we analyze a one-dimensional system constructed from an unconventional symplectic path and exhibiting physically relevant limiting cases. We then show how affine and purely symplectic contributions affect work statistics and irreversibility.
The aim of this chapter is to construct the quantum unitary propagator for Hamiltonians which are quadratic in position and momentum with time-dependent coefficients. We show that the quantum evolution is exactly solvable in terms of the classical flow which is linear. This allows to construct the metaplectic transformations which are unitary operators in L2(Rn)L^2(\mathbb R^n)L2(Rn) corresponding to symplectic transformations. Simple examples of such metaplectic transformations are the Fourier transform, which corresponds to the symplectic matrix J defined in (3.4) and the propagator of the harmonic oscillator, corresponding to rotations in the phase space. The main results of this chapter are computations of the quantum evolution operators for quadratic Hamiltonians acting on coherent states. We show that the time evolved coherent states are still Gaussian states which are recognized to be squeezed states centered at the classical phase space point (see Chap. 8). From these computations we can deduce most of properties concerning quantum quadratic Hamiltonians. In particular we get the explicit form of the Weyl symbols of the metaplectic transformations. These formulae are generalizations of the Mehler formula for the harmonic oscillator. Quadratic Hamiltonians are very important in quantum mechanics because more general Hamiltonians can be considered as non trivial perturbations of time dependent quadratic ones as we shall see in Chap. 5.
In the context of nonequilibrium quantum thermodynamics, variables like work behave stochastically. A particular definition of the work probability density function (pdf) for coherent quantum processes allows the verification of the quantum version of the celebrated fluctuation theorems, due to Jarzynski and Crooks, that apply when the system is driven away from an initial equilibrium thermal state. Such a particular pdf depends basically on the details of the initial and final Hamiltonians, on the temperature of the initial thermal state, and on how some external parameter is changed during the coherent process. Using random matrix theory we derive a simple analytic expression that describes the general behavior of the work characteristic function G(u), associated with this particular work pdf for sudden quenches, valid for all the traditional Gaussian ensembles of Hamiltonians matrices. This formula well describes the general behavior of G(u) calculated from single draws of the initial and final Hamiltonians in all ranges of temperatures.
The dynamical correlations of nonlocal operators in general quadratic open fermion systems is still a challenging problem. Here we tackle this problem by developing a new formulation of open fermion many-body systems, namely, the characteristic function approach. Illustrating the technique, we analyze a finite Kitaev chain with boundary dissipation and consider anyon-type nonlocal excitations. We give explicit formula for the Green's functions, demonstrating an asymmetric light cone induced by the anyon statistical parameter and an increasing relaxation rate with this parameter. We also analyze some other types of nonlocal operator correlations such as the full counting statistics of the charge number and the Loschmidt echo in a quench from the vacuum state. The former shows clear signature of a nonequilibrium quantum phase transition, while the later exhibits cusps at some critical times and hence demonstrates dynamical quantum phase transitions.
The method of Weyl transforms is used to rigorously derive path integral forms for position and momentum transition amplitudes from the time-dependent Schrödinger equation for arbitrary Hermitian Hamiltonians. It is found that all paths in phase space contribute equally in magnitude, but that each path has a different phase, equal to 1/h/ times an ’’effective action’’ taken along it. The latter is the time integral of p⋅q̇−h (p,q), h (p,q) being the Weyl transform of the Hamiltonian operator H, which differs from the classical Hamiltonian function by terms of order h/2, vanishing in the classical limit. These terms, which can be explicitly computed, are zero for relatively simple Hamiltonians, such as (1/2M)[P−eA (Q)]2+V (Q), but appear when the coupling of the position and momentum operators is stronger, such as for a relativistic spinless particle in an electromagnetic field, or when configuration space is curved. They are always zero if one opts for Weyl’s rule for forming the quantum operator corresponding to a given classical Hamiltonian. The transition amplitude between two position states is found to be expressible as a path integral in configuration space alone only in very special cases, such as when the Hamiltonian is quadratic in the momenta.
Polynomial excitations consisting of creation and annihilation operators are studied. Subsystem entanglement is proven produced and preserved by such manipulations. NPT entanglement already present between any two subsystems of a general pure or mixed multipartite Gaussian state is preserved under all polynomial excitations. Nevertheless, these excitations can create NPT entanglement between initially separable subsystems, implying that the number of NPT entangled pairs cannot decrease. Polynomial excitations of a wide class of initially separable pure states are considered. For these states, the method reduces the infinite-dimensional positivity problem to a finite-dimensional one: after partial transposition, NPT entanglement is equivalent to a corresponding bilinear representation matrix not being positive semi-definite. Furthermore, using symplectic diagonalization and spectral decomposition, an explicit transformation is constructed between the Fock-space and position representations. It is applied to quadratic excitations of a three-mode mixed Gaussian state. The behavior of the NPT entanglement is tracked between subsystems as a function of a parameter κκκ, which tunes the non-locality of the excitation, and quantum negativity is evaluated. Under a chosen quadratic Hamiltonian, the resulting unitary time evolution maintains NPT entanglement for long intervals, interrupted only by short periods of PPT states. Finally, a closed analytical expression is derived for the covariance matrix of a general quadratic operator for an arbitrary number of degrees of freedom in phase space. Our framework directly connects mathematical foundations, theoretical modeling, and experimentally feasible implementations.
We have analyzed Dyson's UUU-matrix theory of solving the Schr\"odinger equation in the interaction picture and are able to express the UUU matrix as a dominant term plus an infinite series involving multiple integrals of time. For a certain rather restrictive class of Hamiltonians, our theory is exact for a general time-dependent problem. For other Hamiltonians, we can only obtain approximate expressions for our UUU matrix and hence the wave function. Treating a time-independent problem as a special case of the time-dependent situation with a sudden-switching process, we have shown that our UUU matrix is exact. To demonstrate the working procedures of our theory, we apply it to study the well-known time-independent charged harmonic-oscillator problem and the more general harmonic oscillator with a time-dependent driving force. Compared with other methods, our new theory appears to lead to a result which contains more information than others due to the inclusion of noncommutability properties of operators in the operator Schr\"odinger equation. It has been shown that the classical Feynman path-integral formalism can be deduced from quantum mechanics with the use of the Green's-function operator. It is interesting to note that apart from a step function, the Green's-function operator is the same as that of our U(s){U}^{(s)}U(s) matrix, which is the UUU matrix obtained within the regime of the Schr\"odinger picture for a time-independent Hamiltonian, as a special case of our general time-dependent treatment.