Abstract
A variational principle enabling one to compute individual Floquet states of a periodically time-dependent quantum system is formulated, and successfully tested against the benchmark system provided by the analytically solvable model of a linearly driven harmonic oscillator. The principle is particularly well suited for tracing individual Floquet states through parameter space, and may allow one to obtain Floquet states even for very high-dimensional systems which cannot be treated by the known standard numerical methods.
1 Introduction
The Rayleigh-Ritz variational principle has proven to be of outstanding practical value for the approximate determination of a quantum system’s ground state. For any trial state
where H denotes the system’s Hamiltonian, assumed to be time-independent, and
The purpose of the present paper is to point out that there is also a variational principle which enables one to compute Floquet states of a periodically time-dependent quantum system. Such Floquet states have met with considerable interest recently; among others, they have been invoked for investigating the dynamics of atomic quantum gases in periodically driven optical lattices [2], and they lead to a natural explanation of the spontaneous breaking of time-translation symmetry occurring in so-called Floquet time cystals [3], [4], [5], [6]. Since their quasienergies constitute infinite, ladder-like classes of equally spaced representatives, the Floquet states do generally not possess a natural order; in particular, in most cases there is no “Floquet ground state”. Thus, the idea of the Rayleigh-Ritz principle (1) cannot be transferred one-by-one to Floquet systems. Nonetheless, it will be shown that there exists a similar variational principle which may allow one to compute Floquet states even for systems which are so large that they are no longer amenable to any other technique available so far.
The paper is organized as follows: For the convenience of the reader, the salient features of the Floquet picture are summarized in the subsequent Section 2. The new variational principle for Floquet states then is formulated in Section 3, and tested numerically with the help of an analytically solvable model system in Section 4. An outlook towards future applications of the principle is given in the concluding Secrion 5.
2 The Floquet concept
Consider a quantum system governed by a Hamiltonian which depends periodically on time t with period T,
acting on the system’s Hilbert space
can be factorized to read [7], [8], [9], [10]
where
Generally, the spectral problem posed by
This is not a trivial proposition; of course, this is always the case if
The “stroboscopic” eigenvalue problem (6) already leads to one of the decisive benefits of the Floquet picture. Expanding an initial state with respect to the eigenstates
the combination of Eqs. (3) and (4) gives, for any time t,
where the Floquet functions
inherit the T-periodicity of
Thus, when expanded with respect to the Floquet states
the time evolution (8) of
While the above stroboscopic approach is often found useful for numerical purposes, there also exists another, “extended” viewpoint which is particularly helpful for conceptual considerations. Inserting a Floquet state (11) into the time-dependent Schrödinger equation
one immediately finds
Augmented by the periodic boundary condition (10) to be satisfied by the Floquet functions, this is an eigenvalue problem which does not pose itself on the system’s actual Hilbert space
with
acting on
which appears on the left-hand side of Eq. (13) depends only linearly on this momentum. As a consequence, its quasienergy spectrum is unbounded both from above and from below: Assume that
Then for any integer
likewise are T-periodic eigensolutions,
Thus, from the perspective of the extended Hilbert space each Floquet state (11) evolving in
Therefore, within this extended approach a quasienergy should not be regarded as a number
reflecting the “
The observation that the quasienergy eigenvalue problem (17) plays a role which is conceptually similar to that of the stationary Schrödinger equation now allows one to transfer many notions known from time-independent quantum mechanics to periodically time-dependent quantum systems, such as the Hellmann-Feynman theorem, or Rayleigh-Schrödinger perturbation theory [15]. There is, however, a notable exception: The fact that each Floquet state is equipped with an infinite ladder (21) of quasienergies implies that these states cannot be ordered with respect to the magnitude of their quasienergies, and there is no “lowest” quasienergy. This means that the Rayleigh-Ritz principle (1) has no immediate counterpart in the extended Hilbert space, apparently depriving one of an efficient computational tool. In the follwing section it will be shown how this deficiency can be cured.
3 Variational principle
While the spectrum of the quasienergy operator (16) is unbounded from below, that of its square
where
In order to interpret the physical meaning of the numerical values adopted by the functional
which evolves in the course of one period T into the state
Hence, for any ε the absolute value of the return amplitude after one period T (“raT”) is given by
having used
as an element of
With the estimate (25) holding for any ε, this gives
In particular, if
4 Example
Let us consider a one-dimensional harmonic oscillator which is subjected to a monochromatic force with angular frequency ω, as described in the position representation by the Hamiltonian
where M denotes the mass of the oscillator particle,
To begin with, let us asssume that the driving frequency ω differs from the oscillator frequency
which is
Denoting the familiar eigenfunctions of the unforced oscillator with energy eigenvalues
where
is the classical Lagrangian of the system, evaluated along the trajectory (31). Observing that the integral over this T-periodic function
so that all states exhibit exactly the same ac Stark shift. This is a fairly unusual feature which reflects the integrability of system (29). Thus, apart from a phase factor the Floquet states (32) are given by harmonic-oscillator eigenfunctions which follow the T-periodic oscillations of the classical trajectory (31).
In order to explore whether these Floquet states are correctly recovered by the variational principle (22), one may take
as a natural general ansatz, with real coefficients
Figure 1 displays data computed according to this procedure for the driving frequency
and thus providing an upper bound on the driving amplitude that can be reasonably dealt with in a variational space made up from the lowest
Inserting
Thus, when the variational space is enlarged, larger driving amplitudes become admissible. For instance, when r is increased to
The numerical strategy suggested in this work for following an individual Floquet state through parameter space is not restricted to the Floquet state emanating from the ground state of the system in the absence of the drive, but applies the any state. In order to substantiate this claim, Figure 2 depicts analogous numerical data obtained when tracing the Floquet state originating from the unperturbed oscillator state
5 Conclusion
The customary computational strategies for determining the Floquet states of a periodically time-dependent quantum system rely either on Eq. (6), requiring the computation and diagonalization of the system’s one-cycle evolution operator
The particular model system that has been employed here for testing the new variational principle, the linearly driven harmonic oscillator, certainly is not typical from the Floquet point of view: Being explicitly integrable, its quasienergies (34) and Floquet states (32) can be labeled by the quantum number n of the harmonic-oscillator state to which they are continuously connected when the driving amplitude goes to zero. This is no longer the case for more generic systems, such as periodically forced anharmonic oscillators which possess a classial conterpart exhibiting chaotic dynamics. In such systems one encounters a quasienergy spectrum with a dense net of anticrossings [10], [20], thwarting the notion of continuity. Nonetheless, it is surmised that the idea of “tracing” an individual Floquet state through parameter space will also work for such more realistic systems, helping one to identify those Floquet states which are most important for understanding a given system’s experimentally observable properties.
Finally, it needs to be stressed that the far-reaching progress made recently in the areas of machine learning and hardware design will allow one to solve variational problems even with very large numbers of variational parameters in the near future. Therefore, it is anticipated that the combined use of the variational principle (22), modern hardware, and intelligent algorithms will enable one to investigate truly large Floquet systems which are way beyond the realm of the previously used standard numerical methods.
Funding source: Deutsche Forschungsgemeinschaft
Award Identifier / Grant number: 397122187
Acknowledgments
This work has been supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through Project No. 397122187. The author wishes to thank the members of the Research Unit FOR 2692 for many stimulating discussions.
Author contribution: The author has accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: This work has been supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through Project No. 397122187.
Conflict of interest statement: The author declares no conflicts of interest regarding this article.
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