Testing Quantum Coherence in Stochastic Electrodynamics with Squeezed Schrödinger Cat States

Testing Quantum Coherence in Stochastic Electrodynamics with Squeezed Schrödinger Cat States

Wayne Cheng-Wei Huang (Center for Fundamental Physics, Northwestern University, Evanston, Illinois) and Herman Batelaan (Department of Physics and Astronomy, University of Nebraska-Lincoln).

Atoms 2019, 7(2), 42. Received 15 February 2019; accepted 2 April 2019; published 5 April 2019.

Abstract

The interference pattern in electron double-slit diffraction is a hallmark of quantum mechanics. A long-standing question for stochastic electrodynamics (SED) is whether or not it is capable of reproducing such effects, as interference is a manifestation of quantum coherence. In this study, we used excited harmonic oscillators to directly test this quantum feature in SED. We used two counter-propagating dichromatic laser pulses to promote a ground-state harmonic oscillator to a squeezed Schrödinger cat state. Upon recombination of the two well-separated wavepackets, an interference pattern emerges in the quantum probability distribution but is absent in the SED probability distribution. We thus give a counterexample that rejects SED as a valid alternative to quantum mechanics.

Keywords: interference pattern; stochastic electrodynamics; quantum coherence; squeezed Schrödinger cat state; Kapitza-Dirac effect; parametric excitation.

1. Introduction

Over the past decades, there has been sustained interest in developing classical alternatives to quantum mechanics (QM) with the goal of solving the quantum-classical boundary problem. Despite the proposed classical alternatives, there is a lack of quantitative tests of such theories against QM, mostly because analytic solutions to concrete physical systems such as two-level atoms have not been found. Arguably one of the most developed classical alternatives is stochastic electrodynamics (SED). Studies of SED harmonic systems have found many examples that are in exact agreement with QM. These include the retarded van der Waals force, the ground state distribution of harmonic oscillators, Landau diamagnetism, the Planck spectrum of blackbody radiation, and the Debye specific-heat law for solids. Numerical studies of hydrogen have given some qualitative features but have not led to a clear success. Recently, it was further shown that parametric interaction can give rise to discrete SED excitation spectra that are in excellent agreement with QM predictions.

However, a major drawback to the generality of the SED approach is that none of the investigated effects involves quantum coherence. In light of this, some have proposed studying electron double-slit diffraction within the framework of SED, as interference is a manifestation of quantum coherence. Within the SED community, the proposed view of electron diffraction is that the double-slit poses boundary conditions that modify the classical zero-point electromagnetic field, and in turn, it acts as a guiding wave for free electrons. The appeal of this idea is that the guiding field can be affected by both slits, while the particle passes through only one slit, similar to the idea that has pushed oil droplet analogues. As appealing as the idea may sound, so far, there has been no concrete calculation or simulation demonstrating such an effect because of two major theoretical obstacles: first, the effective spectrum of the zero-point field is unbounded for free electrons; and second, the radiation damping of free electrons gives rise to runaway solutions.

Rather than focusing on the specific theoretical difficulties that are relevant to free electrons, we developed a paradigm that can be used as a direct test for quantum coherence in stochastic electrodynamics. Building on our previous results, we devised a laser excitation scheme to promote a ground-state quantum harmonic oscillator to a squeezed Schrödinger cat state. The Schrödinger cat state of a harmonic oscillator is an analogy of the electron double-slit state, the sum of a left state and a right state, where those indicate the left and right electron slit states in position space. Comparing the QM probability distribution with that of SED harmonic oscillators, we observed some interesting similarities. Nevertheless, the interference pattern is missing in the SED probability distribution.

2. Kapitza-Dirac force on harmonic oscillators

Let us consider the setup in Figure 1. Two counter-propagating laser fields propagate along the x-axis, and the electric fields are linearly polarized along the z-axis (Equation 1): the first field is its vector-potential amplitude times its frequency, times the cosine of its wave number times position minus its frequency times time; the second is the same form with the opposite sign and with the wave travelling the other way. Here the wave numbers are the frequencies divided by the speed of light, and the amplitudes are those of the corresponding vector potentials. The electric and magnetic components of the combined laser field follow by adding the two contributions, the magnetic components carrying the wave numbers rather than the frequencies as prefactors (Equation 2).

Figure 1: two counter-propagating laser fields with two frequencies collaboratively drive the harmonic oscillator with a spatially modulated Kapitza-Dirac force in the direction of wave propagation. The force has a modulation periodicity of two pi times the speed of light divided by the sum of the frequencies, and it oscillates at the difference frequency. A particle subject to the perturbation of the classical zero-point electromagnetic field can get pushed in either direction.

Assuming that the particle is free in the direction of the electric field, that is the z-axis in Figure 1, the cross terms between the electric and magnetic components can give rise to a spatially modulated force in the direction of wave propagation, that is the x-axis in Figure 1. Herein, we term this force the Kapitza-Dirac (KD) force. The KD force can be derived from the equations of motion in which the mass times the rate of change of the velocity along the field direction equals the charge times the electric field, and the force along the propagation direction equals the charge times that velocity times the magnetic field (Equation 3), where the mass and charge are those of the particle.

Now, there are two scenarios. First, if the frequencies of the two laser fields are identical, the KD force will be constant in time, which is also known as the ponderomotive force. Second, if the laser frequencies are different, the KD force will oscillate in time with the sum and difference frequencies. If the charged particle is confined by a harmonic potential with a resonant frequency equal to half the difference of the laser frequencies, the KD force that can resonantly drive the harmonic oscillator takes the form of the squared charge times the product of the two vector-potential amplitudes, divided by the mass, times half the sum of the wave numbers, times the sine of the summed wave number times position, times the cosine of the difference frequency times time (Equation 4, derived in Appendix A). Accordingly, the corresponding time-varying KD potential is the squared charge times the product of the amplitudes divided by twice the mass, times the cosine of the summed wave number times position, times the cosine of the difference frequency times time (Equation 5).

We note that at any given time, a trapping site in the KD potential — that is, a minimum in the potential — will turn into an unstable point after a quarter of the natural period, since the potential polarity is reversed (Equation 6). This feature will later be used to coherently split the ground-state wavepacket of a quantum oscillator.

3. Generation of squeezed Schrödinger cat states

The KD effect for quantum harmonic oscillators can be modeled by adding the KD potential to the unperturbed oscillator Hamiltonian. We replace the continuous-wave laser fields with pulsed fields carrying a Gaussian envelope in time set by a pulse duration (Equation 7), in order to avoid indefinite sequential excitation of the oscillator's ladder levels. Given the appropriate pulse amplitudes and durations, the final population distribution can have a peaked structure. The quantum Hamiltonian is thus the usual kinetic and harmonic terms plus the KD potential multiplied by the squared pulse envelope (Equation 8).

The difference frequency of the laser fields is twice the oscillator's resonant frequency, so the ground-state oscillator will be parametrically excited to the even-symmetry states, which is a prerequisite for cat state generation because a cat state has an even symmetry. We obtained the QM result by numerically solving the Schrödinger equation as in our earlier work. The oscillator's parameters are a mass of 9.11 times 10 to the minus 35 kilograms, a charge of 1.60 times 10 to the minus 19 coulombs, and a resonant frequency of 10 to the 16 radians per second. The laser parameters are chosen to be 2.3 and 0.3 times the resonant frequency, a pulse duration of 5 times 10 to the minus 15 seconds, and equal vector-potential amplitudes of 4.5 times 10 to the minus 8 volt-seconds per metre. The mass is chosen at this unusual value in order to make the computational time for the SED simulation more manageable.

In Figure 2b, the probability trajectory of the excited state is shown. Upon excitation, the ground-state wavepacket is coherently split into two wavepackets, so the oscillator is in a superposition of two macroscopically distinct states. The two wavepackets oscillate back and forth in the harmonic potential. As they recombine, interference fringes emerge in the probability distribution due to the quantum coherence between the two wavepackets. The quantum coherence is readily illustrated by the fringe structure in the oscillator's Wigner function, as shown in Figure 2a. We note that one of the two quadrature uncertainties of each wavepacket, in position or in momentum, is smaller than the corresponding ground state uncertainty, while their product remains the same, half the reduced Planck constant, at all times. This implies that the state generated by the laser excitation is a squeezed Schrödinger cat state, that is, a superposition of two displaced squeezed states with opposite phases.

Figure 2: time evolution of quantum mechanics (QM) and stochastic electrodynamics (SED) probability distributions after laser excitation. Panel a, the Wigner function of the QM oscillator is plotted after the laser excitation, with positive values colour-coded in red and negative values in blue. Quantum coherence between the two well-separated squeezed states is manifested by the fringe structure in the centre. The Wigner function rotates counter-clockwise at the oscillator's resonant frequency. At the moment when the distribution is depicted, the position quadrature uncertainty of the squeezed state is smaller than that of the ground state, while the quadrature uncertainty product satisfies the Heisenberg relation at all times. Panel b, the two wavepackets oscillate back and forth in the harmonic potential, giving rise to a double sinusoidal trajectory of the QM probability distribution; an interference pattern appears when the two wavepackets merge. Panel c, the phase space distribution of the SED oscillator shows two well-separated sub-ensembles, each with a squeezed structure that mimics the QM squeezed state shown in panel a. Panel d, as the SED phase space distribution rotates counter-clockwise, the probability distribution bundles into two "macroscopic" trajectories. In each trial of the SED simulation, there is no knowledge of which macroscopic trajectory a particle will follow unless the initial phase of the background zero-point field is known. No interference-like patterns are found in the SED probability distribution when the two macroscopic trajectories of distributions cross.

For SED oscillators, we first prepared the classical ensemble in a ground state with position and momentum probability distributions identical to those of a quantum oscillator. Under excitation of the same laser pulses, the equation of motion for the SED harmonic oscillator is the harmonic restoring force, plus a radiation damping term proportional to the velocity through a damping coefficient, plus the charge times the vacuum zero-point field, plus the KD force multiplied by the squared pulse envelope (Equation 9), where the damping coefficient is twice the squared charge divided by three times the mass and the cubed speed of light, with the usual Coulomb factor.

Apart from the damping term and the coupling to the zero-point field, this equation is formally equivalent to a quantum Heisenberg equation derived from the quantum Hamiltonian. This suggests that the excitation dynamics in SED should be identical to QM, assuming first that the pulse duration is much shorter than the damping time, and second that the KD force is much stronger than the fluctuating force from the zero-point field (Equation 10). In our simulation, these two conditions are satisfied, and the excitation dynamics in SED and QM are the same as shown in Figure 3a, where the time evolutions of the expectation value of the QM energy and the ensemble average of the SED energy are compared. The two energy trajectories stay overlapped through most of the pulse and only deviate at the end of the excitation. Furthermore, the SED and QM energy distributions have similar shapes, despite the fact that the QM distribution is discrete and the SED distribution is continuous.

Figure 3: the QM and SED energy distributions after laser excitation. Panel a, the ensemble average of the SED oscillator energy is compared with the expectation value of the QM oscillator energy during the course of laser excitation, with the shaded area representing the excitation laser pulse. The pulse duration of 5 times 10 to the minus 15 seconds is much smaller than the SED oscillator damping time of about 3.2 times 10 to the minus 13 seconds; therefore, damping has no significant effect on the oscillator's dynamics during the excitation process. Panel b, energy distributions of QM and SED oscillators are compared at the time when the SED energy reaches its maximum and the radiation damping starts to dominate. The QM energy distribution is discrete and occupies only even energy levels; the SED energy distribution is continuous but has a similar width and average value as the QM distribution. This indicates that the excitation process is identical for QM and SED oscillators.

The probability trajectory of the SED oscillator ensemble along with its phase space distribution are shown in Figure 2c and 2d. The ensemble particle number is 3 times 10 to the fourth. The parametric interaction between the SED oscillator and the laser fields was simulated using the same method as in our earlier work. Like the QM oscillator, upon excitation, the ground-state SED probability distribution also splits into two sub-ensembles that follow two distinct sinusoidal trajectories. The initial phase spectrum of the zero-point field, which is random and considered as the "hidden variable", determines which trajectory a particle will follow in each trial. A detailed comparison between SED and QM probability distributions is given in Figure 4 for two time points: first, when the two QM wavepackets are separated, and second, when they recombine. Although there are some overall similarities between the SED and QM distributions, there are no interference fringes in the SED probability distribution.

Figure 4: comparison between the QM and SED probability distributions at the initial time and after a quarter of the natural period. Panel a, the agreement between QM and SED probability distributions is good when the two macroscopically distinct QM wavepackets are well-separated by a peak-to-peak distance far beyond the ground state width. Panel b, after a quarter of the natural period the two QM wavepackets recombine in the harmonic potential; interference fringes appear in the QM distribution but not in the SED distribution. The fringe periodicity is determined by the wavepacket separation through the relation that the periodicity equals two pi times the reduced Planck constant divided by the mass, the resonant frequency and the separation, which resembles the well-known double-slit diffraction formula. The SED probability distribution captures the outline of the QM distribution as if the quantum coherence between the two QM wavepackets is lost.

4. Discussion and conclusions

While the zero-point electromagnetic field only introduces small radiative corrections to nonrelativistic QM, such as Lamb shifts, it drastically changes the particle dynamics in classical mechanics and leads to the reproduction of QM effects in some classical systems. Our work aimed to investigate to what extent such a classical theory can reproduce QM features by comparing results obtained from SED with those obtained from the QM Hamiltonian. The qualitative difference between the probability distributions of QM and SED in Figures 2 and 4b establishes that SED in its traditional form does not support physical effects that involve quantum coherence. The squeezed Schrödinger cat state used in this work is an analogy of the electron double-slit experiment. The peak-to-peak separation between the two wavepackets determines the fringe periodicity through the relation given above, which mimics the double-slit diffraction formula. Our analysis provides evidence that coherence-like behaviour is absent in SED, and thus, we predict that SED electron double-slit diffraction, if ever calculated, will not show fringes.

On the other hand, our result helps to establish the validity range of SED. We note that the partial agreement between the SED and QM results stems from the formal resemblance between the SED equation of motion and the QM Heisenberg equation, assuming that the laser excitation pulses satisfy certain criteria. While position and momentum are independent dynamic variables in SED, the canonical commutation relation makes them a Fourier pair in QM. This difference makes the distinction between QM and SED in terms of quantum coherence. Therefore, SED may be seen as the decoherence limit of QM. Although the zero-point field bestows a special phase relation between the position and momentum of a SED harmonic oscillator, which leads to the quantum ground-state distributions, the phase relation serves only as initial conditions and does not affect the dynamical evolution of position and momentum during laser excitation. Therefore, we speculate that any mechanism or theoretical operation that restores, or deteriorates, the Fourier relation between position and momentum for SED, or for QM, will make the proper decoherence theory that bridges the gap between SED and QM.

Appendix A. Derivation of the resonant Kapitza-Dirac force

In this appendix we derive the KD force from the equations of motion using the electric and magnetic components of the combined laser field. First, we solve for the velocity along the field direction by integrating the equation of motion, giving minus the charge over the mass times the sum of the two vector-potential terms in sine form (Equation A1). Substituting that velocity into the expression for the force along the propagation direction, we obtain the Lorentz force as the squared charge over the mass, times that sine sum, times the corresponding cosine sum weighted by the wave numbers (Equation A2).

We can see four frequency components in the Lorentz force by expanding this expression (Equation A3): terms at twice each laser frequency, and cross terms at the sum and at the difference of the two frequencies. Because twice the first frequency, twice the second frequency, and the sum frequency are not integer multiples of the oscillator's resonant frequency, we can drop these terms and keep only the parametrically resonant term at the difference frequency, which equals twice the resonant frequency (Equation A4).

The force has a travelling wave profile which can be decomposed into two standing-wave components with even and odd symmetries (Equation A5). The force needs to have a potential profile with even symmetry in order to resonantly drive the oscillator from the ground state at an even multiple of the resonant frequency. This implies that the resonant KD force should have an odd symmetry, and thus takes the form given as Equation (4) in the main text.

Funding and acknowledgements

This research was funded by National Science Foundation grant number PHY-1602755. The authors thank A. M. Steinberg, C. Monroe, and P. W. Milonni for advice. W. C. Huang wishes to give special thanks to Yanshuo Li for helpful discussions. This work utilized high-performance computing resources from the Holland Computing Center of the University of Nebraska. The authors declare no conflict of interest.

(The figures and the numbered reference list are omitted here; the complete text, with its equations set as mathematics and its figures, is at the source.)