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A deceptively simple experiment that involves making precise measurements of the time it takes for a particle to go from point A to point B could spark a breakthrough in quantum physics. The findings could focus attention on an alternative to standard quantum theory called Bohmian mechanics, which posits an underworld of unseen waves that guide particles from place to place.

A new study, by a team at the Ludwig Maximilian University of Munich (LMU) in Germany, makes precise predictions for such an experiment using Bohmian mechanics, a theory formulated by theoretical physicist David Bohm in the 1950s and augmented by modern-day theorists. Standard quantum theory fails in this regard, and physicists have to resort to assumptions and approximations to calculate particle transit times.

“If people knew that a theory that they love so much—standard quantum mechanics—cannot make [precise] predictions in such a simple case, that should at least make them wonder,” says theorist and LMU team member Serj Aristarhov.

ARE WE THERE YET?

It is no secret that the quantum world is weird. Consider a setup that fires electrons at a screen. You cannot predict exactly where any given electron will land to form, say, a fluorescent dot. But you can predict with precision the spatial distribution, or pattern, of dots that takes shape over time as the electrons land one by one. Some locations will have more electrons; others will have fewer. But this weirdness hides something even stranger. All else being equal, each electron will reach the detector at a slightly different time, its so-called arrival time. Just like the positions, the arrival times will have a distribution: some arrival times will be more common, and others will be less so.

But textbook quantum physics has no mechanism for precisely predicting this temporal distribution. “Normal quantum theory is only concerned with ‘where’; they ignore the ‘when,’” says team member and theorist Siddhant Das. “That’s one way to diagnose that there’s something fishy.”

There is a deep reason for this curious shortcoming. In standard quantum theory, a physical property that can be measured is called an “observable.” The position of a particle, for example, is an observable. Each and every observable is associated with a corresponding mathematical entity called an “operator.” But the standard theory has no such operator for observing time. In 1933 Austrian theoretical physicist Wolfgang Pauli showed that quantum theory could not accommodate a time operator, at least not in the standard way of thinking about it. “We conclude therefore that the introduction of a time operator … must be abandoned fundamentally,” he wrote.

MIXING CLASSICAL WITH QUANTUM

But measuring particle arrival times and or their “time of flight” is an important aspect of experimental physics. For example, such measurements are made with detectors at the Large Hadron Collider or instruments called mass spectrometers that use such information to calculate the masses and momenta of particles, ions and molecules.

Even though such calculations concern quantum systems, physicists cannot use unadulterated quantum mechanics all the way through. “You would have no way to come up with [an unambiguous] prediction,” Das says.

Instead they resort to assumptions to arrive at answers. For example, in one method, experimenters assume that once the particle leaves its source, it behaves classically, meaning it follows Newton’s equations of motion.

This results in a hybrid approach—one that is part quantum, part classical. It starts with the quantum perspective, where each particle is represented by a mathematical abstraction called a wave function. Identically prepared particles will have identical wave functions when they are released from their source. But measuring the momentum of each particle (or, for that matter, its position) at the instant of release will yield different values each time. Taken together, these values follow a distribution that is precisely predicted by the initial wave function. Starting from this ensemble of values for identically prepared particles, and assuming that a particle follows a classical trajectory once it is emitted, the result is a distribution of arrival times at the detector that depends on the initial momentum distribution.

Standard theory is also often used for another quantum mechanical method for calculating arrival times. As a particle flies toward a detector, its wave function evolves according to the Schrödinger equation, which describes a particle’s changing state over time. Consider the one-dimensional case of a detector that is a certain horizontal distance from an emission source. The Schrödinger equation determines the wave function of the particle and hence the probability of detecting that particle at that location, assuming that the particle crosses the location only once (there is, of course, no clear way to substantiate this assumption in standard quantum mechanics). Using such assumptions, physicists can calculate the probability that the particle will arrive at the detector at a given time (t) or earlier.

“From the perspective of standard quantum mechanics, it sounds perfectly fine,” Aristarhov says. “And you expect to have a nice answer from that.”

There is a hitch, however. To go from the probability that the arrival time is less than or equal to t to the probability that it is exactly equal to tinvolves calculating a quantity that physicists call the quantum flux, or quantum probability current—a measure of how the probability of finding the particle at the detector location changes with time. This works well, except that, at times, the quantum flux can be negative even though it is hard to find wave functions for which the quantity becomes appreciably negative. But nothing “prohibits this quantity from being negative,” Aristarhov says. “And this is a disaster.” A negative quantum flux leads to negative probabilities, and probabilities can never be less than zero.

Using the Schrödinger evolution to calculate the distribution of arrival times only works when the quantum flux is positive—a case that, in the real world, only definitively exists when the detector is in the “far field,” or at a considerable distance from the source, and the particle is moving freely in the absence of potentials. When experimentalists measure such far-field arrival times, both the hybrid and quantum flux approaches make similar predictions that tally well with experimental findings. But they do not make clear predictions for “near field” cases, where the detector is very close to the source.

BOHMIAN PREDICTIONS

Dissatisfied with this flawed status quo, in 2018 Das and Aristarhov, along with their then Ph.D. adviser Detlef Dürr, an expert on Bohmian mechanics at LMU who died earlier this year, and their colleagues, began working on Bohmian-based predictions of arrival times. Bohm’s theory holds that each particle is guided by its wave function. Unlike standard quantum mechanics, in which a particle is considered to have no precise position or momentum prior to a measurement—and hence no trajectory—particles in Bohmian mechanics are real and have squiggly trajectories described by precise equations of motion (albeit ones that differ from Newton’s equations of motion).

Among the researchers’ first findings was that far-field measurements would fail to distinguish between the predictions of Bohmian mechanics and those of the hybrid or quantum flux approaches. This is because, over large distances, Bohmian trajectories become straight lines, so the hybrid semi-classical approximation holds. Also, for straight far-field trajectories, the quantum flux is always positive, and its value is predicted exactly by Bohmian mechanics. “If you put a detector far enough [away], and you do Bohmian analysis, you see that it coincides with the hybrid approach and the quantum flux approach,” Aristarhov says.

The key, then, is to do near-field measurements, but those have been considered impossible. “The near-field regime is very volatile. It’s very sensitive to the initial wave function shape you have created,” Das says. Also, “if you come very close to the region of initial preparation, the particle will just be detected instantaneously. You cannot resolve [the arrival times] and see the differences between this prediction and that prediction.”

To avoid this problem, Das and Dürr proposed an experimental setup that would allow particles to be detected far away from the source while still generating unique results that could distinguish the predictions of Bohmian mechanics from those of the more standard methods.

PUTTING A SPIN ON IT

Conceptually, the team’s proposed setup is rather simple. Imagine a waveguide—a cylindrical pathway that confines the motion of a particle (an optical fiber is such a waveguide for photons of light, for example). On one end of the waveguide, prepare a particle—ideally an electron or some particle of matter—in its lowest energy, or ground, state and trap it in a bowl-shaped electric potential well. This well is actually the composite of two adjacent potential barriers that collectively create the parabolic shape. If one of the barriers is switched off, the particle will still be blocked by the other that remains in place, but it is free to escape from the well into the waveguide.

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