Tag Archives: Aephraim Steinberg

A demonstration of quantum surrealism

The Canadian Institute for Advanced Research (CIFAR) has announced some intriguing new research results. A Feb. 19, 2016 news item on ScienceDaily gets the ball rolling,

New research demonstrates that particles at the quantum level can in fact be seen as behaving something like billiard balls rolling along a table, and not merely as the probabilistic smears that the standard interpretation of quantum mechanics suggests. But there’s a catch — the tracks the particles follow do not always behave as one would expect from “realistic” trajectories, but often in a fashion that has been termed “surrealistic.”

A Feb. 19, 2016 CIFAR news release by Kurt Kleiner, which originated the news item, offers the kind of explanation that allows an amateur such as myself to understand the principles (while I’m reading it), thank you Kurt Kleiner,

In a new version of an old experiment, CIFAR Senior Fellow Aephraim Steinberg (University of Toronto) and colleagues tracked the trajectories of photons as the particles traced a path through one of two slits and onto a screen. But the researchers went further, and observed the “nonlocal” influence of another photon that the first photon had been entangled with.

The results counter a long-standing criticism of an interpretation of quantum mechanics called the De Broglie-Bohm theory. Detractors of this interpretation had faulted it for failing to explain the behaviour of entangled photons realistically. For Steinberg, the results are important because they give us a way of visualizing quantum mechanics that’s just as valid as the standard interpretation, and perhaps more intuitive.

“I’m less interested in focusing on the philosophical question of what’s ‘really’ out there. I think the fruitful question is more down to earth. Rather than thinking about different metaphysical interpretations, I would phrase it in terms of having different pictures. Different pictures can be useful. They can help shape better intuitions.”

At stake is what is “really” happening at the quantum level. The uncertainty principle tells us that we can never know both a particle’s position and momentum with complete certainty. And when we do interact with a quantum system, for instance by measuring it, we disturb the system. So if we fire a photon at a screen and want to know where it will hit, we’ll never know for sure exactly where it will hit or what path it will take to get there.

The standard interpretation of quantum mechanics holds that this uncertainty means that there is no “real” trajectory between the light source and the screen. The best we can do is to calculate a “wave function” that shows the odds of the photon being in any one place at any time, but won’t tell us where it is until we make a measurement.

Yet another interpretation, called the De Broglie-Bohm theory, says that the photons do have real trajectories that are guided by a “pilot wave” that accompanies the particle. The wave is still probabilistic, but the particle takes a real trajectory from source to target. It doesn’t simply “collapse” into a particular location once it’s measured.

In 2011 Steinberg and his colleagues showed that they could follow trajectories for photons by subjecting many identical particles to measurements so weak that the particles were barely disturbed, and then averaging out the information. This method showed trajectories that looked similar to classical ones — say, those of balls flying through the air.

But critics had pointed out a problem with this viewpoint. Quantum mechanics also tells us that two particles can be entangled, so that a measurement of one particle affects the other. The critics complained that in some cases, a measurement of one particle would lead to an incorrect prediction of the trajectory of the entangled particle. They coined the term “surreal trajectories” to describe them.

In the most recent experiment, Steinberg and colleagues showed that the surrealism was a consequence of non-locality — the fact that the particles were able to influence one another instantaneously at a distance. In fact, the “incorrect” predictions of trajectories by the entangled photon were actually a consequence of where in their course the entangled particles were measured. Considering both particles together, the measurements made sense and were consistent with real trajectories.

Steinberg points out that both the standard interpretation of quantum mechanics and the De Broglie-Bohm interpretation are consistent with experimental evidence, and are mathematically equivalent. But it is helpful in some circumstances to visualize real trajectories, rather than wave function collapses, he says.

An image illustrating the work has been provided,

On the left, a still image from an animation of reconstructed trajectories for photons going through a double-slit. A second photon “measures” which slit each photon traversed, so no interference results on the screen. The image on the right shows the polarisation of this second, “probe." Credit: Dylan Mahler Courtesy: CIFAR

On the left, a still image from an animation of reconstructed trajectories for photons going through a double-slit. A second photon “measures” which slit each photon traversed, so no interference results on the screen. The image on the right shows the polarisation of this second, “probe.” Credit: Dylan Mahler Courtesy: CIFAR

Here’s a link to and a citation for the paper,

Experimental nonlocal and surreal Bohmian trajectories by Dylan H. Mahler, Lee Rozema, Kent Fisher, Lydia Vermeyden, Kevin J. Resch, Howard M. Wiseman, and Aephraim Steinberg. Science Advances  19 Feb 2016: Vol. 2, no. 2, e1501466 DOI: 10.1126/science.1501466

This article appears to be open access.

Interacting photons and quantum logic gates

University of Toronto physicists have taken the first step toward ‘working with pure light’ according to an August 25, 2015 news item on Nanotechnology Now,

A team of physicists at the University of Toronto (U of T) have taken a step toward making the essential building block of quantum computers out of pure light. Their advance, described in a paper published this week in Nature Physics, has to do with a specific part of computer circuitry known as a “logic gate.”

An August 25, 2015 University of Toronto news release by Patchen Barss, which originated the news item, provides an explanation of ‘logic gates’, photons, and the impact of this advance (Note: Links have been removed),

Logic gates perform operations on input data to create new outputs. In classical computers, logic gates take the form of diodes or transistors. But quantum computer components are made from individual atoms and subatomic particles. Information processing happens when the particles interact with one another according to the strange laws of quantum physics.

Light particles — known as “photons” — have many advantages in quantum computing, but it is notoriously difficult to get them to interact with one another in useful ways. This experiment demonstrates how to create such interactions.

“We’ve seen the effect of a single particle of light on another optical beam,” said Canadian Institute for Advanced Research (CIFAR) Senior Fellow Aephraim Steinberg, one of the paper’s authors and a researcher at U of T’s Centre for Quantum Information & Quantum Computing. “Normally light beams pass through each other with no effect at all. To build technologies like optical quantum computers, you want your beams to talk to one another. That’s never been done before using a single photon.”

The interaction was a two-step process. The researchers shot a single photon at rubidium atoms that they had cooled to a millionth of a degree above absolute zero. The photons became “entangled” with the atoms, which affected the way the rubidium interacted with a separate optical beam. The photon changes the atoms’ refractive index, which caused a tiny but measurable “phase shift” in the beam.

This process could be used as an all-optical quantum logic gate, allowing for inputs, information-processing and outputs.

“Quantum logic gates are the most obvious application of this advance,” said Steinberg. “But being able to see these interactions is the starting page of an entirely new field of optics. Most of what light does is so well understood that you wouldn’t think of it as a field of modern research. But two big exceptions are, “What happens when you deal with light one particle at a time?’ and “What happens when there are media like our cold atoms that allow different light beams to interact with each other?’”

Both questions have been studied, he says, but never together until now.

Here’s a link to and citation for the paper,

Observation of the nonlinear phase shift due to single post-selected photons by Amir Feizpour, Matin Hallaji, Greg Dmochowski, & Aephraim M. Steinberg. Nature Physics (2015) doi:10.1038/nphys3433 Published online 24 August 2015

This paper is behind a paywall.