Wave–particle duality quantified for the first time

Physics World ^ | 9/1/2021 | Karmela Padavic-Callaghan

[LibWhacker]

Quantum mechanics

• Research update

Wave–particle duality quantified for the first time

01 Sep 2021

Complementarity A new twist on the double-slit experiment. (Courtesy: Shutterstock/Andrey VP)

One of the most counterintuitive concepts in physics – the idea that quantum objects are complementary, behaving like waves in some situations and like particles in others – just got a new and more quantitative foundation. In a twist on the classic double-slit experiment, scientists at Korea’s Institute for Basic Sciences (IBS) used precisely controlled photon sources to measure a photon’s degree of wave-ness and particle-ness. Their results, published in Science Advances, show that the properties of the photon’s source influence its wave and particle character – a discovery that complicates and challenges the common understanding of complementarity.

The double-slit experiment is the archetypal example of complementarity at work. When a single photon encounters a barrier with two thin openings, it produces an interference pattern on a screen placed behind the openings – but only if the photon’s path is not observed. This interference pattern identifies the photon as a wave since a particle would create only one point of light on the screen. However, if detectors are placed at the openings to determine which slit the photon went through, the interference pattern disappears, and the photon behaves like a particle. The principle of complementarity states that both experimental outcomes are needed to fully understand the photon’s quantum nature.

Signal and idler

The new study adds to this principle by showing that the properties of the slits also matter. In their experiment, the IBS researchers shone so-called “seed beams” of laser light onto two crystals of lithium niobate. Each crystal produces two photons when illuminated: a “signal” photon and an “idler” photon. The researchers sent the signal photon into an interferometer to create interference patterns and quantify the photon’s wave nature, while observing the path of the idler photon to pinpoint its particle character. Because the signal and idler photons are produced together, they form a single quantum state described by both the wave and the particle property measurements.Waves and particles The experimental set-up. (Courtesy: Institute for Basic Science)

By changing the intensity of the seed beams in each crystal, the researchers independently altered the crystals’ chances of emitting photons – a process akin to controlling a photon’s “attraction” to each slit in the classic experiment. When one of the crystals was very likely to emit photons, the pattern the interferometer produced was barely visible, implying that the photon was mostly particle-like. When the crystals’ emission probabilities were equal, the interference pattern was sharp, highlighting the photon’s wave character. “The wave nature of the photon could be extracted as a visibility of the interference pattern,” explains Tai Hyun Yoon, a physicist at IBS and a co-author of the study.

Corroborating theoretical results

In their experiments, Yoon and co-author Minhaeng Cho focused on regimes where the photon was acting partly as a wave and partly as a particle. Previous theoretical studies indicated that the amount of wave-ness and particle-ness in such a system should satisfy a simple equation involving source purity – that is, the likelihood that a particular crystal source will be the one that emits light. The new study is the first complementarity experiment to account for and precisely control this source purity, and it corroborates a prediction made by Xiaofeng Qian and colleagues that source purity µ_s, interference pattern visibility V and path distinguishability P are related through the expression P² + V² = μ_s².

“Having this experimental capability makes it possible to confirm the theoretical structures that we were discussing, to test how the source is controlling a single quantum particle’s wave–particle duality,” says Qian, a physicist at Stevens Institute of Technology in New Jersey, US who was not involved in the present study. “This was a great achievement, that they could produce a single photon state where all the parameters were at their control,” agrees Girish Agrawal, a physicist at Texas A&M University, US and Qian’s collaborator on this earlier theoretical work.

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Double slits with single atoms

The new study also showed that controlling and quantitatively measuring the photon’s wave and particle character can be recast as measuring the entanglement between idler photons and the detectors that identified their path. In this way, researchers connected complementarity to a property of photons that is commonly exploited in practical quantum devices. “This extra controllability [in our set-up] could be an interesting and useful way to quantum engineer states that might be of interest in quantum information,” says Cho.

Besides its possible applied value, the researchers say that their study challenges physicists’ traditional thinking about complementarity. “In the context of pure theory and fundamental experiments, this experiment does add something new,” agrees Peter Milonni, a physicist at the University of Rochester, US who was not an author of the present paper. Qian adds that the experiment quantitatively proves that instead of a photon behaving as a particle or a wave only, the characteristics of the source that produces it – like the slits in the classic experiment – influence how much of each character it has. “This experimental test and the theoretical quantitative analysis really deliver the message that a quantum particle can behave simultaneously, but partially, as both,” he concludes.

[Weirdad]

I have a science background which includes some physics, yet consider things like this behavior of light to be miracles.

Here’s another way of thinking of light that to me makes it even less intuitive:

One particle (photon) can go through two slits at the same time. (That is how one photon (particle) can interfere with itself in order to make a diffraction pattern on the back wall in the classic experiment.

And light was the very first thing we know of that God created. “Let there be light.” Light is a thing that has dual characteristics that seem impossible to exist together. When light is really examined it is not intuitive, we have to resort to deep math and lots of verbage to try to describe it.

So that dual aspect of light helps when people ask (and these tend to be smart people who are bothered the most by this) “If God knows everything, including the future, how am I not a pre-programmed robot who should not be held responsible for what I do, since I must be already destined to do whatever I do?”

(In other words, they are claiming to apparently NOT have free will in a world where God is in control.)

I tell them about light, the first thing made, that has both particle and wave characteristics, two things that do not seem to be able to coexist.

The world and life is like that, like light: God knows all, and yet we DO have free will, that particle can also go through two slits at the same time.

God even refers to himself as light.

I am not claiming it is a pefect analogy.

However light certainly seems to be another way that God reveals his nature in nature.

These things in this universe made by God are true miracles.

[Steely Tom]

Photons have zero rest mass. But anything that has energy also has mass per Einstein’s equation.

As far as small things, that is also relative; to the naked eye, a human cell would be infinitesimally small (invisible), and on average an adult has 300 trillion of those. Bit hard but not impossible to wrap one’s mind around going smaller, to subatomic particles.

One of the things I learned from the David Butler series is that the majority — like 99% — of the mass of ordinary matter is not due to matter, it's due to the energy contained in the forces that hold the quarks together inside protons and neutrons, and the forces that hold the protons together within atomic nuclei. Quarks are held in proximity by the strong nuclear force, which is indeed very strong.

In the Butler lecture series, he said that to separate quarks from one another would require a force of 22 tons, not tons per square inch, but tons per quark.

If you could somehow exert this much force on them in such a way as to separate them, the energy you used to move them apart would be converted into matter, into a string of new quarks that would bridge the gap of separation between the two original quarks.

Isn't that strange and fascinating?

[Steely Tom]

These things in this universe made by God are true miracles.

Yes, and the idea that they happened somehow randomly is more difficult to believe than it is to simply believe in God.

[mjp]

The law of causality still applies. The nature of the system as a whole causes the effect,

[cgbg]

Materialism vs Made by God may seem to be the only possible alternatives, but there are others....

Sometimes when you ask the wrong questions you get the wrong answers....

Imho the concept of the fractal (hologram) is the key....

Nowhere is it written that talking apes are entitled to learn all the secrets of the universe....

[Weirdad]

Indeed

[Steely Tom]

Nowhere is it written that talking apes are entitled to learn all the secrets of the universe....

I don't think it's a matter of entitlement.

We know much more about the secrets of the universe today than we did 100 years ago, or even 10 years ago. That didn't happen because we are entitled to the knowledge, it's because of curiosity on the part of people intelligent enough to find the answers.

[TNoldman]

Quantum theory and God

[Rockingham]

Not as much as it may seem. Surveys show that physicists are less materialist in their personal philosophy than social scientists. Strictly speaking, materialism in modern science is a simplifying assumption, not a tenet of science. After all, as physicists nowadays mostly recognize, their view of reality is best described as panpsychism, not materialism.

[backwoods-engineer]

Why is it so counter intuitive? Anyone who understands the implications of Heaviside’s work knows that there is nothing ‘quantum’ mysterious going on here. Electric and magnetic fields feed each other energy. The fields and the photons are different things. Read Dr. Hanz Schantz’ book on ultrawideband antennas; it’s really more a primer in electromagnetics. Also, he has a paper on the topic that is probably already out. I got to see a preview presentation.

[Steely Tom]

Photons have zero rest mass. But anything that has energy also has mass per Einstein’s equation.

So that little bit of energy-mass is what has the spin! That's amazing to contemplate. Thanks for that insight. I'm afraid I glossed over it at first.

I listened to one video where the narrator said that the concept of quantum spin is the key to the nature of reality itself.

As an undergraduate in physics, my inability to grasp it, and the inability of my professors to explain it, is one of the things that drove me out of physics and into engineering.