Single-photon holography sheds new light on quantum mechanics

Scientists at the University of Warsaw created what they say is the first ever hologram of a single light particle.

A hologram of a single photon is shown reconstructed from raw measurements (left) and theoretically predicted (right). (Image credit: FUW)
A hologram of a single photon is shown reconstructed from raw measurements (left) and theoretically predicted (right). (Image credit: FUW)

IMAGE: A hologram of a single photon is shown reconstructed from raw measurements (left) and theoretically predicted (right). (Image credit: FUW)

As reported in Nature Photonics, scientists at the Faculty of Physics, University of Warsaw (FUW) created what they say is the first ever hologram of a single light particle. The experiment was conducted by Radoslaw Chrapkiewicz and Michal Jachura under the supervision of Wojciech Wasilewski and Konrad Banaszek. They say their successful registering of the hologram of a single photon offers a whole new perspective on quantum phenomena. "We performed a relatively simple experiment to measure and view something incredibly difficult to observe: the shape of wavefronts of a single photon," said Chrapkiewicz.

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When a hologram is created, a well-described, undisturbed light wave (reference wave) is superimposed with another wave of the same wavelength but reflected from a three-dimensional (3D) object (the peaks and troughs of the two waves are shifted to varying degrees at different points of the image). This results in interference and the phase differences between the two waves create a complex pattern of lines. Such a hologram is then illuminated with a beam of reference light to recreate the spatial structure of wavefronts of the light reflected from the object, and as such its 3D shape.

One might think that a similar mechanism would be observed when the number of photons creating the two waves were reduced to a minimum, that is to a single reference photon and a single photon reflected by the object. But that is not the case. The phase of individual photons continues to fluctuate, which makes classical interference with other photons impossible. Since the Warsaw physicists were facing a seemingly impossible task, they attempted to tackle the issue differently: rather than using classical interference of electromagnetic waves, they tried to register quantum interference in which the wave functions of photons interact.

Wave function is a fundamental concept in quantum mechanics and the core of its most important equation: the Schrödinger equation. Physicists are always trying to learn about the wave function of a particle in a given system, since the square of its modulus represents the distribution of the probability of finding the particle in a particular state, which is highly useful.

"All this may sound rather complicated, but in practice our experiment is simple at its core: instead of looking at changing light intensity, we look at the changing probability of registering pairs of photons after the quantum interference," explains doctoral student Jachura.

Why pairs of photons? A year ago, Chrapkiewicz and Jachura used an innovative camera built at the University of Warsaw to film the behavior of pairs of distinguishable and non-distinguishable photons entering a beamsplitter. When the photons are distinguishable, their behavior at the beamsplitter is random: one or both photons can be transmitted or reflected. Non-distinguishable photons exhibit quantum interference, which alters their behavior: they join into pairs and are always transmitted or reflected together. This is known as two-photon interference or the Hong-Ou-Mandel effect.

"Following this experiment, we were inspired to ask whether two-photon quantum interference could be used similarly to classical interference in holography in order to use known-state photons to gain further information about unknown-state photons. Our analysis led us to a surprising conclusion: it turned out that when two photons exhibit quantum interference, the course of this interference depends on the shape of their wavefronts," said Chrapkiewicz.

Quantum interference can be observed by registering pairs of photons. The experiment needs to be repeated several times, always with two photons with identical properties. To meet these conditions, each experiment started with a pair of photons with flat wavefronts and perpendicular polarizations; this means that the electrical field of each photon vibrated in a single plane only, and these planes were perpendicular for the two photons. The different polarization made it possible to separate the photons in a crystal and make one of them 'unknown' by curving their wavefronts using a cylindrical lens. Once the photons were reflected by mirrors, they were directed towards the beamsplitter (a calcite crystal). The splitter didn't change the direction of vertically-polarized photons, but it did diverge diplace horizontally-polarized photons.

In order to make each direction equally probable and to make sure the crystal acted as a beamsplitter, the planes of photon polarization were bent by 45 degrees before the photons entered the splitter. The photons were registered using the state-of-the-art camera designed for the previous experiments. By repeating the measurements several times, the researchers obtained an interference image corresponding to the hologram of the unknown photon viewed from a single point in space. The image was used to fully reconstruct the amplitude and phase of the wave function of the unknown photon.

The experiment conducted by the Warsaw physicists is a major step towards improving our understanding of the fundamental principles of quantum mechanics. Until now, there has not been a simple experimental method of gaining information about the phase of a photon's wave function. Although quantum mechanics has many applications, and it has been verified many times with a great degree of accuracy over the last century, we are still unable to explain what wave functions actually are: are they simply a handy mathematical tool, or are they something real?

"Our experiment is one of the first allowing us to directly observe one of the fundamental parameters of photon's wave function--its phase--bringing us a step closer to understanding what the wave function really is," explains Jachura.

"All of us--I mean physicists--must first get our heads around this new tool. It's likely that real applications of quantum holography won't appear for a few decades yet, but if there's one thing we can be sure of it's that they will be surprising," adds Banaszek.

SOURCE: Faculty of Physics, University of Warsaw; http://www.fuw.edu.pl/press-release/news4619.html

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