Breakthrough discovery in quantum photonics
The past few years have seen significant developments in quantum physics owing to the discovery of PT symmetry. PT symmetry extends the traditional, Hermitian quantum mechanics, by providing a new framework, which increases the number of systems that can be described with a proper quantum theory.
“In the last 20 years, a new kind of quantum mechanics has been proposed: PT-symmetric quantum mechanics. People have been studying this new framework since then but never considered how this extension of quantum mechanics can influence the very quantum nature of things,” Assistant Professor (tenure track) Marco Ornigotti says.
The new study answered two fundamental questions about quantum mechanics: Are quantum effects, and in particular HOM-interference, possible in PT-symmetric systems? Do quantum systems behave the same way in PT-symmetric systems?
“We used a very fundamental quantum property of light, namely 2-photon interference, to show for the first time that the very fact that light is evolving in a PT-symmetric environment, rather than a “traditional” Hermitian one, makes the photon bunching happen earlier than expected,” Ornigotti says.
The experimental measurements were carried out at the University of Rostock in Germany. Ornigotti highlights the contributions of his former colleague, physicist Alexander Szameit, to the study.
“He had a major role in this research,” Ornigotti says.
While PT symmetry is now mainly studied in the field of photonics, it has also been utilised, for example, in the development of superconducting materials and lasers.
Photons are indistinguishable in the quantum world
The experimental setup of the study was based on the well-known Hong-Ou-Mandel (HOM) effect.
“Basically, two photons are combined inside a beam splitter. To monitor their quantum interference, photodetectors are placed at each of the two output ports of the integrated beam splitter,” Ornigotti says,
One of the quantum mechanical phenomena that seem to defy the laws of common sense is that the two photons will always leave the beamsplitter together. Although they start out travelling in different directions entering the beam splitter, they will always leave it together out of the same output port. This is called photon bunching and it´s at the heart of the HOM effect.
When a photon arrives at the detector, it produces a click. If both detectors click at the same time, they exited the systems from different output ports. This mean they are distinguishable.
“On the other hand, if one detector clicks and other one does not, we can be sure that the photons are indistinguishable. In other words, they are bunched together. In this case the quantum properties of the two photons will be manifestly present, and it´s the case we were looking at,” Ornigotti says.
This phenomenon can only be observed when the two photons enter the beamsplitter at exactly the same time.
“We can easily distinguish between the photons if they do not reach the beam splitter at the same time,” Ornigotti says.
PT-symmetry changes the scale of photon bunching
The researchers tested HOM interference using a PT-symmetric beam splitter that consists of two waveguides: one of them experiences losses, the other one doesn´t.
“At this point, something very interesting happened. As soon as the photons entered a lossy environment, their behaviour changed and they bunched together sooner,” Ornigotti says.
Doing a HOM experiment in a Hermitian integrated beam splitter will also result in photon bunching.
“This is nothing new, people do this all the time. If you repeat the experiment with a PT-symmetric beam splitter, like we did in this study, the photons bunch together at a smaller scale,” Ornigotti says.
However, the phenomenon can only be demonstrated under carefully controlled conditions.
“Any lossless system will not do. The experimental setup must be carefully designed, to match the requirements of PT-symmetry,” Ornigotti says.
A light-powered quantum computer would revolutionise the world
In the near future, fundamental research in quantum photonics may go on to transform, for example, computing technology. Existing computers are based on the movement of electrons but it may one day be possible to create a quantum computer that use photons, in other words light, instead of electrons to transmit and process data.
This would solve a number of technological and financial challenges associated with quantum computers. As current quantum computers are extremely sensitive to all external interference, they must be totally isolated and kept at temperatures close to absolute zero. This make them prohibitively expensive.
“The development of a light-powered quantum computer would be a major breakthrough akin to the arrival of electronics and the invention of conventional computers a few decades ago. Such a quantum computer would be capable of performing at room temperature and could be easily adapted to mass production. In our study, we demonstrate two-particle quantum interference in a PT-symmetric system. This discovery may one day be utilised in quantum computers,” Ornigotti concludes.
Text and photos: Jaakko Kinnunen