New Quantum States of Entangled Photons

Quantum model of topological states of two entangled photons. (Source: ITMO)

A research team from ITMO University, with the help of their colleagues from Moscow Institute of Physics and Technology and Poli­tecnico di Torino, has predicted a novel type of topological quantum state of two photons. Scientists have also applied a new, affordable experi­mental method for testing this prediction. The method relies on the analogy: instead of expensive experiments with quantum systems of two or more entangled photons, the researchers have used resonant electric circuits of higher dimensionality described by similar equations. The obtained results can be useful for the engi­neering of optical chips and quantum computers without the need for expensive experi­ments.

Light plays a key role in modern information tech­nologies: with its help, information is transmitted over large distances via optical fibers. In the future, scientists anticipate the invention of optical chips and computers that process infor­mation with the help of photons instead of electrons, as it is done today. This will decrease energy consumption, while also increasing the capa­bilities of computers. However, to turn these predictions into reality, funda­mental and applied research of light behavior at the micro- and nanoscale is needed.

Now, the physicists have theo­retically predicted the formation of a new quantum state of photons: two photons propagating in the array of quantum micro­resonators can form a bound pair and settle down on the edge of the array. A proper experiment demands special nano­structures, as well as special devices to create such quantum state of photons and detect it. Currently, such capabilities are available only to very few research teams worldwide. If conducting a precise experiment is too expensive, it may be useful to come up with a model, or an analogy, which would allow one to test the theo­retical assump­tions without spending too many resources. This is exactly what ITMO University physicists managed to do. They have drawn an analogy between a specific class of quantum systems and classical electric circuits of higher dimen­sionality.

“We connect various points on the board to an external power source and study the system’s response using a multimeter and oscillo­scope,” explains Nikita Olekhno, PhD student at ITMO University. “The result is described by classical equations that in our case coincide with the quantum equa­tions describing two-photon states in the array of qubits. The same equations must have the same solutions, and it doesn’t matter whether it’s a wave function of a quantum particle or an electric potential.”

Of course, the analogy that the scientists have come up with can not entirely replace experiments with quantum systems. However, the classical structure that was developed by the team allows researchers to conduct many experi­ments, providing valuable infor­mation for the field of quantum photonics. The fact that the scientists managed to find such an analogy for quantum systems of many particles for the first time is very promising.

“Theory is always ahead of experi­mental capa­bilities. To be at the forefront of theory, we study subtle effects that we will be able to detect experi­mentally only in several years,” says Maxim Gorlach, head of the project at ITMO University. “We are currently conducting a series of experi­ments in this field by researching topo­logical edge states of more exotic quantum systems and developing ways of their emulation. Such experiments are important both for funda­mental physics and future practical appli­cations.” (Source: ITMO)

Reference: N. A. Olekhno et al.: Topological edge states of interacting photon pairs emulated in a topolectrical circuit, Nat. Commun. 11, 1436 (2020); DOI: 10.1038/s41467-020-14994-7

Link: Topological Photonics, Dept. of Physics and Engineering, ITMO University, Saint Petersburg, Russia

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