A team from the Faculty of Physics, MSU developed a method for creating two beams of entangled photons to measure the delay between them. In the future the results of the study may be used in high-precision measurements, material studies, and informational technologies. The article was published in Optics Letters journal.

David Nikolaevich Klyshko, professor of the Chair of Quantum Electronics at the Faculty of Physics, MSU discovered spontaneous parametric down conversion in 1966 and later on was awarded the State Prize for it together with his colleagues. This discovery marked the beginning of quantum optics, a popular area of physics that studies quantum properties of light. The effect is quite simple: a photon that comes into a crystal is divided into two other photons with the sum of their frequencies equal to the frequency of the original photon. Notably, this process may be observed only in non-linear crystals in which the frequency of photons may change in the course of scattering.

The effect was used in many areas, including the studies of crystals themselves, measurements of efficiency in light-sensitive detectors, and especially in quantum optics where certain success was reached in such fields as quantum cryptography, quantum calculations, and in the elegant effect of quantum teleportation. The fact is that the new photons are entangled: if the polarization of one photon measures, the quantum state of polarization of the second one is altered as well. Any changes in the first photon are immediately “sensed” by the second one. However, this effect cannot be used to exchange information.

In a recent experiment MSU-based scientists under the guidance of the leading research associate of the Chair of Quantum Electronics (Faculty of Physics, MSU) Maria Chekhova tried to generate not separate pairs of entangled photons but bigger amounts – two powerful beams.

“In this case the correlation is not between individual photons but the whole beams, and the question is, what is the precision of this correlation?” explains Pavel Prudkovskii, a co-author of the work. “If we slow one beam down, at what point in time would we notice the desynchronization?”

To answer this questions the scientists had to make photons with different frequencies not to disperse from the crystal at different angles but to form two beams of light and to move together in parallel lines. On order to obtain this effect, lithium niobate crystals that are often used in such experiments had to be grown with a certain structure with a pre-calculated additional non-periodic domain lattice.

In the course of the experiment the scientists made one of the two entangled photon beams to stall a little and to go along an auxiliary path. After that both beams got to the second crystal – the usual lithium niobate. “In this crystal the summation of frequencies took place. If the beams arrive in sync, it is more efficient that in other cases,” commented Prudkovskii. “As a result we get a narrow peak in the summary frequency signal. The width of it is 90 femtoseconds (10 ^-15 sec), and this is our main achievement.

Thus, the scientists managed to experimentally register almost the smallest possible shift between twin beams of entangled photons that may be observed by measurement devices. According to the team, it is possible to further reduce this value, but to do so, the scheme of the experiment should be made more complex. “Right now 90 femtoseconds is a record-setting value, but it can be reduced, and we know what to do for it,” explained Prudkovskii. According to him, the wave period of laser emission is only several femtoseconds, so it is possible to reduce the length of such delay down to a dozen or so.

The results of the study may be used for the development of encrypted communication channels protected from any interruptions or bugging. If a criminal tries to intercept a beam of entangled photons, he or she will have to stall it for some time, and the delay won’t be left unnoticed. Moreover, the registration of a delay in two quantum-entangled beams may be used to detect minor admixtures in substances.