| Professor Alex Lvovsky (left) and PhD student Alexander Ulanov (right) are pictured with their quantum-entanglement setup. (Credit: Russian Quantum Center) |
Scientists from the Russian Quantum Center (Moscow, Russia) have developed a method to restore quantum entanglement
and have tested this method in an experiment in which they restored the
level of quantum correlation between pulses of light in two optical
channels. This research significantly broadens the possibilities of quantum communication and quantum cryptography.1 The work is a step towards building a quantum repeater.
The phenomenon of quantum entanglement (or quantum correlation) plays a
particularly important role in secure quantum communication systems.
However, entangled states are very fragile, breaking easily during
transmission due to noise or optical losses. In addition, the very
creation of entanglement is quite a difficult process. Existing
equipment is only capable of transferring data at a distance of no more
than 100 km. Many research groups are now looking for ways to increase
this limited distance.
Recovery after 20-fold optical loss
Alexander Lvovsky’s group in the Quantum Optics Laboratory at the
Russian Quantum Center conducted a series of experiments in which they
restored the level of quantum correlation between pulses of light after
it was almost completely destroyed after passing through a 20-fold
optical loss. This corresponds to the level of loss in 65 km of ordinary
fiber-optic cable.
A nonlinear optical crystal of periodically poled potassium titanyl
phosphate was used as the source of entangled photons. Picosecond pulses
of light generated by a titanium:sapphire laser were fired at the
crystal. As a result, entangled photon pairs were produced in the
crystal and directed into two different optical channels. In one of
them, the light was subjected to 20-fold attenuation using darkened
glass, causing the level of entanglement to fall almost to zero. Then an
amplification procedure was applied, restoring the quantum properties
of light in the channel to levels close to those that occurred before
the loss.
The procedure is to mix the light pulse in the channel with a single auxiliary photon in a beamsplitter. A single-photon detector is placed at one of the outputs of the beamsplitter. If the detector clicks, this means that the photon has entered the beamsplitter and left. It would seem that the state of the second pulse to enter the beamsplitter (a part of the entangled state) should not change. But, because of paradoxical properties of quantum interference, the state changes in towards the strengthening of its quantum properties.
This phenomenon, discovered by Lvovsky and colleagues in 2002 (doi: 10.1103/PhysRevLett.88.250401), has been named quantum catalysis, because the auxiliary photon, like a chemical catalyst, is not itself involved in the reaction, but changes the state of light in the other channel.
Source: Russian Quantum Center
REFERENCE:
1. Alexander E. Ulanov et al., Nature Photonics (2015); doi: 10.1038/nphoton.2015.195
