First Miniaturized Quantum Logic Gate Created

The device created by the UK researchers, measuring only a few millimeters across, has the capability to process the signals carried by individual photons of light, much in the same way electronic logic gates control electrical signals. While electronic logic gates may be manufactured into tiny silicon chips and incorporated into microprocessors, you should probably know that the previous version of the quantum logical gate occupied a surface measuring several square meters on an optical bench.

Photons of light are believed to be used in the near future as quantum bits of information - qubits - in future quantum computers, thus recreating the first millimetric silicon logic gate chip could represent the first step towards creating the first quantum computer. However, qubits traveling through traditional optical devices such as fiber optic or air lose their quantum properties, because photons hardly interact with each other.

First quantum gate

This means that processing photons through devices such as logic gates is extremely hard, even while being performed in controlled conditions. Nonetheless, this didn't stop researcher Jeremy O'Brien from trying. Back in 2003, O'Brien was able to construct the first quantum logic gate for a single photon. The NOT quantum logic gate he created benefits from two signal inputs. If there is no photon input, then the logical state of the quantum gate remains unchanged and is equivalent to '1' logic, but while a photon is introduced into the gate - '1' logic signal - the gate shift its state from '1' to '0' logic. The reverse state is imposed when the photon input is removed.

As you already probably realized, this was the quantum gate spreading allover the bench table of a whole laboratory. Albeit, computers rely on millions of gates to function properly.

Minimization

With the help of Alberto Politi from the University of Bristol, O'Brien tested with hundreds of versions of the same gate on silicon chips only a millimeter in size. Opposed to the use of mirrors and beam splitters, to make the tiny quantum gate work, O'Brien decided to use transparent silica in which micrometer-wide channels were cast through an industrial process, thus creating coupled waveguides.

Individual gates contained up to six parallel waveguides separated through only a few tens of micrometers. First, the device didn't work because the photons were not interacting with each other, so O'Brien created the waveguides even closer, so that they are separated only by a distance par for that of the wavelength of the photons used as signals.

By doing so, light is allowed to leak and interact through a process known as evanescence, through which light beams are split in two. At some point, the two photons of light had to experience light interference, thus becoming entangled with one another. Three entangling processes must take place inside the CNOT gate, in order to make it work.

O'Brien says that the CNOT gates developed by him are extremely good at entangling photons, but only have a success rate of 1/9, thus only 10 percent of the signals are processed in a correct manner. Theoretically, information processing can be further improved by filtering out the faulty signals with the help of qubits through additional coupled waveguides, to verify whether the resulted signal is correct.

"A very important step towards practical quantum computers, but a very small step," says O'Brien describing his invention.