New computer takes quantum leap

Researchers at the University of Rochester have built a simple computer that combines the computing power of quantum mechanics with the ease of manipulating light.

The device is said to prove that a specific quirk of atoms, which lets scientists conduct huge computations almost instantly, can be mimicked by light.

The device mimics quantum interference, a property that makes quantum computers exponentially faster at tasks such as breaking encryption codes or searching huge databases. Instead of interference, conventional computers use electrons to perform tasks sequentially.

The new device proves that using light interference is just as effective as quantum interference in retrieving items from a database.

‘There’s a big push to explore information processing based on quantum mechanics,’ said Ian Walmsley, professor of optics at the University of Rochester. ‘What we’ve shown here is that if you have a quantum computer that is based entirely on quantum interference, we can build you a computer that is equally efficient, based entirely on light interference. And light is a whole lot easier to manipulate than quantum systems.’

One of the biggest limitations of quantum computers had always been thought to be their need for entanglement—a condition where different particles become linked, sharing many similar properties. Entanglement is difficult to achieve, and so far it has not been done for more than a few particles at a time.

Scientists then found that entanglement may not be necessary for operations such as database searches if quantum interference was used. When Walmsley heard this, he was sure he could build a computer that used light interference instead of subatomic particle interference.

Walmsley’s device uses a piece of transparent tellurium dioxide called an acousto-optic modulator, which acts as the database by storing the information in the form of acoustic waves. A transducer vibrates against one side of the modulator; sending waves through it much like a stereo speaker would send sound waves through the air.

The waves slightly compress some parts of the modulator and slightly expand others, creating a pattern in which the database information resides.To search the database, Walmsley directs a beam of light toward the modulator. The light is first split into two, with one part travelling through a prism so that a range of different frequencies of light shines on the modulator.

Each frequency shines through a different compressed or expanded part of the tellurium dioxide, which bends that frequency of light the way a straw appears bent when sticking out of a glass of water. The range of frequencies is then recombined into a single beam. By mixing the new beam with the original beam that entered the device, a single frequency will emerge as having been altered by its trip through the database.

With Walmsley’s device, 50 different frequencies of light shine through the modulator, and if the 20th frequency is the altered one, then Walmsley knows that the bit of information he was searching for is located at position 20 in the database.

A conventional computer would have had to check 20 times to find the location. If the database in question were a phone book, the search for a single phone number could take a conventional computer several million searches, while a light-based device could pinpoint the number in just one.