Feature

The future of quantum IT

The word "quantum" can conjure images of scientists working on technology that will one day allow time travel, or, at the very least, permit travel to alternate realities.

Although such developments may currently be confined to an episode of Dr. Who, the use of quantum information technology in business computing will become a reality within the next ten years, says Andrew Shields, leader of the quantum information group at Toshiba's Cambridge Research lab.

Shields and his team have made a breakthrough that they say will allow information to be captured and returned to the user in a meaningful form after having been processed by a quantum IT program.

Shields says this would allow future computers to process complex computations in four months. This may seem like a long time, but it would take a 1Ghz computer of today's standards longer than the age of the universe to perform the same calculations.

In traditional computing, a bit is a fundamental unit of information, represented in binary as 0 or 1 and stored as an "on" or "off" signal on an electronic circuit. If you search a database for a particular result, the computer program methodically analyses each row of data in the database, bit by bit, for a positive match. Where there is a match, the database returns a value of "true" (where true is the binary state "1") or "false" (binary state "0") for no match.

In a quantum computer, the fundamental unit of information is a qubit. Qubits are stored on particles of light called photons. A qubit can exist not only in a state corresponding to the logical values 0 or 1, but also in states corresponding to a blend or superposition of these values.

When searching a database in quantum computing, the program would be able to "see" all results at once from a superposition instantly, rather than having to move through each row.

"The net effect of quantum computing is that data can be processed much quicker than under a traditional computing model," says Shields.

The other benefit of quantum computing is that it can enable secure communications over optical fibre links. This is where scientists at Toshiba labs believe they have made progress.

They say they have developed the first practical semiconductor device that can count the particles or photons in light signals.

The new device is a significant step towards viable quantum computers and communication systems, which exploit the particle-like properties of light.

Counting photons is necessary when sending secret digital keys over long distances.

"A simple semiconductor device that can count the photons in a light signal is important for several quantum applications. You can prevent eavesdropping on optical fibre links by counting if the photons have been interfered with," says Shields.

Lack of a suitable photon number resolving detector has been a major obstacle to real-world deployment of quantum technologies, says Shields.

In principle, quantum key distribution provides a secure means for transmitting secret keys between two parties on fibre optical networks. However, the QKD systems developed so far are vulnerable to hacking.

The weak laser diode used to generate single photon pulses that carry the quantum keys, will sometimes generate pulses with multiple photons.

As a result, an eavesdropper could split off one of these extra photons and measure it, while leaving the other photons in the pulse undisturbed, thus determining part of the key while remaining undetected.

Furthermore, an eavesdropper can determine the entire key, by blocking the single-photon pulses and allowing only the multi-photon pulses to travel through the fibre.

"Using these new methods for QKD we can distribute more secret keys per second, while at the same time guaranteeing their unconditional security. This enables QKD to be used for a number of important applications such as encryption of high bandwidth data links," says Shields.

Until now the most common semiconductor detector, the avalanche photo-diode, has been able to register only the presence or absence of one or more photons.

The detector, developed by Toshiba, however, is able to count the number of individual photons in a pulse. It is the first practical device with this capability.

The breakthrough is a result of a new technique developed by Toshiba to detect weak photon-induced avalanches. The electrical current caused by a single photon in a semiconductor is too weak to be detected quickly.

Avalanche photo-diodes work by amplifying this current a million-fold using an avalanche effect. Usually, however, the strength of the final current does not depend on the number of photons that initiated it.

The Toshiba device can detect photon-induced avalanches 20 times weaker than conventionally, and the strength of which scale with the incident number of photons.

It is a step in the right direction for quantum computing but there are many challenges that must be overcome before quantum computing becomes mainstream.

"We need lots of new technology for quantum computing to take off, such as quantum memory," says Shields.

Shields says that quantum memory can store information in quantum states. However, this is something the industry does not currently have. Quantum logic gates and sensors that can accurately detect quantum states are also pre-requisites.

Nevertheless, Shields remains confident that with the developments of his lab, it will be only a matter of time before we are all taking a quantum leap forward in computing.





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This was first published in July 2008

 

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