LED fires one photon at a time

By Eric Smalley, Technology Research News

The weird nature of quantum physics makes perfectly secure communications possible. The technology has existed in the laboratory for several years -- all that remains is figuring out how to make it practical.

Scientists at Toshiba Research and the University of Cambridge have taken an important step in that direction by making an electronic device that emits single photons on demand. The device could boost the transmission rates of secret communications and would be smaller and easier to use than similar light sources.

Quantum cryptography uses strings of individual photons, which are the indivisible particles of light, to make the mathematical keys that scramble secret messages. The keys are long, random numbers in the form of bits, the ones and zeros of digital communications. Ordinary communications transmits bits as light pulses, but because each light pulse contains many photons an eavesdropper could siphon some of them off to record the series of pulses to get a copy of the key without being detected.

However, when the keys' bits are encoded in the quantum states of individual photons -- like how they are polarized -- eavesdroppers can't escape detection. Because a photon cannot be split, an eavesdropper can't look at it without stopping it from reaching the intended receiver. And an eavesdropper can't cover his tracks by making copies of the photons he intercepts because he cannot reliably recreate their quantum states, which means the sender and receiver can compare notes to see that some of the photons have been altered.

When a sender and receiver know they have an uncompromised key, the sender can use it to encrypt messages that only the receiver can unscramble.

Making practical quantum cryptographic systems requires light sources that produce one photon at a time. A candle flame emits about one hundred thousand trillion photons per second, many at the same time.

Even the dimmest possible ordinary light source occasionally emits two photons at once. "We can control and trigger the emission time of the photons," said Andrew Shields, a group leader at Toshiba Research Europe in Cambridge, England.

Single-photon light sources are not new, but previous devices have all been triggered by lasers. "This is a cumbersome and expensive arrangement that would be difficult to achieve outside the laboratory," said Shields. "The new device is driven by a voltage so [it] is more robust, compact and would be cheaper to manufacture."

The researchers' single-photon source, a special type of light emitting diode (LED), contains a layer of quantum dots surrounded by layers of semiconductor material. Each quantum dot, which is a speck of semiconductor material about 20 nanometers in diameter, holds a single electron when a voltage is applied to the device. When the negatively- charged electron combines with a positively-charged hole in the quantum dot, it releases the energy as a single photon. A nanometer is one millionth of a millimeter.

The diode is capped by a metal layer with a series of small openings that block all but a single quantum dot per opening. By pulsing electrical current through the device, the researchers cause the quantum dots to emit a photon per pulse.

The device can theoretically emit a photon every half a nanosecond, said Shields. A nanosecond is one billionth of a second. But in practice the researchers' diode does not emit a photon with every pulse.

"The efficiency has not been optimized in this prototype, so [it] is quite low," said Shields. "If we use a cavity structure to direct more of the light out of the device in a certain direction, we can expect efficiencies exceeding 10 percent."

Ten percent efficiency could be good enough for practical devices. A potentially bigger hurdle is the cold temperatures needed to run the diode. The researchers' prototype operates at five degrees Kelvin, or -268 degrees Celsius.

"We have already seen efficient emission from quantum dots at temperatures exceeding [-73 degrees Celsius], for which cryogen-free thermal-electric cooling can be used," said Shields. "We hope to be able to push this further to room temperature."

A single-photon source that is triggered by an electrical current would be much more practical than an optically triggered single-photon source, said Gerard Milburn, a physics professor at the University of Queensland in Australia. "The control circuits could be integrated into the device producing the photons and processing their detection."

Without single-photon sources, researchers have to use privacy amplification techniques to ensure that transmitted bits remain secret, which results in less efficient transmission rates, said Richard Hughes, a physicist at Los Alamos National Laboratory.

This new light source technology could lead to higher secret bit rates if it could be made into a practical device, he said. Making an electrically-driven device is a big step in that direction, "but it would also be important for a practical device to operate at a temperature that would not require the user to deal with cryogens."

The researchers next steps are to increase the efficiency and raise the operating temperature of the single-photon diode, said Shields. "There are technological challenges to overcome, but we think we know the solutions. We think we can make a useful device within three years," he said.

Shields' research colleagues were Zhiliang Yuan, Beata E. Kardynal and R. Mark Stevenson of Toshiba Research, Charlene J. Lobo, Ken Cooper and David A. Ritchie of the University of Cambridge, and Neil S. Beattie and Michael Pepper of both institutions. They published the research in the December 13, 2001 online issue of the journal Science. The research was funded by Toshiba Corporation in the Engineering and Physical Sciences Research Council of the UK.

Timeline:   <3 years
Funding:   Corporate; Government
TRN Categories:   Optical Computing, Optoelectronics and Photonics; Quantum Computing; Semiconductors
Story Type:   News
Related Elements:  Technical paper, "Electrically captured in Single Photon Source," Science, online December 13, 2001


December 19, 2001

Page One

LED fires one photon at a time

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Temperature changes laser color


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