Physicists in the US have created a device that can emit single photons of the right shape and colour for use in quantum information. The advance is another step in the development of practical quantum-computing and quantum-cryptography systems.

Quantum computing exploits the peculiar laws of quantum physics to process certain calculations much faster than any of today's computers, whereas quantum cryptography uses those laws to prevent eavesdropping on secure communications. Both rely on the transmission of quantum information, and one of the best media for transmitting quantum information is single photons.

However, photons also come with some practical difficulties. Transferring quantum information over long distances requires telecommunications optical fibres, which work most efficiently at infrared wavelengths. Storing quantum information involves devices called quantum memories, which prefer photons at visible or near-visible wavelengths and with a certain "shape" or intensity profile. As a result, researchers have been trying to develop devices that convert telecommunications-band photons to photons that are compatible with quantum memories.

The past several years has seen a number of research groups come up with methods to manipulate either a single photon's shape or its wavelength. Now, however, Matthew Rakher and others at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, have devised a way to do both at once. "Our work provides a method to take telecommunications-band single photons, which are ideal for transmission, and change their wavelength and shape so that they can be stored in visible-wavelength quantum memories," says Rakher.

Stronger and faster

To release single photons, Rakher and colleagues use a quantum dot, which is a semiconductor version of a single atom. A quantum dot has discrete energy levels and can reliably emit a single photon every time it is excited, usually with infrared wavelengths of about 1300 nm. The NIST researchers collect these photons in a fibre optic and direct them to a crystal, where they are combined with a stronger and faster laser pulse that has a wavelength of about 1550 nm. This prompts a process known as sum-frequency conversion, which translates the photon's wavelength to a visible 710 nm. What is more, the single photons adopt the laser pulse's tighter shape.

Wolfgang Lange at the University of Sussex in the UK praises the results of the NIST researchers, but notes that more work needs to be done. "In particular, the efficiency of the source should be enhanced," he says. "But [their demonstration] is a very important step forward on the way to the perfect single-photon source, bridging the gap between quantum dots and devices for transferring, processing and storing quantum information."

Rakher thinks that the next step for his group is to change the wavelength and shape of the single photons to specifically match the requirements of quantum memory. "This will be a crucial step in developing quantum-dot single-photon sources for use in quantum-information applications," he says.

The research is due to be published in Physical Review Letters.