Cambridge semiconductor photon detector can pick out individualsSteve BushResearchers at Toshiba Research Europe (TREL), on the Cambridge Science Park, havedeveloped a low-noise semiconductor photo-detector capable of registering individualphotons.
At the heart of the Fet-like device is a layer of quantum dots, sited a few nanometresabove the Fet's conduction channel.
The geometry is such that any single electron entering or leaving any dot makes aclearly discernable change in channel current.
Photon to electron conversion takes place in semiconductor material in the vicinity ofthe dots. Here the incoming photons knock electrons from atoms to form electron-holepairs.
"The carriers can be created in several places and either the electrons or holescan be directed to the quantum dots. In the experimental device the photons generatecarriers in the channel semiconductor and the holes migrate to the dots. The electronsmove away with the channel current,"said Andrew Shields, team leader at Toshiba.
At the moment each dot can only accept one hole (lose one electron). "The fieldsdirect any holes to dots that have not yet lost an electron,"said Shields.
A gate electrode in the structure is provided to allow the dots to be‘reset'. The active area of the experimental device is 2 x 1?m and it hasaround 100 quantum dots.
Operation has only been tested at liquid helium temperatures. "200K or roomtemperature operation is desirable and should be possible," said Shields.
Not all photons are detected in the experimental device but shields expects quantumefficiency to improve dramatically in later versions.
Quantum Cryptography Cryptographic techniques are available that are considered to be secure enough for most applications by most people.
Where they fall down is when a new numerical key has to be sent to allow a recipient to unlock a coded message. The key is vulnerable to eavesdropping on route to the recipient.
Quantum cryptography allows a key to be sent over an ‘open' optical path in such a way that it either arrives unobserved, or it is indelibly corrupted by the eavesdropping process. And this is guaranteed by the laws of quantum mechanics.
No other form of key distribution with this level of security is yet known.
The key is encoded by setting the polarisation or phase of individual photons in a stream, one bit per photon. As photons cannot be split, any photon that has been illegitimately observed will not arrive at the legitimate recipient.
If a hacker measures the photons and then re-transmits them, quantum mechanics predicts that the quantum bits (or qubits) are disturbed by manipulation in a way that can be detected downstream.
At the recipient, receiver error rates can be important. ‘Privacy amplification' can form a secure key from data with a few per cent errors. At tens of per cent, it becomes impossible to distinguish the errors from the presence of an eavesdropper and quantum key distribution fails.