| WIKI | FAQ | Tech FAQ | |
 http://csua.com/feed/ | |
| ||||||
| 7/9 |
| 2005/5/26-27 [Science/Physics] UID:37845 Activity:low |
5/26 LED capable of emitting exactly one photon each time:
http://www.newscientist.com/article.ns?id=dn7420
\_ Two questions: 1) The idea is that the receiver will know that a
message is intercepted because a photon can only be received by
either the interceptor or the receiver, not both. But what if the
intercepter then injects a different photon with the same wavelength
back to the stream right away? Then the receiver won't notice,
right? 2) Regardless of #1, isn't it more important to prevent a
message from being intercepted, than to know that the message has
been intercepted after it happens? It doesn't help much to find out
that someone has intercepted your SSN when it is transmitted this
way.
\_For quantum criptography you need a PAIR of coupled particles,
one for the sender and one for the reciever. Then both
sides make spin measurements on either the x & z axis randomly. By\
comparing a subset of those measurements you can determine if some-\
one was listening in. The rest is used for a key. -scottyg
\_ You don't send your SSN that way, you send a random string of
bits over the quantum channel, throw away the ones that get
read by the evesdropper, and use the remaining bits as the key
to a one time pad that you use to send the SSN over a classical
channel.
\_ This answers #2, thanks. What about #1?
\_ Not an expert, but wouldn't there be a machine-perceptible
delay as the intercepting device receives the photon and stores
its identifying info, then recalibrates and sends a different
photon with the same wavelength to the originally designated
receiving end? |
| 7/9 |
|
| www.newscientist.com/article.ns?id=dn7420 Advertising The first photon gun capable of firing single particles of light over opt ical fibres was unveiled on Tuesday. The breakthrough may remove one of the final obstacles keeping perfectly secure messages from being sent ov er standard telephone fibres. Encryption techniques change each character in a message in a way that ca n be reversed by a receiver who possesses the relevant key. But sending the key to the receiver is just as troublesome as sending the message as it too can be intercepted - a problem known as key distribution. Twenty years ago, North American physicists Giles Brassard and Charles Be nnett outlined a way to send a key without anyone being able to eavesdro p Their idea rests on the notion that a message sent using quantum part icles - such as photons - is so fragile that measuring the photons chang es their properties. So anybody listening in to a transmission would des troy it - which the sender and receiver would easily notice. But so-called quantum encryption works only if the key is sent using indi vidual photons, rather than the pulses of many photons that are used for communication today. Too many photons In the last year, a number of companies have begun selling quantum encryp tion kits that create single photons by reducing the intensity of a lase r beam so that it produces pulses each containing less than one photon, on average. But there always remains a small probability that any pulse will contain two or more photons. This is a potentially serious weakness because a hacker could intercept t he extra photons without the sender and receiver being any the wiser. Now Andrew Shields and colleagues at Toshibas Cambridge Research Laborato ry in the UK have developed a light-emitting diode (LED) that allows a d ata transfer rate of 1 megabit per second. And crucially, the photon gun works at the same light wavelength as comme rcial optical fibres - at 13 micrometers. It could be commercially avai lable within two to three years, says Shields. Exotic clusters The device is essentially a standard LED made of gallium arsenide but con taining a layer of quantum dots - exotic clusters of indium arsenide eac h containing just a few thousand atoms. In a conventional LED, electrons in the central layer combine with holes - or absences of electrons - re leasing a photon in the process. In the new device, this recombination takes place only inside the quantum dots which emit photons of a wavelength similar to their size. So the s ize of the dots determines the wavelength at which the device operates. A masking layer then allows only the light from a single dot to escape, ensuring that the device emits only one photon at a time. This device should finally close the security loophole in the current qua ntum encryption techniques. We are in the process of building our own qu antum encryption equipment, says Shields. It will make the process of communicating using the quantum properties of light much more efficient, notes Will Stewart, chairman of Innos, a sil icon research and development company in the UK. The Toshiba team unveiled the device at the Quantum Electronics and Laser Science Conference in Baltimore, US. |