13 Dec 2001 - Cambridge, UK
Single Photon Emitting Diode Prevents Hacking Of Fibre-Optic Networks
Researchers at Toshiba Research Europe Limited (Cambridge, UK) have developed a new type of light emitting diode (LED) that fires out photons (the particles, or quanta, of light) one at a time. One of its applications is quantum cryptography, a secure form of optical communications that is safe from hacking.
Working in collaboration with the University of Cambridge, the Toshiba team has found that by incorporating semiconductor nano-technology into an LED they can trigger the emission of single photons at regulated times. Their advance is reported in the prestigious research journal, Science on 4 January 2002.
Dr Andrew Shields, who leads this work at TREL says: “A single photon source is a building block for a wide range of applications in quantum information technology, of which secure optical communications is the most immediate. In the future we may see that quantum effects enable many new optical technologies, rather like the laser did a few decades ago.”
Toshiba has been developing single photon technology for quantum cryptography, a technique to authenticate users of an optical network, or for two parties on the network to form a shared secret key. Such keys can subsequently be used for the encryption of data sent on the network between the two parties. Confidentiality of network communications is of great importance for e-commerce, as well as in the financial, healthcare and government sectors.
An attraction of quantum cryptography is that fundamental laws of nature (Quantum Mechanics) guarantee its security. These laws dictate that any attempt by a hacker to read the single photon signals can be detected by the sender and intended recipient. In contrast to methods based on codes, the keys formed by quantum cryptography can, in principle, be completely uncrackable. Another advantage is that it allows the key to be changed each time, reducing the risk due to misappropriation of a stolen key.
Until now, quantum cryptography has not been fail-safe, because conventional light-emitting diodes and lasers sometimes (unavoidably) produce two or more photons, which could allow a hacker to determine parts of the key without detection. Thus the single photon LED is crucial to ensuring the unconditional security of quantum cryptography.
Technical background
(i) Single Photons
The idea that light consists of indivisible units, or quanta, was first introduced by Planck in 1900. Planck found he could accurately describe the form of the emission spectra of a hot object, if he assumed the light was emitted in multiples of a certain quantum. However, at the time, there was no apparent explanation for this mathematical fix, and Planck later distanced himself from the idea. Despite this the assumption proved to be very successful. In 1905 Einstein gave further credibility to quantum theory by showing it could also explain the photoelectric effect.
One photon corresponds to an extraordinarily ‘dim’ optical signal, as can be gauged from the fact that an ordinary 100W light bulb emits 10,000,000,000,000,000,000 (10 billion billion) photons every second (approximately). The human eye is an extremely sensitive detector, capable of detecting a single photon. However, the brain averages out these single photon signals, as otherwise dimly lit objects would show speckle.
(ii) Single Photon Emitting Diode
Quantum cryptography requires a source that generates single photons at regularly spaced time intervals. However, conventional light emitting diodes (LEDs) and laser diodes emit photons at completely random times. This means that under pulsed operation, they produce pulses with a statistical mixture of one, two or more photons. Since this is true even if the laser or LED is very strongly attenuated, multi-photon pulses are unavoidable with conventional light sources.
Multi-photon pulses render quantum cryptography insecure because a hacker could measure one of the photons in the pulse, while allowing the others to pass undisturbed. This would allow the hacker to gain information about the key while remaining undetected. A single photon source is therefore crucial for secure optical communications.
Light emitting diodes convert electrical energy into optical. A current applied to the device, injects electrons and holes (holes are positively charged particles in the semiconductor — they are actually the absence of an electron) into the emissive region, where recombine to emit photons (lots of them).
At the heart of the Single Photon Emitting Diode lies a tiny volume of semiconductor, called a quantum dot. The quantum dot, which typically measures about 15nm in diameter and 5nm in height, is so small that it can capture at most only two electrons and two holes from the applied current pulse. Recombination of a single electron and a single hole in the dot results in the emission of a single photon. We control the level of the applied current pulses so that the dot captures on average one electron and one hole, and thus produces one photon per pulse. If the dot by chance captures two electron-hole pairs, it will emit two photons. However, since the extra photon is emitted at a different wavelength, we can block it using a filter. Thus, in this mode of operation, we can ensure that only one photon is emitted per applied current pulse. On the other hand, by setting a larger current pulse height, so that the dot captures two electron-hole pairs, we can configure the device to produce pairs of photons at distinct wavelengths.
(iii) Quantum Cryptography
Cryptography is best known as a way of keeping the contents of a message secret. We usually think of the main practitioners of cryptography as being spies and diplomats who wish to communicate secretly. In the future, however, many people predict the widespread use of cryptography as one of the enabling technologies of e-commerce and the network society in general. Confidentiality of network communications, for example, is of great importance for e-commerce and other network applications. However, the applications of cryptography in e-commerce go beyond simple confidentiality. In particular, cryptography allows the network business and customer to verify the authenticity and integrity of their transactions. If the trend to a global electronic marketplace continues, better cryptographic techniques will have to be developed to protect business transactions.
Sensitive information sent over an open network may be scrambled into a form that cannot be understood by a hacker or eavesdropper. This is done using a mathematical formula, known as an encryption algorithm, which transforms the bits of the message into an unintelligible form. The intended recipient has a decryption algorithm for extracting the original message. There are many examples of information on open networks, which need to be protected in this way, for instance, bank account details, credit card transactions, or confidential health or tax records.
In order to allow different users to use the same algorithm, the algorithm is used in conjunction with a secret key, a long sequence of binary numbers, which is known only by the legitimate users. Only users sharing the same key will be able to decrypt each other's encrypted messages. Since the key allows access to the encrypted information, it is of paramount importance that it is kept secret.
Before two parties can send information securely, they must first exchange a secret key. This however presents a dilemma, sometimes called the Catch 22 of Cryptography — how can the two parties exchange a key secretly before they can communicate in secret? Even if the sender and receiver found a channel that they believed to be secure, they still could not be sure, with total certainty, that the key was transmitted securely.
Quantum cryptography is a technique for two parties to form a key on an open optical network. Such keys can subsequently be used for the encryption of data sent on the network between the two parties, or alternatively, be used for an ‘uncrackable’ type of encryption called the Vernam cipher. An attraction of quantum cryptography is that fundamental laws of nature (Quantum Mechanics) guarantee its security. It allows the detection of unauthorised eavesdropping, as well as providing a guarantee of security when there is no eavesdropper present. This is not possible using any other form of key distribution, which relies either upon the difficulty of factorising large numbers, or the assumed privacy of the network.
In optical quantum cryptography the bits used to form the key are carried by single photons travelling either, along an optical fibre, or in an optical free space link. Information can be encoded on the photons in a variety of ways, such as by their polarisation or phase. Because the information is carried by a single photon, it is not possible for a hacker to tap in and remove part of the signal. Since single photons don't split, if the hacker measures the photons on the fibre, they will not be received at the other end, alerting the intended recipient to the presence of the hacker. Furthermore, the technique is also secure from a slightly more sophisticated type of eavesdropping where the hacker first measures the photons and then retransmits them. This is because the laws of quantum mechanics tells us quantum bits (or qubits) of information, such as encoded single photons, have the peculiar characteristic that they are disturbed by measurement. This fact allows the legitimate receiver of the message to test whether it has been intercepted or altered by a hacker on the channel.
TREL are developing a nano-technology for the generation and detection of single photons and applying these to secure optical communications. They have already developed a single photon detector based on nano-technology with advantages for quantum cryptography. Their latest advance uses similar technology to generate the single photon signals.
Terms and Conditions
