Cryptography used to be of interest only to spies and the security services. However, the rise of e-commerce means that everyday actions, such as withdrawing cash from ATM machines or buying goods on the Internet, are mediated by cryptographic techniques that would be the wonder of WWII code breakers. The widespread deployment of such techniques shows that they are basically sound. However, all these cryptographic techniques rely on keys and ensuring the security of these keys is a problem. Quantum cryptography (QC) is a technique for distributing keys in a way that would provide, in theory at least, total protection against hackers and eavesdroppers. The basic principle is to encode the 0s and 1s of a classical cryptography key on the polarisation state of individual photons. An eavesdropper attempting to 'read' one of these photons risks changing its polarisation state, thus revealing his/her presence. As this idea depends on creating an optical link operating at the single photon level, barriers to its implementation were considerable. Progress has been steady and transmission distances over fibre now exceed 100 km. The article looks at the latest developments in QC.
Cryptography used to be of interest only to spies and the security services. However, the rise of e-commerce means that everyday actions, such as withdrawing cash from ATM machines or buying goods on the Internet, are mediated by cryptographic techniques that would be the wonder of WWII code breakers. The widespread deployment of such techniques shows that they are basically sound. However, all these cryptographic techniques rely on keys and ensuring the security of these keys is a problem. Quantum cryptography (QC) is a technique for distributing keys in a way that would provide, in theory at least, total protection against hackers and eavesdroppers. The basic principle is to encode the 0s and 1s of a classical cryptography key on the polarisation state of individual photons. An eavesdropper attempting to 'read' one of these photons risks changing its polarisation state, thus revealing his/her presence. As this idea depends on creating an optical link operating at the single photon level, barriers to its implementation were considerable. Progress has been steady and transmission distances over fibre now exceed 100 km. The article looks at the latest developments in QC.
In our modern era of telecommunications and the Internet, information has become a valuable commodity. Sometimes it must therefore be protected against theft - in this case, loss of secret information to an eavesdropper. Most of today's transactions are protected using encryption unproven to be secure against a computational attack by a classical computer and, in fact, the standardly used encryption algorithms are provably vulnerable to the mind-boggling parallelism of a quantum computer, should one ever be physically realized. Enter quantum cryptography. Underlying nearly all forms of encryption is the necessity for a truly secret key, a random string of zeros and ones; the basic notion of quantum cryptography is to employ single photon transmissions (or the closest attainable approximation to these) to distribute the random key material, while removing the threat of an undetected eavesdropper. Now, nearly twenty years since the seminal quantum cryptography paper by Bennett and Brassard (Bennett C H and Brassard G 1984 Proc. IEEE Int. Conf. on Computers, Systems, and Signal Processing (Bangalore) (New York: IEEE) pp 175-9), we take a look at several state-of-the-art implementations, and glimpse how future quantum cryptosystems might look. We start with papers from three of the world's leading experimental quantum cryptography efforts: Stucki et al and Bethune and Risk describe working systems for quantum key distribution (QKD) over telecommunications fibres (at 1550 nanometres and 1300 nanometres, respectively). The former's achievement of quantum key exchange over 67 kilometres of optical fibre is a world record, as is the experimental demonstration by Hughes et al of daylight free-space QKD over a 10 km atmospheric range. Next, Lütkenhaus and Jahma explore the possible vulnerabilities of such systems (which employ attenuated laser pulses instead of actual single photon states) to conceivable future eavesdropping technologies. Enzer et al have implemented a totally new protocol, using polarization-entangled photons, which in some circumstances can tolerate higher error rates than the traditional one of Bennett and Brassard; moreover, the use of entanglement provides a means of `automatic source verification'. Finally, looking to the future, Elliott gives a provocative view of how these technologies may be merged into network operation, and Shapiro describes a method to combine a novel source of entangled photons with a means to transfer the photons' quantum state to trapped-atom quantum memories. If realized, these systems could presage the world's first quantum network. Focus on Quantum Cryptography Contents Quantum key distribution over 67 km with a plug&play system D Stucki, N Gisin, O Guinnard, G Ribordy and H Zbinden Autocompensating quantum cryptography Donald S Bethune and William P Risk Practical free-space quantum key distribution over 10 km in daylight and at night Richard J Hughes, Jane E Nordholt, Derek Derkacs and Charles G Peterson Quantum key distribution with realistic states: photon-number statistics in the photon-number splitting attack Norbert Lütkenhaus and Mika Jahma Entangled-photon six-state quantum cryptography Daphna G Enzer, Phillip G Hadley, Richard J Hughes, Charles G Peterson and Paul G Kwiat Building the quantum network Chip Elliott Architectures for long-distance quantum teleportation Jeffrey H Shapiro Ground to satellite secure key exchange using quantum cryptography J G Rarity, P R Tapster, P M Gorman and P Knight Method for decoupling error correction from privacy amplification Hoi-Kwong Lo Paul G Kwiat University of Illinois at Urbana-Champaign, USA
Quantum cryptography was designed to provide a new approach to the problem of distributing keys for private-key cryptography. The principal idea is that security can be ensured by exploiting the laws of quantum physics and, in particular, by the fact that any attempt to measure a quantum state will change it uncontrollably. This change can be detected by the legitimate users of the communication channel and so reveal to them the presence of an eavesdropper. In this paper I explain (briefly) how quantum key distribution works and some of the progress that has been made towards making this a viable technology. With the principles of quantum communication and quantum key distribution firmly established, it is perhaps time to consider how efficient it can be made. It is interesting to ask, in particular, how many bits of information might reasonably be encoded securely on each photon. The use of photons entangled in their time of arrival might make it possible to achieve data rates in excess of 10 bits per photon.
Quantum Cryptography is based on the use of subatomic particles (photons) and their intrinsic quantum properties (photon polarization) to develop an unbreakable system. It is not possible to measure the quantum state of any system without affecting the system. This property provides secure transmission of key between sender and receiver. Quantum Cryptography is a way to combine the relative ease and convenience of key exchange in public key cryptography with the ultimate security of a one-time pad. Quantum techniques for key distribution, the classically impossible task of distributing secret information over an insecure channel whose transmissions are subject to inspection by an eavesdropper, between parties who share no secret key initially. Quantum Cryptography uses the principles of Quantum Mechanics to implement a cryptographic system. The key problem which is solved by using quantum techniques is that of eavesdropping detection. The bits are represented as qubits, physically modeled by photons, and communicated over a quantum channel. The polarization states of photons represent 0's and 1's. Keyword: Asymmetric key, Cryptography, Protocols, Quantum, symmetric key
The practical implementation of quantum information technologies requires, for the most part, highly advanced and currently experimental procedures. One exception is quantum cryptography, or quantum key distribution, which has been successfully demonstrated in many laboratories and has reached an advanced level of development. It will probably become the first commercial application of quantum information. In quantum key distribution, Alice and Bob exploit a quantum channel to create a secret shared key comprising a random string of binary digits. This key can then be used to protect a subsequent communication between them. The principal idea is that the secrecy of the key distribution is ensured by the laws of quantum physics. Proving security for practical communication systems is a challenging problem and requires techniques that are beyond the scope of this book. At a fundamental level, however, the ideas are simple and may readily be understood with the knowledge we have already acquired. Quantum cryptography is the latest idea in the long history of secure (and not so secure) communications and, if it is to develop, it will have to compete with existing technologies. For this reason we begin with a brief survey of the history and current state of the art in secure communications before turning to the possibilities offered by quantum communications. The history of cryptography is a long and fascinating one. As a consequence of the success or, more spectacularly, the failure of ciphers, wars have been fought, battles decided, kingdoms won, and heads lost. In the information age, ciphers and cryptosystems have become part of everyday life; we use them to protect our computers, to shop over the Internet, and to access our money via an ATM (automated teller machine). One of the oldest and simplest of all ciphers is the transposition or Caesarean cipher (attributed to Julius Caesar), in which the letters are shifted by a known (and secret) number of places in the alphabet. If the shift is 1, for example, then A is enciphered as B, B→C, · · ·, Y→Z, Z→A. A shift of five places leads us to make the replacements A→F, B→G, · · ·, Y→D, Z→E.
Quantum cryptography has been proven to be the only practical method for fast distribution of secret keys needed for encryption. It enables absolutely secure information transfer from one party to another and allows for the detection of an eavesdropper without sacrificing security. This project involved designing and building a quantum cryptography device. This device has applications in many fields, including internet security as well as military communications. The data are encoded on single photons, which are sent through free space over a distance of about 30 cm. The electronics, which control the device however, are equally suitable for operation with fibre optic cables. The device was designed to operate in dark conditions within a laboratory. Currently, no research group has demonstrated transmission speeds faster than 8 kb/s. The aim of this project was to achieve bit rates faster than 10 kb/s, while operating under strict economic constraints. Although quantum cryptography was not demonstrated in the laboratory due to problems with the detector, the emission end, which generates single photons of light with one of two different polarisations, was shown to work as it was designed. The current design has limited commercial future due to the dark conditions under which the communication must take place. The project however has demonstrated that quantum cryptography is getting closer to becoming a reality.
Quantum cryptography is a technique for establishing a fundamentally secure key for cryptography using the encoding of the key on single photon states in such a way that an eavesdropper cannot acquire any information about the key without impairing detectable errors in the communication channel [1]. In the recent years there has been a remarkable progress in experimental implementations at 850 nm and 1300 nm wavelengths with secret key generation at kbits/s up to 48 km distance. However, for 1550 nm wavelength, where fiber losses are as smallest, progress has been hampered by the lack of good single photon detectors.