Modern economic and social activity is characterised by the increasing amounts of information which are being transmitted over a variety of communication channels. Growth in data transmission is particularly associated with extended use of fibre-optic communications and the growth of internet access to both commercial and private premises.
It
is a basic expectation of users of such communication channels that their
transactions are entirely confidential or 'secure'. There is a particular
need to ensure the correct transmission of information relevant to the
growing volume of commercial transactions which utilise modern communications
channels. The essential need is for the development of techniques which
can ensure that 'eavesdroppers' cannot intercept transmitted messages
- so that the information can be transmitted securely. The technology
which is used to ensure secure transmission of data is termed cryptography.
In recent years, effort has been given to two new kinds of cryptography
- quantum cryptography and chaos cryptography. Research achievement related
to both these topics have been made at the Optoelectronics Research Group
in the University of Wales, Bangor.
In this article, attention is focused on the rather spectacular results which have emerged from research on quantum information processing - particularly in regard to so-called quantum teleportation.
Research
in Electronic Engineering and Computer Systems
To
set the scene, however, an outline is given of the research environment
within the School of Electronic Engineering and Computer Systems (SEECS)
at the University of Wales, Bangor. It should be mentioned at the outset
that the research effort in Bangor is distinctly international. Much of
the work is undertaken in collaboration with research in laboratories,
particularly in Europe, the Americas, Japan and Australia, with strong
contacts existing with groups in other parts of the world. Research students
and staff are similarly drawn from all parts of the globe. Research visitors
also add to the rich tapestry of international links which facilitate
much of the research activities in Bangor.
The research effort in SEECS, Bangor is organised into two main themes: 'Materials & Sensors', and 'Information Systems'. The former activity includes work on semiconductor polymer, magnetic and bio-electronic materials, and devices with uses in a range of applications, from biotechnology and medicine through to data storage and electrical insulation. A particularly prestigious project is work on developing a 'biofactory on a chip' technology for miniaturised manipulation of bio-particles in order, for example, to distinguish between cancerous and non-cancerous cells.
The 'Information Systems'
area encapsulated the convergence between modern day communications and
computing. Activities span advanced radio-navigation through optoelectronics
and quantum information processing, chaotic and classical control engineering
to software engineering.
So, for example, work is under way on computer software aspects of World Wide Web internet development. With the growth of web-based 'electronic business' or e-commerce, the needs for secure data transmission is strongly emphasised. In parallel developments of optical communications systems, there is a need to achieve secure transmissions of the high volume of data which can be carried by such communications systems. As one approach to attaining this requirement, there has been world-wide interest and activity directed at using the properties of chaotic systems, and particular efforts have been made to harness chaos in lasers.
With a view to wide applications of this technique, a particular focus of activity in this field has been the development of chaotic data encryption techniques suitable for use in optical fibre and free-space optical communications systems. The chaotic transmitter and receiver in such systems would be chaotic lasers, and specifically chaotic semiconductor lasers. Chaos can be obtained in such lasers in a number of ways, but the Bangor work has concentrated on utilising the effects of optical feedback, wherein a small amount of the light emitted by the laser is reflected back into the laser - simply by using a mirror - a so-called external cavity laser diode arrangement. This arrangement can be easily used to generate chaos in the light emission. The experiments performed at Bangor provide 'proof of principle' of the synchronisation procedure of chaotic laser diodes. The experiments have involved 'free-space' communication between chaotic laser diodes, where the synchronisation was effected optically. The work provides the basis for examining in detail the practicalities of implementing chaotic optical data encryption.
Quantum
Information Processing
In
1993, Charles Bennett (IBM, TJ Watson Research Center) and colleagues
theoretically developed a method for quantum teleportation. Now, a team
of physicists from Caltech, Aarhus University, and Dr. Sam Braunstein
of the University of Wales, Bangor have successfully achieved quantum
teleportation of optical coherent states.
Quantum teleportation is similar to the far-fetched 'transporter' technology used in the television series 'Star Trek'. "Quantum teleportation involves the utter destruction of an unknown physical entity and its reconstruction at a remote location," says Professor H. Jeff Kimble, the leader of the research group at Caltech, who with Braunstein, conceived the experiment. Using a phenomenon known as 'quantum entanglement', the researchers force a photon of light to project its unknown state onto another photon, with only a miniscule amount of information being sent between the two. This is the first time quantum teleportation has been performed with a high degree of 'fidelity', which means that the output reproduces the input with good accuracy. Quantum teleportation was announced earlier last year by two independent labs in Europe, but the low-fidelity results achieved in these experiments could also be explained away by standard (classical) optics, without invoking teleportation at all. There has been much progress in the field, but not an actual demonstration until now.
In the October 23 1998 issue of Science, the physicists described how they used squeezed-state entanglement to teleport light. In previous teleportation experiments (announced over the last year by separate research groups in Austria and Rome), only two-dimensional discrete variables (e.g. the polarization states of a photon, or the discrete levels of a two-level atom) were teleported. In this recent experiment, however, every state, or the entire quadrature phase amplitude, of the light beam was teleported. In the Science article, the researchers explain that teleporting optical fields may someday be appropriate for the use in communication technology.
How
did they do it?
Caltech
researcher Jeff Kimble explains that the key to their achievement is using
quantum entanglement, which Einstein once termed 'spooky action at a distance'.
The main source of light for their experiment comes from a single-frequency
Tirsapphire laser at 860nm. The researchers first generate an Einstein-Podolsky-Rosen
(EPR) beam via parametric down-conversion, which results in a squeezed
two-mode optical field. The beam is then split in two, and even though
the identical beams are spatially separate, their photons remain entangled.
Each beam is then sent to a different destination. "This squeezing
is the key to producing entanglement between continuous variables (in
this case, quadratures of the electric field)", explains Dr. Samuel
Braunstein.
One of the two EPR beams is directed at a sending station, 'Alice'. At the same time, an input beam is sent to Alice by a third party, 'Victor', the verifier. Alice uses homodyne detectors to make to make Bell-state measurements of the quadrature phase amplitudes for the combined input beam and the EPR state, destroying the input beam during the process. Alice sends the resulting electrical current (her Bell-state measurements) over a classical communication channel to a receiving station - 'Bob'.
Bob uses the classical information sent by Alice to modify the second EPR beam. Because the two EPR beams are entangled, Bob is able to reconstruct an output beam similar to the input beam sent to Alice, via a simple phase-space displacement of the second EPR beam. Victor, the verifier, receives the output from Bob, and measures the overlap between the input and output beams as given by the fidelity (F). If the two states are orthogonal, F=0, and if the states are exactly the same (perfect teleportation), F=1. "In our case, the distance was only a metre, but the scheme would work just as well over much larger distances," says Braunstein.
"This particular scheme is a little like holography," describes Braunstein. "The EPR beams play the role of reference beams, and the classical information transmitted by Alice to Bob plays that of the hologram. One difference is that instead of merely producing an image of some object, you reconstruct the object itself. Another difference is that the quantum 'hologram' is worthless without Bob's half of the entanglement."
Why use quantum entanglement? In 'classical' teleportation, the upper limit for fidelity is 0.5. Kimble explains that their results are significant because they prove that quantum entanglement allows a fidelity greater than 0.5 to be achieved. "The EPR beams enable Alice and Bob to beat the classical limit. What we've done is take an unknown input state, and have Bob recreate that state to be more similar to the input than what is possible classically," says Kimble.
According to Kimble, the way to better fidelity is to focus on improving the efficiency of quantum entanglement. In the Science article, the physicists state that in an ideal case, perfect EPR correlations for loss-less propagation and detection will result in perfect fidelity.
Possible
Applications
"It
is worthwhile for society to learn how to process information quantum
mechanically," says Kimble. "In recent years, there have been
remarkable theoretical studies on the capabilities of quantum mechanics,
on the processing and distribution of information." He estimates
that in 100 years, society will be using quantum information in a number
of ways to develop quantum computers and quantum tele-communications systems
that will overcome limitations set by classical physics. "Image a
quantum internet, with vast computational powers, and perhaps the closest
we could come to foolproof security in communications," adds Braunstein.
Currently, companies such as Hewlett-Packard and IBM are investing in
research teams such as Kimble's, and exploring ways in which quantum mechanics
may provide the key to the future of communications.
The work on quantum teleportation exemplifies the international dimension to research activities at the School of Electronic Engineering and Computing Systems, University of Wales, Bangor. We applaud the role which will be played by international graduates in further exciting developments in our portfolio of research activities. We offer a single Welsh word to the world: 'Croeso' - welcome.
Author
Professor K Alan Shore
School of Electronic Engineering & Computer Systems
University of Wales, Bangor
