Information society

Where is the quantum revolution?

Is the marriage of information sciences and quantum physics in the design of supercomputers and inviolable communication networks just a dream or a fast-approaching reality? Albeit promising, the progress made working in the laboratory and on a small scale has not removed the technological obstacles that remain before we will see the dawning of this new “revolutionary” age of the information and communication society.

Grey matter in action at the Rudolph Grimms research group at the Institute for Quantum Optics & Information (IQOQI – AT). The group is a pioneer in research on Bose-Einstein condensates. © C.Lackner/IQOQ Grey matter in action at the Rudolph Grimms research group at the Institute for Quantum Optics & Information (IQOQI – AT). The group is a pioneer in research on Bose-Einstein condensates. © C.Lackner/IQOQ
Close-up of the first ion trapper at the IQOQI (AT) that succeeded, in 2005, in experimenting with and physically defining the quantum bit information vector, marking major progress in evolving quantum information processing. © C.Lackner Close-up of the first ion trapper at the IQOQI (AT) that succeeded, in 2005, in experimenting with and physically defining the quantum bit information vector, marking major progress in evolving quantum information processing. © C.Lackner
Micrometric view of a 2-qubit circuit based on supraconducting circuits exhibiting the Josephson effect (see rectangle). Stored in internal loops in the form of quantum electrical currents associated with a magnetic flow, the information is collected by the circuits shown in white that “read” the magnetic content. © Hans Mooij, TU Delft (NL) Micrometric view of a 2-qubit circuit based on supraconducting circuits exhibiting the Josephson effect (see rectangle). Stored in internal loops in the form of quantum electrical currents associated with a magnetic flow, the information is collected by the circuits shown in white that “read” the magnetic content. © Hans Mooij, TU Delft (NL)
Ion-trapping device developed by the SCALA project. One of the paths of quantum information processing involves using electromagnetic fields that act as ion attractors and succeeding in getting them to “jump” from one trapping zone to another. © Ion Trap Group, Imperial College – London/Dan Crick Ion-trapping device developed by the SCALA project. One of the paths of quantum information processing involves using electromagnetic fields that act as ion attractors and succeeding in getting them to “jump” from one trapping zone to another. © Ion Trap Group, Imperial College – London/Dan Crick

William D. Phillips, 1997 Nobel Laureate in Physics, believes that quantum information processing “represents an even more radical break than that between today’s computers and the abacus”. David Deutsch, professor of physics at Oxford University and a pioneer of quantum data processing, believes that “the computer age has not yet really begun”. What these researchers foresee is a sea change: a move to the quantum world as the use of the unique properties of this science opens up new avenues that are inconceivable for conventional technologies.

Although Europe is investing only €8 million a year in this research (compared with €75 million by the United States), it has managed to create a genuine research community within which a multitude of research projects covering all areas of quantum information processing are coordinated. For example, with funding of €38 million under the 6th Frame - work Programme, the Quantum Information Processing and Communication (QIPC) cluster brings together three integrated projects (SCALA, EuroSQIP, QAP) relating to quantum data processing, eight smaller projects combining information processing, communication and information sciences, and a vast integrated project (SECOQC) seeking to develop a secure global communication network based on quantum cryptography.

The strength and weakness of quantum data processing

The quantum computer represents primary information in a radically different way to a conventional computer. While the information unit of the latter is the “bit’ with a value of “0” or “1”, expressed by the blocking or the passing of a microelectronic current, the quantum computer is based on “quantum bits” or “qubits” of a value of both “0” and “1”. Whether transferred on a photon, an ion, an atom or any other object governed by the laws of quantum mechanics, the qubit “superposes” this duality of state throughout the calculation.

The calculating power of such a computer is therefore exponential depending on the number of qubits: the process takes place for the two possible values and for all the qubits. This dissociates memory and power. The live memory of an office computer is counted in billions of bits, while a quantum computer of several hundred qubits would in theory be able to encode a quantity of information equal to the number of atoms in the universe.

However, this superposition – the key to the power of these computers – is also their Achilles’ heel. If a qubit comes into contact with an external element or an uncontrolled interaction takes place between two qubits, the superposition disappears. This phenomenon is known to physicists as decoherence. The superposition must be maintained for the calculation to take place in parallel for all the possible combinations of states offered by the qubits as a whole.

For a few qubits more

The more qubits there are, the more they are in danger of interacting, and the more dif ficult it is to implement the technologies to control these interactions and retain the superpositioning. Between 1998 and 2006 a number of US teams produced experimental computers of 2, 7 and finally 12 qubits, controlled by nuclear magnetic resonance (NMR) and using liquid crystals. “It is probably impossible to exceed around 20 qubits with this approach,” believes Göran Wendin, coordinator of EuroSQIP. It is not therefore the approach adopted by the QIPC community that is focusing, among other things, on qubits provided by supraconductor circuits or impurities trapped in solid-state semi-conductors. Producing a machine based on these principles would already be major progress, even if the number of qubits remains limited. EuroSQIP anticipates between five and eight qubits in the medium term, with the hope of extending this to 128 in the future. Other avenues pursued by the SCALA and QAP projects plan to trap ions and atoms for transferring qubits. There are many potential technological solutions and the challenge for QIPC is to explore the most promising.

Securing communications

At the heart of quantum computer technology, superposition is also central to entanglement, the basis for applying quantum physics to the field of communication. By causing two particles to interact, a quantum relation is created that renders their states interdependent, regardless of the distance that separates them. This dependency is used by researchers to secure a communication as the entanglement relation is, theoretically, totally inviolable. All it takes is for a wandering eye to observe one of the two particles and the superposition of the quantum states of the two particles vanishes!

As these entangled quantum states are the key to encoding, if you lose them you lose the key. The securing method consists of sharing a couple of previously entangled particles between the emitter and the receiver. The emitter then uses the quantum state of the particle to encode the message sent through a classical channel. This state is instantly teleported to that of the receiver who simply uses the transmitted key to decrypt the message.

Long distances are impossible

The first challenge facing quantum communication lies in the difficulty of transmitting the particles and their quantum states – that is, entangled particles – over long distances.

Photons transported by optical fibres cannot, on the basis of current knowledge, travel more than 100 km, with laboratory experiments having failed to produce results beyond 50 km. One possibility currently being studied envisages introducing quantum relays as a means of segmenting the communication channels and limiting distances to a few dozen kilometres.

Other studies are looking at transmitting photons using optical beams in the atmosphere (or in space). The success of a European team, in 2007, in transferring an entangled photon over a distance of 144 km and using this quantum link to generate an encoding key opens up promising prospects. The aim is to achieve feasibility at the level of a satellite link, although the step to an exploitable technology has not yet been taken.

The techniques involved are many and complex, requiring extreme precision and with as yet incomplete theoretical bases. Experimentation also remains complex and costly. Yet all over the world scientists want to believe in this field of research and are investing hundreds of millions of euros every year. Is it the formidable technological potential alone that motivates this funding or is it also a scientific fascination with the keys to the gate between two major sciences of the 20th century: quantum physics and information technology? A marriage which, according to Jozef Gruska, professor at Masaryk University in Brno (CZ) and a pioneer of information processing in Europe, “promises considerable progress in the information sciences and in our understanding of the quantum nature of our universe.”

François Rebufat


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Vision of the future

“Current experiments are concerned with processors of between two and four qubits which use solid supraconductors. The hope is to give birth to 3- or 4-qubit processors within the next five years, expanding to 10 qubits within the next decade. Other teams are working on semiconductor-based supports, a rival technology that offers interesting prospects. There is no telling what stage this research will have reached within the next 20 years. It is possible that computers of between 20 and 50 qubits will be available by then or that quantum information processing will have been abandoned…”

“Whatever the case, research on quantum technologies still has much to look forward to. Instruments such as quantum coprocessors, communication relays, detectors or quantum sensors are opening the door to new possibilities for developing tools to supplement or improve conventional devices.” (Göran Wendin, coordinator of the EuroSQIP project)



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A networked quantum computer

Researchers working on the SECOQC project want to use entanglement to develop a distributed network architecture that would spread the calculating load between several computers. Each qubit would transmit its state to the others by means of a shared entanglement, independently of the computer on which it is found. While   conventional distributed network architectures (Grids) make it necessary to continually rethink various computer structures, the networking of quantum computers is coherent with the functioning of these computers. As a quantum calculation is the controlled interaction of a set of qubits, it matters little whether these qubits are present  in the same computer or distributed within a network linked by the entanglement of qubits. The network then acts as one giant computer.



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