Tuesday, 1 July 2008

Will the QC kill the PC?



Quantum computers could become a reality very soon, opening up some fantastic possibilities - including teleportation, says Richard Gray

  • It might be the science of the very small, but quantum computing is on the verge of solving some giant problems. For more than 30 years, physicists have dreamed of harnessing the power of atoms to produce computers that would far outstrip the capabilities of the silicon microchips used in today's PCs.

    Is the PC about to be replaced?

    Will the PC soon become a thing of the past?

    These machines would have the ability to perform calculations that would take normal computers millions of years, while carrying out a vast number of tasks simultaneously. They promise to make as big an impact on information technology as the transistor.

    Despite the dazzling potential of such technology, quantum computers have remained little more than a possibility on paper. Building a quantum computer has proven far more challenging than the theory, leading scientists to doubt whether they would ever find a practical purpose.

    But the world's foremost experts in the field are now revising their predictions and believe that we could see quantum computers within years.

    Their excitement has been fed by experiments that demonstrated some of the practical uses of quantum computers, showing that it will at last be possible to turn the theory into reality.

    "A few years ago, I would have said that quantum computing would be of little use for anything practical," says Professor Anton Zeilinger, a quantum physicist at Vienna University, who is regarded as one of the godfathers of quantum computing.

    "But now I am far more optimistic. It has been a huge surprise for those of us in the field. I believe that in 20 years at the most, quantum computers will be used in everyday life on people's desktops."

    The power of quantum computers rests in the strange way that matter behaves at the atomic and sub-atomic levels. Particles at this scale, such as electrons and the nuclei of atoms, adhere to laws that are outside the normal realms of physics.

    In the quantum world, a particle can be in two places at once. It can even be in two different states at the same time, either as a particle or as a wave of radiation.

    It is a mind-boggling concept, but physicists believe that, by harnessing the unique ability of small particles, quantum computers could carry out many calculations simultaneously.

    Traditional computers shuffle information in the form of binary numbers, the digits 1 and 0, which are remembered by the "on" and "off" positions of tiny switches, or "bits", on the circuit boards. Quantum computers use atoms and subatomic particles as the switches that perform the memory and processing tasks.

    The difference is that in quantum computing, the switches can be "on" and "off" at the same time. This means the basic component, the "qubit", can be involved in multiple calculations, while its strange properties also allow such computers to skip the step-by-step operations that current PCs use.

    Quite how much more powerful this could make a quantum computer has still to be seen, but some scientists have estimated that even a very simple 30-qubit computer would be around 1,000 times faster than most desktop PCs.

    The benefits of having this kind of technology at your fingertips are becoming clear. The first is speed - by crunching through excruciatingly complex calculations in seconds such powerful computers can open up new uses for technology.

    Information would no longer need to be carefully logged in structured databases in order to be easily found, as it does now. Instead of searching the internet for key words attached to images and video - which is how YouTube or Google work - quantum computers would be able to search for the images and video themselves.

    "Take a telephone book as an example," explains Prof Zeilinger. "If you know the name of someone you are looking for, it is relatively easy to find their phone number. On the other hand, if you only have the phone number, searching for the person's name is extremely difficult, as you have to go through every entry and check it. That is what traditional computers would have to do.

    "A quantum computer can search through unsorted data like this with great ease because of its ability to perform the complex algorithms needed."

    The field of video gaming could also be transformed. By exploiting the multiple states of qubits, quantum video game consoles could generate a truly random aspect to gameplay, producing a more realistic experience. They could also generate new types of games that rely upon betting against which state the qubits will take.

    Quantum computing, however, also has some worrying implications for security. With the ability to carry out difficult calculations on large numbers, quantum computers would be able to crack one of the cornerstones of computer security - cryptography. Conventional computers struggle with the difficult calculations needed to break codes: to test all of the possible answers in a "brute force" attack would take far too much time and processing power. But a quantum computer that tried the different answers simultaneously could easily defeat this security.

    This has worried many in the financial and business sectors, where confidentiality and protection against hackers is crucial. Some companies have begun offering new types of security, called quantum cryptography, as an ultra-secure way of sending information.

    As well as providing a new way of generating the secret keys needed to unlock encoded information, the technique also allows the detection of eavesdroppers by exploiting another quirk of the quantum world - that when you try to look at a quantum particle, it will alter its state.

    By encoding messages in quantum states, an eavesdropper who tries to intercept the message will change its state - so the sender and receiver can tell if it has been tinkered with. One of the key tests of the method was carried out in Britain - to a pub in Malvern. Prof Zeilinger did another demonstration in a Viennese sewer for an Austrian bank.

    As the threat posed by internet viruses and hackers to people's personal computers increases, quantum cryptography could become a standard feature of desktop computers to ensure safe internet communication.

    Professor Seth Lloyd, a quantum mechanical engineer at the Massachusetts Institute of Technology, believes this property of quantum computing has opened up another new possibility, that is of growing concern to internet users.

    His research has revealed a way of using quantum computing to keep personal information private. Currently, internet sites and search engines can keep large amounts of information about people's computer and search practices.

    "If you use what I am calling quantum private queries, it would allow you to ask a question of a search engine like Google, but keep your own information private. If they try to keep your information, you will know about it. It will allow computer users to know no one else is snooping on their information," said Professor Lloyd.

    Among the breakthroughs that have brought the prospect of a quantum computer closer is the ability to harness a process called "entanglement", in which two particles become connected so that the fate and movements of one depends on the other.

    In the Newton Medal Lecture this month, Professor Zeilinger told the Institute of Physics that these entangled particles could provide a valuable tool in communicating between quantum computers.

    His team has managed to set a new record for the distance over which they entangled two particles - around 90 miles apart, on two of the Canary Islands. It could mean that quantum computers could use entanglement to send and receive information.

    Professor David Deutsch, a physicist at the University of Oxford who is one of the main pioneers of quantum computing, said he was more hopeful than ever that they would become everyday devices. Another new technique, which splits the quantum computation into two phases, had changed the way scientists thought quantum computers would have to be built.

    "I used to say that quantum computers would be built in several decades," he said, "but now I think it will be more like years. This particular technique is going to make it drastically easier to make them as it brings tremendous improvements in efficiency.

    "I now expect to see simple quantum computers in only a matter of years."


    • Typical personal computers calculate 64 bits of data at a time. A 64- qubit quantum computer would be about 18 billion billion times faster.

    • A working quantum computer could be so mind-bogglingly powerful that it would solve in seconds certain problems that would take the fastest supercomputer millions of years to complete.

    • Consumers, credit card companies and high-tech firms rely on cryptography to protect sensitive information. The basis for encryption systems is that computers would need thousands of years to factor a large number, making it very difficult to do. But a QC could break the most complicated encryption in hours.

    • One of the more bizarre properties of QCs was identified by a team at the University of Illinois at Urbana-Champaign. They presented the first demonstration of "counterfactual computation", inferring information about an answer, even though the computer itself did not run.

    • Quantum computers could also take advantage of another quantum property, teleportation. Teleportation allows information about one particle to be transmitted to another particle some distance away. A quantum computer could use teleportation instead of wires to move bits around inside itself.

    • Quantum computation has captured the imagination of the scientific community, recasting some of the most puzzling aspects of quantum physics - once pondered by Einstein, Schrödinger and others - in the context of advancing computer science. Roger Highfield

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