Quantum computers - why would you want one?

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Supercomputers, which usually take up a whole room, are already in demand for things like forecasting weather and climate, designing aircraft and computer chips. Yet something called a quantum computer is emerging which could give everyone that sort of grunt on their desktop.

Cracking the code

One much-discussed need for a really slick computer is cryptography or code-cracking. This is not just spy stuff. We move money about on the Internet all the time: we are told the transactions are protected by uncrackable security codes. Many of these encryptions rely on very large numbers – like numbers with 400 digits. To crack the code, these have to be broken down into the smaller prime numbers which create the big number when you multiply them together.

With today's computers this takes just about forever, so we can rely on such encryptions to keep our money safe. But it seems quantum computers could crack them within a few hours or even a few minutes. What then? Even tougher codes that will need even faster computers? Will future quantum computers spend a lot of time chasing their own tails?

Moore's Law hits the wall

The most famous, or perhaps the most notorious statement in computer science is Moore's law, named after a famous computer pioneer. It is not really a 'law', it is rather an observation of what has happened, and what we might expect to continue to happen, at least in the immediate future.

This 'law' says that the number of components which computer-chip makers can squeeze onto a chip for data storage or processing doubles every 18 months or so, as design and manufacturing methods improve. Certainly this has been going on for more than three decades. Where a few thousand transistors or capacitors or resistors would fit in 1975 – on a piece of silicon the size of your fingernail – we can now place hundreds of millions. Vastly more information is being stored and calculations now done billions of times a second, mightily increasing the power of computers.

For this to happen, the chip makers must make those components even smaller. Every 18 months or so they are cut in half in size, which is another way of expressing Moore's law.

Into the nanoworld and beyond

The smallest dimensions of a chip, such as the width of the connecting wires, were ten microns (10 millionths of a metre) or more 30 years ago.

Nowadays 100 nanometres (100 billionths of a metre) or less is the typical size. These electronic fragments have become smaller than viruses and a thousandth the width of a human hair. And they continue to dwindle in size, as their masters push for ever better performance to satisfy customers.

But this cannot go on for ever. If we keep driving in that direction, sooner or later we will run into trouble. Some nasty 'quantum uncertainties' will show up and the chips will not behave as they should. By 2020, according to Moore's law, the circuit elements would be as small as atoms. Long before then, the bits of electric charges that store information and drive processing power will start to leak away.

And it is probably impossible to manufacture circuit elements so small anyway. Even the current generations of computer chips are straining the ingenuity of the chip engineers, and the costs of building manufacturing plants have become astronomical.

The good news is that there is a way out of this dead end. We can drop, straight into the nanoworld rather than creeping down step by step (Box 1: Into the nanoworld). In that way we can take advantage of the peculiarities of quantum physics, rather than having to work our way round them. That path leads to the quantum computer.

Bits and qubits

To see how it might happen, we should compare the vision of a quantum computer with the computer that sits on your desk. In that machine, and in the biggest supercomputer in the world, information is stored very simply as strings of numbers. In fact, there are only two sorts of numbers, 0s and 1s. These are known as 'bits', short for 'binary digits'.

Clever coding now lets us reduce all sorts of information, such as ordinary numbers, words, sounds, pictures and movies to such strings of numbers, and process that information by adding, subtracting and comparing the number chains. Each bit can be stored, permanently or temporarily, in a tiny box, as a dab of electric charge in a capacitor or a tiny fragment of magnetism on a circle of magnetic film. Something in the box means a 1; an empty box represents a 0.

A quantum computer does much the same thing, but it uses nano-sized particles, such as atoms, as the storage boxes. These are called quantum bits or qubits. For example an atom spinning one way would represent a 1, spinning the other way would be a 0.

Quantum weirdness

The difference between our everyday computer and a quantum computer is that the nanoworld lets a qubit be both a 0 and a 1 at the same time. This peculiar behaviour is called quantum superposition. There is a certain probability that the qubit holds a 1 and another probability it is recording a 0. You have to interrogate the qubit to find out, but that will disturb it, and stop it from taking any further part in any computation. That is something else unexpected that quantum physics demands.

Now we get the real payout from all this odd behaviour. As a consequence of superposition, you need a huge amount of information to describe the state of even a small number of qubits. That information doubles for each qubit you add to the assembly. Just 50 qubits would demand more than a billion numbers to describe their collective contents. Put another way, a collection of 50 qubits could store a vast amount of information, far more than any everyday computer memory can hold.

It does not stop there. In an everyday computer, the program has to operate on its stored information in sequence, one bit at a time. Even so, that can allow for billions of calculations every second. A quantum computer can process all the information in all the qubits simultaneously – geeks call this parallel processing. Imagine having millions of desk tops running side by side rather than just one, all working on the same problem. Yet a quantum computer will need only one processor.

The consequence of all this is devastating processing speed when compared with the 'classic' computers of today. The hare against the tortoise many times over. A really tough problem like the big number factorisation highlighted above, that would take a supercomputer years or decades to crack, can be crunched – at least in theory – by a quantum computer in very little time at all.

The quantum computer era is more than a glow on the horizon, but the dawn is still some distance off (Box 2: What are we up to here in Australia?). But quantum computers will start to affect our lives one day, perhaps a decade from now (Box 3: So when will we have one?). There is no fundamental reason why we should not have them, though we will need to think about their uses before that day arrives.

Related Nova topics:

Nanotechnology – taking it to the people

Nanoscience – working small, thinking big

Box 1: Into the nanoworld

Since then microscopes, working first with light and later with electrons, have revealed an immense and intricate micro-world. A hundred times smaller than our speck of dust we find living things like bacteria, a few microns (millionths of a metre across), viruses are ten times smaller again, molecules of complex chemicals like DNA are another power of ten down.

As we descend in size, we enter the unsettling nanoworld, where everything is a bit odd. Its inhabitants are uncountable numbers of individual atoms and small molecules of compounds, with sizes of a few nanometres (billionths of a metre) or less. On this scale, a bacterium is huge, a vast blimp more than thousand nanometres across. Even a passing virus would block out the Sun.

The way of life is very different down here. Quantum physics rules. In our everyday world, we can measure with precision both where something is and how fast it is going. Not so in the nanoworld. It is all fuzzy. The more you know about the speed of an atom, the less you know about its location. The more precise you are about when an event takes place, the more uncertain will be your measurement of any associated energy. It sounds crazy, but that is how things are.

This inescapable uncertainty poses real problems for visitors from above. We have no trouble up here making the stream of electrons that constitute an electric current stay inside the wire carrying it. But as quantum uncertainties begin to intrude, we grow less certain about just where the electrons are. Are they inside the wire or outside? This means that electric charge can leak easily from nanosize boxes, whereas the micron-size ones on a computer chip hold such charges securely.

Related sites

• 'Exact uncertainty' brought to quantum world (New Scientist, 27 April 2002)
• Nanoscience – working small, thinking big (Nova: Science in the news, Australian Academy of Science)

Box 2: What are we up to here in Australia?

With a number of universities active here, the effort is being coordinated through the Centre for Quantum Computer Technology, set up in 2000.

The research programs are diverse, as they have to be in such a new area. Some concentrate on the theory of quantum computing. Others are trying out a range of possible practical systems that quantum computers might use. Some of these employ semiconductor materials like silicon, as today's computers do; others are trying to use particles of light (known as photons) to store and compute the data.

Here are a few recent highlights:

• Researchers have found a way to trap up to ten 'ions' (atoms which have lost some electrons) in a vacuum chamber where they can be controlled and manipulated by a laser. Such ions could serve as the qubits that will store data in a quantum computer.

• Scientists have been investigating new ways to generate photons as current photon production methods are expensive and inefficient. Experiments are being carried out using engineered crystals, mirrors, lenses and beam splitters for photon production.

• In another research program, the aim is to build a quantum computer from the 'bottom up', by placing atoms on a surface which is then incorporated into nano-scale structures. This contrasts with attempts to make large devices even smaller, as is happening in computer technology today.

• Ion implantation is another subject under intense study. Advanced technologies can precisely place single atoms of elements such as phosphorus onto a surface. Researchers expect that they can get these to interact and behave as qubits for storage of data or as the devices that do the computing.

Related sites

• Griffith physicists make quantum leap in race for a supercomputer (Griffith University, Australia)

• Shining light in quantum computing (The University of Queensland, Australia)

• Quantum computing (The Science Show, 7 May 2005, Australian Broadcasting Corporation)

• A quantum leap in computing (The University of Melbourne, Australia)

Box 3: So when will we have one?

But before you can get your hands on a quantum computer, there are still a lot of practical questions to be solved.

How can we stop the qubits from being accidentally bumped and so spilling out an answer before the calculation is complete?
Information stored in qubits can be protected from any data handling errors by employing quantum error correction schemes. These schemes have been demonstrated in quantum computing systems consisting of up to 8 qubits.

What is the best way to interact with the qubits, to feed information in and get answers out?
Pulses of radio waves, like those in a magnetic resonance imaging (MRI) machine are working promisingly there, though the interchange is still quite slow.

How many qubits will we actually be able to assemble and work together?
As few as 300 qubits would be sufficient to outperform computational simulations of physical and chemical systems currently achieved with conventional computers. However, millions of qubits are required to provide a public key encryption system that is superior to the current one.

Related sites

• First 'commercial' quantum computer solves sudoku puzzle (Scientific American, 13 February 2007)

• NIST demonstrates data 'repair kit' for quantum computers (National Institute of Standards and Technology, USA)

• Stable quantum mechanical memory and a small quantum processor created (What's Next in Science and Technology, USA)

• A quantum leap in computing (MSNBC.com, USA)

• New hurdle for quantum computing (Science A Go Go, Australia)

Activities

• Internships in public science education (University of Wisconsin, USA)
  • Nanoscale activity: Cutting it down to nano – students learn about the nanoscale through discussions and hands-on activities.

• NanoSense (UK)
  • Size matters: Introduction to nanoscience – provides an introduction to nanoscience, concepts of size and scale and the properties of the nanoscale.

• The New York Times (USA)
  • When Moore is less for microprocessors – students examine Moore's law and how microprocessors work.

• TryEngineering (USA)
  • Give binary a try! – students learn to read a binary clock.

Further reading

• 2020 Computing: Champing at the bits
• 2020 Computing: Milestones in scientific computing
• 2020 Computing: Everything, everywhere
• 2020 Computing: Exceeding human limits
• 2020 Computing: The creativity machine

Useful sites

What is quantum nanoscience? (The University of Queensland, Australia)

Provides a summary to quantum nanotechnology.
http://www.physics.uq.edu.au/people/milburn/qnsresearch/QNSSchool.htm

Australian Broadcasting Corporation

• Diamonds offer cool computer solution (News in Science, 20 June 2008)
  Proposes the use of diamonds in quantum computers.
  http://www.abc.net.au/science/articles/2008/06/20/2278896.htm?site=science&topic=latest

• Quantum entanglement (The Lab, 18 November 2004)
  Explores the area of quantum entanglement and its applications.
  http://www.abc.net.au/science/features/quantum/default.htm

How does a quantum computer work? (How Stuff Works, USA)

Provides an overview to quantum computers.
http://computer.howstuffworks.com/quantum-computer.htm

Scientific American

• Quantum computing with molecules
  Describes the progress in the development of quantum computers.
  http://www.media.mit.edu/physics/publications/papers/98.06.sciam/0698gershenfeld.html

• What makes a quantum computer so different (and so much faster) than a conventional computer?
  Explains the difference between a quantum computer and a conventional computer.
  http://www.sciam.com/askexpert_question.cfm?articleID=000B4C6B-EE1A-1214-AE1A83414B7F0000&sc=I100322

It's a small world (Chemistry World, UK)

Presents the latest developments in the nanotechnology world.
http://www.rsc.org/chemistryworld/restricted/2004/February/smallworld.asp

The quantum computer (California Institute of Technology, USA)

Provides a brief introduction to quantum computers.
http://www.cs.caltech.edu/~westside/quantum-intro.html

Glossary

cryptography. The science or study of encoding and decoding messages. For more information see Cryptography (Webopedia, USA).

factorisation. Resolution of an object (a number, a polynomial or a matrix) into factors, which when multiplied together give the original object.

ion implantation. Process by which the ions of a material are placed on the surface of a solid. The process modifies the physical properties of the solid.

parallel processing. The simultaneous processing of a task by two or more computer systems; also referred to as parallel computing.

photon. A photon is the smallest unit of light energy.

quantum decoherence. The process that takes place when a quantum system interacts with its environment.

quantum error correction. A method used in quantum computing to minimise the impacts of quantum decoherence and other quantum noise.

quantum superposition. A phenomenon where an object exists in more than one state simultaneously.

qubit. A unit of information in quantum computing.