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What are Quantum Computers?

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What are Quantum Computers?

Quantum computers have the potential to perform certain calculations billions of times faster than any silicon-based computer.

Scientists have already built basic quantum computers that can perform certain calculations; but a practical quantum computer is still years away.

Computers have become more compact and considerably faster in performing their task, the task remains the same: to manipulate and interpret an encoding of binary bits into a useful computational result.  A bit is a fundamental unit of information, classically represented as a 0 or 1 in your digital computer.  Each classical bit is physically realized through a macroscopic physical system, such as the magnetization on a hard disk or the charge on a capacitor.  A document, for example, comprised of n-characters stored on the hard drive of a typical computer is accordingly described by a string of 8n zeros and ones.  Herein lies a key difference between your classical computer and a quantum computer.  Where a classical computer obeys the well understood laws of classical physics, a quantum computer is a device that harnesses physical phenomenon unique to quantum mechanics to realize a fundamentally new mode of information processing.

In a quantum computer, the fundamental unit of information (called a quantum bit or qubit), is not binary but rather more quaternary in nature.  This qubit property arises as a direct consequence of its adherence to the laws of quantum mechanics which differ radically from the laws of classical physics.  A qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states.

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For problems with all four properties, it will take an average of (n + 1)/2 guesses to find the answer using a classical computer. The time for a quantum computer to solve this will be proportional to the square root of n. That can be a very large speedup, reducing some problems from years to seconds. It can be used to attack symmetric ciphers such as Triple DES and AES by attempting to guess the secret key. But it is also easy to defend against, by doubling the size of this key. There are also more complicated methods for secure communication, such as using quantum cryptography.

There are currently no other practical problems known where quantum computers give a large speedup over classical computers. Research is continuing, and more problems may yet be found.


Problems with quantum computing

One of the major obstacles of quantum computing is the problem of decoherence, which causes the unitary character (and more specifically, the invertibility) of quantum computational steps to be violated. Decoherence times for candidate systems, in particular the transverse relaxation time T2 (terminology used in NMR and MRI technology), typically range between nanoseconds and seconds at low temperature. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much quicker than the decoherence time. If the error rate is small enough, it is possible to use quantum error correction, which corrects errors due to decoherence, thereby allowing the total calculation time to be longer than the decoherence time. An often cited (but rather arbitrary) figure for required error rate in each gate is 10−4. This implies that each gate must be able to perform its task 10,000 times faster than the decoherence time of the system.

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So, many experimentalists do not give up. The current challenge is not to build a full quantum computer right away but rather to move from the experiments in which we merely observe quantum phenomena to experiments in which we can control these phenomena. This is a first step towards quantum logic gates and simple quantum networks.

Can we then control nature at the level of single photons and atoms? Yes, to some degree we can! For example in the so called cavity quantum electrodynamics experiments, which were performed by Serge Haroche, Jean-Michel Raimond and colleagues at the Ecole Normale Superieure in Paris, atoms can be controlled by single photons trapped in small superconducting cavities[6]. Another approach, advocated by Christopher Monroe, David Wineland and coworkers from the NIST in Boulder, USA, uses ions sitting in a radio-frequency trap[7]. Ions interact with each other exchanging vibrational excitations and each ion can be separately controlled by a properly focused and polarised laser beam.

Experimental and theoretical research in quantum computation is accelerating world-wide. New technologies for realising quantum computers are being proposed, and new types of quantum computation with various advantages over classical computation are continually being discovered and analysed and we believe some of them will bear technological fruit. From a fundamental standpoint, however, it does not matter how useful quantum computation turns out to be, nor does it matter whether we build the first quantum computer tomorrow, next year or centuries from now. The quantum theory of computation must in any case be an integral part of the world view of anyone who seeks a fundamental understanding of the quantum theory and the processing of information.

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