Quantum Computer - New Technology for the New Century

Thus, it is no longer necessary to conclude that the neutron went through one slit or the other, Schrodinger’s Cat is not necessarily dead if it is not alive…. There now exists a further intermediate logical state. The adoption of this quantum logic can provide an explanation of sorts for the world of quantum strangeness, but only at the expense of giving up the logic that applies to every thing else. Most physicists regard this as an acceptable schizophrenia. After all, one has to use ordinary logic to argue for the application of quantum logic.” (1)

Any computer no matter how complex consists of a bunch of little objects that can be in two positions 1 or 0. Allow them to interact and any pattern – a mathematical equation, a novel, a symphony, a painting, or a movie – can be stored and processed. (2)

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 (especially quantum interference) to realize a fundamentally new mode of information processing. (3)

The first chapter, The Future in Theory, in the book, The Next Fifty years, edited by John Brockman, is contributed by Lee Smolin, a physicist who is working on the unification of the fundamental forces (4). He has discussed seven big questions, which were faced by the physicists in the 1950s (about fifty years back) and has listed seven fundamental questions which are unanswered at present. The first of these seven unanswered questions is:

“Is quantum theory true as presently formulated or will it need to be modified, either to have a sensible physical interpretation or to unify it with relativity and cosmology?”

Deliberating on this question, Smolin wrote, “At present, powerful new techniques are being developed that promise to greatly extend the regime over which the quantum theory has been experimentally tested – techniques chiefly in aid of developing quantum computers. These are macroscopic devices that use quantum effects, such as superposition and entanglement, to do computations impossible for ordinary computers.”

Theory of quantum mechanics applies to the atomic and subatomic particles. If a computer can be built on the quantum mechanical principles (logic), which produces accurate and usable results in the macroscopic world, the deterministic world governed by gravity, it may be considered as a validation of the notion for unifying the fundamental forces of nature. This is probably to which Smolin alluded in his statement.

Although the technology required for building a quantum computer is not yet fully developed, there is a great deal of optimism for its realization in the near future. In response to an interviewer’s (Filiz Peach) question, David Deutsch, one of the pioneers of the theory of quantum computer and a distinguished quantum physicist at the Centre for Quantum Computation at the Clarendon Laboratory, Oxford University, remarked, “..I am not involved in any of the experimental work, except as a spectator. I work only in the theory. I can only say that I am extremely impressed by the power of the experimental techniques that are now available. These people routinely manipulate individual atoms and individual photons, and engineer interactions between them and measure them with extraordinary precision, and they are optimistic about the possibility of building working quantum computers. At the moment the most powerful quantum computer in the world probably has 3 or 4 qubits. One would probably need several hundreds to perform any quantum computation that was useful as such,” (5).

A classical digital computer works on the logic that a given statement is either true or false. These states can be represented by an electrical switch in ‘on’ or ‘off’ position and numerically by 1 or 0 in binary system. The quantum logic is different from the classical logic. The Schrödinger cat can be dead, alive, or dead and alive at the same time depending on how it is observed. Similarly, an electron can be here, there, and here and there at the same time. The classical logic is deterministic while the quantum logic is probabilistic. The development of the classical computer has almost reached its practical limit beyond which it is probably not possible to go with the present ideas and technologies used in the architecture of classical computers. The idea of using quantum mechanical principles to develop entirely a new type of computer thus seems appropriate and timely.

A problem that arose at the time when the theory of quantum computation was being conceived, related to the nature of the underlying theory. The physical laws that govern the behavior and properties of the circuit at the atomic level are quantum mechanical. Will the quantum computer work in the macroscopic ambience? Feynman reassured that a quantum computer would work by producing an abstract model, in 1982. In 1985, Deutsch published a seminal paper in which he showed that any physical process, in principle, could be modeled perfectly by a quantum computer. Deutsch and Eckert (6) remarked at the end of the twentieth century, “As far as the elegance of the theory goes, it turns out that the quantum theory of computation hangs together better, and fits in far more naturally with fundamental theories in other fields, than its classical approximation was ever expected to.”


The fundamental unit of information in a quantum computer is called quantum bit or ‘qubit’ in contrast to ‘bit’ in the classical computer. A qubit can, not only, exist as 1 or 0 as a classical bit but also in states which are a blend or ‘superposition’ of these classical states. “The ability to be in multiple states at the same time is called superposition,” (7). This feature of the quantum physics had baffled and still confound many including physicists also, particularly those who are still influenced by the classical logic, and way of thinking. Discussing this ‘weird’ fact of the microscopic world, Johnson remarked, “..while a switch in a conventional computer can be either on or off, representing 1 or 0, a quantum switch can paradoxically be in both states at the same time…If that seems impossible, don’t despair. The physicists find it as puzzling as the rest of us. It is just the way nature seems to work,” (8)

A new lexicon has also been created for handling unfamiliar phenomenon and processes such as “superposition”, “interference”, “decoherence”, “qubit”, “NOT’ gate, “Controlled NOT”, “controlled NOT NOT”, “NAND” gates etc. This will not be a serious problem because we did learn the languages of the classical computers and many of their strange acronyms such as “RAM” (random access memory), etc. in due time, became part of our household vocabulary.

According to Wikipedia (free encyclopedia), “A quantum computer can be implemented using any small particles that can have two states. Quantum computers might be built from atoms that are both excited and not excited at the same time. They might be built from photons of light that are in two places at the same time. They might be built from protons and neutrons that have a spin of ‘up’ and ‘down’ at the same time. A microscopic molecule can contain many thousands of protons and neutrons. It might be used as a quantum computer with many thousands of qubits…If a quantum computer were based on the protons and neutrons in a molecule, it might be too small to see, but could factor integers with many thousands of bits. A classical computer running known algorithm could also factor those integers. But to do it before the sun burns out, it would have to be larger than the known universe. That would be somewhat inconvenient to build,” (7).

Once it was realized that the development of a quantum computer was theoretically feasible, methods of its testing were investigated. The earliest algorithm for testing was provided by Shor, which enabled factoring large numbers (of the order of 10^200) in a matter of seconds. Its practical application is in the field of encryption (coding) where “..one common (and best) encryption code … relies on the difficulty of factoring very large composite numbers into their primes. A computer which can do this easily is naturally of great interest to numerous government agencies,” (3). Also, “many of society’s secrets, from classified military documents to the credit card numbers sent over the Internet are protected using codes based on the near-impossibility of factoring large numbers,” (9).

Decoherence and its Consequences

Proposals to explain quantum physics in computers with many times the power of today’s supercomputers depends on the ability to maintain many quantum switches in coherent superposition. ()

A quantum computer has the advantage over the conventional computer by virtue of its ability to ‘superpose’ information from all its various quantum states because it works on the quantum mechanical principles. The most important precaution that is required during its processing the information is that it does not ‘collapse’ into the classical regime. If it does it will not only lose its advantage but also might produce wrong output. The undesired transition from quantum mechanical regime to the classical regime is called decoherence.

One of the weirdest aspects of quantum mechanics is, as I have already mentioned, that a microscopic entity (an atom or electron, for example) can exist in many different states at the same time before it is actually observed. Such an entity in its quantum mechanical existence may be on the floor, at the table, at the table and the floor, and any where in between at the same time. The act of ‘observation’ or ‘measurement’ causes these numerous states to coalesce (collapse) into one state that we actually observe in the macroscopic world.

If the collapse occurs prematurely in a quantum computer, it may malfunction. A great deal of research is in progress for correcting this mishap. If an error occurs during the processing of the information, the computer should be able to detect it internally (without allowing the collapse to occur) and then correct it internally. It has to be made self-detecting and self-correcting, so to say. It’s a big challenge but theory shows it can be done.

Historical Brief and Timeline

The seeds of quantum computer were planted by Richard Feynman, a Nobel Laureate. He pondered on this problem long before he propounded his fundamental thesis in his 1982 paper. Among the early ground breakers were Charles H. Bennett of the IBM Thomas J. Watson Research Center, Paul A. Benioff of Argonne National Laboratory in Illinois, David Deutsch of the University of Oxford, among others.

A landmark in quantum computing and testing was developed in 1994 by Peter Shor of AT&T as noted above, which is now called Shor’s algorithm. The National Institute of Standards and Technology and the California Institute of Technology contemplated shielding a quantum system from environmental influence, e.g., magnetic fields, in 1995, which was followed by the University of California at Berkeley, MIT, Harvard University, and IBM using nuclear magnetic resonance (NMR), in 1996.

The feasibility of ‘quantum teleportation’ was proposed in 1993 by an international team of researchers. The idea arose from the theorem (Einstein-Podolsky-Rosen) that describes “how two particles which come into contact become entangled and part of the same quantum system…The idea is actually put into practice nearly six years later, by researchers at the University of Innsbruck in Austria.”

There is a great deal of research going on in this field. Although development of a quantum computer is still in its infancy, the ways and capacity of classical computing will become a relic of the recent past and will be replaced by quantum computation in the early part of the twenty first century.

I will conclude with the following quotation from Julian Brown (10):

At key points in human history, civilization took a leap forward because people discovered a new way of exploiting nature. Tool making, farming, the industrial revolution, and the information revolution were all triggered by the discoveries of new ways of manipulating nature. All of these advances transformed the way humans live. Quantum computation, Deutsch argues, could turn out to be as significant in its effects on human civilization.

References

1.Barrow, John D., “What is Mathematics?” in “The World treasury of Physics, Astronomy, and Mathematics,” ed. Timothy Ferris, Little, Brown and Company, Boston, 1991, pp. 541-558.
2.Johnson, George, “A Shortcut Through Time: The Path to the Quantum Computer,” Alfred A. Knopf, New York, 2003, p.28.
3.“The Quantum Computer,” http:www.cs.caltech.edu/~westside/quantum-intro.htm
4.̶ 0;The Next Fifty Years,” ed. John Brockman, Vintage Books, A Division of Random House, Inc., New York, 2002, p.6, 8.
5.“David Deutsch,” http:/www.qubit.org/people/david?Articles/philosophyNow.htm. (This interview appeared in Philosophy NOW, 30, December 2000).
6.Deutsch, D. and Artur Eckert, “Machines, Logic and Quantum Physics,” arXiv: math.HO/9911150 v1, 19 Nov 1999.
7.“Quantum Computer,” http://en.wikipedia.org/wiki/Quantum_Compute.
8.Ref. 2, p.6.
9.Ref. 2, p.83.
10.Brown, Julian, Minds, Machines, and the Multiverse: The Quest for the Quantum Computer,” Simons and Schuster, New York, 2000, p. 38.