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Quantum computing : Possible data carrier for the next generation of supercomputers
:: 29 May, 2007
The electron might have just caught up with the photon as a possible data carrier for the next generation of supercomputers. Scientists have demonstrated the ability to control a current so precisely that they can transmit electrons one at a time. More research is needed, but the achievement could someday be a key part of building a quantum computer on a chip.
In the race to create ever smaller and faster computers, quantum computing has become the ultimate goal among scientists, promising speeds and capacities far above present technologies using a tiny fraction of the power and space that conventional machines need.
The key to this new technology is quantum physics, which allows subatomic particles to exist in multiple states simultaneously and therefore perform several tasks in the same time it takes existing computers to make a single calculation. The enduring problem in attempting to create such a computer has been how to control the flow of photons or electrons carrying the information. Photons are fairly docile, but transporting them requires complex arrays of lasers, beam splitters, polarizers, and the like. Electrons require just a conductor, but they are unruly, which complicates the creation of supremely delicate quantum states.
Now, researchers have demonstrated a tiny device that can emit single electrons through a conducting medium called a two-dimensional electron gas (2DEG), every nanosecond or so. The 2DEG allows the electrons to pass undisturbed, so they can act as quantum bits, or qubits, more elaborate versions of the individual data bits in conventional computer systems. The team, at L'École Normale Supérieure in Paris, describes in today's issue of Science how they created an extremely tiny electrical insulator called a quantum dot, which allows electrons through to the 2DEG one at a time whenever it receives a tickle of electricity.
The device represents a step toward quantum computing, says physicist and co-author Christian Glattli. The next step is to begin to control the movement of qubits. Glattli says the difficulty of such control will be overcoming the signal noise generated when single electrons are emitted quickly.
Also needed is an electron detector to receive the qubits, "a challenging thing," says Stephen Giblin, a physicist at the U.K.'s National Physical Laboratory in Middlesex who wrote an accompanying Perspective in Science. Building such a detector will require a 10-fold increase in speed for single-electron detectors, he says. Nevertheless, quantum computers that use electrons are worth pursuing, he adds. Although these computers would be "messy compared to photonic systems," Giblin says, they could be fabricated easily in large numbers and would be easier to integrate with conventional electronics.
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What is a Quantum Computer?
Behold your computer. Your computer represents the culmination of years of technological advancements beginning with the early ideas of Charles Babbage (1791-1871) and eventual creation of the first computer by German engineer Konrad Zuse in 1941. Surprisingly however, the high speed modern computer sitting in front of you is fundamentally no different from its gargantuan 30 ton ancestors, which were equipped with some 18000 vacuum tubes and 500 miles of wiring! Although 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 (especially quantum interference) 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. In other words, a qubit can exist as a zero, a one, or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state. This may seem counterintuitive because everyday phenomenon are governed by classical physics, not quantum mechanics -- which takes over at the atomic level
The Potential and Power of Quantum Computing
In a traditional computer, information is encoded in a series of bits, and these bits are manipulated via Boolean logic gates arranged in succession to produce an end result. Similarly, a quantum computer manipulates qubits by executing a series of quantum gates, each a unitary transformation acting on a single qubit or pair of qubits. In applying these gates in succession, a quantum computer can perform a complicated unitary transformation to a set of qubits in some initial state. The qubits can then be measured, with this measurement serving as the final computational result. This similarity in calculation between a classical and quantum computer affords that in theory, a classical computer can accurately simulate a quantum computer. In other words, a classical computer would be able to do anything a quantum computer can. So why bother with quantum computers? Although a classical computer can theoretically simulate a quantum computer, it is incredibly inefficient, so much so that a classical computer is effectively incapable of performing many tasks that a quantum computer could perform with ease. The simulation of a quantum computer on a classical one is a computationally hard problem because the correlations among quantum bits are qualitatively different from correlations among classical bits, as first explained by John Bell. Take for example a system of only a few hundred qubits, this exists in a Hilbert space of dimension ~1090 that in simulation would require a classical computer to work with exponentially large matrices (to perform calculations on each individual state, which is also represented as a matrix), meaning it would take an exponentially longer time than even a primitive quantum computer.
Richard Feynman was among the first to recognize the potential in quantum superposition for solving such problems much much faster. For example, a system of 500 qubits, which is impossible to simulate classically, represents a quantum superposition of as many as 2500 states. Each state would be classically equivalent to a single list of 500 1's and 0's. Any quantum operation on that system --a particular pulse of radio waves, for instance, whose action might be to execute a controlled-NOT operation on the 100th and 101st qubits-- would simultaneously operate on all 2500 states. Hence with one fell swoop, one tick of the computer clock, a quantum operation could compute not just on one machine state, as serial computers do, but on 2500 machine states at once! Eventually, however, observing the system would cause it to collapse into a single quantum state corresponding to a single answer, a single list of 500 1's and 0's, as dictated by the measurement axiom of quantum mechanics. The reason this is an exciting result is because this answer, derived from the massive quantum parallelism achieved through superposition, is the equivalent of performing the same operation on a classical super computer with ~10150 separate processors (which is of course impossible)!!
Early investigators in this field were naturally excited by the potential of such immense computing power, and soon after realizing its potential, the hunt was on to find something interesting for a quantum computer to do. Peter Shor, a research and computer scientist at AT&T's Bell Laboratories in New Jersey, provided such an application by devising the first quantum computer algorithm. Shor's algorithm harnesses the power of quantum superposition to rapidly factor very large numbers (on the order ~10200 digits and greater) in a matter of seconds. The premier application of a quantum computer capable of implementing this algorithm lies in the field of encryption, where one common (and best) encryption code, known as RSA, relies heavily 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 that use RSA -- previously considered to be "uncrackable" -- and anyone interested in electronic and financial privacy.
Encryption, however, is only one application of a quantum computer. In addition, Shor has put together a toolbox of mathematical operations that can only be performed on a quantum computer, many of which he used in his factorization algorithm. Furthermore, Feynman asserted that a quantum computer could function as a kind of simulator for quantum physics, potentially opening the doors to many discoveries in the field. Currently the power and capability of a quantum computer is primarily theoretical speculation; the advent of the first fully functional quantum computer will undoubtedly bring many new and exciting applications.
Release link: http://www.cs.caltech.edu/~westside/quantum-intro.html
Tags: supercomputer , Quantum Computer , photon , electron , subatomic particles ,
