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Quantum Information Research at NIST: Goals and Vision |
Quantum information systems can transcend the physical limits of today’s computing and communications technologies. Transistors and other electronic components have been shrinking in size for many years. When they get close to the size of single atoms, they will be miniaturized out of a job. Atomic-sized circuits cannot be made to function in conventional ways, in part because of the inability to dissipate heat and in part because they do not behave like their larger counterparts. Thus, at the smallest scales, scientists need to take advantage of a different set of design rules. These are the rules of quantum mechanics, nature’s instruction book for the smallest particles of matter and energy. First developed by Albert Einstein, Niels Bohr, and other physicists during the early years of the 20th century, quantum mechanics is the most fundamental and successful set of principles and equations known at this time for predicting the behavior of particles such as atoms and electrons, and electromagnetic radiation such as light and radio waves. Quantum mechanics plays an important role in many modern technologies. It describes a world where energy is measured and exchanged only in discrete, measurable units, or quanta. It is also a world of counterintuitive “weirdness” where objects behave in exotic ways—existing in two places at once, for example—that have no precedent in everyday, macroscopic life.
Today’s digital information systems represent 1s and 0s using tiny electrical switches, which are either on or off, or the orientation of a magnet up or down, or the presence or absence of light. The information in such a device or light signal is called a bit. In quantum information processing, various quantum mechanical states of individual particles or systems are used as quantum bits (qubits). For instance, ions (charged atoms) may have different “spin” states that can represent 0 and 1. Spin can be thought of as the direction of a little compass needle inside the ion, with north and south poles. “Spin up,” corresponding to 0, has a greater energy than “spin down,” corresponding to 1. Similarly, single photons (the smallest quantities of light) can be transmitted in different orientations, or directions of their electric field, to represent 0 and 1. Amazingly, qubits can be a combination of both 0 and 1 at the same time, a property called superposition. Qubits also can be correlated with each other, even at a distance—a property called entanglement. It is these unusual properties that give quantum information its power. Superposition
Qubits can process far more information than today’s digital bits because they can exist in a “superposition” of two quantum states that, at a given moment, has some combination of both 1 and 0 at the same time. A qubit can be in any one of an infinite number of possible superposition states at a given time, as long as it is not being measured. In the graphic below, this is represented as any one of many possible ion spin directions in between up and down (right image). Superposition states always collapse to 0 (left image) or 1 (middle image) when the qubit is measured. The fate of a superposition state in which the spin is depicted as horizontal is, when measured, spin up 50 percent of the time and spin down 50 percent of the time.
To understand why superpositions are so important, compare the processing power of a hypothetical three-qubit quantum computer to a conventional three-bit computer. Three conventional bits can store just one of eight numbers from 0 to 7 in binary code (see below). Three qubits can store all eight (23) such numbers at once thanks to the “magic” of superposition—an exponential increase. This also means that, effectively, eight calculations could be carried out at virtually the same time using just three qubits. This is a built-in capability for parallel processing, using far fewer bits than would be needed for simultaneous computations in today’s classical computers. With a few hundred qubits, a quantum computer could have far more power than even a network of the world’s best supercomputers working together. Entanglement Conventional computers use programmed logic operations and specialized circuits to perform calculations and solve problems. These are the software and hardware equivalents of “if/then” statements. For instance, a particular logic operation that combines two bits to get one result might work as follows: If either bit has the value 0, then the output is 1; otherwise, the output is 0. Through a quantum version of a logic operation on two qubits, the qubits become entangled. If one of these qubits is measured, its fate is correlated with that of the other qubit, even if the two are widely separated. Einstein called entanglement “spooky action at a distance.” Entanglement sounds supernatural but in fact can occur spontaneously when two atoms, for example, are in close proximity. The atoms’ properties and behavior become linked in predictable ways, and may remain so even if the atoms are physically moved apart. Entangled atoms can be compared to dance partners who do not touch each other but somehow synchronize their movements. While real dancers may do this through subtle communication, the qubit partners somehow do this without communicating, even though the dance steps may differ each time. This is what makes it “spooky.”
Entanglement needs to be precisely controlled to be useful in information processing, a difficult task. Scientists have learned how to control entanglement of small numbers of atoms and photons. The effect can be visualized as a set of gears moving two atoms in tandem (see image above). There are no real gears in quantum entanglement, however; the atoms just “know” what to do on their own. Controlled entanglement is a unique quantum resource that offers, for example, a way of transmitting data or performing controlled interactions on distant quantum bits, as long as a classical communications channel is also available. |
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Date
created: 4/11/06
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