Indiana University       Research & Creative Activity       September 2000 • Volume XXIII Number 2


Quantum Mechanics
Meets
the Information Age

by Mary Hrovat

Zhe-Yu "Jeff" Ou, associate professor of physics at IUPUI, combines the field of optics and quantum mechanics in his exploratins into quantum information processing. Photo Tyagan Miller.

 

T he computer on your desk right now can quickly and easily make calculations, organize data, and produce graphics in ways that not long ago required massive equipment, slow processors, and laborious human participation every step of the way. Likewise, Zhe-Yu “Jeff” Ou’s work at IUPUI currently requires a table full of lasers and expensive equipment. In the future, however, the applications his research may yield could be as commonplace as today’s spreadsheet software.

Ou, associate professor of physics at Indiana University Purdue University Indianapolis, works in the field of quantum optics and quantum communication—in other words, using photons (units of light) to carry and process information.

In classical optics, the laser is a workhorse. Lasers emit a narrow beam of light at a single wavelength, and the waves are related to each other in a particular way called coherence. Lasers are a very well-behaved source of light—well-behaved, that is, if you consider light as only a wave. When you weaken a laser light source so that you can see individual photons arriving, the photons show up randomly, like drops from a light rain falling on random spots in a pail. Ou and Yajun Lu, one of Ou’s graduate students, are working on a source of photons that do not arrive randomly.

Ou uses a quantum process by which a photon is turned into a pair of photons of lower frequency. His goal is to create a two-photon state that is as coherent and as well-behaved on the photon level as the precise waves of laser light. Because these two photons will be used to transmit information, understanding their behavior is crucial to developing applications for quantum information processing. Ou’s well-behaved photons, when available, could be the foundation for technologies such as quantum cryptography and quantum computing.

In this experimental setup from Ou's lab, a blue photon is split to produce two infrared photons for the two-photon state. Courtesy photo.

Quantum computing relies on a property of quantum particles known as the superposition of states. In the macroscopic world that we’re familiar with, a switch, for example, can be either on or off, and we know which one it is. Objects can spin counterclockwise or clockwise, and again we know which direction they are turning. Quantum particles, on the other hand, can be in two mutually exclusive states at the same time. Probability describes what the chances are that when a particle is observed, it will be in one state or another, but until the observation is made, the particle exists in both states. This is called a superposition of states.

A quantum computer would be exponentially faster than a classical computer because the bits in a quantum computer could be in more than one stateconcurrently. Rather than doing calculations sequentially, as in a traditional computer, the computer would carry out calculations simultaneously. Ou calls this “parallel processing.”

The bits of computer memory are usually seen as having one of two values, either a zero or a one. A photon can be both zero and one simultaneously. If you have 100 photons, or 100 bits, there are 2^100 possible combinations of zeros and ones, or 2^100 pieces of information that can be stored in those 100 bits. In a quantum computer, a large number of calculations can be done in just a single operation, rather than waiting for each operation to be completed separately.

Quantum computers are still largely hypothetical. To make even a single logic gate requires a great deal of expensive equipment. “A quantum system needs to be isolated,” explains Ou, so that it doesn’t interact with the macroworld of physics to which we are accustomed. Keeping everything quantum mechanical is the most difficult thing about quantum computing, according to Ou. Isolation presents a dilemma when you are trying to make calculations—to be useful, a quantum computer needs to get input from, and provide output to, the rest of the world.

Quantum interference is crucial to resolving this dilemma. One of the quantum properties of light is particle-wave duality, which is evident in the way we discuss light as wave and particle at the same time. This duality is also evident in the way we talk about interference, or the way waves interact by adding to or subtracting from each other, even when we’re talking about particles like photons.

Quantum interference is like a converter that takes information from the world that we are familiar with, introduces it to the quantum world where calculations are performed at super high speeds, then converts the quantum answer into something we can observe. The answer will appear as an interference pattern of bright and dark areas, with the dark areas corresponding to zeros and the bright areas to ones, providing information that can be translated into an answer we understand. The coherent two-photon state that Ou is pursuing is essential for this interference pattern to appear.

Quantum computing “is totally different from classical computing,” says Ou, “so you have to understand the rules.” Only some types of calculation will be suitable for quantum computers; one example is cracking today’s cryptographic keys.

When two or more particles are in a quantum-mechanical state, each particle’s properties can be correlated with the others. This gives rise to a property called non-locality, in which a measurement performed on one particle can instantaneously determine properties of other particles in the same quantum-mechanical state, even when they are separated by substantial distances. Ou’s early research involved theoretical studies on non-locality, a conundrum of quantum physics that has puzzled researchers since the days of Einstein. As it turns out, the concept may have a practical application in cryptography.

Quantum cryptography uses pairs of entangled photons to provide a cryptographic key. Secure communication relies on a key that is used to encrypt and decode data; the key is shared only by the sender and the receiver of the data. Typically, a key is based on factoring composite numbers made of large prime numbers. To break a key requires numbers of calculations so huge that, with current computers, they would require processing times equal to the age of the universe. But as processors become faster, the time required to break a key will decrease.

“In principle, current protocols for keys are breakable if you have a powerful enough computer,” says Ou. “It’s only safe because the technology is not catching up yet.”

Quantum cryptography offers a system that is unbreakable in practice and in principle. The security of a key transmitted via quantum cryptography is “ensured by the laws of physics,” says Ou. “Quantum mechanics prohibits breaking the code.”

To send a message using quantum cryptography, the sender would transmit the encrypted message along normal communication channels and would use pairs of entangled photons to carry information about the key. As quantum physics says, the information about these two entangled photons is contained only in their combined properties. The information contained in the state of the photons provides the key for encrypting and decoding the message, and this information is available only to the sender and the receiver. Any attempt to intercept the information would be evident because it would change the correlation between the two particles.

Most of the information contained in the two photons is used to exchange the key, with about 1 percent of the information used to check for errors that would indicate that the key had been intercepted. To use this method and to make sure that the sender and receiver have the same information about the key again requires quantum interference and the coherent two-photon source Ou is working on.

Experimental work such as Ou’s is at a very small scale: a pair of bits, a single logic gate. And much of the work in quantum computing and communication is still theoretical. So far, most of that work is being done at universities and national laboratories. Ou predicts that industry won’t become active in this research until there are some breakthroughs by academic researchers. But he believes quantum communication will become possible in this century, and his lab is laying the foundation.

“Once the first machine is made, improvements can come very quickly,” says Ou, citing the traditional computer as an example. “Once there’s a breakthrough, it goes very fast.”

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