# Texas A&M Physicists Devising What May Be The Next Generation Of Computers

COLLEGE STATION —

Texas A&M physicists are devising computer codes and prototypes of quantum computers, which may be the next generation of computers.

An ordinary computer may seem very sophisticated, but in fact, it is only programmed to gobble large series of ones and zeros and spit out other large series of ones and zeros.

It is that simple because a combination of zeros and ones can make up all possible numbers. All possible operations on these numbers – for example addition, subtraction or division – are also simple in principle because they rely on logical rules of combinations of zeros and ones.

This basic structure of zeros and ones is limited, however, because only one number is processed at a time.

“The most dramatic example that has been tackled by computer scientists so far is factorization,” says Suhail Zubairy, visiting professor of physics at Texas A&M. “You have a large number and you want to factorize it. If you take a thousand-digit number, on the fastest computer today, factorizing it would take a few billion years.”

Factoring can be important to encryption of numbers, which keeps your credit card number secret when making computer-transacted purchases.

In order to avoid infinitely long calculations and further optimize current computers, scientists have started devising a new way to simultaneously process many numbers. This is possible for a quantum computer.

In an ordinary computer, every number is represented by a series of bits, with each bit taking the value zero or one. Zero or one means that a switch is set to off or on, respectively. One bit makes zero or one, two bits make the numbers zero, one, two or three (00, 01, 10 or 11), three bits make one of the numbers zero to seven (000, 001, 010, 011, 100, 101, 110, 111), and so on.

In a quantum computer, each bit, which is now called a “qubit,” or quantum bit, can be simultaneously zero and one. This strange property of quantum physics permits three qubits to hold all of the numbers zero through seven at the same time.

The bottom line: In an operating quantum computer, there is both zero and one in each qubit, while there is either zero or one in each bit of an ordinary computer.

“For instance, a calculation using three qubits might involve all of the numbers zero through seven simultaneously, but at the end of the calculation, I get only one number out, as with a usual computer,” says David Church, professor of physics at Texas A&M. “The one number I get could be random, anywhere between zero and seven, which does not seem to help me at all. Which number is the right answer to the calculation?”

This apparent paradox is solved by the way calculation is performed, called “quantum interference.” When the final result is read, the single correct answer is very probable, not random. In a quantum computer, zero or one might be associated with the internal energies of an ion – a charged atom – which act as a qubit.

The ion can have low internal energy – called zero – or higher internal energy – called one – or exist in an intermediate stage. The intermediate stage corresponds to a qubit that is both zero and one.

“Laser pulses are applied to each ion to put it in the intermediate stage,” Church says. “By studying the light emitted by these ions, it is possible to derive the actual number they represent.

“In the case of three bits, three ions would hold the numbers zero through seven, and each ion would interact separately with the laser light,” he says.

The study of quantum computing using singly charged ions is being pursued at the National Institute for Standards and Technology in Boulder, Colo., at Los Alamos National Laboratory in Los Alamos, N.M., and in Europe.

Church has been collaborating with the Lawrence Livermore National Laboratory in Livermore, Calif., to study the possibility of quantum computing using highly charged – instead of singly charged – ions.

“We have used the facilities of Livermore Laboratory to confine and cool highly charged ions,” Church says. “It turns out that these ions have many internal energy states that are accessible to lasers. Simulations also show that when sufficiently cooled, these ions form regular arrays.”

Church says that work on quantum computing is just “lifting off the ground” and is highly interdisciplinary, involving at once physicists, chemists, computer scientists, engineers and mathematicians. He is currently teaching a course in quantum computing, along with Walter Daugherity, senior lecturer of computer science at Texas A&M.

Contact: David A. Church, (979) 845-2841 or church@physics.tamu.edu.

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