Scientists make seven-bit quantum leap in computer research

Scientists at MIT and the Department of Energy's Los Alamos National Laboratory have made yet another experimental leap forward in the quest for a functional quantum computer capable of solving large mathematical problems or cracking secret codes faster than today's fastest supercomputers.

Using nuclear magnetic resonance (NMR) techniques, researchers created a seven-qubit quantum computer within a single drop of liquid. This latest advance could lead to computers that use quantum bits, or qubits, for information processing.

The laws of quantum physics allow quantum particles to exist in multiple states; quantum particles can represent both a zero and one at the same time. This concept allows bits -- in this case qubits -- to be encoded at speeds beyond what is possible in a classical digital computer.

The advance and proposal for benchmark experiments on which to base future quantum computer performance are described in a paper in the March 23 issue of Nature by Emanuel Knill, Raymond Laflamme and Rudy Martinez at Los Alamos and Ching-Hua Tseng of MIT.

If functional quantum computers can be built, they will be valuable in factoring large numbers and therefore extremely useful for decoding and encoding secret and confidential information. Their arrival could spell trouble for Internet users.

"Of course, if we had an operational quantum computer today, nothing on the Internet would be safe," Dr. Laflamme said. "Our current methods of encrypting secret or personal data, like the RSA public key encryption algorithm currently used in web browsers, would be nearly worthless."

Even with this latest advance, the world is still years away from a functional quantum computer. Nonetheless, this recent development is a strong indication that quantum computing is quickly moving from the realm of science fiction into reality.

"What we find particularly intriguing," said Dr. Laflamme, "is that this latest advance seems to follow Moore's Law." Moore's Law says that the density of transistors on integrated circuits, and in turn the calculating speed of the computer, doubles every 18 months. The birth of the three-qubit quantum computer came roughly 18 months ago at Los Alamos. "I think it's a bit premature, however, to really assume it follows Moore's Law, but who knows what future technological developments will do. Of course, if Moore's Law is at work here, then we could have a 30-qubit quantum computer in less than five years."

A 30-qubit quantum computer would be roughly equivalent to a conventional computer running at 10 teraops, or trillions of operations per second. The fastest supercomputers in the world have achieved peak speeds of three teraops.

The quantum computer uses NMR to manipulate particles in the atomic nuclei of molecules of transcrotonic acid, a simple fluid consisting of molecules made up of six hydrogen and four carbon atoms. The particles are like tiny bar magnets spinning in a magnetic field that can be "lined up" by applying an electromagnetic pulse from the nuclear magnetic resonance device. This lining up of spinning particles in positions either parallel or counter to the magnetic field allows the quantum computer to mimic the information encoding of bits -- zeros and ones -- in classic digital computers.

Quantum computer scientists believe they may some day be able to use NMR pulses of just the right frequency to manipulate or flip the quantum states of particles with sufficient reliability to create a functional quantum computer.

Quantum computers were first proposed in the early 1990s as a way to factor large numbers. The first three-qubit quantum computer was achieved at Los Alamos in 1998 using NMR and a trichloroethylene molecule.

A version of this article appeared in MIT Tech Talk on March 29, 2000.

Topics: Computer science and technology, Physics

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