Alternative quantum bits go natural

By Eric Smalley, Technology Research News

One of the first hurdles to developing practical quantum computers is coming up with devices that let researchers precisely control qubits, the individual atoms, electrons or photons that are the basic building blocks of quantum computers.

Controlling qubits usually involves minute, finely tuned and precisely aimed laser, microwave or magnetic pulses. These control operations are very difficult for even a single qubit and the task of controlling as few as 10 is daunting. Ultimately, practical quantum computers will require systems that can control at least several hundred qubits.

A team of researchers based at the University of California at Berkeley has come up with an encoding scheme that sidesteps the problem. The trick is making qubits out of the natural interactions of two or more particles rather than changing the behavior of individual particles. The researchers are proposing to fit computer logic to the natural actions of qubits, rather than forcing qubits to do conventional logic operations.

There are two basic requirements quantum computers must satisfy in order to perform all the binary logic operations of ordinary computers: controlling all possible quantum mechanical states of each physical qubit and quantum mechanically linking two or more physical qubits to form a Controlled-Not (CNOT) logic gate. A CNOT gate has a control bit and a target bit. If the control bit is 1, it flips the target bit from 0 to 1 or 1 to 0. If the control bit is zero, it leaves the target bit alone.

These requirements, outlined in a 1995 paper, have become a sort of bible of universal quantum computation, said Daniel Lidar, an assistant professor of chemistry at the University of Toronto.

The trouble is, it's very difficult to make physical systems that can satisfy these requirements, he said. "In... quantum dots, for example, implementing the single-qubit operations can be very hard," he said.

In a quantum computer whose qubits are individual electrons trapped inside quantum dots, which are microscopic specks of semiconductor material, flipping a bit from a 0 to a 1 or a 1 to a 0 requires extreme accuracy, said Lidar. "You need to apply a very, very local, microscopically accurate magnetic field," he said.

This is where the Berkeley encoding scheme comes in. Rather than forcing physical qubits to perform these difficult operations, the researchers propose to use "what we call the natural talents of the physical system," said Lidar. "The paradigm shift that we're proposing is that you start with whatever is natural for [a given] system," he said. "You investigate whether the system as such is capable of implementing universal [quantum] computation."

In a quantum dot system, one natural operation is switching information between two neighboring quantum dots, said Lidar. "If you have two physical qubits -- two electrons on two separate quantum dots -- [you can swap] the wave functions of these two electrons. It's a lot easier to perform than these single-qubit operations," he said.

The catch is that these natural operations don't translate directly to the necessary quantum computing functions.

"If you want to just use the naturally available interactions in the system, you'll have to play some tricks," said Lidar. "You're going to have to represent your logical zeros and ones in terms of some entangled combinations of [quantum mechanical] states of these physical qubits," he said.

And at some level the quantum computer still has to perform the same operations that implement universal quantum computation. "Again you use single qubit gates and a CNOT, but these single-qubit gates no longer operate on the physical... qubits, rather they operate on the encoded qubits," said Lidar.

The downside to encoding is that quantum computers will need at least twice as many physical qubits as non-encoded systems require.

"The trade-off in encoding is that you're using a number of physical qubits in order to encode one logical qubit," said Lidar. "Whether the net balance is positive, that's something that's going to depend on a particular implementation," he said. "What is easier to do, engineer a difficult operation or give access to more physical qubits?"

The encoding scheme grew out of research on decoherence-free subspaces, which protect qubits from decoherence. Decoherence occurs when energy from the environment knocks a physical qubit out of its quantum mechanical state. Limiting decoherence is one of the principal challenges to developing practical quantum computers.

The encoding research shows how this idea can be extended to general notions of fault-tolerant quantum computation, said Seth Loyd, an associate professor of mechanical engineering at the Massachusetts Institute of Technology.

"This scheme is potentially useful, [but] whether this or any other scheme will lead to large-scale quantum computation remains to be seen," he said.

In order to test the encoding scheme, researchers will need to build prototype solid-state quantum computers with between two and four qubits and test them first using the standard paradigm, said Lidar. That probably won't happen for another five years, he said.

Many researchers say that it will be at least two decades before practical quantum computers are developed.

Lidar's research colleagues were Dave Bacon, Julia Kempe and K. Birgitta Whaley of the University of California at Berkeley and David P. DiVincenzo of IBM Research. The research was funded by the Army Research Office, the National Security Agency, and the Advanced Research and Development Activity.

Timeline:   >20 years
Funding:  Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Encoded Universality in Physical Implementations of a Quantum Computer," posted to the Los Alamos National Laboratory e-Print archive Feb. 27, 2001 ; Technical paper "Quantum computation," Science, October 13, 1995


April 18, 2001

Page One

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Alternative quantum bits go natural

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