Quantum bit withstands noise

By Eric Smalley, Technology Research News

The everyday world we see around us rests on a foundation of atoms and subatomic particles and the interactions among them. Separated from the world at large, these particles behave according to a very different set of rules than the laws of physics we experience.

Physicists have been able to study quantum mechanics by isolating particles for fleeting moments, and what they have found has led some to devise schemes to make extraordinarily powerful computers by harnessing the bizarre behavior of the particles.

The main challenge to making useful quantum computers is being able to isolate the delicate particles from environmental energies, or noise, like radio waves, magnetic fields, and light for more than small fractions of a second.

"Because it is virtually impossible to isolate a real-world quantum system from its environment, decoherence is practically ubiquitous," said Lorenza Viola, a postdoctoral fellow at the Los Alamos National Laboratory. Decoherence happens when noise from the environment intrudes on quantum particles' isolation, changing the quantum mechanical properties used to store information in quantum computing.

A team of researchers from Los Alamos National Laboratory and the Massachusetts Institute of Technology has come up with a way of fending off decoherence by using environmental noise rather than trying to block it.

Quantum computers use isolated atoms or subatomic particles to form quantum bits. Qubits, like bits in classical computing, have two positions that can represent the ones and zeros of binary logic, which is the basis of nearly all computers. For instance, an electron can be used as a qubit because, like a top, it can spin in one of two directions: spin up or spin down.

One of the strange qualities of quantum particles is that when a particle is isolated from its environment, which by definition means it cannot be observed directly, it acts differently from a particle that is not isolated and can be observed. An isolated particle is in superposition, which is some unknown mixture of all possible states. For example, an electron in superposition could be 1 percent spin up and 99 percent spin down or 50 percent spin up and 50 percent spin down.

The advantage of using a string of qubits in superposition to represent data is that it can effectively represent many numbers at once. A string of seven qubits could represent all 128 of the seven-digit combinations of spin up and spin down. Ten qubits could represent 1,024 combinations, and 15 qubits, 32,768 combinations at once. Classical bits can represent the same number of combinations, but must go through the combinations one at a time to find, for example, a combination that represents a solution to a problem.

One way of preserving qubits long enough for them to perform these useful computations is to control environmental noise in a way that leaves sheltered zones where some qubits can be protected. "If one has a bunch of qubits and, say, the noise affects all but the first qubit, then the information carried by qubit one is clearly preserved," said Viola.

One way of doing this is by making one logical qubit out of the interactions, or waves generated by several particles, rather than protecting the data in one physical qubit.

Quantum particles behave like both particles and waves. The geometrical shape of a particle's wave, its wave function, can be symmetrical, just like the left and right sides of a person. Interacting particles have a common wave shape, and when noise from the environment affects all of the particles equally, the shape of their collective wave contains symmetries.

The Los Alamos and MIT team has made a type of sheltered zone called a noiseless subsystem that makes a qubit out of the symmetries in the collective wave function of a set of three carbon atoms.

Using the wave function symmetries of quantum particles to store information requires more than one particle to represent a logical qubit, but that qubit preserves quantum information in the face of noise better than a qubit made of a characteristic like spin in a single particle. In fact, the noise that produces wave symmetries would be enough to destroy any single-particle qubit.

An earlier scheme demonstrated by the researchers at the National Institute of Standards and Technology (NIST) also uses sheltered zones and makes logical qubits from wave symmetries. The decoherence-free subspaces scheme uses noise to produce symmetries that do not otherwise affect the underlying set of particles. The scheme is also less complicated than noiseless subsystems, but it requires more precisely controlled noise, said Viola. They also require at least four particles per qubit, while the noiseless subsystem needs only three.

The noise that produces the wave symmetries in the noiseless subsystem scheme does change the quantum characteristics like spin in the underlying set of particles, which makes them more difficult to produce than decoherence-free subspace qubits, said Daniel Lidar, an assistant professor of chemistry at the University of Toronto.

On the other hand, noise that affects the particles is more common than noise that does not, which means noiseless subsystem qubits are potentially easier to sustain in the real world, said Viola. "Assuming the ability to identify or engineer quantum devices with the correct symmetries, noiseless subsystems would allow for more options in implementing robust [quantum] memories simply because noiseless subsystems are more common than decoherence-free subspaces," she said.

In addition, noiseless subsystems can be combined with a broader range of quantum error correction codes than decoherence-free subspaces, Viola said. Error correction codes catch errors that occur when one or more bits accidentally change from a one to a zero or vice versa.

The research is a significant step towards robust scalable quantum computing, said Lidar. As expected, noiseless subsystems produce a significant increase in coherence time for noise of arbitrary strength, he said. "The result holds for engineered noise, and hence its utility in real life remains to be seen," he said.

Although fewer particles are needed to encode qubits using noiseless subsystems than decoherence-free subspaces, the encoding process is somewhat more complicated, Lidar said. "A very valuable lesson learned from the [research] is that the encoding/decoding steps can take a significant amount of time and contribute to coherence degradation," he said.

The issue of number of particles versus ease of encoding presents researchers with a trade-off to consider when choosing between noiseless subsystems and decoherence-free subspaces, said Paul Kwiat, a physics professor at the University of Illinois at Urbana Champaign. "The trade-off of which is easier to deal with will depend on the particular system used for quantum computing."

Though the concept of noiseless subsystems might be important in developing quantum computers, the particular noiseless subsystem the Los Alamos and MIT researchers produced is not practical because it was created using Nuclear Magnetic Resonance (NMR) techniques, he said. NMR, which is also used for medical imaging, uses strong magnetic fields to align atoms.

NMR quantum computing "is now accepted not to be true quantum computing" because NMR quantum computers cannot be made with more than a few qubits, and they also cannot produce quantum entanglement, said Kwiat.

Particles in superposition can also be linked, or entangled, so that they share one or more quantum characteristics like spin; if environmental noise knocks an electron out of superposition and into the spin up position, an electron that was entangled with it also leaves superposition in the same spin up position, regardless of the physical distance between them. Entanglement would give practical quantum computers the ability to do many calculations efficiently.

The technique can be used in other quantum computer architectures, including trapped ions and solid-state quantum chips, according to Viola.

Practical applications of quantum information processing for cryptography and simulating quantum mechanics could be achieved in the next few years, said Viola. However, full-blown quantum computers that are able to, for example, improve on current capabilities for factoring large numbers are probably more than 20 years off, she said.

Viola's research colleagues were Emanuel Knill and Raymond Laflamme of Los Alamos National Laboratory and Evan M. Fortunato, Marco A. Pravia and David G. Cory of the Massachusetts Institute of Technology. They published the research in the September 14, 2001 issue of the journal Science. The research was funded by the Department of Energy, the National Security Agency (NSA), the Army Research Office and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   2 years, > 20 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Experimental Realization of Noiseless Subsystems for Quantum Information Processing," Science, September 14, 2001


September 26, 2001

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Quantum bit withstands noise

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