Quantum computing uses quantum bits, or qubits, to encode information in the form of ones and zeros. |
Scientists have taken the next major step toward quantum computing, which
will use quantum mechanics to revolutionize the way information is processed.
Quantum computers will capitalize on the mind-bending properties of quantum
particles to perform complex calculations that are impossible for today’s
traditional computers.
Using high-magnetic fields, Susumu Takahashi, assistant professor at the University
of Southern California (USC) Dornsife College of Letters, Arts, and Sciences,
and his colleagues managed to suppress decoherence, one of the key stumbling
blocks in quantum computing.
“High-magnetic fields reduce the level of the noises in the surroundings so
they can constrain the decoherence very efficiently,” Takahashi says.
Decoherence has been described as a “quantum bug” that destroys fundamental
properties that quantum computers would rely on.
The research appears online in Nature.
Quantum computing uses quantum bits, or qubits, to encode information in the
form of ones and zeros. Unlike a traditional computer that uses traditional
bits, a quantum computer takes advantage of the seemingly impossible fact that
qubits can exist in multiple states at the same time, which is called “superposition.”
While a bit can represent either a one or a zero, a qubit can represent a
one and a zero at the same time due to superposition. This allows for
simultaneous processing of calculations in a truly parallel system, skyrocketing
computing ability.
Though the concepts underpinning quantum computing are not new, problems
such as decoherence have hindered the construction of a fully functioning
quantum computer.
Think of decoherence as a form of noise or interference, knocking a quantum particle
out of superposition—robbing it of that special property that makes it so
useful. If a quantum computer relies on a quantum particle’s ability to be both
here and there, then decoherence is the frustrating phenomenon that causes a
quantum particle to be either here or there.
University of British Columbia researchers calculated all sources of
decoherence in their experiment as a function of temperature, magnetic field,
and by nuclear isotopic concentrations, and suggested the optimum condition to
operate qubits, reducing decoherence by approximately 1,000 times.
In Takahashi’s experiments, qubits were predicted to last about 500 microseconds
at the optimum condition—ages, relatively speaking.
Decoherence in qubit systems falls into two general categories. One is an
intrinsic decoherence caused by constituents in the qubit system, and the other
is an extrinsic decoherence caused by imperfections of the system—impurities
and defects, for example.
In their study, Takahashi and his colleagues investigated single crystals of
molecular magnets. Because of their purity, molecular magnets eliminate the
extrinsic decoherence, allowing researchers to calculate intrinsic decoherence
precisely.
“For the first time, we’ve been able to predict and control all the
environmental decoherence mechanisms in a very complex system—in this case a
large magnetic molecule,” says Phil Stamp, University of British Columbia
professor of physics and astronomy and director of the Pacific Institute of
Theoretical Physics.
Using crystalline molecular magnets allowed researchers to build qubits out
of an immense quantity of quantum particles rather than a single quantum object—the
way most proto-quantum computers are built at the moment.
“This will obviously increase signals from the qubit drastically so the
detection of the qubit in the molecular magnets is much easier,” says
Takahashi, who conducted his research as a project scientist in the Institute of Terahertz
Science and Technology and the Department of Physics at the University of California,
Santa Barbara.
Takahashi has been at USC Dornsife since 2010.
