July 4, 2012
Scientists Make Strides Toward Quantum Computing
Lee Rannals for redOrbit.com - Your Universe Online
Harvard scientists claim that they have solved a problem faced in quantum computing by using diamonds.
One challenge quantum computing has faced is creating quantum bits that exist in a solid-state system at room temperature. Most systems rely on complex and expensive equipment designed to trap an atom or electron in a vacuum, and then cool the entire system to nearly absolute zero, or –459.67° Fahrenheit.
The Harvard team used a pair of impurities in laboratory-grown diamonds to create quantum bits, or qubits, and store information in them for nearly two-seconds.
Although two-seconds doesn't seem like a long time, it is actually an increase of nearly six orders of magnitude over the life span of earlier systems.
The scientists wrote in the journal Science that this is a first step in the eventual construction of a functional quantum computer.
"What we´ve been able to achieve in terms of control is quite unprecedented," Professor of Physics Mikhail Lukin, leader of the research, said. "We have a qubit, at room temperature, that we can measure with very high efficiency and fidelity."
He said the work is limited only by technical issues, so it would be feasible to increase the life span into the range of hours.
"At that point, a host of real-world applications become possible," Lukin said.
He said he envisions the system being used in applications that include "quantum cash," which is a theoretical payment system for bank transactions and credit cards that rely on coding of quantum bits to keep counterfeiters at bay. Another application, according to Lukin, would be for "quantum networks," which is a highly secure communications method that uses quantum bits to transmit data.
"This research is an important step forward in research toward one day building a practical quantum computer,” graduate student Georg Kucskoo, who works in Lukin´s lab and is one of two first authors of the paper, said. "For the first time, we have a system that has a reasonable timescale for memory and simplicity, so this is now something we can pursue."
During the initial experiments, the team used diamonds that contained 99 percent carbon-12 atoms, which have no spin. However, the remainder was made up of carbon-13 atoms, which is a tricky isotope that contains a spin in the atom's nucleus.
“The nuclear spin of the carbon-13 makes an ideal quantum bit, because they are very isolated,” Lukin said. “Because they interact with so few outside forces, they have relatively long coherence times. Of course, the same properties that make them ideal qubits also make them difficult to measure and manipulate.”
The team decided that rather than trying to find a way to measure the spin of the carbon atoms, they would use the nitrogen-vacancy (NV) centers, which are atomic-scale impurities in lab-grown diamonds, to do it for them.
They developed a new technique to create crystals that were even more pure, and then bombarded the crystal with nitrogen to create the NV center.
The interaction resulted in the NV center mirroring the state of the carbon atom, which means the researchers can encode a bit of information into the spin of the atom, then "read" that data by monitoring the NV center.
“The system we´ve developed uses this very local probe, the NV center, to allow us to monitor that spin,” Lukin said. “As a result, for the first time, we can encode a bit of information into that spin, and use this system to read it out.”
However, encoding information into the spin of the carbon-13 atom and reading it using the NV center is only a first step. The team had to determine how to take advantage of the atom's quantum properties as well.
Being able to be in two states at the same time is a key principle in quantum computers. Traditional computers encode bits of information as either zero or one, while quantum computers rely on atomic-scale quantum mechanics to give quantum bits both values at once.
That property allows quantum computers to perform multiple computations in parallel, making them more powerful than traditional computers.
The first step, according to Lukin, is to cut the connection between the NV center and the carbon atom by using massive amounts of laser light.
The second step is that the diamond crystal is bombarded with a specific set of radio frequency pulses, which suppresses the interaction between the carbon-13 atom and nearby atoms.
“By limiting interactions with the carbon-13 atom, we can extend the life of the qubit and hold the data for longer,” Lukin said. “The end result is that we´re able to push the coherence time from a millisecond to nearly two seconds.”