BOSTON—The quantum world and the everyday world of human experience are supposed to be two different realms. Quantum effects, as demonstrated in the lab, are usually confined to the tiniest scales. They last for imperceptibly brief instants. And they appear mostly in highly controlled systems operating at cryogenic temperatures near absolute zero.
But experimental physicists are pushing across the assumed divide between the quantum and the ordinary by demonstrating quantum effects in more familiar environments. Now a group of researchers has furthered that cause by encoding quantum information into a room-temperature solid for time spans that can be ticked off on a stopwatch. The new quantum memory scheme can store information for more than a second, which extends by orders of magnitude the lifetime of information encoded as a quantum bit, or qubit, on a particle at ordinary temperatures. The American, German and British researchers have only just submitted the research to a peer-reviewed journal, but here in late February they presented their findings to a meeting of the American Physical Society.
A qubit, much like an ordinary bit in commonplace electronic devices, has a 0 state and a 1 state. But unlike a classical bit, a qubit can be in a so-called superposition of 0 and 1. That property, along with other phenomena such as quantum entanglement, means that quantum computers based on qubits would be phenomenally powerful—that is, if a practical machine could ever be built.
But that power comes at a price. A qubit can easily be corrupted by outside influences such as heat and magnetic fields. Physicists have produced long-lived qubits by all but eliminating such noise, confining individual atoms to vacuum traps or cooling them nearly to absolute zero. But some research groups have been trying to design qubits that can operate in solid-state systems at room temperature—to make a qubit, in short, that can survive in the world of the bit.
In the latest advance on that front, the research groups of Mikhail Lukin of Harvard University and Ignacio Cirac of the Max Planck Institute of Quantum Optics in Garching, Germany, and their colleagues encoded long-lived quantum information in the spin of a single-atom impurity in a synthetically produced diamond. Spin is a quantum property akin to the pointing of a particle's internal bar magnet, either up or down, representing 1 or 0.
The experimental quantum-grade diamond is 99.99 percent pure carbon 12, the most common isotope of the element. But the crystal also contains a small amount of the heavier isotope carbon 13 as well as implanted nitrogen ions that form defects in the diamond lattice known as nitrogen vacancy centers. Both impurities have certain quantum benefits.
Each, for example, features an intrinsic spin with a special talent. The nitrogen ion has an associated electron whose spin state is readily detectable by shining laser light on the nitrogen vacancy center. The carbon 13's nuclear spin state remains stable for long intervals.
The researchers figured out a way to combine these two attributes. Their approach uses the carbon 13 to store information for long periods of time and the nitrogen ion as a readout.
The scientists located an area in the diamond where a carbon 13 and a nitrogen ion are only about two nanometers apart. At that distance the spin of the nitrogen ion's electron and that of the nearby carbon nucleus couple together—the electron acts as a tiny magnetometer that reflects the carbon 13's nuclear spin state. By hitting the nitrogen vacancy center with laser light, the researchers can measure the electron's spin and, by extension, the spin of the carbon 13 nucleus.
The catch is that the electron's spin is not as stable as the nuclear spin of the carbon atom; it fluctuates on the millisecond timescale. And once the electron changes its spin, the information in the qubit is lost. "A single electronic spin flip completely destroys the coherence of our nucleus," said Georg Kucsko, a member of Lukin's research group who presented at the meeting. To keep the electron's flip-flopping from affecting the nucleus, the researchers continually reset the electron's spin with green laser light, essentially turning off the interaction between electron and nucleus when that interaction is not needed.
"In order to make things better, you make it worse," says solid-state quantum physicist Fedor Jelezko of the University of Ulm in Germany, who was not part of the new research. "If the electron flips very fast, the nucleus will not see it anymore. It creates some average field that does not fluctuate."
Using that tactic, along with a sequence of radio-frequency pulses to suppress interactions with other carbon nuclei in the diamond, the researchers were able to store quantum information at ambient temperatures for nearly two seconds. That is a significant leap from previous experiments, where storage times in single qubits have generally been measured in microseconds. "There were really no examples before of such long coherence times, except perhaps in trapped atoms or ions," Jelezko says. And the vacuum traps and laser-cooling apparatuses required for those laboratory setups would be prohibitively complex for some applications.
"The demonstration of a single-qubit quantum memory with seconds of storage time at room temperature is certainly exciting," adds Nick Vamivakas, an optical physicist at the University of Rochester who was not part of the research team. "I am not aware of another system that is being investigated for quantum information purposes that has demonstrated memory lifetimes on the second timescale."
Kucsko added that even better storage times ought to be attainable by stabilizing the diamond's temperature to eliminate thermal drifts that limit the qubit's life span. Purifying the diamond even further, thereby reducing the number of secondary carbon 13 impurities that can interact with the qubit, would also help. Lukin noted that with such improvements, storage times measured in hours might even be possible. "We have really an excellent qubit at room temperature which combines memory, control and measurement—all three things," he said. "We're actually quite excited about this new development."