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."