Quantum computers are, as the statement from MIT expresses it, “largely hypothetical” devices that could in principle perform some calculations much more rapidly than conventional computers, exploiting quantum superposition, which describes a quantum particle’s counter-intuitive ability to, in some sense, inhabit more than one physical state at the same time.
But superposition is fragile, and finding ways to preserve it is one of the chief obstacles to developing large, general-purpose quantum computers. As reported in Nature, MIT researchers describe a new approach to preserving superposition in a class of quantum devices built from synthetic diamonds. The work could ultimately prove an important step toward reliable quantum computers.
In most engineering fields, the best way to maintain the stability of a physical system is feedback control. The problem with using this technique to stabilise a quantum system is that measurement destroys superposition. So quantum-computing researchers have traditionally had to do without feedback.
“Typically, what people do is to use what’s called open-loop control,” says Paola Cappellaro, the Esther and Harold Edgerton Associate Professor of Nuclear Science and Engineering at MIT and senior author on the new paper. “You decide a priori how to control your system and then apply your controller and hope for the best — that you knew enough about your system that the control you applied will do what you thought it should. Feedback should be more robust, because it lets you adapt to what’s going wrong.”
In the Nature paper, Cappellaro and her former PhD student Masashi Hirose, who graduated last year and is now with McKinsey and Company in Tokyo, describe a feedback-control system for maintaining quantum superposition that requires no measurement. “Instead of having a classical controller to implement the feedback, we now use a quantum controller,” Cappellaro explains. “Because the controller is quantum, I don’t need to do a measurement to know what’s going on.”
The vehicle for the observed quantum states in this work is diamond, with missing atoms in the lattice (a ‘vacancy’) and [occasional] nitrogen atoms substituting for a carbon. A nitrogen atom next to a vacancy in the lattice is a “nitrogen-vacancy (NV) centre”.
The MIT statement (read it here) goes on to outline how the reported work uses magnetic fields and pulses of microwaves to set and interrogate the quantum states of the electrons associated with the “NV centre” – and how increased stability can be obtained without erasing the information the system is designed to encode.
In experiments, the researchers found that, with their feedback-control system, an NV-center quantum bit would stay in superposition about 1,000 times as long as it would without it.
“[Cappellaro] sheds light on a method, coherent feedback, which was discussed in the literature, in theory, a while back but has never been experimentally explored,” says Jörg Wrachtrup, a physics professor at the University of Stuttgart, in Germany. “What is extremely good in there is that she’s showing — once you do it right, and once you find the right algorithm, which she did — how easy it is in the end to protect the electron spin against spin flip or dephasing.”
“The main advantage of this technique compared to previously reported results, like protection of spin using echoes, is robustness against noise,” adds Fedor Jelezko, a physics professor at Ulm University, in Germany. “The technique demonstrated by the Cappellaro group is less sensitive to the time scale of the noise. I believe that applications of this technique will appear soon, as demonstrations of new protocols applied to quantum metrology and quantum computing.”
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