Quantum feedback in synthetic diamond

The concept of quantum superposition is fragile but could be achievable using feedback control in synthetic diamonds, thanks to work by researchers at Massachusetts Institute of Technology. Their efforts could be an important step towards a reliable quantum computer. [M. Hirose and P. Cappellaro, Nature (2016) 532, 77 DOI: 10.1038/nature17404]

At its heart quantum computing abhors observation and so, one would think, would preclude feedback control simply because the very act of testing and nudging the system changes the quantum state of the system by virtue of quantum theory. "Typically, we use open-loop control," explains MIT's Paola Cappellaro. "You first decide how to control your system and then apply your controller and hope for the best. But, feedback should be more robust, because it lets you adapt to what's going wrong."

Cappellaro and Masashi Hirose have now demonstrated feedback control that can maintain quantum superposition without interfering measurements. "Instead of having a classical controller to implement the feedback, we now use a quantum controller," Cappellaro says. "Because the controller is quantum, I don't need to do a measurement to know what's going on." That controller is a nitrogen-vacancy center - a nitrogen atom that has displaced a carbon atom in the lattice adjacent to a vacancy.

The electrons next to the NV center can be spin up or spin down or a superposition of the two, thus the center can act as a "qubit". The NV center brings several advantages because they are an intrinsic feature of the physical structure so need not be trapped with sophisticated hardware. Also, they are light emitters so their state can be "read" relatively easily. Moreover, those photons may themselves be in superposition so offer a way to transfer the quantum information.

In a proof of principle with a single qubit, the team used microwave radiation to first put the electronic spin into superposition and radio waves to put the nucleus of the nitrogen atom into a specific spin state of its own. A second, lower-power dose of microwaves "entangles" these spins into such a state that might be used in a quantum calculation. After computation, another blast of calibrated microwaves with different circular polarization disentangles the nucleus and the NV center. If there were an error in the computation (a simple spin flip or dephasing in this case), that blast which is controlled and known will adjust, or correct, the NV center, if there were no error it will remain the unaltered. This feedback-control system, sustained the NV-center qubit in superposition for a thousand times as long as it would exist without it.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".