Feedback technique used on diamond 'qubits' could make quantum computing more practical

April 6, 2016 by Larry Hardesty
Researchers have designed a feedback-control system for maintaining quantum superposition that requires no measurement. Masashi Hirose (pictured) adjusts optical components on the the setup used to perform the experiment. Credit: Greg Hren

Quantum computers are largely hypothetical devices that could perform some calculations much more rapidly than conventional computers can. They exploit a property called superposition, which describes a quantum particle's counterintuitive 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. In today's 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. You make a measurement—the current trajectory of an airplane, or the temperature of an engine—and on that basis produce a control signal that nudges the system back toward its desired state.

The problem with using this technique to stabilize a quantum system is that measurement destroys superposition. So 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 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."

Vacant expression

Cappellaro and Hirose's system uses a so-called nitrogen-vacancy center in diamond. A pure diamond consists of carbon atoms arranged in a regular latticework structure. If a carbon nucleus is missing from the lattice where one would be expected, that's a vacancy. If a nitrogen atom takes the place of a carbon atom in the lattice, and it happens to be adjacent to a vacancy, that's a nitrogen-vacancy (NV) center.

Associated with every NV center is a group of electrons from the adjacent atoms, which, like all electrons, have a property called spin that describes their magnetic orientation. When subjected to a strong magnetic field—from, say, a permanent magnet positioned above the diamond—an NV center's electronic spin can be up, down, or a quantum superposition of the two. It can thus represent a quantum bit, or "qubit," which differs from an ordinary computer bit in its ability to take on not just the values 1 or 0, but both at the same time.

NV centers have several advantages over other candidate qubits. They're an intrinsic feature of a physical structure, so they dispense with the complex hardware for trapping ions or atoms that other approaches require. And NV centers are natural light emitters, which makes it relatively easy to read information from them. Indeed, the light particles emitted by an NV center may themselves be in superposition, so they provide a way to move quantum information around.

Local control

Like electrons, atomic nuclei have spin, and Cappellaro and Hirose use the spin state of the nitrogen nucleus to control the NV center's electronic spin. First, a dose of microwaves puts the electronic spin into superposition. Then a burst of radio-frequency radiation puts the nitrogen nucleus into a specified spin state.

A second, lower-power dose of microwaves "entangles" the spins of the nitrogen nucleus and the NV center, so that they become dependent on each other. At this point, the NV qubit could, together with other qubits, be enlisted to perform a computation. But in their experiments, Cappellaro and Hirose were evaluating a single qubit, so they could test only the most rudimentary computational operation: the not gate, which flips a bit's value.

Because the spins of the nitrogen nucleus and the NV center are entangled, if anything goes wrong during the computation, it will be reflected in the spin of the nitrogen nucleus.

After the computation is performed, a third dose of microwaves—whose polarization is rotated relative to that of the second—disentangles the nucleus and the NV center. The researchers then subject the system to a final sequence of microwave exposures. Those exposures are calibrated, however, so that their effect on the NV center depends on the state of the . If an error crept in during the computation, the microwaves will correct it; if not, they'll leave the NV center's state unaltered.

In experiments, the researchers found that, with their feedback-control system, an NV-center bit would stay in about 1,000 times as long as it would without it.

Explore further: Quantum engineering

More information: Coherent feedback control of a single qubit in diamond, Nature, DOI: 10.1038/nature17404

Related Stories

Quantum engineering

August 13, 2014

It can be difficult to distinguish between basic and applied research in the nascent field of quantum engineering. One person's exploration of quantum systems like atoms and electrons yields another's building block for quantum ...

Magnetism at nanoscale

August 3, 2015

As the demand grows for ever smaller, smarter electronics, so does the demand for understanding materials' behavior at ever smaller scales. Physicists at the U.S. Department of Energy's Ames Laboratory are building a unique ...

A qubit candidate shines brighter

December 29, 2014

In the race to design the world's first universal quantum computer, a special kind of diamond defect called a nitrogen vacancy (NV) center is playing a big role. NV centers consist of a nitrogen atom and a vacant site that ...

Recommended for you

Two teams independently test Tomonaga–Luttinger theory

October 20, 2017

(Phys.org)—Two teams of researchers working independently of one another have found ways to test aspects of the Tomonaga–Luttinger theory that describes interacting quantum particles in 1-D ensembles in a Tomonaga–Luttinger ...

Using optical chaos to control the momentum of light

October 19, 2017

Integrated photonic circuits, which rely on light rather than electrons to move information, promise to revolutionize communications, sensing and data processing. But controlling and moving light poses serious challenges. ...

Black butterfly wings offer a model for better solar cells

October 19, 2017

(Phys.org)—A team of researchers with California Institute of Technology and the Karlsruh Institute of Technology has improved the efficiency of thin film solar cells by mimicking the architecture of rose butterfly wings. ...

Terahertz spectroscopy goes nano

October 19, 2017

Brown University researchers have demonstrated a way to bring a powerful form of spectroscopy—a technique used to study a wide variety of materials—into the nano-world.

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Hyperfuzzy
not rated yet Apr 06, 2016
OK, I don't get it. Feedback is either the field, particle motion as current, or potential. So how do you feedback a theoretical concept? So this sounds like a bang bang controller. But if you want to call it a "quantum" controller, meh!
Stephen_Crowley
not rated yet Apr 10, 2016
I'm with ya. Sounds like a candidate for a variational optimal control solution with partial observations

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.