The potential power of a quantum computer comes from the qubit – a unit of information which quantum physics allows to be in a “superposition” of all possible values between 1 and 0. But quantum superpositions are so fragile that even taking a measurement can cause them to collapse. How to keep superpositions stable is one of the central questions of quantum computing research.
Now Paola Cappellaro, an Associate Fellow in CIFAR’s Quantum Information Science program and a researcher at the Massachusetts Institute of Technology, has found a way to use the quantum properties of diamonds to stabilize superpositions. She developed the technique with her former PhD student, Masashi Hirose, and they reported it earlier this month in the journal Nature.
The simplest way to keep a system based on classical physics stable is through feedback. For instance, to keep a room at a stable temperature you install a thermostat, which gives the system a nudge when it gets too hot or too cold. But feedback systems require some sort of measurement – and the fragility of quantum systems makes that difficult.
Cappellaro’s technique tackles the problem by using another quantum effect called entanglement, in which the state of one quantum particle affects the state of another. The technique was demonstrated using qubits associated with imperfections within diamond crystals.
In a perfect diamond, carbon atoms are arranged in an evenly-spaced crystalline lattice. But in an imperfect diamond, some of the carbon atoms will be missing, creating a so-called “vacancy.” At other locations, a carbon atom may be replaced by a nitrogen atom. If the two are adjacent, it’s called a “nitrogen vacancy (NV) centre.”
The NV centre comprises electrons that have a quantum property known as spin, which can be up or down or some combination of the two – thus allowing it to function as a qubit, the basic unit of quantum information. The nucleus of the nitrogen atom also has a spin. When it’s entangled with the NV centre, the spin of the nitrogen nucleus can be used to “read” or “write” information stored in the spin of the NV – without the sort of intrusive measurement that would normally destabilize a quantum system.
The interaction between the electronic and nuclear spins, modulated by external microwave impulses, creates entanglement between the two spins. The result is a qubit that can remain in superposition a thousand times longer than it would without the control system in place.
The diamond-based systems Cappellaro and Hirose have been studying function only as a single qubit, so their computational power is limited. But the researchers believe the work may be a vital first step with a wide array of potential applications, from quantum sensing to quantum communications.
“Once we have long-lasting quantum bits, then we can start trying to connect them – it’s a way to scale up your system,” says Cappellaro. “The work is more general than just building quantum computers. It could be powerful in many different situations.”