Quantum computing advance—diamonds are a qubit’s best friend

Another week, another quantum computing story. Quantum computing is a very hot topic in the world of physics at the moment because, for the first time, we can actually contemplate the sort of control required to make a quantum computer. It happens to also be true that a truly scalable quantum computer would have applications in the real world as well—code breaking, and simulating quantum systems are two common examples.

The key to quantum computing is the ability to manipulate and maintain quantum states. Unfortunately, these quantum states are hard to create, hard to manipulate, and very, very hard to stabilize. Now, researchers have demonstrated that qubits based on the electronic and nuclear states of nitrogen vacancies in diamond may be the way of the future.

Diamond is supposed to be a material made purely of carbon. Each carbon atom attaches itself to four other carbon atoms that sit at the apexes of an imaginary pyramid. However, if nitrogen is introduced during growth, it will become incorporated into the diamond. But nitrogen can only attach to three carbon atoms, so associated with every nitrogen is a gap where a carbon atom would normally sit. These vacancies distort the electronic structure of diamond so that each vacancy is associated with an electron that is free to move around the neighboring atoms. Moreover, that electron is more strongly coupled to the nuclear states of the surrounding atoms, meaning that, if one manipulates the state of the electron, then one can also place a nucleus in a well-defined state.

Researchers have taken advantage of the mobility of these electrons, using them to act as a local communications channel between qubits. The qubits used by the researchers were the spin orientation of two C¹³ nuclei that happened to be adjacent to a vacancy. They manipulated the state of the electron associated with the vacancy using microwaves, and then watched the radio frequency response of the adjacent nuclei.

They were able to entangle the two nuclei in a controlled manner and, perhaps more surprisingly, that entanglement lasted for milliseconds. Furthermore, it was also possible to entangle the electron with the two nuclei; that three-way entanglement lasts for a few hundred microseconds.

Provided the qubit state can be transfered to photonic qubits, then it should be possible to scale the entanglement up so that it can involve more than a single nitrogen vacancy—having the entanglement lasts for milliseconds helps in this regard. Furthermore, qubits based on nitrogen vacancies aren't too hard to scale, since it is a solid-state material—no vacuum pumps required, thank you very much. Finally, the longevity of the entanglement should also enable the development of a refreshable quantum RAM.