Diamonds are a Qubit’s Best Friend

A Nitrogen Vacancy in a diamond crystal isolates the nuclear spin yet makes it accessible via the hyperfine coupling.

A while ago, I looked into the chance that there would ever be a quantum computer for the rest of us. The biggest obstacle for this is the ultra-low temperature regime that all current quantum computing realizations require.  Although a long shot, I speculated that high temperature super-conductors may facilitate a D-Wave-like approach at temperature regimes that could be achieved with relatively affordable nitrogen cooling. Hoping for quantum computing at room temperature seemed out of the question. But this is exactly the tantalizing prospect that the recent qubit realization within a diamond’s crystal structure is hinting at – no expensive cooling required at all. So, ironically, the future quantum computer for the rest of us may end up being made of diamond.

A qubit requires a near perfectly isolated system i.e. essentially any interaction with the environment destroys the quantum information by randomly transitioning the pure qubit quantum state to a mixed ensemble state (a random superposition mixture of wavefunctions).  The higher the temperature, the more likely are these unwanted interactions via increased Brownian and thermal background radiation, a process know as decoherence.  Solid state qubit realizations are therefore always conducted at temperatures close to absolute zero, and require expensive Helium cooling.  Even under these conditions, qubits realized on superconducting chips don’t survive for very long. Their typical coherence time is measured in micro-seconds. On the other hand, ion-based systems can go for several minutes. While this is an obvious advantage, the challenge in using this design for quantum computing is the ability to initialize these systems into a known state, and the read-out detection sensitivity.  But great strides have been made in this regard, and in the same Science issue that the diamond results were presented, this article has been published that demonstrates a suitable system that exhibits quantum information storage for over 180 seconds.

All these coherence times are very sensitive to even the slightest temperature increase i.e. every millikelvin matters.  (This graph illustrates this for the first commercially available quantum computing design).

Key is to not have too many point defect.

It is in this context that the result of a successful qubit storage in a diamond lattice is almost breathtaking. A coherence time of over one second at room temperatureHarvard is not know for cutting edge experimental quantum computing research, so this result is surprising in more than one respect.

The diamond in question is artificially made and needs to contain some designer irregularites (but not too many of them):  These point defects replace a carbon atom in the diamonds crystal grid with a nitrogen one. If there are no other nitrogen vacancies nearby, the nuclear spin of this atom is very well isolated. Rivaling one would otherwise require close to absolute zero temperatures. On the other hand, this atom’s extra electron can readily interact with EM fields, and this is eactly what the researchers exploited. But there is more to it.

The really intriguing aspect is that this nuclear spin qubit in turn can be made to interact with the spin states of the excess electron, and the coherence times of both can be individually enhanced by suitably tuned laser exposure. The different coupling mechanisms are illustrated in the I came across this popular science write-up that does an excellent job in explaining this (long time readers know that I am rather critical of what usually passes as popular science, so I am delighted when I find something that I can really recommend).

The original paper concludes that additional coherence enhancing techniques could yield  jaw-dropping qubit storage of up to 36 hours at room temperature.

Of course, when everything else fails, physicists can always fall back on this novel approach, a song designed to scare a qubit to never come out of its coherent state:

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