Scalable Quantum Computer Chip Technique Revealed
One of the more interesting runners in the race to build scalable quantum computers is the idea of using point-like defects in a diamond lattice that have been filled with a nitrogen atom. The nitrogen interloper provides an extra electron which can be used to generate photons or to store quantum information.
The big advantage of these so-called nitrogen vacancies is that they’re easy to see (because they can be made to emit photons) which means they can be relatively easily addressed. They are also well isolated from many types of environmental interference and so can store qubits for relatively long periods of up to several hundred microseconds.
But the problem is how to make them en masse. Until now, the fastest way was to fire nitrogen atoms one by one through an aperture into a thin layer of diamond. That makes for slow going if you need hundreds of thousands of them in a single layer.
Now David Toyli at the University of California, Santa Barbara, and few buddies have demonstrated a much faster technique. Their approach is to cover the diamond with a thin layer of resist, through which they then blast an array of holes using electron beam lithography.
In the next step, they then bombard the many-holed resist with accelerated nitrogen ions. So any nitrogen ions that pass through the holes created by the electron beams end up burying themselves in the diamond layer, creating a nitrogen vacancy under each hole.
Toyli and co have carried out a proof-of-principle experiment to create a micrometer scale 60 x 60 array of nitrogen vacancies in a thin layer of diamond. They say that this creates nitrogen vacancies at a rate of 1000 per second, orders of magnitude faster than the current technique.
The team say they can see these vacancies by making them photoluminesce and have even used them to store qubits and measured how long they remain coherent. In these first experiments, the coherence times are relatively poor, just a few microseconds rather than the few hundred that are known to be possible. The team says this can be improved with better control over the number of nitrogen ions that end up being implanted under each hole in the resist. In this experiment, some 30 ions ended up in a volume of diamond normally expected to carry only on ion.
Overall, this looks like an interesting step forward. An array of nitrogen vacancies that are easily addressable and coherent for hundreds of microseconds is a ready-made quantum register of the kind that quantum computer scientists have been crying out for.
Which means that the odds have suddenly improved of nitrogen vacancy technology winning this race and making scalable quantum computers possible for the first time.
Ref: arxiv.org/abs/1007.0240: Chip-Scale Nanofabrication Of Single Spins And Spin Arrays In Diamond
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