Gone in 45 nanoseconds – but a new opportunity for quantum control?

A new study has successfully measured the coherence of electron spin – the period of time in which the particle’s elusive quantum state can be read and manipulated – for an electron trapped in conditions that could form the basis of a future quantum internet.

The study, reported in the journal Physical Review Letters, was carried out by researchers at the Universities of Cambridge and Saarbrücken. It reveals the coherence time of an electron trapped in a silicon-based colour centre within a microscopic fragment of diamond. This is a gap, manufactured inside the diamond’s lattice structure, and designed to snare an electron so that it can be manipulated.

At just 45 nanoseconds, the time period for which the electron’s spin is visible seems a miniscule fraction, but for scientists trying to bring this under control, it is, in relative terms, an age.

The “spin” of a particle is its intrinsic angular momentum and can point either up or down. Physicists at numerous leading research universities, including Cambridge, are currently engaged in research which is trying to utilise spin to develop advanced quantum technologies.

In the future, electron spin could be used to represent data and move large amounts of information much faster than is currently possible. This means that better control of spin might well underpin future computing, enable the creation of an entirely new quantum network (or quantum internet), and provide the foundations for a huge range of other technologies, such as advanced sensing devices.

One problem that hinders scientists who are attempting to gain greater command over electron spin for this purpose, however, is that spins in solids cannot be seen, or manipulated, for very long. After a tiny fraction of a second has passed, the spin’s quantum state decays beyond the point of visibility. Therefore, it needs to be retained for long enough for information about the spin to be registered and manipulated.

In the new study, the researchers successfully demonstrated the extent of the coherence of an electron trapped in a “silicon-vacancy” – an impurity in the lattice of carbon atoms that make up diamond. A silicon-vacancy centre provides highly promising conditions for the manipulation of electron spin.

Building on previous research, the researchers put the electron into a “superposition” state, using a technique which involves targeting it with two lasers with carefully-tuned frequencies. In this quantum state, the spin of the electron is potentially both up and down, and it is useful because it provides a basic position from which they can then observe and measure changes using laser pulses. The vision for future spin-based technologies involves creating chains of electrons whose spin will change relative to one another based on this initial superposition concept.

When applied to the electron in the silicon vacancy centre, the method achieved a coherence period of tens of nanoseconds – a fraction of time which, for scientists trying to control spin, is actually ample.

Dr Mete Atature, a researcher at the Cavendish Laboratory and St John’s College, University of Cambridge, who led the study with Professor Christoph Becher in Saarbrcüken, said: “This is incremental research, but it essentially deals with the elephant in the room for these colour centres, which was whether there was long-living coherence for the electron spin or not, and whether we had time to see its quantum state?”

“Arguably this is the most pressing challenge for these colour centres right now. We established that we can not only access the electron spin states, but also sustain an arbitrary superposition of them for 45 nanoseconds. When you bear in mind that it will take us picoseconds to execute laser-based operations to manipulate the spin, it becomes clear that just a fraction of this period is required. So this gives us a lot of possibilities to work with.”

The vacancy centre was created by substituting a silicon atom and a gap in place of two neighbouring carbon atoms in the carbon lattice of a fragment of diamond. Research earlier this year showed that a silicon-based vacancy has the potential to be used for this purpose because the photons – or light particles – emitted by an electron trapped in such conditions are sufficiently bright, and on a sufficiently narrow bandwidth, to be attractive for various applications. The research adds to a growing realisation among scientists that silicon-vacancy centres could provide advantageous conditions for spin and photon control, simultaneously.

“Now we know that silicon vacancies provide an alternative colour centre that has spin coherence, optical detectability and superior optical qualities,” Atature added. “The next challenge is to see if we can extend this spin coherence time by various techniques and, in parallel, see if we can entangle the spin with a single photon with sufficiently high fidelity.”