Atomic clocks in silicon donors boost up electrons for quantum information

A method to dramatically enhance the amount of time a quantum system can remain in two states has been applied in the context of silicon-based quantum information, taking researchers a step closer to the development of quantum computers.

Quantum computers are expected to offer great improvements and promise huge impacts in the fields of computing, sensors, communications and more. However, these systems require delicate control of tiny individual quantum systems, involving groups of electrons, atoms and photons, which is challenging.

Quantum technologies exploit strange phenomena called superposition, the ability of a particle to exist in two different states, or places, at the same time, and entanglement, a type of connection between two quantum particles which exists even if they are far apart.

The ability for the system to remain in a superposition for long periods of time is of critical importance to encoding information and creating a quantum computer.

In a paper published in Nature Nanotechnology, a research team led by a group from the London Centre for Nanotechnology at UCL, suggests a possible solution to one of the main problems for quantum technology, “decoherence”, an uncontrollable loss of superposition over time.

Trapped ions and donors in silicon are the two major competitors in the pursuit of longer coherence times. Trapped ions encode information in nuclear spins and have coherence times of up to 10s. This is partly due to the use of ``clock transitions”, a simple method originally from atomic clocks and frequency standards which makes the system insensitive to magnetic field fluctuations. The disadvantage of trapped ions however, is that each operation - or gate time – takes a long time, microseconds rather than nanoseconds, so therefore cannot be used where in situations where fast computation is needed.

In this study, the researchers investigated bismuth-doped silicon. Donors in silicon have both a nuclear and electron spin. These spins, and therefore their quantum states, can be separately controlled or affected by both the application of an external magnetic field and by their interactions with each other. Information can be encoded in states that show mixed behaviour of the two spins.

In most cases, this combined system sees its characteristics dominated by that of the electron with reduced coherence time around 10 microseconds yet faster gate times around 20 nanoseconds. However, for bismuth donors as studied here, clock transitions can also arise.

With these clock transitions, the stability of the electron spin is boosted up and an increase in coherence time by two orders of magnitude, to nearly 3 seconds, was measured.

Gary Wolfowicz, lead author of the paper said, "Clock transitions are very elegant; you just have to tune the quantum state of the system once and it becomes naturally insensitive to various sources of noise, making it more stable against decoherence – and it is practically very easy to use.”

This technology is still at an early stage and is currently comparable to some memories in early computers; however, in the future, these developments could have a huge impact on quantum devices.

Journal Link: Atomic clock transitions in silicon-based spin qubits, Nature Nanotechnology. DOI: 10.1038/nnano.2013.117

Figure: At the clock transition, superpositions in bismuth donors become insensitive (in yellow) to magnetic field fluctuations. The coherence time (qubit lifetime) of 3s is the highest ever measured in electron spins.