Modified coherence of quantum spins in a damped pure-dephasing model

—by Mattias Johnsson

EQUS researchers have developed a simple model to understand and model solid-state defect systems and to preserve quantum states in noisy environments.

When working with quantum systems, the ideal case is a system that is fully isolated from the environment and therefore retains coherence for arbitrarily long times, while also being accessible for manipulation and interrogation.  Sadly, this is never the case in real life, so interactions between the system and the environment must be taken into account. But because the environment consists of everything that isn't the quantum system, doing this accurately is impossible.  An alternative approach is to treat the environment as a 'bath'—a sea of identical simple systems that interact with the quantum system of interest according to a chosen coupling scheme. In this limit, interesting results may be obtained.

The EQUS team considered what would happen to a set of many spin-1/2 particles (or, equivalently, a single particle with arbitrarily high angular momentum) when they interact with a bath of bosons, with the additional wrinkle that the bosons also interact with their own thermal bath.  This is interesting if we consider the loss of coherence of a quantum system as it interacts with an environment as the environment 'measuring’ the state of the quantum system.  From this perspective, it is the environment gaining information about the quantum system that slowly destroys the quantum state.  If that information is then lost by the bosonic bath into its own thermal bath on a fast enough timescale, the bosonic bath 'forgets' the quantum state it has measured and the quantum state is preserved.

We looked at how the coupling strengths between the spins and the bosonic bath, and the decay rate of the bosonic bath into its thermal bath, affected information loss from the quantum spin system.  We also investigated how introducing structure to the bosonic bath (by allowing the bosons to couple to each other with various strengths) affected the rate of information loss.  We found that random couplings between the bosons (surprisingly!) resulted in a non-random bath structure, with one privileged mode separated from all the others.  It is therefore possible achieve quantum coherent control of the spin system by manipulating the bath, allowing some types of quantum information processing to be carried out, even in the presence of a noisy environment.

We also applied our theoretical framework to solid-state systems with defects, such as nitrogen–vacancy centres in diamond.  Nitrogen–vacancy centres have applications in quantum sensing and information storage, but many of their properties are not known with high accuracy.  Our model provides a way to get a handle on some of these properties.

The publication of our model wraps up this particular research project.  It's been on-going for several years, partly because of people moving on to other things and/or institutions, which meant we've had to work remotely (especially during covid), and partly because the focus kept changing as we discovered new things.  So, it's really nice to have reached a place where we are all happy to stop investing new directions and instead sign off and finish the project!

Read the full paper here: https://doi.org/10.1103/PhysRevB.105.094308.  Mattias is a Research Fellow at Macquarie University.  This work is part of EQUS' Designer Quantum Materials research program.