Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - DECRESIM (A Chemical Approach to Molecular Spin Qubits: Decoherence and Organisation of Rare Earth Single Ion Magnets)

Teaser

All electronics “runs on quantum mechanics”, from semiconductors and lasers to giant-magnetoresistance-based hard drives and memories. A number of spectacular technologies even depend very explicitly of quantum effects, from medical MRIs to quantum dots. Nevertheless...

Summary

All electronics “runs on quantum mechanics”, from semiconductors and lasers to giant-magnetoresistance-based hard drives and memories. A number of spectacular technologies even depend very explicitly of quantum effects, from medical MRIs to quantum dots. Nevertheless, there is ample room for research, development and innovation in the next-generation quantum technologies such as quantum computation, quantum key distribution and quantum metrology, which all revolve around the coherent manipulation of the wave function and the concept of quantum two-state system or qubit.
Very different physical, chemical or even biological systems can embody a qubit, but not all are equally well suited. For decades, a large number of physicists –including giants such as Haroche and Cirac– have performed experiments on trapped ions and resonant cavities, performing quantum manipulations that often were beyond the capabilities of quantum dots or SQUIDs. However, in recent times the spectacular results using Nitrogen-Vacancy (NV) defects in a diamond matrix, such as room-temperature entanglement with storage times (at room-temperature!) of the order of 1 milisecond, have shown the potential of systems in the domain of Chemistry and Materials Science. In the last few years, the study of quantum effects in a chemical context is also becoming a hot topic for biophysicists, as some natural biological processes –most notably, photosynthesis in plants and geolocation in birds– have been found to be quantum coherent.
Whatever the hardware, the key to quantum technologies are basic quantum effects: quantum superposition and quantum correlations (e.g. the so-called “cat states”). In real systems, and specially in the solid state, these quantum states are very fragile: uncontrolled interaction with the environment destroy, they lose any existing quantum superposition and/or quantum correlations. This phenomenon, called decoherence, is a major obstacle for quantum aplications. As a result, it will not be possible to exploit the advantages of solid state systems, such as stable circuits or scalability, that would make them disruptive technologies, until we have a realistic model for decoherence.
Much is at stake. Understanding and eventually controlling the processes that give rise to quantum decoherence will lay the foundations of any conceivable advance in present or future quantum technologies, but the fundamental questions we are dealing with are also formidable: Do we really understand why quantum effects are not persistent in time? Is quantum mechanics valid in the macroscopic world? Of course, to give practical answers we will need to choose and focus on a particular qubit architecture. In our case we will focus on molecular spin qubits, as this molecular approach appears to be very promising to chemically design and manipulate spins in solid matter.
A major advantage of molecular spin qubits over other candidates stems from the power of chemistry for a tailored and inexpensive synthesis of systems for their experimental study. Molecular Magnetism has produced an array of tools to study, design and fine-tune magnetic molecules, and, in particular, Single-Molecule Magnets (SMMs) that vastly outnumbers the variations that can be performed on a particular type of crystalline defect in diamond. Experimentally, magnetic molecules have already been used to perform experiments on coherent oscillations, and there have been recent theoretical studies about coherently manipulating a single qubit with electric fields. Note that Quantum Tunneling of the Magnetization, which can be seen as a particular case of decoherence, is the main phenomenon in SMMs. Moreover, SMMs typically have a doubly (near-)degenerate ground state with a large separation to the first excited state, that is, a qubit, and a crucial parameter to describe this qubit is the tunneling splitting breaking its near degeneracy. Thus, SMMs have a potential to be crucial in understand

Work performed

Overall, our work has progressed very satisfactorily, with Web of Knowledge reporting 24 papers published in the years 2015-2018, including several high-impact journals (IF>8) such as Nature (1), Chemical Science (2) and Journal of Chemical Physics Letters (2), overall attracting 200 citations so far.
Obviously, some of our publications in the period 2015-2018 result from work that predates DECRESIM. Likewise, many of the advances in these first 30 months has not been published yet. In particular, at the end of the 30 month period we have under peer review an article on “Mononuclear rare-earth nanomagnets: A source of spin-qubits obtained by chemical design” in Nature Chemistry, another on “Spin states, vibrations and spin relaxation in molecular nanomagnets and spin qubits: a critical perspective” in Chemical Science (already published as Advance Article at the time of this writing) both of them advancing on objectives of the DECRESIM project.
Nevertheless, we can easily point out ways in which our work is fulfilling the goals of the project. Indeed, we are finding systems, conditions and recipes for improved quantum coherence and qubit organisation. Likewise, our theoretical framework is getting more sophisticated and our computational capabilities are getting more powerful. One needs to note that I encountered some difficulties with the attraction/retention of sufficiently talented PhDs/postdocs, and thus some contracts were delayed. I judged this was safer than hiring people that were not up to the requirements. While this also delayed some results, it did not severely impede the development of the project, since we were fortituously able to substitute some of these contracts by free collaborations, as detailed below.

Final results

Here we need to emphasize our contribution “Determining Key Local Vibrations in the Relaxation of Molecular Spin Qubits and Single-Molecule Magnets” (JPCL, 2017) which has attracted an immediate attencion in a hot field, receiving approximately 1 citation/month, including citations from works in high-profile journals such as Nature and Chemical Science. This is directly derived from Task 1.2 (Expand the Hamiltonian to include the coupling with the spin and oscillator baths), and it is a major achievement which we think will have impact both in the field of molecular spin qubits and in the field of single-ion magnetism. We’re still working on this line and on its connection to Task 1.3 (Expand the Hamiltonian to include the coupling with the spin and oscillator baths), which we now realize has room for a much more ambitious approach, since it also has ramifications into coherent energy propagation. This research line might be one of the main results of the project in the long term.

Website & more info

More info: https://www.uv.es/gaita/decresim.