Ar:N2_2 - a non-universal glass

The bias energies of various two-level systems (TLSs) and their strengths of interactions with the strain are calculated for Ar:N$_2$ glass. Unlike the case in KBr:CN, a distinct class of TLSs having weak interaction with the strain and untypically small bias energies is not found. The addition of CO molecules introduces CO flips which form such a class of weakly interacting TLSs, albeit at much lower coupling than are typically observed in solids. We conclude that because of the absence of a distinct class of weakly interacting TLSs, Ar:N$_2$ is a non-universal glass, the first such system in three dimensions and in ambient pressure. Our results further suggest that Ar:N$_2$:CO may show universal properties, but at temperatures lower than $\approx 0.1$ K, much smaller than typical temperature $\approx 3$ K associated with universality, because of the untypical softness of this system. Our results thus shed light on two long standing questions regarding low temperature properties of glasses: the necessary and sufficient conditions for quantitative universality of phonon attenuation, and what dictates the energy scale of $\approx 3$ K below which universality it typically observed.Comment: 6 pages, 2 figures, results for excitations densities of states are added, presentation improve

Identification of strong and weak interacting two level systems in KBr:CN

Tunneling two level systems (TLSs) are believed to be the source of phenomena such as the universal low temperature properties in disordered and amorphous solids, and $1/f$ noise. The existence of these phenomena in a large variety of dissimilar physical systems testifies for the universal nature of the TLSs, which however, is not yet known. Following a recent suggestion that attributes the low temperature TLSs to inversion pairs [M. Schechter and P.C.E. Stamp, arXiv:0910.1283.] we calculate explicitly the TLS-phonon coupling of inversion symmetric and asymmetric TLSs in a given disordered crystal. Our work (a) estimates parameters that support the theory in M. Schechter and P.C.E. Stamp, arXiv:0910.1283, in its general form, and (b) positively identifies, for the first time, the relevant TLSs in a given system.Comment: minor modifications, published versio

A call for frugal modelling: two case studies involving molecular spin dynamics

As scientists living through a climate emergency, we have a responsibility to lead by example, or to at least be consistent with our understanding of the problem, which in the case of theoreticians involves a frugal approach to modelling. Here we present and critically illustrate this principle. First, we compare two models of very different level of sophistication which nevertheless yield the same qualitative agreement with an experiment involving electric manipulation of molecular spin qubits while presenting a difference in cost of $>4$ orders of magnitude. As a second stage, an already minimalistic model involving the use of single-ion magnets to implement a network of probabilistic p-bits, programmed in two different programming languages, is shown to present a difference in cost of a factor of $\simeq 50$. In both examples, the computationally expensive version of the model was the one that was published. As a community, we still have a lot of room for improvement in this direction

Decoherence from dipolar interspin interactions in molecular spin qubits

The realization of spin-based logical gates crucially depends on magnetically coupled spin qubits. Thus, understanding decoherence when spin qubits are in close proximity will become a roadblock to overcome. Herein, we propose a method free of fitting parameters to evaluate the qubit phase memory time Tm in samples with high electron spin concentrations. The method is based on a model aimed to estimate magnetic nuclear decoherence [P. C. E. Stamp and I. S. Tupitsyn, Phys. Rev. B 69, 014401 (2004)]. It is applied to a ground-spin J=8 magnetic molecule 1 displaying atomic clock transitions, namely [HoIII(W5O18)2]9−, which remarkably increase Tm at unusually high electron-spin concentrations. Our approach unveils the causes that limit the coherence reached at the clock transitions in challenging systems such as 1, where recent models fail

Molecular spins for quantum computation

Spins in solids or in molecules possess discrete energy levels, and the associated quantum states can be tuned and coherently manipulated by means of external electromagnetic fields. Spins therefore provide one of the simplest platforms to encode a quantum bit (qubit), the elementary unit of future quantum computers. Performing any useful computation demands much more than realizing a robust qubit¿one also needs a large number of qubits and a reliable manner with which to integrate them into a complex circuitry that can store and process information and implement quantum algorithms. This 'scalability' is arguably one of the challenges for which a chemistry-based bottom-up approach is best-suited. Molecules, being much more versatile than atoms, and yet microscopic, are the quantum objects with the highest capacity to form non-trivial ordered states at the nanoscale and to be replicated in large numbers using chemical tools

Theoretical evaluation of lanthanide binding tags as biomolecular handles for the organization of single ion magnets and spin qubits

Lanthanoid complexes are amongst the most promising compounds both in single ion magnetism and as molecular spin qubits, but their organization remains an open problem. We propose to combine Lanthanide Binding Tags (LBTs) with recombinant proteins as a path for an extremely specific and spatially-resolved organisation of lanthanoid ions as spin qubits. We develop a new computational subroutine for the freely available code SIMPRE that allows an inexpensive estimate of quantum decoherence times and qubit–qubit interaction strengths. We use this subroutine to evaluate our proposal theoretically for 63 different systems. We evaluate their behavior as single ion magnets and estimate both decoherence caused by the nuclear spin bath and the interqubit interaction strength by dipolar coupling. We conclude that Dy3+ LBT complexes are expected to behave as SIMs, but Yb3+ derivatives should be better spin qubits.Lanthanoid complexes are amongst the most promising compounds both in single ion magnetism and as molecular spin qubits, but their organization remains an open problem. We propose to combine Lanthanide Binding Tags (LBTs) with recombinant proteins as a path for an extremely specific and spatially-resolved organisation of lanthanoid ions as spin qubits. We develop a new computational subroutine for the freely available code SIMPRE that allows an inexpensive estimate of quantum decoherence times and qubit–qubit interaction strengths. We use this subroutine to evaluate our proposal theoretically for 63 different systems. We evaluate their behavior as single ion magnets and estimate both decoherence caused by the nuclear spin bath and the interqubit interaction strength by dipolar coupling. We conclude that Dy3+ LBT complexes are expected to behave as SIMs, but Yb3+ derivatives should be better spin qubits