Engineering coherent interactions in molecular nanomagnet dimers

Abstract

Proposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr 7 Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates. INTRODUCTIONAn information processing device whose elements are capable of storing and processing quantum superposition states (a quantum computer) would support algorithms for useful tasks such as searching 1 and factoring 2 that are much more efficient than the corresponding classical algorithms, 3 and would allow efficient simulation of other quantum systems. 4 One of the key challenges in realizing a quantum computer lies in identifying a physical system that hosts quantum states sufficiently coherently, and provides appropriate interactions for implementing logic operations.5 Among the molecular spin systems that have been proposed as qubit candidates are N@C 60, 6-9 organic radicals

Cr7Ni Wheels: Supramolecular Tectons for the Physical Implementation of Quantum Information Processing

“…This has to be strong enough to obtain a gate time shorter than the phase memory time of qubits, otherwise the information will be lost through decoherence. However, it cannot be too strong, otherwise the gate time would be shorter than the time needed to manipulate a single spin-in pulsed EPR spectroscopy this time is around 10 ns [41].…”

Supramolecular Complexes for Quantum Simulation

“…These are characterized by a S = 1/2 ground state, and coherence times above 10 µs have been demonstrated on properly engineered variants [12]. Rabi oscillations within the computational doublet (corresponding to single-qubit gates) have already been performed on ensembles of MNMs [13][14][15], as well as two-qubit operations in dimers of permanently coupled molecular qubits [16,17]. However, an important and much more challenging step towards the realization of a digital quantum simulator is the physical implementation of two-qubit entangling gates on a scalable architecture.…”

Measuring Spin⋅⋅⋅Spin Interactions between Heterospins in a Hybrid [2]Rotaxane

“…[7] Quantifying weak interactions between very different spins is challenging.I n cases where the interaction can easily be measured, for example by magnetometry or continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy, [8] the interaction will be too large to permit implementation of both one-and two-qubit gates in the same molecule. [3] CW EPR spectroscopy can still be au seful tool to investigate dipolar interactions;byobserving the line broadening of aN triplet in at wo-qubit structure,Z hou et al showed the existence of ad ipolar interaction that they could estimate in conjunction with spin density calculations. [9] On the other hand, DEER spectroscopy, [10] while ap owerful tool to measure weak interactions by directly manipulating two weakly interacting spins with specific microwave pulses,i s limited as the bandwidth of the microwave source must encompass the resonant frequency of both electron spins.…”

“…This magnetic interaction would give agate time of 125 ns, which falls in the correct range to implement atwo-qubit gate with 1. [3] Unfortunately,w hile RIDME is very good at measuring the interaction between different spins it only addresses one of the two spins.T herefore,i fh eterospin systems were to be used in quantum algorithms there remains an eed for EPR spectrometers capable of pulsing at two distinct frequencies.…”

“…Recently,A rdavan et al have shown using double electronelectron resonance (DEER) spectroscopy that the interaction between electron spins in {Cr 7 Ni}r ings can be controlled to fall within the range needed for two qubit gates. [3] These studies were performed on dimers of {Cr 7 Ni}rings [4] where the S = 1/2 spins in each half are identical.…”