Chemical Control of Spin Propagation between Heterometallic Rings

Abstract

We present a synthetic, structural, theoretical, and spectroscopic study of a family of heterometallic ring dimers which have the formula [{Cr(7)NiF(3)(Etglu)(O(2)CtBu)(15)}(2)(NLN)], in which Etglu is the pentadeprotonated form of the sugar N-ethyl-D-glucamine, and NLN is an aromatic bridging diimine ligand. By varying NLN we are able to adjust the strength of the interaction between rings with the aim of understanding how to tune our system to achieve weak magnetic communication between the spins, a prerequisite for quantum entanglement. Micro-SQUID and EPR data reveal that the magnetic coupling between rings is partly related to the through-bond distance between the spin centers, but also depends on spin-polarization mechanisms and torsion angles between aromatic rings. Density functional theory (DFT) calculations allow us to make predictions of how such chemically variable parameters could be used to tune very precisely the interaction in such systems. For possible applications in quantum information processing and molecular spintronics, such precise control is essential.

Engineering coherent interactions in molecular nanomagnet dimers

“…Previously we have used simple di-imines, 23 producing dimers in which we can vary the inter-ring interaction through distance and through the torsion angle within the bridging group. The short di-imine linker leads to inter-ring interaction energies of the order of h × 0.3 GHz to h × 10 GHz, giving rise to large splittings in the continuous wave (CW) ESR spectrum compared with the monomer.…”

“…The short di-imine linker leads to inter-ring interaction energies of the order of h × 0.3 GHz to h × 10 GHz, giving rise to large splittings in the continuous wave (CW) ESR spectrum compared with the monomer. 23 Interactions on this scale would give rise to inter-qubit gate times that are shorter than the duration of single-qubit gates, precluding the possibility of multi-qubit pulsed ESR experiments.…”

“…This observation has motivated various efforts to synthesise dimers including, for example, N@C 60 -N@C 60, 20,21 radical-radical, 10,11 N@C 60 -molecular magnet (Kaminski D. et al in preparation) and molecular magnet-molecular magnet. [22][23][24][25] However, the design of a dimer specifically to host two-qubit experiments should take account of the importance of three key time scales relative to one another: T 2 , the individual qubit phase relaxation time, must be longest; h/J, which is characteristic of the duration of two-qubit gates (where J is the inter-qubit interaction energy and h is Planck's constant) should be intermediate; and the single-qubit manipulation time should be the shortest. In practice, phase relaxation times in heterometallic antiferromagnetic rings are in the 1-10 μs range at low temperatures, 18,19 and they can be manipulated in a typical pulsed electron spin resonance (ESR) apparatus on the 10-ns timescale; thus an interaction offering h/J in the 100-ns range could be exploited, for example, in a multiqubit experiment to generate controlled entanglement.…”

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

“…The resulting wheel has the formula [Cr 7 NiF 3 (Etglu)(O 2 C t Bu) 15 (H 2 O)], abbreviated as {Cr 7 Ni} , where the polyalcohol proligand is completely deprotonated and the five alkoxide groups replace five fluoride anions [42]. The most striking feature of this {Cr 7 Ni} wheel is the presence of a labile terminal water molecule bound to the nickel(II) site, which can be easily displaced by reaction with oligopyridines or related polyazines and polyazoles [41][42][43][44][45] Figure 2. In particular, the dimer of formula [{Cr 7 NiF 3 (Etglu)(piv) 15 } 2 (4,4 -bpy)] constitutes the first example of the rational design of entangled double qubits [44].…”

“…Therefore, an overall downward field shift of the inflection points (Hc) in the isothermal magnetisation curves at 0.04 K occurs along this series of dimers following the trend 30 (pyr) > 5 (4,4′-bpy) > 4 kOe (bpe), thus reflecting the weakening of the intramolecular magnetic coupling constant [−J = 1.4 (pyr), 0.22 (4,4′-bpy), and 0.18 cm −1 (bpe)]. In fact, the gap between inflection points in the magnetization curves is directly related to the energy gap between the ground singlet (S = 0) and excited triplet (S = 1) pair states of the dimer [∆EST = E(S = 0) − E(S = 1) = J] [43][44][45]. Yet, these inter-ring magnetic interactions across the aromatic bridging ligands within the ring dimers would give rise to gate times that are shorter than the manipulation time of a single qubit, thus precluding their examination as candidates for a quantum gate through pulsed EPR experiments.…”