Engineering Electronic Structure to Protect Phase Memory in Molecular Qubits by Minimising Orbital Angular Momentum

Abstract

Using electron spins within molecules for quantum information processing (QIP) was first proposed by Leuenberger and Loss (1), who showed how the Grover algorithm could be mapped onto a Mn12 cage (2). Since then several groups have examined two-level (S = ½) molecular spin systems as possible qubits (3-12). There has also been a report of the implementation of the Grover algorithm in a four-level molecular qudit (13). A major challenge is to protect the spin qubit from noise that causes loss of phase information; strategies to minimize the impact of noise on qubits can be categorized as corrective, reductive, or protective. Corrective approaches allow noise and correct for its impact on the qubit using advanced microwave pulse sequences (3). Reductive approaches reduce the noise by minimising the number of nearby nuclear spins (7-11), and increasing the rigidity of molecules to minimise the effect of vibrations (which can cause a fluctuating magnetic field via spin-orbit coupling) (9,11); this is essentially engineering the ligand shell surrounding the electron spin. A protective approach would seek to make the qubit less sensitive to noise: an example of the protective approach is the use of clock transitions to render spin states immune to magnetic fields at first order (12). Here we present a further protective method that would complement reductive and corrective approaches to enhancing quantum coherence in molecular qubits. The target is a molecular spin qubit with an effective 2S ground state: we achieve this with a family of divalent rare-earth molecules that have negligible magnetic anisotropy such that the isotropic nature of the electron spin renders the qubit markedly less sensitive to magnetic noise, allowing coherent spin manipulations even at room temperature. If combined with the other strategies, we believe this could lead to molecular qubits with substantial advantages over competing qubit proposals.<br>

Comparison of tetravalent cerium and terbium ions in a conserved, homoleptic imidophosphorane ligand field

“…Determining the chemical and physical basis for the stability of molecular lanthanide complexes with unusual oxidation states is critical for enabling the development of technical applications such as topological insulators, magnetocaloric refrigerants, and single molecular magnets or qubits for quantum information science (QIS). [19][20][21][22][23][24][25][26][27] These QIS applications stem from the lanthanides' inherent anisotropy and magnetic properties through the contribution of f, d, and s character. These magnetic contributions are inuenced through both modulation of the ligand eld about the lanthanide ion as well as the identity and oxidation state of the lanthanide ion itself.…”

“…Recently, non-traditional divalent lanthanides, such as Tb, Dy, and La, have been shown to exhibit promising magnetic behaviour for QIS technology. 21,22 Thus, expanding accessible oxidation states is important for developing new design principles for emerging magnetic and quantum technologies. Recent gas-phase work suggests that the range of accessible oxidation states in the condensed phases may extend beyond tetravalent ions to pentavalent lanthanide ions as well.…”

Identical anomalous Raman relaxation exponent in a family of single ion magnets: towards reliable Raman relaxation determination?

“…4,32,33 Moreover, deep understanding of relaxation mechanisms for SIMs which do not possess pure mJ ground states is also important from the point of view of molecular qubit candidates design, demonstrated for several non-dysprosium complexes so far. [34][35][36][37][38][39][40] Herein we present a thorough magnetic and EPR study of three lanthanide-based SIMs from the family of [Ln(phendo)4](NO3)3•xMeOH (Ln = Gd (Gd), Er (Er), Yb (Yb), phendo = 1,10-phenanthroline-N,N'-dioxide). The complexes show four-bladed propeller-like geometry enforced by the helicene-type phendo ligands.…”

Nuclear-spin-pattern control of electron-spin dynamics in a series of V(iv) complexes

“…[27][28][29][30] However, in spin-rich environments, lifetimes are typically less than 100 ms (ref. 31) and more frequently less than 10 ms. 19,23, A recent breakthrough in the study of paramagnetic defects in SiC might hold the key to longer relaxation times in nuclear spin-rich baths (Fig. 1).…”