Will Spin-Relaxation Times in Molecular Magnets Permit Quantum Information Processing?

Abstract

Using X-band pulsed electron spin resonance, we report the intrinsic spin-lattice (T1) and phase coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M = Ni and Mn, phase coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3 µs at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.Certain computational tasks can be efficiently implemented using quantum logic, in which the informationcarrying elements are permitted to exist in quantum superpositions [1]. To achieve this in practice, a physical system that is suitable for embodying quantum bits (qubits) must be identified. Some proposed scenarios employ electron spins in the solid state, for example phosphorous donors in silicon [2], quantum dots [3], heterostructures [4] and endohedral fullerenes [5,6], motivated by the long electron-spin relaxation times exhibited by these systems. An alternative electron-spin based proposal exploits the large number of quantum states and the non-degenerate transitions available in high spin molecular magnets [7,8]. Although these advantages have stimulated vigorous research in molecular magnets [9,10,11], the key question of whether the intrinsic spin relaxation times are long enough has hitherto remained unaddressed. Here we show, using pulsed electron spin resonance experiments on heterometallic wheels, that the relaxation times in molecular magnets can significantly exceed the duration of coherent manipulations, a prerequisite for the deployment of these systems in quantum information applications.Molecular magnets comprising clusters of exchanged coupled transition metal ions have been studied extensively in recent years [12]. They can exhibit a substantial ground state spin with a large and negative zerofield splitting (ZFS), leading to a spontaneous magnetic moment parallel to the easy axis. In the absence of a magnetic field, the configurations in which the moment is 'up' or 'down' relative to the easy axis are degenerate, and this bistable nature has stimulated interest in the application of magnetic clusters as classical [13] or quantum [7,8,9, 11] information elements.Molecules in this class have been synthesised with widely varying properties, from the S = 10 highly axial Mn 12 -acetate [14], to the diamagnetic ring Cr 8 F 8 Piv 16 [15,16]. A key recent chemical advance is the the development of procedures for magnetically 'doping' a diamagnetic cluster to synthesise paramagnetic molecules in a systematic and controllable way [17]. Thus, substituting a Cr 3+ (s = 3/2) by a Mn 2+ (s = 5/2) or a Ni 2+ (s = 1) generates the S = 1 Cr 7 Mn or the S = 1/2 Cr 7 Ni respectively.Many clusters have been investigated using thermodynamic probes such as magnetization [18] and heat capacity [19], and spectroscopic probes such as neutr...

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