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Using molecules as quantum information elements enables chemistry to be used to assemble large arrays [1]. A potential molecular building block is Sc@C₈₂, which is formed by inserting a scandium atom into a fullerene cage [2, 3]. This could be used as a molecular qubit because it has an electron spin with a relatively long coherence time, T₂ = 13 ms [4]. Pulsed electron spin resonance (ESR) can be used to manipulate this qubit [5], flipping the spin in just 40 ns. 

Sc@C₈₂ is not isotropic, so its ESR response depends on its orientation. Understanding the orientation-dependence of this response is necessary if Sc@C₈₂ is to be used in a QIP array. We solved this problem by comparing ESR measurements with density functional theory (DFT) simulations. Six groups from the UK and the USA worked together to achieve this.

Analyzing ESR measurements revealed the anisotropic g- and hyperfine tensors of Sc@C₈₂. The molecules in the experiment were randomly aligned, so it was not possible to relate these tensors to the coordinate frame of the molecule. DFT was used to predict the hyperfine tensor in the coordinate frame of the molecule, and comparing this with the ESR result allowed us to orient the measured tensors. The paper describing this work was downloaded 500 times within the first 51 days of appearing on the website of the journal Nanotechnology [6].

The figure shows molecular orbitals and spin density plot of the stable C_2v isomer of Sc@C₈₂ obtained with density functional theory. Semi-occupied molecular orbitals(SOMO) on the left with isovalue of 0.03 e/ų and yellow and blue indicating opposite signs of the wavefunction. The spin density is shown on the right as an isosurface within which the spin is greater than 0.01 Bohr magneton.

[1] A N Khlobystov , D A Britz, A Ardavan, G A D Briggs, Phys. Rev. Lett. 92, 245507 (2004).

[2] H Shinohara, H Sato, M Ohkohchi, Y Ando, T Kodama, T Shida, T Kato, Y Saito, Nature 357, 52 (1992).

[3] C S Yannoni, M Hoinkis, M S Devries, D S Bethune, J R Salem, M S Crowder, R D Johnson, Science 256, 1191 (1992).

[4] G W Morley, D. Phil. Thesis, University of Oxford (2005).

[5] J J L Morton, A M Tyryshkin, A Ardavan, K Porfyrakis, S A Lyon, G A D Briggs, Phys. Rev. A 71, 012332 (2005).

[6] G W Morley, B J Herbert, S M Lee, K Porfyrakis, T J S Dennis, D Nguyen-Manh. R Scipioni, J van Tol, A P Horsfield, A Ardavan, D G Pettifor, J C Green and G A D Briggs, Nanotechnology 16, 2469 (2005).