Coherent State Transfer between an Electron and Nuclear Spin inN15@C60

Abstract

Electron spin qubits in molecular systems offer high reproducibility and the ability to self assemble into larger architectures. However, interactions between neighbouring qubits are 'always-on' and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both controllably turn on/off dipolar interactions between neighbouring spins and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and 15 N nuclear spin in 15 N@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radiofrequency pulses and measure a nuclear spin coherence lifetime of over 100 ms.Hybrid quantum computing schemes aim to harness the benefits of multiple quantum degrees of freedom through the coherent transfer of quantum information between them. Such transfer has previously been shown between light and atomic ensembles [1,2], as well as electron to nuclear spin states in nitrogen vacancies [3] and 31 P donors [4], and progress is being made towards coupling electron spin ensembles to superconducting qubits [5, 6]. Common motivations for state transfer between electron to nuclear spin qubits include the much longer decoherence times typically exhibited by the nuclear spin, and also the weaker dipolar interaction between nuclear spins which allows interactions between neighbouring qubits to be effectively turned off [3, 4,[7][8][9]. Both effects can be attributed to the relatively weak nuclear magnet moment, typically 3 orders of magnitude smaller than an electron spin. Thus, a powerful hybrid model for quantum computing is one where the electron spin qubit (which is more readily polarised and more quickly manipulated) is used for initialisation and processing, while the nuclear spin is used as a memory. The presence of the electron spin also offers considerable advantages for the readout of a single qubit, either of the electron spin state directly [10, 11], or a quantum nondemolition measurement of the nuclear spin [12,13].Endohedral fullerenes (atoms held within a carbon cage) offer promise as a molecular qubits due to their exceptionally long electron decoherence times [14][15][16] and convenient coupling to a local nuclear spin. In particular, N@C 60 has been used to demonstrate polarisation transfer from the electron to the nuclear spin and subsequent 'bang-bang' decoupling [17,18], as well as generation of pseudo-entanglement between the electron and nuclear spin [19]. The advantages of a molecular system include the ability to produce larger arrays that can be engineered to control electron dipolar interaction [9,[20][21][22]. Molecular approaches offer self assembly of highly reproducible structures [23,24], but are limited by the 'always-on' nature of dipolar interactions between neighbouring electron spins. In this Lette...

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