R. G. Herb Condensed Matter Seminars

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Events During the Week of November 27th through December 3rd, 2016

Monday, November 28th, 2016

High-fidelity entangling gate for double-quantum-dot spin qubits
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: John Nichol, University of Rochester
Abstract: Electron spins in semiconductors are promising qubits, because their long coherence times enable nearly a billion coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. We discuss a new entangling gate between two double-quantum-dot spin qubits in GaAs, which uses a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking, we measure single-qubit gate fidelities of approximately 99%, and through self-consistent quantum measurement, state, and process tomography, we measure an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault tolerant quantum information processing.
Host: Eriksson
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Tuesday, November 29th, 2016

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Wednesday, November 30th, 2016

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Thursday, December 1st, 2016

Controlling Spin Qubits in Diamond with a Mechanical Resonator
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Evan MacQuarrie, Cornell University
The spin state of the nitrogen-vacancy (NV) center in diamond offers a promising platform for the development of quantum technologies and investigations into spin dynamics at the nanoscale. With lengthy coherence times even at room temperature, NV centers present one path towards quantum information in the solid state and enable precision metrology with atomic scale spatial resolution. The NV center spin state can be coherently manipulated with resonant magnetic fields, electric fields, or, at cryogenic temperatures, optical fields. Here, we demonstrate direct mechanical control of an NV center spin by coherently driving magnetically-forbidden spin transitions with the resonant lattice strain generated by a bulk-mode mechanical resonator [1,2]. We then employ this mechanical driving to perform continuous dynamical decoupling and extend the inhomogeneous dephasing time of a single NV center spin [3]. Finally, we experimentally demonstrate that a spin-strain coupling exists within the NV center room temperature excited state and theoretically analyze a dissipative protocol that uses this newly discovered coupling to cool a mechanical resonator [4]. The methods of mechanical spin control developed here unlock a new degree of freedom within the NV center Hamiltonian that may enable new sensing modes and could provide a route to diamond-mechanical resonator hybrid quantum systems.

[1] E. R. MacQuarrie, et al, Phys. Rev. Lett. 111, 227602 (2013).

[2] E. R. MacQuarrie, et al, Optica 2, 233 (2015).

[3] E. R. MacQuarrie, et al, Phys. Rev. B 92, 224419 (2015).

[4] E. R. MacQuarrie, et al, arXiv:1605.07131 (2016).
Host: Eriksson
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Friday, December 2nd, 2016

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