Hybrid Circuit Quantum Electrodynamics Landau-Zener Interferometry Nanowire Synthesis Nitrogen Vacancy Centers in Diamond Single Charge Coherence Spin-Momentum Locking in Topological Insulators Spin-Orbit Qubits Ultra-Coherent Spin Qubits

Landau-Zener Interferometry

The Nitrogen Vacancy (NV) center is a defect formed in diamond by one substitutional nitrogen atom and an adjacent vacancy. The NV forms a ground state spin triplet that can be coherently controlled at room temperature using electromagnetic fields. Due to its energy level structure, NV fluorescence is spin-state dependent, allowing simple routes for optical initialization and readout. For these reasons, the NV center is one of the prominent candidates for room temperature quantum information processing.

We use a confocal microscope (a) to perform a fluorescence imaging of diamond samples. NV centers are excited by an off-resonant 532 nm laser and subsequently emit fluorescence with a 637 nm zero-phonon line (b), resulting in bright fluorescence spots in the confocal image (c). To create a qubit, we isolate the states ms = 0, -1 from the state ms = +1 by applying an external magnetic field to induce Zeeman splitting (b). Because of the spin-selective transitions and an extra shelving state 1A1 associated with the ms = -1, +1 states, the fluorescence is suppressed when the NV center is in ms = -1. It is precisely this mechanism that allows for initialization and readout of the spin state. Rabi oscillations can be observed by applying microwave radiation to the NV (d). Inhomogeneous dephasing of the qubit due to coupling with proximal electronic and nuclear spins can be overcome using spin-echo, which allows for refocusing of the qubit state (e), thereby extending the coherence time of the qubit.

Hyperfine coupling of the electronic spin with the intrinsic 14N and proximal 13C nuclear spins has been demonstrated at room temperature [1][2] with a coherence time exceeding one second [3]. This electronic-nuclear spin coupling can serve as a basis for quantum registers and quantum memory.

While NV centers can be conveniently manipulated and read out, the host material, diamond, imposes a fundamental constraint on the readout efficiency. Given the high refractive index of diamond, the collection efficiency is limited to a very shallow critical angle of total internal reflection. We are working to overcome this limitation by fabricating a solid immersion lenses (SILs) on the surface of bulk diamonds (f). These hemispherical SILs are milled with high-energy gallium ions and are positioned such that the NV center of interest is at the origin of the sphere (g), eliminating total internal reflection, thereby improving the overall collection efficiency.

We are also investigating NV centers at cryogenic temperatures where the transitions between the triplet ground state and each of the excited states can be individually addressed via resonant optical excitation. With resonant excitation, it is possible to perform single-shot readout of a qubit state [4], which will be crucial for the realization of quantum error correction protocols.

[1] M. V. G. Dutt et al., Science 316, 1312 (2007)
[2] G. D. Fuchs et al., Nature Phys. 7, 789 (2011)
[3] P. Maurer et al., Science 336, 1283 (2012)
[4] L. Robledo et al., Nature 477, 574 (2011)

Project Publications

Cavity-mediated entanglement generation via Landau-Zener interferometry

C. M. Quintana, K. D. Petersson, L. W. McFaul, S. J. Srinivasan, A. A. Houck, J. R. Petta
Phys. Rev. Lett. 110, 173603 (2013)

Interplay of charge and spin coherence in Landau-Zener-Stuckelberg-Majorana interferometry

Hugo Ribeiro, J. R. Petta, Guido Burkard
Phys. Rev. B 87, 235318 (2013)

Coherent adiabatic spin control in the presence of charge noise using tailored pulses

Hugo Ribeiro, Guido Burkard, J. R. Petta, H. Lu, A. C. Gossard
Phys. Rev. Lett. 110, 086804 (2013)

Harnessing the GaAs nuclear spin bath for quantum control

Hugo Ribeiro, J. R. Petta, Guido Burkard
Phys. Rev. B 82, 115445 (2010)

A coherent beam splitter for electronic spin states

J. R. Petta, H. Lu, A. C. Gossard
Science 327, 669 (2010)