Novel techniques for control and transduction of solid-state spin qubits
Rosenfeld, Emma Louise
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CitationRosenfeld, Emma Louise. 2021. Novel techniques for control and transduction of solid-state spin qubits. Doctoral dissertation, Harvard University Graduate School of Arts and Sciences.
AbstractOptically-addressable electronic spin defects in the solid state are promising candidates for realization of quantum sensing and quantum information processing (QIP), exhibiting long coherence times at elevated temperatures. However, entangling pairs of spin qubits on demand remains an ongoing challenge due to the local nature of the magnetic dipole interactions. In this thesis, we present experimental and theoretical progress towards realizing entanglement between distant color centers, with a focus on mechanical quantum transducers.
First, inspired by protocols to leverage plentiful optically-dark electron spins in the diamond as a bus between distant nitrogen vacancy (NV) centers, we characterize a three-spin cluster consisting of two electron S = 1/2 spins and a single NV center, with all-to-all coupling. We observe coherent flip-flop dynamics between electron spins in the solid state using the NV as an atomic probe, and further employ the NV center to demonstrate initialization of the dark spin pair. Such a quantum register is rare to find in the diamond, as defect fabrication techniques are not precise to the ~nm length scale required for engineering coherent magnetic dipole interactions. In response, in the rest of this thesis, we develop hybrid quantum systems, which are more controlled and reproducible given current fabrication technology. Specifically, we consider spin qubits coupled to magnetically functionalized mechanical oscillators external to the diamond, which can act as quantum transducers between distant spins. With further system improvements, this could lead to reproducible, coherent quantum interconnects between remote electron spins in the solid state, enabling scalable NMR quantum information processing at elevated temperatures.
At the frequencies and temperatures of interest, these mechanical oscillators are in highly thermal states, introducing a large noise source given by the thermal fluctuation which must be mitigated. Therefore, we propose and analyze an efficient, heralded scheme that employs a parity measurement in a decoherence free subspace to enable fast and robust entanglement generation between distant spin qubits mediated by a hot mechanical oscillator. We find that high-fidelity entanglement at cryogenic and even ambient temperatures is feasible with realistic parameters, and show that the entangled pair can be subsequently leveraged for deterministic controlled-NOT operations between nuclear spins.
In a physical realization, a coherently coupled spin-mechanics platform is both desirable and a challenge to implement: the high-Q resonator must exhibit large zero point motion and magnetic gradients to maximize the coupling strength, while long spin coherence times are also required. To address this formidable challenge experimentally, we present two novel systems combining magnetic oscillators with NV spin defects in diamond. First, a rare-earth micromagnet is magnetically levitated above a yttrium barium copper oxide (YBCO) superconductor, and coupled to NV spins in a diamond nearby. Working in the field-cooled regime, we measure center-of-mass resonator mode frequencies exceeding 1000 Hz, with quality factors approaching one million. As the observed spin-phonon coupling strength of 0.05 Hz is limited by geometric constraints from our support structure, we introduce an improved geometry, in which the relative NV-micromagnet distance can be arbitrarily small, which in turn is expected to increase the coupling strength by multiple orders of magnitude.
While our levitated magnetomechanics approach minimizes dissipation through isolation from the environment, in some applications of hybrid quantum systems, a solid state geometry is advantageous. We develop an additional hybrid quantum system, consisting of nanofabricated arrays of magnetically-functionalized silicon nitride nanobeams coupled to NV centers in a scanning diamond nanopillar. At room temperature, we measure mechanical quality factors approaching one million and frequencies in the MHz regime, and observe preliminary results consistent with coupling to NV centers using a T2-limited dynamical decoupling sensing protocol.
In both platforms, with modest reductions in the spin-magnet distance, improvements in the quality factor, and extension of the NV coherence time to previously observed bulk values, coherent spin-mechanics coupling is within reach. Such a device could enable distant, coherent coupling between solid state spin qubits, and even eliminate the need for optical addressing of the spins through single-shot mechanical readout. Looking forward, this thesis thus paves the way toward novel, solid-state, scalable and integrated QIP architectures for a wide variety of solid-state spin qubits at elevated temperatures.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37368496
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