No abstract

Although geometric phases in quantum evolution were historically overlooked, their active control now stimulates strategies for constructing robust quantum technologies. Here, we demonstrate arbitrary single-qubit holonomic gates from a single cycle of non-adiabatic evolution, eliminating the need to concatenate two separate cycles. Our method varies the amplitude, phase, and detuning of a two-tone optical field to control the non-Abelian geometric phase acquired by a nitrogen-vacancy center in diamond over a coherent excitation cycle. We demonstrate the enhanced robustness of detuned gates to excited-state decoherence and provide insights for optimizing fast holonomic control in dissipative quantum systems.Besides its central role in the understanding of contemporary physics [1,2], the quantum geometric phase is gaining recognition as a powerful resource for practical applications using quantum systems [3][4][5]. The manipulation of nanoscale systems has progressed rapidly towards realizing quantum-enhanced information processing and sensing, but also revealed the necessity for new methods to combat noise and decoherence [6][7][8]. Due to their intrinsic tolerance to local fluctuations [9,10], geometric phases offer an attractive route for implementing high-fidelity quantum logic. This approach, termed holonomic quantum computation (HQC) [3,[11][12][13][14][15], employs the cyclic evolution of quantum states and derives its resilience from the global geometric structure of the evolution in Hilbert space. Arising both for adiabatic [16] and non-adiabatic [17] cycles, geometric phases can be either Abelian (phase shifts) or non-Abelian (matrix transformations) [18] by acting on a single state or a subspace of states, respectively.Recently, non-Abelian, non-adiabatic holonomic gates using three-level dynamics [19] were proposed to match the computational universality of earlier adiabatic schemes [3,[11][12][13], but also eliminate the restriction of slow evolution. By reducing the run-time of holonomic gates, and thus their exposure to decoherence, this advance enabled experimental demonstration of HQC in superconducting qubits [20], nuclear spin ensembles in liquid [21], and nitrogen-vacancy (NV) centers in diamond [22,23]. However, these initial demonstrations were limited to fixed rotation angles about arbitrary axes, and thus required two non-adiabatic loops of evolution, from two iterations of experimental control, to execute an arbitrary gate [20][21][22][23]. Alternatively, variable angle rotations from a single non-adiabatic loop can be achieved using Abelian geometric phases [14,24] or hyperbolic secant pulses [25][26][27], but these approaches are complicated by a concomitant dynamic phase. To address these shortcomings, non-Abelian, non-adiabatic single-loop schemes * awsch@uchicago.edu [28,29] were designed to allow purely geometric, arbitrary angle rotations about arbitrary axes with a single experimental iteration.In this Letter, we realize single-loop, single-qubit holonomic gates by implementing th...

The interaction of solid-state electronic spins with deformations of their host crystal is an important ingredient in many experiments realizing quantum information processing schemes. Here, we theoretically characterize that interaction for a nitrogen-vacancy (NV) center in diamond. We derive the symmetry-allowed Hamiltonian describing the interaction between the ground-state spin-triplet electronic configuration and the local strain. We numerically calculate the six coupling-strength parameters of the Hamiltonian using density functional theory, and propose an experimental setup for measuring those coupling strengths. The importance of this interaction is highlighted by the fact that it enables to drive spin transitions, both magnetically allowed and forbidden, via mechanically or electrically driven spin resonance. This means that the ac magnetic field routinely used in a wide range of spin-resonance experiments with NV centers could in principle be replaced by ac strain or ac electric field, potentially offering lower power requirements, simplified device layouts, faster spin control, and local addressability of electronic spin qubits.

We describe fabrication and testing of composite flux qubits combining Nb-and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π-phase shifters fabricated using well-established Nbbased technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlO x /Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without πjunction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.