SESSION 11: Solid-state qubits and cavity

Chair: (Emily Pritchett (HRL Laboratories))
10:15am-11:00amSimon Gustavsson, Massachusetts Institute of Technology
Dynamical control techniques with superconducting qubits
Abstract. Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation (T1). Here, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons - quasiparticles - in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability. In a separate experiment, we investigate qubit dephasing (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. We have made the first quantitative spectroscopy of this noise for both thermal (i.e., radiation from higher temperature stages) and coherent photons (residual photons from the readout and control pulses), and we uniquely identify thermal shot noise as the dominant source of dephasing. Furthermore, by improving the filtering, we are able to reduce the residual photon population to 0.0004, resulting in T2 echo times approaching 100 us.
11:00am-11:30amAaron Jones, HRL Laboratories
Experimental measurement of leakage-error in exchange-only SiGe quantum dot qubits by extending randomized benchmarking
Abstract. Randomized benchmarking is a common method for quantifying qubit gate error, but has questionable validity or reliability for some physically relevant error sources. The focus of this talk will be leakage out of the computational subspace of a decoherence-free subsystem, which can introduce additional benchmarking decay and unreliable error estimates. Though techniques have been developed to characterize leakage, it is not clear how best to use that information to inform computational error rates. Here we develop an extension to the randomized benchmarking protocol that estimates both computational and leakage errors by means of preparing and tracking different final states of the benchmarking sequence. Using this protocol, we experimentally measure the leakage error per gate in an exchange-only SiGe triple quantum dot at rates well below that of state-preparation-and-measurement (SPAM) error and pulse error from electrical noise. These leakage rates are in close agreement with a noise model with electrical and hyperfine-induced magnetic noise terms.
11:30am-12:00pmNa Young Kim, University of Waterloo
Engineered hopping integrals in exciton-polariton quantum simulators
Abstract. Microcavity exciton-polaritons are hybrid quantum quasi-particles as an admixture of cavity photons and quantum-well excitons. We engineer exciton-polariton-lattice systems, where we seek the beauty of non- zero momentum boson order arising from the intrinsic open-dissipative nature of the condensate as well as the topology of lattices. In this work, we quantify the hopping integrals of the lowest-band exciton-polaritons in terms of two physical parameters: nearest-neighbor site distance, d (3, 4,5 and 7 𝜇m), and detuning values Δ ( - 19 ~ 9 meV) in engineered two-dimensional honeycomb lattices. The artificial lattices are formed by an etching-overgrowth technique to module the cavity layer thickness to induce a photon confinement. The lattice potential depths vary 3-5 meV at different Δ values, and we construct the band structures of the exciton- polaritons via a low-power angle-resolved photoluminescence spectroscopy. The hopping integrals of nearest- neighbor and next-nearest neighbor sites in the lowest bands are extracted from the measured band structures by the tight-binding Hamiltonian fittings.

SQuInT Chief Organizer
Akimasa Miyake, Assistant Professor

SQuInT Co-Organizer
Mark M. Wilde, Assistant Professor LSU

SQuInT Administrator
Gloria Cordova
505 277-1850

SQuInT Founder
Ivan Deutsch, Regents' Professor

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