As the field of superconducting quantum computing advances from the few-qubit stage to larger-scale processors, qubit addressability and extensibility will necessitate the use of 3Dintegration and packaging. While 3D integration is well-developed for commercial electronics, relatively little work has been performed to determine its compatibility with high-coherence solid-state qubits. Of particular concern, qubit coherence times can be suppressed by the requisite processing steps and close proximity of another chip. In this work, we use a flip-chip process to bond a chip with superconducting flux qubits to another chip containing structures for qubit readout and control. We demonstrate that high qubit coherence (T1, T2,echo>20μs) is maintained in a flip-chip geometry in the presence of galvanic, capacitive, and inductive coupling between the chips.
We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detectthe resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.
The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). In this work, we revisit the design and fabricationof the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and coherence times in excess of 40 us at its flux-insensitive point. Qubit relaxation times across 21 qubits of widely varying designs are consistently matched with a single model involving ohmic charge noise, quasiparticle fluctuations, resonator loss, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, reaching T2 ~ 80 us , approximately the 2T1 limit. In addition to realizing a dramatically improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary state-of-art qubits based on transverse qubit-resonator interaction.
We present a systematic study of the first excited-state population in a 3D transmon qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version ofthe protocol developed by Geerlings et al. [1], we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates to 0.1%, near the resolution of our measurement. We verified this result using a flux qubit with ten-times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T_{eff} = 35 mK. Assuming T_{eff} is due solely to hot quasiparticles, the inferred qubit lifetime is 108 us and in plausible agreement with the measured 80 us.
We report high-fidelity, quantum nondemolition, single-shot readout of a superconducting transmon qubit using a DC-biased superconducting low-inductance undulatory galvanometer(SLUG)amplifier. The SLUG improves the system signal-to-noise ratio by 7 dB in a 20 MHz window compared with a bare HEMT amplifier. An optimal cavity drive pulse is chosen using a genetic search algorithm, leading to a maximum combined readout and preparation fidelity of 91.9% with a measurement time of Tmeas = 200ns. Using post-selection to remove preparation errors caused by heating, we realize a combined preparation and readout fidelity of 94.3%.
We have incorporated a single crystal silicon shunt capacitor into a
Josephson phase qubit. The capacitor is derived from a commercial
silicon-on-insulator wafer. Bosch reactive ionetching is used to create a
suspended silicon membrane; subsequent metallization on both sides is used to
form the capacitor. The superior dielectric loss of the crystalline silicon
leads to a significant increase in qubit energy relaxation times. T1 times up
to 1.6 micro-second were measured, more than a factor of two greater than those
seen in amorphous phase qubits. The design is readily scalable to larger
integrated circuits incorporating multiple qubits and resonators.