Coherence and Decay of Higher Energy Levels of a Superconducting Transmon Qubit

We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive π pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20  μs for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (Air Force Contract FA8721-05-C-0002)United States. Army Research Office (Contract W911NF-14-1-0078)National Science Foundation (U.S.) (Grant PHY-1415514)Engineering and Physical Sciences Research Counci

Large-area NbN superconducting nanowire avalanche photon detectors with saturated detection efficiency

Superconducting circuits comprising SNSPDs placed in parallel—superconducting nanowire avalanche photodetectors, or SNAPs—have previously been demonstrated to improve the output signal-to-noise ratio (SNR) by increasing the critical current. In this work, we employ a 2-SNAP superconducting circuit with narrow (40 nm) niobium nitride (NbN) nanowires to improve the system detection efficiency to near-IR photons while maintaining high SNR. Additionally, while previous 2-SNAP demonstrations have added external choke inductance to stabilize the avalanching photocurrent, we show that the external inductance can be entirely folded into the active area by cascading 2-SNAP devices in series to produce a greatly increased active area. We fabricated series-2-SNAP (s2-SNAP) circuits with a nanowire length of 20 μm with cascades of 2-SNAPs providing the choke inductance necessary for SNAP operation. We observed that (1) the detection efficiency saturated at high bias currents, and (2) the 40 nm 2-SNAP circuit critical current was approximately twice that for a 40 nm non-SNAP configuration.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (United States. Air Force Contract FA8721-05-C-0002

Thermal and Residual Excited-State Population in a 3D Transmon Qubit

Remarkable advancements in coherence and control fidelity have been achieved in recent years with cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has remained a long-standing fundamental and technical challenge. In this context, we present a systematic study of the first-excited-state population in a 3D transmon superconducting qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al., 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 at approximately 0.1%. 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[subscript eff] is due solely to hot quasiparticles, the inferred qubit lifetime is 108  μs and in plausible agreement with the measured 80  μs.United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (United States. Air Force Contract FA8721-05-C-0002)United States. Army Research Office (Grant W911NF-14-1-0078)National Science Foundation (U.S.) (Grant PHY-1415514