A single crystal niobium thin film was grown using molecular beam epitaxy on a c-plane sapphire wafer. Several samples were fabricated into dc resistivity test devices and coplanarwaveguide resonator chips using the same microfabrication procedures and solvent cleans. The samples were then subject to different acid cleaning treatments using different combinations of piranha, hydrofluoric acid, and buffered oxide etch solutions. The different samples expressed changes in dc resistivity in the normal and superconducting states such that the low temperature resistivities changed by more than 100\%, and the residual resistivity ratio dropped by a factor of 2. The internal quality factor of coplanar waveguide resonators measured near 5~GHz also showed significant variation at single photon powers ranging from 1.4×106 to less than 60×103. These changes correlate with the formation of surface crystallites that appear to be hydrocarbons. All observations are consistent with hydrogen diffusing into the niobium film at levels below the saturation threshold that is needed to observe niobium hydrides.
Low-loss superconducting microwave devices are required for quantum computation. Here, we present a series of measurements and simulations showing that conducting losses in the packagingof our superconducting resonator devices affect the maximum achievable internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, we sampled different electromagnetic energy volumes for the resonators affecting Qi. When the backside of the sapphire substrate of the resonator device is adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi is found as the electromagnetic sampling volume is increased. This is a result of induced currents in large surface resistance regions and dissipation underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, we decrease the ohmic losses and increase the maximum Qi for the larger size resonators.
We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lowerthe superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current for voltage biases below the superconducting gap when compared to standard Al/AlOx/Al junctions. The energy relaxation time constant for an Al/AlOx/Al/Ti 3D transmon was T1 = 1 {\mu}s, over two orders of magnitude shorter than the measured T1 = 134 {\mu}s of a standard Al/AlOx/Al 3D transmon. Intentionally adding disorder between the Al and Ti layers reduces the proximity effect and subgap current while increasing the relaxation time to T1 = 32 {\mu}s.
We have embedded two fixed-frequency Al/AlOx/Al transmons, with ground-to-excited transition frequencies at 6.0714 GHz and 6.7543 GHz, in a single 3D Al cavity with a fundamental modeat 7.7463 GHz. Strong coupling between the cavity and each transmon results in an effective qubit-qubit coupling strength of 26 MHz and a -1 MHz dispersive shift in each qubit’s transition frequency, depending on the state of the other qubit. Using the all-microwave SWIPHT (Speeding up Waveforms by Inducing Phases to Harmful Transitions) technique, we demonstrate the operation of a generalized controlled-not (CNOT) gate between the two qubits, with a gate time τ_g=907 ns optimized for this device. Using quantum process tomography we find that the gate fidelity is 83%-84%, somewhat less than the 87% fidelity expected from relaxation and dephasing in the transmons during the gate time.
Photon number splitting is observed in a transmon coupled to a superconducting quasi-lumped-element resonator in the strong dispersive limit. A thermal population of 5.474 GHz photonsat an effective resonator temperature of T = 120mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator.
Two-tone spectroscopy using independent coupler and probe tones reveals an Autler-Townes splitting in the thermal n = 1 photon peak.
The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit-resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.
We have investigated the decoherence of quantum states in two Al/AlOx/Al Cooper-pair boxes coupled to lumped element superconducting LC resonators. At 25 mK, the first qubit had anenergy relaxation time T1 that varied from 30 us to 200 us between 4 and 8 GHz and displayed an inverse correlation between T1 and the coupling to the microwave drive line. The Ramsey fringe decay times T2* were in the 200-500 ns range while the spin echo envelope decay times Techo varied from 2.4-3.3 us, consistent with 1/f charge noise with a high frequency cutoff of 0.2 MHz. A second Cooper-pair box qubit with similar parameters showed T1=4-30 us between 4-7.3 GHz, and that the T1 and the coupling were again inversely correlated. Although the lifetime of the second device was shorter than that of the first device, the dependence on coupling in both devices suggests that further reduction in coupling should lead to improved qubit performance.
We have observed the Autler-Townes doublet in a superconducting Al/AlOx/Al transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperatureof 22 mK. Using pulsed microwave spectroscopy, the three lowest transmon levels are isolated, eliminating unwanted effects of higher qubit modes and cavity modes. The long coherence time (~40 us) of the transmon enables us to observe a clear Autler-Townes splitting at drive amplitudes much smaller than the transmon level anharmonicity (177 MHz). Three-level density matrix simulations with no free parameters provide excellent fits to the data. At maximum separation, the fidelity of a dark state achieved in this experiment is estimated to be 99.6-99.9%.
We report on the quadrupling of the transition spectrum of an Al/AlOx/Al Cooper-pair box (CPB) charge qubit in the 4.0-7.3 GHz frequency range. The qubit was coupled to a quasi-lumpedelement Al superconducting resonator and measured at a temperature of 25 mK. We obtained good matches between the observed spectrum and the spectra calculated from a model Hamiltonian containing two distinct low excitation energy two-level systems (TLS) coupled to the CPB. In our model, each TLS has a charge that tunnels between two sites in a local potential and induces a change in the CPB critical current. By fitting the model to the spectrum, we have extracted microscopic parameters of the fluctuators including the well asymmetry, tunneling rate, and a surprisingly large fractional change (30-40%) in the critical current (12 nA). This large change is consistent with a Josephson junction with a non-uniform tunnel barrier containing a few dominant conduction channels and a TLS that modulates one of them.