Initial experimental results on a superconducting-qubit reset based on photon-assisted quasiparticle tunneling

The resolutions of the optical microscope and the optical lithography are both limited by the well-known Rayleigh limit. Rabi oscillation is a coherent nonlinear process that can modulate the population distribution between two energy states and also modulate the resonant fluorescence spectrum. If we have a gradient electric field amplitude in the space, the Rabi frequency for different position is also different. The spatial distribution of the population in the excited state can be modulated and the spatial information of the atoms can also be encoded in the resonant fluorescence spectrum. If the gradient of the field is large enough, the pattern generated can be subwavlength and the atoms with subwavelength distances can also be extracted. Here we present a review on both the subwavelength photolithography and microscopy via Rabi oscillations.

A theory of the nonlinear interaction between a single quantum dot (QD) and electromagnetic fields, accounting the QD-depolarization (local-field) has been developed. QD excitation by classical and quantum electromagnetic fields such as coherent state of light, Fock qubit and gaussian excitation has been considered. As a result, for the case of the QD illuminated by coherent states of light, we predict the appearance of two oscillatory regimes in the Rabi oscillations. In the first one, signatures of Rabi oscillations are found to be suppressed: the population inversion is negative and the conventional collapse-revivals phenomenon is absent, while in the second one the collapse and revivals are appeared, showing significant difference as compared to those predicted within the standard Jaynes-Cummins theory. Under the pulsed excitation, Rabi oscillation dynamics is found to be strongly depended on the input pulse area and duration.

We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels after applying a sequence of π pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times τ1, τ2 for three devices in the temperature range from 20 to 300 mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support this with experimental data. The signal-to-noise ratio of the temperature measurement depends strongly on τ1, which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the π pulses. The effective qubit temperature follows closely that of the cryostat in the range from 100 to 200 mK. We present a numerical model of the qubit population distribution and find hat it compares favorably with the experimental results. Finally, we compare our technique with previous work on qubit thermometry and discuss its application prospects.

We discuss some recent experimental results on charge transfer into excited states in collisions between multiply charged ions and Li atoms. Beams of He2+, Cq+, Oq+ (q = 1, 2, 3) ions in the energy range of (10-200)q keV were crossed with a thermal Li beam produced by an oven. From measurements of the radiation emitted by the decay of the excited ion states we were able to determine absolute cross sections for electron capture into specific n, l-states. The wavelength range covered is about 10-700 nm. For Cq+ and Oq+, we also used a molecular hydrogen target. Comparing the results for both targets, we can explain the differences as a result of the difference in ionization potential. The importance of metastable fractions in the Cq+, Oq+ ion beams is discussed.

We report a comparative study on microwave control lines for a transmon qubit using (i) flexible stripline transmission lines and (ii) semi-rigid coaxial cables. During each experiment, we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the wiring configurations. Each measurement run spanned 70–250 h of the measurement time, and four separate cooldowns were performed so that each configuration could be tested twice. From these datasets, we observe that changing the microwave control lines from coaxial cables to flexible stripline transmission lines does not have a measurable effect on coherence compared to thermal cycling the system or random coherence fluctuations. Our results open up the possibility of large-scale integration of qubit control lines with integrated component with planar layouts on flexible substrate.

We consider two non-interacting two-level emitters that are coupled weakly to a one-dimensional non-linear wave guide. Due to the Kerr-like non-linearity, the wave guide considered supports -- in addition to the scattering continuum -- a two-body bound state. As such, the wave guide models a bath with non-trivial mode structure. Solving the time-dependent Schr\"odinger equation, the radiation dynamics of the two emitters, initially prepared in their excited states, is presented. Changing the emitter frequency such that the two-emitter energy is in resonance with one of the two-body bound states, radiation dynamics ranging from exponential decay to fractional populations to Rabi oscillations is observed. Along with the detuning, the dependence on the separation of the two emitters is investigated. Approximate reduced Hilbert space formulations, which result in effective emitter-separation and momentum dependent interactions, elucidate the underlying physical mechanisms and provide an avenue to showcase the features that would be absent if the one-dimensional wave guide did not contain a non-linearity. Our theoretical findings apply to a number of experimental platforms and the predictions can be tested with state-of-the-art technology. In addition, the weak-coupling Schr\"odinger equation based results provide critical guidance for the development of master equation approaches.

Ultrafast excitation process in NaNH 3: a combined theoretical and experimental analysis

High-resolution spectroscopy of highly excited electronic states by pulsed-cw double resonance in a molecular beam. A study of the NO(E-A) system

Arguments are presented to resolve experimental discrepancies with the theory of photon-assisted tunneling. A standing-wave model is suggested as a more realistic picture of the rf fields in a thin-film tunnel junction. By including the effects of this model in the theories of Tien and Gordon and of Werthamer, we obtain qualitative agreement with data on both the Josephson effects and the photon-assisted tunneling effects in the same junction. In an experiment with a junction sufficiently small to maintain a uniform rf field, we find excellent agreement with the unmodified theory. The low-frequency equivalence of the photon-assisted tunneling phenomenon and classical rf detection is also pointed out and used to demonstrate that the magnitude of the rf voltage appears correctly in the theory. We conclude that the Werthamer theory of rf-induced effects in superconducting tunnel junctions is on very firm experimental ground.

Progress towards the realization of quantum computers requires persistent advances in their constituent building blocks-qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems1-3. Electrons, ubiquitous elementary particles of non-zero charge, spin and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits through either motional or spin states depends critically on their material environment3-5. Here we report our experimental realization of a qubit platform based on isolated single electrons trapped on an ultraclean solid neon surface in vacuum6-13. By integrating an electron trap in a circuit quantum electrodynamics architecture14-20, we achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are implemented to measure the energy relaxation time T1 of 15 μs and phase coherence time T2 over 200 ns. These results indicate that the electron-on-solid-neon qubit already performs near the state of the art for a charge qubit21.

Herein we develop an Al/AlO x /Al trilayer process, feasible to fabricate complex circuits with wiring crossovers, for the preparation of Al junctions and phase qubits. The AlO x layer is obtained by in situ thermal oxidation, which provides high-quality junction tunnel barriers. The Al junctions show a considerably low leakage current and the Josephson critical current density can be conveniently controlled in the range of a few to above 100 A/cm2, which is favorable in the phase qubit application. Macroscopic quantum tunneling, energy spectrum, energy relaxation time, Rabi oscillation, and Ramsey interference of the Al phase qubits are measured, demonstrating clearly quantum coherent dynamics with a timescale of 10 ns. Further improvements of the coherent dynamic properties of the device are discussed.

We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.

We present experimental measurements and analysis of leakage errors occurring during resonant digital control of a superconducting qubit. By increasing the amplitude of the digital pulse trains and therefore decreasing the duration of the control gates, from 100 to 40 ns for a π-gate, the leakage error rate measured per Clifford gate in a randomized benchmarking test increases from 4.3×10−4 to 2.4×10−3 and becomes the dominant source of single-qubit gate errors for our qubit; these error rates are 1–2 orders of magnitude larger than we measure when controlling the same qubit using traditional, shaped-analog signals. Simulations show the dominant leakage mechanism arises from the increased spectral power of the pulse trains at the frequency ω12 corresponding to excitations from the first excited state |1⟩ to the second excited state |2⟩. Our measurements demonstrate the fundamental limits to resonant digital control of low-anharmonicity qubits and outline the trade-off between reducing gate times while preserving gate fidelity. We discuss possible strategies for mitigating this issue in future digital control implementations.

We study the Rabi flopping of the population between the ground and excited 2p states of the hydrogen atom, induced by intense short laser pulses of different shapes and of carrier frequency ω=0.375 a.u. which resonantly couples the two states, and manifestations of this dynamics in the energy spectra of photoelectrons produced in the subsequent ionization of the atom from the excited state. It is found that, for Gaussian, half-Gaussian and rectangular pulses, characterized by the same pulse area, the final populations take the same values and the spectra consist of similar patterns having the same number of peaks and approximately the same separation between the prominent edge (Autler–Townes) peaks. The additional analysis in terms of dressed states showed that the mechanism of formation of multiple-peak structures during the photoionization process is the same regardless of the pulse shape. These facts disprove the hypothesis proposed in earlier studies with Gaussian pulse, that the multiple-peak pattern appears due to dynamic interference of the photoelectrons emitted with a time delay at the rising and falling sides of the pulse, since the hypothesis is not applicable to either a half-Gaussian pulse that has no rising part or a rectangular pulse whose intensity is constant.

The ground state tunneling splitting of the 2-pyridone2-hydroxypyridine dimer