S. Wünsch

Anisotropic rare-earth spin ensemble strongly coupled to a superconducting resonator.

Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent qubit interconversion are optically active spins, magnetically coupled to a superconducting resonator. We report on an on-chip cavity QED experiment with magnetically anisotropic Er(3+)∶Y2SiO5 crystals and demonstrate collective strong coupling of rare-earth spins to a lumped element resonator. Moreover, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without the aid of a cavity.

Losses in coplanar waveguide resonators at millikelvin temperatures

We study the loss rate for a set of λ/2 coplanar waveguide resonators at millikelvin temperatures (20–900 mK) and different applied powers (3⋅10−19–10−12 W). The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent.

Weak continuous monitoring of a flux qubit using coplanar waveguide resonator

We study a flux qubit in a coplanar waveguide resonator by measuring transmission through the system. In our system with the flux qubit decoupled galvanically from the resonator, the intermediate coupling regime is achieved. In this regime, dispersive readout is possible with weak back action on the qubit. The detailed theoretical analysis and simulations give good agreement with the experimental data and allow us to make the qubit characterization.

Magnetic hysteresis effects in superconducting coplanar microwave resonators

Institut fu¨r Mikro- und Nanoelektronische Systeme,Karlsruher Institut fu¨r Technologie, Hertzstrasse 16, 76187 Karlsruhe, Germany(Dated: February 28, 2012)We performed transmission spectroscopy experiments on coplanar half wavelength niobium res-onators at a temperature T = 4.2K. We observe not only a strong dependence of the quality factorQ and the resonance frequency f

Nonthermal response of YBa2Cu3O7−δthin films to picosecond THz pulses

The photoresponse of YBaCuO thin film microbridges with thicknesses between 15 and 50 nm was studied in the optical and terahertz frequency range. The voltage transients in response to short radiation pulses were recorded in real time with a resolution of a few tens of picoseconds. The bridges were excited by either femtosecond pulses at a wavelength of 0.8 μm or broadband (0.1–1.5 THz) picosecond pulses of coherent synchrotron radiation. The transients in response to optical radiation are qualitatively well explained in the framework of the two-temperature model with a fast component in the picosecond range and a bolometric nanosecond component whose decay time depends on the film thickness. The transients in the THz regime showed no bolometric component and had amplitudes up to three orders of magnitude larger than the two-temperature model predicts. Additionally THz field-dependent transients in the absence of DC bias were observed. We attribute the response in the THz regime to a rearrangement of vortices caused by high-frequency currents.

Strong coupling of anEr3+-dopedYAlO3crystal to a superconducting resonator

Quantum memories are integral parts of both quantum computers and quantum communication networks. Naturally, such a memory is embedded into a hybrid quantum architecture, which has to meet the requirements of fast gates, long coherence times and long distance communication. Erbium doped crystals are well suited as a microwave quantum memory for superconducting circuits with additional access to the optical telecom C-band around 1.55 {\mu}m. Here, we report on circuit QED experiments with an Er3+:YAlO3 crystal and demonstrate strong coupling to a superconducting lumped element resonator. The low magnetic anisotropy of the host crystal allows for attaining the strong coupling regime at relatively low magnetic fields, which are compatible with superconducting circuits. In addition, Ce3+ impurities were detected in the crystal, which showed strong coupling as well.

Real-time measurement of picosecond THz pulses by an ultra-fast YBa2Cu3O7−d detection system

The temporal evolution of picosecond THz pulses generated at ANKA, the electron storage ring of the Karlsruhe Institute of Technology, has been measured in real-time using an ultra-fast YBa2Cu3O7−δ detection system. YBa2Cu3O7−δ thin-film detectors with 30 nm thickness were patterned to microbridges (2 μm long, 4.5 μm wide) and embedded into a planar log-spiral THz antenna. The detectors were glued on a silicon lens and installed in an ultra-fast readout system with a temporal resolution of 15 ps (full width at half maximum). Detector responses as short as 17 ps were recorded showing very good agreement with the expected storage ring bunch lengths.

Energy relaxation time in NbN and YBCO thin films under optical irradiation

For a systematic study of energy relaxation processes in thin NbN and YBCO films on sapphire substrates, a frequency domain technique has been set up and employed. The magnetron sputtered NbN films of 3 nm to 22 nm thickness and pulsed-laser deposited YBCO films with thicknesses between 20 nm and 45 nm were excited by amplitude-modulated optical radiation (? = 850 nm). The response spectra were analyzed on basis of the two-temperature model of the energy dynamics in the interacting electron and phonon subsystems at quasi-equilibrium conditions. An increase of the energy relaxation time with increasing film thickness has been obtained for both NbN and YBCO thin film samples.

High-Speed Y–Ba–Cu–O Direct Detection System for Monitoring Picosecond THz Pulses

A high-speed YBa₂Cu₃O_7-δ direct detection system was developed to monitor terahertz picosecond pulses in the time domain. High-T_C superconducting thin-film YBa₂Cu₃O_7-δ microbridges with critical temperatures of T_C = 85 K were embedded into a planar log-spiral antenna to couple the broadband terahertz radiation (0.1 -2 THz) of several picosecond pulsed sources. The YBa₂Cu₃O_7-δ detectors were installed in a liquid nitrogen cryostat equipped with 18 GHz effective bandwidth readout electronics. THz pulses generated at the electron storage rings ANKA and UVSOR-II have been resolved with a temporal resolution of 30 ps (full width at half maximum) limited by the readout electronics bandwidth. Beam dynamic effects of bursting coherent synchrotron radiation were successfully monitored.

Highly Responsive Y–Ba–Cu–O Thin Film THz Detectors With Picosecond Time Resolution

High-temperature superconducting YBa2Cu3O7 - δ (YBCO) thin-film detectors with improved responsivities were developed for fast time-domain measurements in the THz frequency range. YBCO thin films of ≈ 30 nm thickness were patterned to micro- and nanobridges and embedded into planar log-spiral THz antennas. The YBCO thin-film detectors were characterized with continuous wave radiation at 0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO nanobridges. Pulsed measurements in the THz frequency range were performed at the electron storage ring ANKA from the Karlsruhe Institute of Technology (KIT). Due to the high responsivities of the nanobridges no biasing was required for the detection of the coherent synchrotron radiation pulses achieving very good agreement between the measured pulse shapes and simulations.