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We demonstrate a cryogenic electro-optic sampling (EOS) setup that allows for the measurement of microwave signals at arbitrary positions on a cryogenic chip-scale device. We use a Josephson Arbitrary Waveform Synthesizer (JAWS) to generate quantum-accurate voltage signals and measure them with the EOS setup, allowing for the calibration of its response, yielding traceability of the microwave measurements to a quantum standard. We use the EOS setup to determine the time-domain response of ultrafast cryogenic photodiodes and the electrical reflection coefficient, i.e., the S11 scattering parameter, in a superconducting transmission line. Finally, we introduce an optical femtosecond pulse source which can be used to study the fidelity of superconducting transmission lines and terminations, as well as reflections from elements like Josephson junction arrays imbedded in them., Comment: 16 pages

Cascade multiplication is a common technique to enhance the sensitivity of photon detectors. In this study, we demonstrate novel cascade-amplified superconducting detectors utilizing arrays of Josephson junctions. The mutual coupling between junctions induces avalanche-like switching of multiple junctions upon photon absorption, leading to cascade amplification of the readout voltage. We present two prototypes featuring either low-Tc linear Nb/NbxSi1-x/Nb arrays, or high-Tc stacked intrinsic Josephson junctions. Both devices exhibit clear antenna effects in microwave directivities, indicating good impedance matching and absorption efficiency. The combination of high absorption efficiency and large cascade amplification has the potential to produce broadband THz sensors with sensitivity exceeding 10^13 V/W., Comment: 9 pages, 3 figures

We have developed a cryogenic characterization platform for ultrafast photodiodes, whose time domain responses are extracted by electro-optic sampling using femtosecond laser pulses in a pump-probe configuration. The excitation of the photodiodes with the pump beam and the electro-optic sampling crystals with the probe beam are realized in a fully fiber-coupled manner. This allows us to place the characterization platform in almost any temperature environment. As application example, we characterize the time-domain response of commercial p-i-n photodiodes with a nominal bandwidth of 20 GHz and 60 GHz at temperatures of 4 K and 300 K and in a large parameter range of photocurrent and reverse bias. For these photodiodes, we detect frequency components up to approximately 250 GHz, while the theoretical bandwidth of our sampling method exceeds 1 THz. Our measurements demonstrate a significant excitation power and temperature dependence of the photodiodes' ultrafast time responses, reflecting, most likely, changes in carrier mobilities and electric field screening. Since our system is an ideal tool to characterize and optimize the response of fast photodiodes at cryogenic temperatures, it has direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators., Comment: 17 pages, 4 figures

We report the levitation of a superconducting lead-tin sphere with 100 micrometer diameter (corresponding to a mass of 5.6 micrograms) in a static magnetic trap formed by two coils in an anti-Helmholtz configuration, with adjustable resonance frequencies up to 240 hertz. The center-of-mass motion of the sphere is monitored magnetically using a dc superconducting quantum interference device as well as optically and exhibits quality factors of up to 2.6e7. We also demonstrate 3D magnetic feedback control of the sphere's motion. The setup is housed in a dilution refrigerator operating at 15 millikelvin. By implementing a cryogenic vibration isolation system we can attenuate environmental vibrations at 200 hertz by approximately seven orders of magnitude. The combination of low temperature, large mass and high quality factor as well as adjustable resonance frequencies provides a promising platform for testing quantum physics in previously unexplored regimes with high mass and long coherence times., Comment: Updated to published version

Magnetic media remain a key in information storage and processing. The continuous increase of storage densities and the desire for quantum memories and computers pushes the limits of magnetic characterisation techniques. Ultimately, a tool which is capable of coherently manipulating and detecting individual quantum spins is needed. The scanning tunnelling microscope (STM) is the only technique which unites the prerequisites of high spatial and energy resolution, low temperature and high magnetic fields to achieve this goal. Limitations in the available frequency range for electron spin resonance STM (ESR-STM) mean that many instruments operate in the thermal noise regime. We resolve challenges in signal delivery to extend the operational frequency range of ESR-STM by more than a factor of two and up to 100GHz, making the Zeeman energy the dominant energy scale at achievable cryogenic temperatures of a few hundred millikelvin. We present a general method for augmenting existing instruments into ESR-STMs to investigate spin dynamics in the high-field limit. We demonstrate the performance of the instrument by analysing inelastic tunnelling in a junction driven by a microwave signal and provide proof of principle measurements for ESR-STM., Comment: 8 pages, 7 figures

We recently demonstrated a 1$^{\textrm{st}}$-order axial gradiometer SQUID system, which is operated in a liquid He dewar with negligible noise contribution. The achieved close to SQUID-limited measured coupled energy sensitivity $\varepsilon_{c}$ of $\sim 30\,h$ corresponds to a white field noise below 180~aT~Hz${^{-1/2}}$. In order to further improve the SQUID noise performance, the junction capacitance was reduced by decreasing its lateral size from $2.5~\mu$m to below $1~\mu$m. This was realized by extending the fabrication process for submicrometer-sized Josephson Junctions based on the HfTi self-shunted junction technology to an SIS process with AlO$_{\textrm{x}}$ as the insulating layer. We achieved energy sensitivities of 4.7$\,h$ and 20$\,h$ at 4.2~K for uncoupled and coupled SQUIDs, respectively. We also investigated the temperature dependence of the noise of the uncoupled SQUIDs and reached an energy sensitivity of 0.65$\,h$ in the white noise regime at 400~mK., Comment: 4 pages, 7 figures, ISEC 2019, version containing all the proofs (provided to IEEE TAS prior to publication) which should have been published by IEEE TAS

We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity $S_\mu^{1/2}$ upon operation in intermediate magnetic fields $B$ (tens of mT), applied perpendicular to the substrate plane. Our so far best SQUID can be continuously operated in fields up to $B\approx\pm50\,\rm{mT}$ with rms flux noise $S_{\Phi,\rm w}^{1/2}\leq250\,\rm{n\Phi_0/Hz^{1/2}}$ in the white noise regime and spin sensitivity $S_{\mu}^{1/2}\leq29\,\rm{\mu_B/Hz^{1/2}}$. Furthermore, we demonstrate operation in $B=0.5\,\rm{T}$ with high sensitivity in flux $S_{\Phi,\rm w}^{1/2}\approx680\,\rm{n\Phi_0/Hz^{1/2}}$ and in electron spin $S_{\mu}^{1/2}\approx79\,\rm{\mu_B/Hz^{1/2}}$. We discuss strategies to further improve the nanoSQUID performance., Comment: to appear in Apl. Phys. Lett