The direct current superconducting quantum in terference device (dc SQUID) sensor represents one of the most established applications of superconductor technology due to its sensitivity to magnetic flux. In this work, we present superconducting “fine-pitch” input coils with sub-micrometer parameters fabricated with electron beam lithography (EBL) that will be integrated into existing Nb/Al-AlOx/Nb-based SQUID sensor designs. The aim is to reduce inductive losses of the signal to-SQUID coupling without compromising the overall device layout. In a SQUID current sensor, maximization of the inductive coupling constant k between the signal input coil and the SQUID loop results in a low coupled energy sensitivity ϵc = ϵ/k2 where ϵ is the intrinsic energy sensitivity. The fine-pitch coils are expected to increase the inductive coupling constant k as well as extend the range of input coil inductance for our existing devices. The SQUID energy sensitivity ϵ ∼ √CJJ can be lowered by reducing the Josephson junction (JJ) capacitance CJJ. We are developing a process, also based on EBL, to fabricate window-type JJs with sub-micrometer lateral size to obtain reduced JJ capacitances. Details concerning design aspects of the components, their fabrication and characterization results are provided in this paper.

We study a certain type of anomalous Hall current in magnetically and electrically biased bulk GaAs samples under the formation of Landau levels. The currents are generated by ultrafast optical excitation of spin-polarized carriers and detected by time-resolved measurements of the simultaneously emitted terahertz (THz) radiation. Due to the requirements of simultaneous magnetic and electric driving fields we refer to these currents as magnetic- and electric-field-induced anomalous Hall currents (BE-AHC). We find that the BE-AHC peaks for optical transitions between the band extrema of Landau levels of valence and conduction bands. These discrete features are attributed to the energy dependence of the geometric phases being responsible for anomalous transport effects in a semiconductor band structure. Surprisingly, we even detect the discrete Landau band transitions at room temperature, most likely due to the ultrafast local probing realized in our experiment. An analysis of the THz spectra using a model, based on the Boltzmann transport equation of optically excited carriers, shows that electron and hole contributions lead to complex current dynamics. While the cyclotronic motion of electrons results in a dip in the THz spectral response of BE-AHC, it causes a peak in the spectral response of normal Hall currents. Additionally, our experimental results strongly suggest that the Landau levels of the valence band play a significant role in the generation of BE-AHC. We expect that our results will initiate further studies on Berry-phase effects in Landau bands.

Superconducting circuits are one of the leading technologies for the realization of quantum computers. However, achieving practical, fault-tolerant quantum computers requires solutions to many technological challenges. Metrology for qubits and their peripherical devices could accelerate this development and help gaining more confidence in scientific achievements. The Quantum Electronics Department of PTB is – together with national and international partners – actively pursuing this metrological support and this poster will highlight some of the corresponding activities at PTB: (i) Concepts of voltage metrology, such as variants of the Josephson arbitrary waveform synthesizer (JAWS), are adopted at PTB for the control of superconducting qubits in order to realize quantum accurate synthesizers with a minimum of amplitude and phase noise. First waveforms at GHz frequencies, that are generated from a JAWS circuit, have been realized. Such devices could help improving qubit operation. (ii) Parametric amplifiers, that do not contain dissipative elements, enable amplification at the quantum limit of added noise. These devices are ideally suited to amplify read out signals of superconducting qubits. PTB is working on travelling-wave parametric amplifiers (TWPAs) and resonant amplifiers. The latest devices of PTB show 20 dB gain over a bandwidth of 4 GHz and, thus, are extremely promising for qubit readout. (iii) Using the properties of a two-level system, superconducting qubits can be employed as quantum-accurate and extremely sensitive power sensors at cryogenic temperatures. So far PTB has fabricated Transmon qubits and employed qubit spectroscopy and the AC Stark shift for the demonstration of power sensing. The ultimate goal will be to have dedicated circuit elements, that can be placed everywhere in a superconducting circuit, enabling quantum-accurate power sensing at this position. (iv) Opto-electronic techniques based on femtosecond lasers allow for quasi non-invasive measurements of high-frequency signals with an unprecedented bandwidth. Based upon these techniques, PTB has constructed a cryogenic sampling oscilloscope with a theoretical bandwidth exceeding 1 THz. This oscilloscope has been used for the characterization of ultrafast photodiodes at 4 K and will be further enhanced for in-situ waveform measurements for the optimization of superconducting circuits. The poster will summarize these topics, showing the latest results and future prospects.

We combine a cryogenic BiCMOS integrated circuit, which generates high-speed return-to-zero (RTZ) pulses, with a superconducting Josephson junction array. The BiCMOS circuit acts as a cryogenic pulse pattern generator, delivering data rates of 30 Gb/s, while consuming 302 mW at 4 K. Each electrical pulse of the serializer effectively transfers one magnetic flux quantum through every Josephson junction, so that the average output voltage of the array produces well-defined plateaus (Shapiro steps) in its current-to-voltage characteristic. To the best of our knowledge, this is the first integration of a Josephson junction array with a cryogenic BiCMOS chip. The presented results pave the way toward a hybrid and fully integrated Josephson arbitrary waveform synthesizer (JAWS) that can generate ultra-low-noise signals for quantum voltage metrology and quantum information systems.

With increasing applications of quantum systems operating at high frequencies, the role of integrated microwave components for quantum circuits becomes more and more important. In this article, we present low-temperature superconducting bandpass-bandstop diplexers for a new generation of quantum AC-voltage standards operating at 1 GHz. Based on stringent requirements, the diplexers are designed to provide low passband loss, good stopband rejection, and otherwise a broad ideally matched spectrum with excellent spurious-free performance. A second-order 1 GHz superconducting bandpass-bandstop diplexer is fabricated and characterized for verification. To this end, a cryogenic setup and on-chip calibration standards are implemented for S-parameter measurements. The experimental results exhibit low insertion loss as low as 0.03 dB in the passband section and high rejection in the bandstop branch exceeding 43 dB. Furthermore, the diplexer features wide spurious-free response extending to more than $14 \cdot \mathrm{f}_{0}$ and occupies a very compact area of only $1.8 \text{mm} \times 1.2 \text{mm}$.

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.

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 use the characterization platform at different temperatures, ranging from cryogenic to room temperature. As an 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 a direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators.

The magneto-volume effect is a well-established yet frequently overlooked phenomenon in magnetic materials that may affect a wide range of physical properties. Our study explores the influence of the magneto-volume effect on the transient reflectivity of MnSi, a well-known chiral magnet with strong magnetoelastic coupling. We observe a unipolar reflectivity transient in the paramagnetic phase, contrasting with a bipolar response in phases with magnetic long-range order. Comparing our findings with thermal expansion from literature, we establish that the bipolar response originates in the magneto-volume effect which dominates the thermal expansion and influences the optical reflectivity. Our results highlight not only that the magneto-volume effect must be considered when discussing transient reflectivity measurements of magnetic materials but also that such measurements permit to study the characteristic time scales of the magneto-volume effect itself, contributing to a deeper understanding of this often-neglected phenomenon.

We discuss the flip-chip mounting process of photodiodes and fiber sleeves on silicon substrates to meet the increasing demand for fabrication of highly integrated and hybrid quantum circuits for operation at cryogenic temperatures. To further increase the yield and success rate of the flip-chip procedure, the size of the gold stud bumps, and flip-chip parameters were optimized. Moreover, to connect optical fibers to the photodiodes in an optimal position, the fiber sleeves were aligned with specially fabricated alignment circles before applying thermocompression with the flip-chip machine. The mounted photodiodes were tested at both room temperature and cryogenic temperature, and we find that mechanical imperfections of the sleeve-ferrule combination limit the overall alignment accuracy. The experimental results show that our flip-chip process is very reliable and promising for various optical and electrical applications and, thus, paves the way for fabrication of hybrid chips, multi-chip modules and chip-on-chip solutions, which are operated at cryogenic temperatures.

We report on electro-optic sampling (EOS) of microwave signals under cryogenic conditions employing a fully fiber-coupled measurement scheme. We use this setup to characterize the time response of ultrafast photodiodes at a temperature of 4 K. We additionally integrate a Josephson Arbitrary Waveform Synthesizer (JAWS) to the EOS platform, which allows us to calibrate the EOS response, yielding traceability of the microwave measurements to a quantum standard. Finally, we determine the electrical reflection coefficient, i.e., the S11 scattering parameter, in a Niobium transmission line using EOS at different positions. Our work denotes an important step for traceable in-situ microwave measurements in superconducting circuits.

A key challenge for the realization of future skyrmion devices comprises the controlled creation, annihilation and detection of these topologically non-trivial magnetic spin textures. In this study, we report an all-optical approach for writing, deleting, and reading skyrmions in the cubic chiral magnet Fe$_{0.25}$Co$_{0.75}$Si based on thermal quenching. Using focused femtosecond laser pulses, patches of a skyrmion state are created and annihilated locally, demonstrating unprecedented control of thermally metastable skyrmions in a bulk compound. The skyrmion state is read-out by analyzing the microwave spin excitations in time-resolved magneto-optical Kerr effect measurements. Extracting the magnetic field and laser fluence dependence, we find well-separated magnetic field regimes and different laser fluence thresholds for the laser-induced creation and annihilation of metastable skyrmions. The all-optical skyrmion control, as established in this study for a model system, represents a promising and energy-efficient approach for the realization of skyrmions as magnetic bits in future storage devices, reminiscent of magneto-optical storage devices in the past.

We discuss the flip-chip mounting process of photodiodes and fiber sleeves on silicon substrates to meet the increasing demand for fabrication of highly integrated and hybrid quantum circuits for operation at cryogenic temperatures. To further increase the yield and success rate of the flip-chip procedure, the size of the gold stud bumps, and flip-chip parameters were optimized. Moreover, to connect optical fibers to the photodiodes in an optimal position, the fiber sleeves were aligned with specially fabricated alignment circles before applying thermocompression with the flip-chip machine. The mounted photodiodes were tested at both room temperature and cryogenic temperature, and we find that mechanical imperfections of the sleeve-ferrule combination limit the overall alignment accuracy. The experimental results show that our flip-chip process is very reliable and promising for various optical and electrical applications and, thus, paves the way for fabrication of hybrid chips, multi-chip modules and chip-on-chip solutions, which are operated at cryogenic temperatures.

Dataset of the publication “Excitonic anomalous currents in semiconductor quantum wells”, by C. Ngo, S. Priyadarshi, H. T. Duc, M. Bieler, and T. Meier, published in Physical Review B 108, 165302 (2023) ( https://doi.org/10.1103/PhysRevB.108.165302 )The zip file includes a brief description and the data on which the plots of figures 2 – 10 are based, and the codes used for the numerical evaluations (k.p and semiconductor Bloch equations).

Anomalous currents refer to electronic currents that flow perpendicularly to the direction of the accelerating electric field. Such anomalous currents can be generated when Terahertz fields are applied after an optical interband excitation of GaAs quantum wells. The underlying processes are investigated by numerical solutions of the semiconductor Bloch equations in the length gauge. Excitonic effects are included by treating the manybody Coulomb interaction in time-dependent Hartree-Fock approximation and additionally also carrier-phonon scattering processes are considered. The band structure and matrix elements are obtained from a 14-band k · p model within the envelope function approximation. The random phase factors of the matrix elements that appear due to the separate numerical diagonalization at each k-point are treated by applying a smooth gauge transformation. We present the macroscopic Berry curvature and anomalous current transients with and without excitonic effects. It is demonstrated that the resonant optical excitation of excitonic resonances can significantly enhance the Berry curvature and the anomalous currents.

The anomalous Hall currents (AHC) and spin Hall currents are variants of the Hall effect, with a current flowing perpendicular to an external electrical bias due to the Berry curvature and scattering processes. In this work, we study AHC in bulk GaAs under the formation of Landau levels, which give rise to quantized current responses in the case of two-dimensional GaAs systems. We show that an onset of quantization can also be reached in three-dimensional GaAs systems in the ultrafast (fs to ps) regime. This quantization appears as an enhancement of the AHC for ultrafast excitation of optical transitions between valance and conduction band Landau levels. An analysis of the shape of the resulting THz pulses for different excitation conditions suggests that both, Berry curvature and scattering contribute to the AHC.

We investigate the microwave spin excitations of the cubic chiral magnet Fe$_{0.75}$Co$_{0.25}$Si as driven by the thermal modulation of magnetic interactions via laser heating and probed by time-resolved measurements of the magneto-optical Kerr effect. Focusing on the topologically nontrivial skyrmion lattice state, the dynamic properties in thermodynamic equilibrium are compared with those of a metastable state prepared by means of rapid field cooling. In both cases, we find precessional and exponential contributions to the dynamic response, characteristic of a breathing mode and energy dissipation, respectively. When taking into account the universal scaling as a function of temperature, the precession frequencies in the equilibrium and metastable skyrmion state are in excellent quantitative agreement. This finding highlights that skyrmion states far from thermal equilibrium promise great flexibility, for instance with respect to temperature and field scales, both for possible microwave applications and the study of fundamental properties.

We study ultrafast anomalous Hall currents in bulk GaAs generated using optical femtosecond excitation and magnetic fields up to 7 T. The currents' dynamics are resolved by measuring the simultaneously emitted THz radiation. The resulting current amplitude shows maxima at excitation photon energies, which agree well with interband transitions between valence and conduction band Landau levels. This behavior is observed for both cryogenic and room temperature. While our work helps to better understand the anomalous Hall effect, it also demonstrates a method to detect and possibly employ Landau levels at elevated temperatures.

Photonic integration technologies are key to scale-up superconducting quantum computers. Here, we identify suitable classical optical links to control and read out the qubits in cryostats and resolve the power dissipation issue of superconducting computing platforms. Recent results and future solutions are shown.

All envisaged practical implementations of cryogenic processors, including quantum computers and classical processors based on single flux quantum (SFQ) signals, require massive data transfer from and to classical high performance computers (HPCs). Cryogenic computing has recently become a very hot topic, including superconducting quantum computers (QCs), and classical processors based on single flux quantum (SFQ) signals. All envisaged practical implementations of cryogenic processors require massive data transfer from and to classical HPCs. The project aCryComm aims to develop building blocks for cryogenic photonics interconnects and eventually enable this challenging data transfer. The long-term goal is the development of an open-access platform to integrate classical optical interfaces based on low-loss silicon photonics, plasmonics, and nano light sources together with superconducting photonic and electronic devices, including SFQ-based co-processors for HPCs and for QCs.