Detlef Born

Radio-frequency method for investigation of quantum properties of superconducting structures

We implement the impedance measurement technique (IMT) for characterization of interferometer-type superconducting qubits. In the framework of this method, the interferometer loop is inductively coupled to a high-quality tank circuit. We show that the IMT is a powerful tool for studying the response of an externally controlled two-level system to different types of excitations. Conclusive information about the qubits is obtained from a readout of the tank properties.

Multiphoton transitions between energy levels in a phase-biased Cooper-pair box

We investigated both theoretically and experimentally dynamic features of a phase-biased charge qubit consisting of a single-Cooper-pair transistor closed by a superconducting loop. The effective inductance of the qubit was probed by a high-quality tank circuit. In the presence of a microwave power, with a frequency of the order of the qubit energy level separation, an alteration of the qubit inductance was observed. We demonstrate that this effect is caused by the redistribution of the qubit level population. The excitation of the qubit by one-, two-, and three-photon processes was detected. Quantitative agreement between theory and experimental data was found.

Toward high-sensitivity and high-resolution submillimeter-wave video imaging

Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350±40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz.

YBCO Nanobridges: Simplified Fabrication Process by Using a Ti Hard Mask

An innovative fabrication procedure has been developed to obtain YBa2Cu3O7-x (YBCO) nanobridges from c-axis oriented films. The novelty regards the use of a thin titanium mask used in the patterning process. The use of the Ti makes simpler the fabrication procedure guaranteeing high quality devices, as demonstrated by transport measurements. Critical temperatures and critical current densities scale with the width of the microbridge down to 200 nm in agreement with most results available in literature. The actual properties of the devices, performances, yield and reproducibility, along with an accurate control on doping, may allow the use of these micro-bridges for nanoscale experiments.

Development of passive submillimeter-wave video imaging systems for security applications

Passive submillimeter-wave imaging is a concept that has been in the focus of interest as a promising technology for security applications for a number of years. It utilizes the unique optical properties of submillimeter waves and promises an alternative to millimeter-wave and X-ray backscattering portals for personal security screening in particular. Possible application scenarios demand sensitive, fast, and flexible high-quality imaging techniques. Considering the low radiometric contrast of indoor scenes in the submillimeter range, this objective calls for an extremely high detector sensitivity that can only be achieved using cooled detectors. Our approach to this task is a series of passive standoff video cameras for the 350 GHz band that represent an evolving concept and a continuous development since 2007. The cameras utilize arrays of superconducting transition-edge sensors (TES), i. e. cryogenic microbolometers, as radiation detectors. The TES are operated at temperatures below 1 K, cooled by a closed-cycle cooling system, and coupled to superconducting readout electronics. By this means, background limited photometry (BLIP) mode is achieved providing the maximum possible signal to noise ratio. At video rates, this leads to a pixel NETD well below 1K. The imaging system is completed by reflector optics based on free-form mirrors. For object distances of 3–10 m, a field of view up to 2m height and a diffraction-limited spatial resolution in the order of 1–2 cm is provided. Opto-mechanical scanning systems are part of the optical setup and capable frame rates up to 25 frames per second. Both spiraliform and linear scanning schemes have been developed. Several electronic and software components are used for system control, signal amplification, and data processing. Our objective is the design of an application-ready and user-friendly imaging system. For application in real world security screening scenarios, it can be extended using image processing and automated threat detection software.

Progress report on Safe VISITOR: approaching a practical instrument for terahertz security screening

As reported before,1, 2 Safe VISITOR (Safe VISible, Infrared and Terahertz Object recognition) is a German project to build a passive security camera which visualizes sub-mm wavelengths using cooled bolometer arrays. This camera could be used for a variety of application scenarios, such as airport screenings or to protect military camps. In all cases, a practical instrument requires ease of use, in particular a flexible installation and a straightforward usage by the security personnel. Here we present a new generation of Safe VISITOR designed to meet these requirements. The main condition for an effective operation is a high frame rate of the imager. Safe VISITOR is able to record videos up to 10 Hz, using a small array of superconducting bolometers in combination with an opto-mechanical scanner. The required cooling of the detector array is provided by a commercial pulse tube cooler with a second, self-contained cooling stage. The cooling cycle is completely automated; after 10 hours of initial cooling from room temperature the system can operate quasi-continuously. For imaging, a 50 cm diameter optics is used which is able to provide an object resolution of approximately 1.5 cm at 8 m distance. For a flexible installation, the object distance can be tuned manually between 7 and 10 m. Additionally, video streams from two commercial cameras are fused with the sub-mm stream: a CCD for visible light and a microbolometer for far infrared (14 μm). This combines the ability of identification of the person under test with the unprecedented temperature resolution at infrared and the almost perfect transmission at sub-mm. To assist a security official, all image data are displayed in various graphic renditions by a unified system software.

Development of passive submillimeter-wave video imaging systems

Passive submillimeter wave imaging is a concept that has been in the focus of interest as a promising technology for security applications for a number of years. It utilizes the unique optical properties of submillimeter waves and promises an alternative to millimeter-wave and X-ray backscattering portals for personal security screening in particular. Possible application scenarios demand sensitive, fast, and fleixible high-quality imaging techniques. Considering the low radiometric contrast of indoor scenes in the submillimeter range, this objective calls for an extremely high detector sensitivity that can only be achieved using cooled detectors. Our approach to this task is a series of passives standoff video cameras for the 350 GHz band that represent an evolving concept and a continuous development since 2007. The cameras utilize arrays of superconducting transition-edge sensors (TES), i.e. cryogenic microbolometers, as radiation detectors. The TES are operate at temperatures below 1K, cooled by a closed-cycle cooling system, and coupled to superconducting readout electronics. By this means, background limited photometry (BLIP) mode is achieved providing the maximum possible signal to noise ratio. At video rates, this leads to a pixel NETD well below 1K. The imaging system is completed by reflector optics based on free-form mirrors. For object distances of 3–10m, a field of view up to 2m height and a diffraction-limited spatial resolution in the order of 1–2cm is provided. Opto-mechanical scanning systems are part of the optical setup and capable frame rates up to 25 frames per second. Both spiraliform and linear scanning schemes have been developed.

Coherent Rabi response of a charge-phase qubit under microwave irradiation

We report on radio-frequency rf measurements of the charge-phase qubit being under continuous microwave irradiation in the state of weak coupling to a radio-frequency tank circuit. We studied the rf impedance dependence on the two important parameters such as the power of microwave irradiation, whose frequency is close to the gap between the two lowest qubit energy levels, and the temperature of the internal heat bath. We have found that backaction effects of the qubit on the rf tank and vice versa, tank on the qubit, lead to a negative as well as a positive real part of the qubit impedance Re Z seen by the tank. We have implemented noise spectroscopy measurements for direct impedance readout at the extreme points corresponding to maximum voltage response and obtained absolute values of about 0.017 for the negative and positive Re Z. Our results demonstrate the existence and persistence of the coherent single- and multiphoton Rabi dynamics of the qubit with both negative and positive dynamic resistances inserted into the tank in the temperature range of 10–200 mK.

Low noise cryogenic system for the measurement of the Casimir energy in rigid cavities

We report on preliminary results on the measurement of variations of the Casimir energy in rigid cavities through its influence on the superconducting transition of in-cavity aluminium (Al) thin films. After a description of the experimental apparatus, we report on a measurement in the presence of thermal photons, discussing its implications for the zero-point photons case. Finally, we show preliminary results for the zero-point case.

Progress in passive submillimeter-wave video imaging

Since 2007 we are developing passive submillimeter-wave video cameras for personal security screening. In contradiction to established portal-based millimeter-wave scanning techniques, these are suitable for stand-off or stealth operation. The cameras operate in the 350GHz band and use arrays of superconducting transition-edge sensors (TES), reflector optics, and opto-mechanical scanners. Whereas the basic principle of these devices remains unchanged, there has been a continuous development of the technical details, as the detector array, the scanning scheme, and the readout, as well as system integration and performance. The latest prototype of this camera development features a linear array of 128 detectors and a linear scanner capable of 25Hz frame rate. Using different types of reflector optics, a field of view of 1×2m2 and a spatial resolution of 1–2 cm is provided at object distances of about 5–25m. We present the concept of this camera and give details on system design and performance. Demonstration videos show its capability for hidden threat detection and illustrate possible application scenarios.