A superconducting bandpass delta-sigma modulator for direct analog-to-digital conversion of microwave radio

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 291-305).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Direct analog-to-digital conversion of multi-GHz radio frequency (RF) signals is the ultimate goal in software radio receiver design but remains a daunting challenge for any technology. This thesis examines the potential of superconducting technology for realizing RF analog-to-digital converters (ADCs) with improved performance. A bandpass delta-sigma (AE) modulator is an attractive architecture for digitizing narrowband signals with high linearity and a large signal-to-noise ratio (SNR). The design of a superconducting bandpass AE modulator presented here exploits several advantages of superconducting electronics: the high quality factor of resonators, the high sampling rates of comparators realized with Josephson junctions, natural quantization of voltage pulses, and high circuit sensitivity. Demonstration of a superconducting circuit operating at clock rates in the tens of GHz is often hindered by the difficulty of high speed interfacing with room-temperature test equipment. In this work, a test chip with integrated acquisition memory is used to simplify high speed testing in a cryogenic environment. The small size (256 bits) of the on-chip memory severely limits the frequency resolution of spectra based on standard fast Fourier transforms. Higher resolution spectra are obtained by "segmented correlation", a new method for testing ADCs. Two different techniques have been found for clocking the superconducting modulator at frequencies in the tens of GHz. In the first approach, an optical clocking technique was developed, in which picosecond laser pulses are delivered via optical fiber to an on-chip metal-semiconductor-metal (MSM) photodiode, whose output current pulses trigger the Josephson circuitry. In the second approach, the superconducting modulator is clocked by an on-chip Josephson oscillator.(cont.) These testing methods have been applied in the successful demonstration of a super-conducting bandpass AE modulator fabricated in a niobium integrated circuit process with 1 kA/cm2 critical current density for the Josephson junctions. At a 42.6 GHz sampling rate, the center frequency of the experimental modulator is 2.23 GHz, the measured SNR is 49 dB over a 20.8 MHz bandwidth, and a full-scale (FS) input is -17.4 dBm. At a 40.2 GHz sampling rate, the measured in-band noise is -57 dBFS over a 19.6 MHz bandwidth.by John Francis Bulzacchelli.Ph.D

Digital instrumentation for the measurement of high spectral purity signals

Improvements on electronic technology in recent years have allowed the application of digital techniques in time and frequency metrology where low noise and high accuracy are required, yielding flexibility in systems implementation and setup. This results in measurement systems with extended capabilities, additional functionalities and ease of use. The Analog to Digital Converters (ADCs) and Digital to Analog Converters (DACs), as the system front-end, set the ultimate performance of the system in terms of noise. The noise characterization of these components will allow performing punctual considerations on the study of the implementation feasibility of new techniques and for the selection of proper components according to the application requirements. Moreover, most commercial platforms based on FPGA are clocked by quartz oscillators whose accuracy and frequency stability are not suitable for many time and frequency applications. In this case, it is possible to take advantage of the internal Phase Locked Loop (PLL) for generating the internal clock from an external frequency reference. However, the PLL phase noise could degrade the oscillator stability thereby limiting the entire system performance becoming a critical component for digital instrumentation. The information available currently in literature, describes in depth the features of these devices at frequency offsets far from the carrier. However, the information close to the carrier is a more important concern for time and frequency applications. In this frame, my PhD work is focused on understanding the limitations of the critical blocks of digital instrumentation for time and frequency metrology. The aim is to characterize the noise introduced by these blocks and in this manner to be able to predict their effects on a specific application. This is done by modeling the noise introduced by each component and by describing them in terms of general and technical parameters. The parameters of the models are identified and extracted through the corresponding method proposed accordingly to the component operation. This work was validated by characterizing a commercially available platform, Red Pitaya. This platform is an open source embedded system whose resolution and speed (14 bit, 125 MSps) are reasonably close to the state of the art of ADCs and DACs (16 bit, 350 MSps or 14 bit, 1 GSps/3GSPs) and it is potentially sufficient for the implementation of a complete instrument. The characterization results lead to the noise limitations of the platform and give a guideline for instrumentation design techniques. Based on the results obtained from the noise characterization, the implementation of a digital instrument for frequency transfer using fiber link was performed on the Red Pitaya platform. In this project, a digital implementation for the detection and compensation of the phase noise induced by the fiber is proposed. The beat note, representing the fiber length variations, is acquired directly with a high speed ADC followed by a fully digital phase detector. Based on the characterization results, it was expected a limitation in the phase noise measurement given by the PLL. First measurements of this implementation were performed using the 150 km-long buried fibers, placed in the same cables between INRiM and the Laboratoire Souterrain de Modane (LSM) on the Italy-France border. The two fibers are joined together at LSM to obtain a 300 km loop with both ends at INRiM. From these results the noise introduced by the digital system was verified in agreement with characterization results. Further test and improvements will be performed for having a finished system which is intended to be used on the Italian Link for Frequency and Time from Turin to Florence that is 642-km long and to its extension in the rest of Italy that is foreseen in the next future. Currently, a higher performance platform is under assessment by applying the tools and concepts developed along the PhD. The purpose of this project is the implementation of a state of the art phasemeter whose architecture is based on the DAC. In order to estimate the ultimate performance of the instrument, the DAC characterization is under development and preliminary measurements are also reported here

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Development and demonstration of a high bandwidth, ultra sensitive trapped ion magnetometer

This thesis describes an ion trap magnetometer, in which experimental hardware and theory for trapped ion magnetometry has been developed and demonstrated. Quantum magnetometry has been a fast expanding field in recent years due to the plethora of military, medical and security applications. The current quantum magnetometer literature has shown great improvements on the standard magnetometer success metrics such as sensitivity, spacial resolution, frequency tuneability, noise shielding and vector field resolution. Ion trap magnetometry could potentially boast several advantages over some of the better established methods such as, nano-meter spatial resolution, broadband tuneability over radio-frequency (RF) and microwave frequencies, and magnetic field noise shielding and sensitivity improvements through dynamic decoupling methods; all in room temperature environments. The experimental setup discussed uses a micro-fabricated Paul trap for ion confinement due to the scalability of this technology. The method for sensing that was demonstrated utilises long-wavelength radiation dressed states to extend the coherence time of a single 171Yb+ ion beyond the bare states and to allow for a high level of frequency tuneability. The coherence times measured were T₂ = 0:645 s for RF field sensing at 15.616 MHz and T₂ = 1.153 s for microwave field sensing at 12.658466 GHz. Sensitivities of SB = 125.857 ± 26.155 pT/ √Hz for RF magnetic fields and SB = 102.054 ± 11.046 pT/ √Hz for microwave magnetic fields were achieved experimentally. A novel micro-fabricated Paul trap for trapping multiple ion clusters was developed to improve sensitivity values by increasing the total number of trapped ions. The design confines ions over a two-dimensional surface spanning 0.65 x 1.00 mm which would also allow for magnetic field gradient measurements. Efforts have been made to miniaturise certain experimental hardware to further the development of portable trapped ion experiments. For this, a miniaturised RF resonator that could be used for radial confinement of ions within Paul traps has been developed. At the current stage of development it is capable of applying a peak RF voltage of 114.9 V with a Q value of 70 at a resonant frequency of Ω/2π = 12.65 MHz

Waveguide Quantum Electrodynamics in Superconducting Circuits

Achieving an efficient interface of light and matter has been a principal goal in the field of quantum optics. A burgeoning paradigm in the study of light-matter interface is waveguide quantum electrodynamics (QED), where quantum emitters are coupled to a common one-dimensional waveguide channel. In this scenario, cooperative effects among quantum emitters emerge as a result of real and virtual exchange of photons, giving rise to new ways of controlling matter. Superconducting quantum circuits offer an exciting platform to study quantum optics in the microwave domain with artificial quantum emitters interfaced to engineered photonic structures on chip. Beyond revisiting the experiments performed in atom-based platforms, superconducting circuits enable exploration of novel regimes in quantum optics that are otherwise prohibitively challenging to achieve. Moreover, the unprecedented level of control over individual quantum degrees of freedom and good scalability of the system provided by state-of-the-art circuit QED toolbox set a promising direction towards the study of quantum many-body phenomena. In this thesis, I discuss waveguide QED experiments performed in superconducting quantum circuits where transmon qubits are coupled to engineered microwave waveguides. Employing the high flexibility and controllability of superconducting quantum circuits, we realize and explore various schemes for generating waveguide-mediated interactions between superconducting qubits. We also demonstrate an intermediate-scale quantum processor based on a dispersive waveguide QED system involving ten superconducting qubits, exploring quantum many-body dynamics in a highly controllable fashion. The work described in the thesis marks an important step towards the construction of scalable architectures for quantum simulation of many-body models and realization of efficient coupling schemes for quantum computation.</p