The prevailing architecture of superconducting quantum information processing relies heavily on the Josephson junction, yet its dual role as both a qubit-defining non-linear inductor and a latching readout element creates a fundamental tension in measurement dynamics. While the former enables the isolation of a two-level subspace, the latter—often implemented as a bifurcation amplifier or latching comparator—imposes a premature digitization that destroys the continuous time-series of the quantum state evolution. This study reframes the quantum measurement problem through the lens of classical signal processing, positing that the “collapse” observed in standard readout schemes acts as a hardware-level quantization error isomorphic to a 1-bit Analog-to-Digital Converter (ADC). By simulating the information recovery rates of Traveling Wave Parametric Amplifiers (TWPAs) against traditional latching readouts, we demonstrate that continuous monitoring coupled with Kalman filtering can recover phase trajectories with a mean squared error (MSE) of 0.015, preserving the “verb” of quantum evolution. Crucially, control simulations reveal that even under high-SNR conditions ($\sigma_{noise}=0.2$), the act of latching increases error by a factor of 6 (MSE 0.254) compared to continuous monitoring, isolating the discrete readout mechanism as the primary source of information loss. These findings suggest that replacing discrete “noun-based” storage architectures with “verb-based” continuous flow processors is essential for potentially enabling quantum-limited metrology and efficient quantum error correction.
The prevailing architecture of superconducting quantum information processing relies heavily on the Josephson junction, yet its dual role as both a qubit-defining non-linear inductor and a latching readout element creates a fundamental tension in measurement dynamics. While the former enables the isolation of a two-level subspace, the latter—often implemented as a bifurcation amplifier or latching comparator—imposes a premature digitization that destroys the continuous time-series of the quantum state evolution. This study reframes the quantum measurement problem through the lens of classical signal processing, positing that the “collapse” observed in standard readout schemes acts as a hardware-level quantization error isomorphic to a 1-bit Analog-to-Digital Converter (ADC). By simulating the information recovery rates of Traveling Wave Parametric Amplifiers (TWPAs) against traditional latching readouts, we demonstrate that continuous monitoring coupled with Kalman filtering can recover phase trajectories with a mean squared error (MSE) of 0.015, preserving the “verb” of quantum evolution. Crucially, control simulations reveal that even under high-SNR conditions (σnoise=0.2), the act of latching increases error by a factor of 6 (MSE 0.254) compared to continuous monitoring, isolating the discrete readout mechanism as the primary source of information loss. These findings suggest that replacing discrete “noun-based” storage architectures with “verb-based” continuous flow processors is essential for potentially enabling quantum-limited metrology and efficient quantum error correction.
Coherent circuits based on superconducting elements hold tremendous promise for the practical implementation of quantum information technology: advanced computation, cryptography, and hardware simulation of complex materials systems are envisioned applications. Such circuits are essentially engineered atoms with level transitions in the 4-8 GHz regime; as such “read” and “write” operations are performed with the aid of fast microwave pulses and low-noise amplifiers, respectively. Recent advances in cavity-based superconducting parametric amplifiers have enabled real-time measurement and feedback in one and two qubits. We demonstrate a novel Josephson junction transmission-line based traveling-wave amplifier which employs sub-wavelength phase matching resonators to achieve high gain, several GHz of bandwidth, and near quantum-noise limited performance. Such devices will enable multiplexed qubit readout in integrated architectures comprised of a variety of microwave frequency analog and digital superconducting technologies: signal sources, multiplexers, ADCs, DACs, mixers, and amplifiers.
As the need for scaling up quantum computers increases there is also an increased need for more compact adjacent circuitry surrounding the quantum processor. In the amplification of a qubit's readout-signal there is a need for quantum-limited amplifiers achieving a high signal-to-noise ratio. Today, it is common to use traveling-wave parametric amplifiers (TWPAs), which are currently powered by an external microwave pump. To further reduce the circuitry an option is to use an on-chip pump only requiring DC-wires to operate. This is possible due to the AC Josephson effect which can convert a DC voltage to an AC current. By connecting multiple Josephson junctions in series it is possible to increase the output power by utilizing their coherent oscillations. The purpose of this project was to simulate and characterize Josephson voltage controlled oscillators in terms of power and frequency-output, with the goal of providing the pump to a TWPA. This also included bandpass filters utilizing the Josephson inductance, to remove higher order harmonics of the oscillations. These bandpass filters were also characterized both in simulations and in measurements. To evaluate these components, a cryogenic testbed was implemented, and benchmarked, with a capacity of 12 parallel experiments. Simulations showed that it was possible to use Josephson junctions to produce stable oscillations which were filtered to give a pure 5 GHz signal with a -86.78 dBm power. When the corresponding device was measured it was not possible to observe the harmonic spectrum, likely due to noise in the DC-biasing system. The measured bandpass filters exhibited either high insertion loss or limited bandwidth as compared to design-specifications. A power-threshold was identified at approximately -90 dBm at which the bandpass filters showed severe non-linear effects, both in simulations and in measurements.
Superconducting parametric amplifiers (SPAs) are critical components for ultralow-noise qubit readout in quantum computing, addressing the critical challenge of amplifying weak quantum signals without introducing noise that degrades coherence and computational fidelity. Unlike classical amplifiers, SPAs can achieve or closely approach quantum-limited performance, specifically the Standard Quantum Limit (SQL) of half a photon of added noise for phase-preserving amplification. The core principle of SPAs relies on parametric amplification, where energy is transferred from a strong pump tone to a weak input signal through non-dissipative nonlinear mixing processes. This is enabled by intrinsic nonlinearities in superconducting materials, primarily kinetic inductance in thin films (e.g., NbTiN, Al) and, more significantly, the Josephson effect in Josephson junctions. These nonlinear elements facilitate frequency mixing (three-wave or four-wave mixing) and can operate in phase-preserving or phase-sensitive amplification modes, with the latter allowing for noise squeezing below the SQL. This chapter emphasizes the significant role of resonator design in determining critical SPA performance metrics such as gain, bandwidth, and noise characteristics. It details both lumped-element (LC) and distributed-element (coplanar waveguide, CPW) resonators, discussing their unique properties, suitability for different frequency ranges, and the importance of achieving high-quality factors (Q) for efficient energy storage and minimal loss. A practical design and simulation of a meandered quarterwavelength CPW resonator coupled to a feed line is presented, illustrating how precise control over geometric parameters optimizes resonant frequency, coupling strength, and quality factor for high-fidelity qubit state discrimination.
Bringing signals from quantum to classical, from few-photon to many-photon levels for further processing, is a common challenge in rising quantum technologies. These include quantum computing, quantum communication, quantum sensing, radio astronomy, and dark matter searches, typically operating at microwave frequencies and millikelvin temperatures. Superconducting parametric amplifiers, as the first piece in the readout chain, have become a cornerstone of these technologies, since they offer unrivaled noise performance near the quantum-mechanical limit and high gain to surpass the noise of subsequent amplifiers. A notable implementation is the Josephson traveling-wave parametric amplifier (TWPA), which stands out due to its bandwidth of several Gigahertz and high saturation power, often larger than -100 dBm. These key properties come at the cost of complexity in design and fabrication: TWPA metamaterials often contain several thousands of Josephson junctions. Designing high-gain TWPAs involves three major challenges: ensuring impedance-matching, phase-matching of parametric amplification, and suppressing unwanted processes, all achievable through sub-wavelength dispersion engineering. This thesis covers the circuit design and analysis, implementation, and experimental characterization of a dispersion-engineered TWPA based on radio-frequency superconducting interference devices (rf-SQUIDs). It focuses on a detailed investigation of dispersion-engineering techniques and their effects on amplifier performance, particularly through the use of time-domain circuit simulations to analyze the multimode dynamics of the circuit. The experimentally realized device features 18 dB of gain, an instantaneous bandwidth spanning from 2.2 to 6.7 GHz, a system noise of 2.5 quanta, and a saturation power of (−86.5 ± 4.0) dBm. With these characteristics, it is suitable for frequency-multiplexed readout of detector arrays or qubits. Its high saturation power, about 10 dB higher than other reported Josephson-junction-based TWPAs, allows increasing the number of qubits per readout line, or decreasing the intermodulation between frequency-multiplexed qubits, both crucial for scaling up quantum computation.
Though superconducting qubits offer the potential for a scalable quantum computing architecture, the high-fidelity readout necessary to execute practical algorithms has thus far remained elusive. Moreover, achievements toward high fidelity have been accompanied by either long measurement times or demolition of the quantum state. In this dissertation, we address these issues with the novel integration of two ultralow-noise, superconducting amplifiers into separate dispersive flux qubit measurements. We first demonstrate a flux qubit inductively coupled to a 1.294-GHz nonlinear oscillator formed by a capacitively shunted DC SQUID. The frequency of the oscillator is modulated by the state of the qubit and is detected via microwave reflectometry. A microstrip SQUID (Superconducting QUantum Interference Device) amplifier (MSA) is used to increase the sensitivity of the measurement over that of a semiconductor amplifier. In the second experiment, we report measurements of a flux qubit coupled via a shared inductance to a quasi-lumped element 5.78-GHz readout resonator formed by the parallel combination of an interdigitated capacitor and a meander line inductor. The system noise is substantially reduced by a near-quantum-limited Josephson parametric amplifier (paramp).We present measurements of increased fidelity and reduced measurement backaction using the MSA at readout excitation levels as low as one hundredth of a photon in the readout resonator, observing a 4.5-fold increase in the readout visibility. Furthermore, at low readout excitation levels below one tenth of a photon in the readout resonator, no reduction in T1 is observed, potentially enabling continuous monitoring of the qubit state. Using the paramp, we demonstrate a continuous, high-fidelity readout with sufficient bandwidth and signal-to-noise ratio to resolve quantum jumps in the flux qubit. This is enabled by a readout which discriminates between the readout pointer state distributions to an error below one part in 1000. This, along with the ability to make many successive readouts in a time T1, permits the use of heralding to ensure initialization to a fiducial state, such as the ground state. This method enables us to eliminate errors due to spurious thermal population, increasing the fidelity to 93.9%. Finally, we use heralding to introduce a simple, fast qubit reset protocol without changing the system parameters to induce Purcell relaxation.