Parametric frequency modulation is a standard tool in superconducting circuits for activating tunable interactions and implementing quantum gates. Here, we engineer dissipation in a flux-tunable transmon qubit by using sideband modulation to bring it into resonance with a lossy resonator, opening an on-demand Purcell decay channel. We find that pulsing this channel on and off does not simply lower the time-averaged decay rate; instead, it reorganizes the dissipation spectrum into a structured interference pattern. A Chebyshev-propagator model for the repeated on/off block reproduces the measured spectra and reveals a close structural correspondence to N-slit Fraunhofer diffraction, with each on-window acting as a temporal aperture. By varying the pulse duration and duty cycle, we demonstrate control over the spacing, contrast, and envelope of the dissipation spectrum. These results establish pulsed parametric modulation as a direct method for shaping engineered dissipation in superconducting circuits and provide a new control knob for open quantum system dynamics.
Parametric frequency modulation is a standard tool in superconducting circuits for activating tunable interactions and implementing quantum gates. Here, we engineer dissipation in a flux-tunable transmon qubit by using sideband modulation to bring it into resonance with a lossy resonator, opening an on-demand Purcell decay channel. We find that pulsing this channel on and off does not simply lower the time-averaged decay rate; instead, it reorganizes the dissipation spectrum into a structured interference pattern. A Chebyshev-propagator model for the repeated on/off block reproduces the measured spectra and reveals a close structural correspondence to N-slit Fraunhofer diffraction, with each on-window acting as a temporal aperture. By varying the pulse duration and duty cycle, we demonstrate control over the spacing, contrast, and envelope of the dissipation spectrum. These results establish pulsed parametric modulation as a direct method for shaping engineered dissipation in superconducting circuits and provide a new control knob for open quantum system dynamics.
We present a superconducting circuit device that provides active, on-demand, tunable dissipation on a target mode of the electromagnetic field. Our device is based on a tunable “dissipator” that can be made lossy when tuned into resonance with a broadband filter mode. When driven parametrically, this dissipator induces loss on any mode coupled to it with energy detuning equal to the drive frequency. We demonstrate the use of this device to reset a superconducting qubit’s readout cavity after a measurement, removing photons with a characteristic rate greater than 50μs−1 . We also demonstrate that the dissipation can be driven constantly to simultaneously damp and cool the cavity, effectively eliminating thermal photon fluctuations as a relevant decoherence channel. Our results demonstrate the utility of our device as a modular tool for environmental engineering and entropy removal in circuit quantum electrodynamics. Published by the American Physical Society 2024
Superconducting transmon qubits form the backbone of today's leading quantum processors, where high field dispersive readout relies fundamentally on the shape and temporal structure of microwave control pulses. While transmon qubits offer improved robustness against charge noise, their performance is still strongly influenced by environmental coupling, particularly the Purcell effect and by the spectral characteristics of the readout pulses used for state preparation and logic operations. In this study, we present a rigorous comparative analysis of qubit coherence dynamics using measurement under four relevant configurations: a single-step rectangular pulse without Purcell decay, a single-step pulse subject to Purcell-induced relaxation, a two-step shaped pulse without Purcell effects, and a two-step shaped pulse incorporating Purcell decay. By solving the time-dependent Lindblad master equation for a three-level transmon system, we illustrate how readout pulse shaping and Purcell filtering independently and jointly influence qubit population dynamics, excited-state lifetime, and the final-state fidelity. The results demonstrate that two-step readout pulse shaping significantly improves coherence by suppressing overshoot and reducing higher-level population transfer, while the inclusion of Purcell mitigation further stabilizes the dynamics and enhances fidelity. The most notable observation is that the combined application of pulse shaping and Purcell filtering produces the most stable and highest fidelity qubit state discrimination. This unified perspective constitutes the central novelty of our work. Such a comparative framework, integrating pulse engineering with environmental decay modeling, has received limited explicit treatment in prior literature. The findings provide critical insights for optimizing both readout protocols and quantum hardware design in large scale superconducting quantum systems.
Superconducting circuits are a powerful platform for studying and controlling the interaction between light and matter at the quantum level. In these systems, superconducting qubits act as artificial atoms (with matter-like degrees of freedom), whereas microwave resonators play the role of oscillators hosting photons (with photonic-like degrees of freedom). A major advantage of this platform is the ability to tailor the electromagnetic environment to which the qubits are coupled. This degree of control makes it possible to realize well-known quantum optics models, like the Jaynes-Cummings model, as well as more complex systems, like quantum electrodynamics close to a bandgap. In this thesis, we engineer structured electromagnetic environments and couple superconducting qubits to them to experimentally study their interaction. By coupling multiple microwave resonators together, we create a metamaterial. When an atom interacts with the metamaterial, an atom-photon bound state emerges. We observe the dynamics of the creation and dissolution of the atom-photon bound state, as well as the spectral content of its photonic component, for the first time to our knowledge.Our metamaterial is intrinsically nonlinear because each resonator is formed by an array of Josephson junctions to serve as an inductor. When the system is pumped, its phase diagram exhibits multimode dissipative phase transitions with timescales on the order of hundred seconds. There is ongoing theoretical debate about whether such phase transitions can occur in one-dimensional systems, a question we address and resolve in this work.Furthermore, we engineer qubit decay using two different approaches. In the first one, we engineer the decay of two coupled atoms to two waveguides by using a symmetric–antisymmetric coupling configuration. This approach enables selective coupling to bright and dark states of the system. In the second one, we suppress the Purcell decay of a qubit using a compact lumped-element Purcell filter. In addition, we characterize the Purcell decay by employing an auxiliary superconducting qubit as a sensitive power detector directly coupled to the feedline waveguide used for the qubit readout.In summary, this thesis presents three different methods for controlling light–matter interactions in structured quantum systems, including decay suppression with both Purcell filters and symmetry-selective couplings. The thesis also introduces new experimental techniques for probing atom-photon bound states and provides direct evidence of collective phenomena in driven-dissipative quantum metamaterials.
We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved with a tunable coupling design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasistatic tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single-photon exchange in 6 ns. Qubit coherence times exceeding 20 μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon nonconserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80%.
Parametric modulation, valued for its versatility, is widely employed in superconducting circuits for quantum simulations and high-fidelity two-qubit gates. Conventionally, the qubit coupling strength is determined by the amplitude of the parametric flux pulse, which affects the qubit parameters dramatically. In this paper, we propose and implement a phase-modulation scheme to tune the interaction strength via adjustment of the relative phase between the parametric flux pulses applied to two coupled qubits. We characterize this modulation for sideband couplings, at both sweet and off-sweet spots, achieving a broad range of coupling strengths, as confirmed by both population dynamics and spectroscopy methods. This approach enables phase-controlled modulation of coupling strength, providing a promising candidate for parametrically driven quantum simulations and gate operations.
Josephson junctions are a fundamental building block of superconducting quantum circuits. They provide the nonlinearity required to realize quantum bits and enable a wide range of nonlinear phenomena central to quantum technologies. This thesis explores two superconducting systems in which this nonlinearity plays a central role. In the first part, we investigate a superconducting resonator terminated by a superconducting quantum interference device (SQUID), an arrangement of two Josephson junctions connected in parallel. The SQUID provides a tunable Kerr nonlinearity and allows the resonator to be driven parametrically. Under the combined action of the parametric drive, Kerr nonlinearity, and dissipation to the environment, the state of the resonator can undergo abrupt changes as the pump frequency or amplitude is varied. We interpret these changes within the framework of dissipative phase transitions (DPTs). By implementing a finite-component thermodynamic limit, we experimentally and theoretically investigate both first- and second-order DPTs in this system. We characterize their steady-state and dynamical signatures, including squeezing below vacuum, phase coexistence, hysteresis, spontaneous symmetry breaking, and critical slowing down. Finally, we exploit the enhanced susceptibility near the second-order critical point to implement a frequency-estimation protocol. We show that the estimation precision scales quadratically with the effective system size, demonstrating that finite-component DPTs can provide a quantum advantage in a sensing protocol. In the second part, we investigate a novel architecture for the readout of superconducting qubits. Conventional dispersive readout relies on a transverse interaction between a transmon and a resonator, which can lead to Purcell decay and measurement-induced state transitions, ultimately limiting the readout fidelity. To overcome these limitations, we implement a nonlinear coupling by galvanically connecting the transmon and resonator through a Josephson junction. This interaction provides a strong cross-Kerr coupling for clear state discrimination, but also introduces a detrimental transverse interaction. We experimentally show that the exchange part of the transverse interaction can be suppressed by adding a capacitor in parallel with the coupling junction while preserving the strong cross-Kerr coupling. In addition, we show that the same circuit acts as an intrinsic Purcell filter, protecting the qubit against Purcell decay. The coupling junction also induces a Kerr nonlinearity in the resonator, which we exploit to implement a bifurcation readout with a 99.4 % fidelity in a 68 ns integration time. This performance is achieved without a dedicated Purcell filter or a near-quantum-limited amplifier. Overall, this thesis demonstrates the engineering of nonlinear superconducting circuits to investigate dissipative phase transitions, exploit criticality for enhanced quantum sensing, and develop a novel qubit readout architecture enabling fast, high-fidelity measurement.