Superconducting qubits are one of the most mature approaches to realize quantum computers. Unfortunately, existing design approaches use full-wave computational electro-magnetics methods to model these devices in inefficient ways that become impractical if even a few qubits are considered. This work uses a field-based formalism to derive a simple formula for the exchange coupling rate between multiple qubits, which is a key design parameter for creating large-scale quantum architectures. This approach is relevant for the common situation of transmon qubits dispersively coupled via an electromagnetic resonator or environment. Then the accuracy of this approach is validated by comparing it to existing modeling approaches on a realistic superconducting circuit topology.
Superconducting qubits are one of the most mature approaches to realize quantum computers. Unfortunately, existing design approaches use full-wave computational electro-magnetics methods to model these devices in inefficient ways that become impractical if even a few qubits are considered. This work uses a field-based formalism to derive a simple formula for the exchange coupling rate between multiple qubits, which is a key design parameter for creating large-scale quantum architectures. This approach is relevant for the common situation of transmon qubits dispersively coupled via an electromagnetic resonator or environment. Then the accuracy of this approach is validated by comparing it to existing modeling approaches on a realistic superconducting circuit topology.
Superconducting qubits are one of the most mature platforms for achieving practical quantum computers, but significant performance improvements are still needed. To improve the engineering of these systems, three-dimensional (3D) full-wave computational electromagnetics analyses are increasingly being turned to. Unfortunately, existing analysis approaches often rely on full-wave simulations using eigenmode solvers that are typically cumbersome, not robust, and computationally prohibitive if devices with more than a few qubits are to be analyzed. To improve the characterization of superconducting circuits while circumventing these drawbacks, this work begins the development of an alternative modeling framework that we illustrate in the context of evaluating the qubit-qubit exchange coupling rate between transmon qubits. This quantity is a key design parameter that determines the entanglement rate for fast multiqubit gate performance and also affects decoherence sources like qubit crosstalk. Our modeling framework uses a field-based formalism in the context of macroscopic quantum electrodynamics, which we use to show that the qubit-qubit exchange coupling rate can be related to the electromagnetic dyadic Green's function linking the qubits together. We further show how the needed quantity involving the dyadic Green's function can be related to the impedance response of the system that can be easily and efficiently computed with classical computational electromagnetics tools. We demonstrate the validity and efficacy of this approach by simulating four practical multiqubit superconducting circuits and evaluating their qubit-qubit exchange coupling rates. We validate our results against a 3D numerical diagonalization method and against experimental data where available. We also demonstrate the impact of the qubit-qubit exchange coupling rate on qubit crosstalk by simulating a multicoupler device and identifying operating points where the qubit crosstalk becomes zero.
Superconducting qubits have emerged as a leading platform for realizing quantum computers. Accurate modeling of these devices is essential for predicting performance, improving design, and optimizing control. Many modeling approaches currently rely on lumped circuit approximations or other simplified treatments that can be limited in resolving the interplay between the qubit dynamics and the electromagnetic circuitry, leading to significant experimental deviations from numerical predictions at times. To address many of these limitations, methods that self-consistently solve the Schrödinger equation for qubit dynamics with the classical Maxwell's equations have been developed and shown to accurately predict a wide range of effects related to superconducting qubit control and readout. Despite these successes, these methods have not been able to consider multi-qubit effects that give rise to qubit-qubit entanglement. Here, we address this by rigorously deriving how multi-qubit coupling effects between transmon qubits can be embedded into Maxwell-Schrödinger methods. To support this, we build on earlier first-principles derivations of Maxwell-Schrödinger methods for the specific case of two transmon qubits coupled together through a common electromagnetic system in the dispersive regime. To aid in validating aspects of the Maxwell-Schrödinger framework, we also provide a new interpretation of Maxwell-Schrödinger methods as an efficient simulation strategy to capture the class of non-Markovian open quantum system dynamics. Our results demonstrate that these effects can give rise to strong classical crosstalk that can significantly alter multi-qubit dynamics, which we demonstrate for the cross-resonance gate. These classical crosstalk effects have been noted in cross-resonance experiments, but previous quantum theory and device analysis could not explain their origin.
Superconducting qubits are one of the leading platforms for creating practical quantum computers. However, these architectures face significant engineering challenges as they scale up and move towards devices capable of error correction. To aid these efforts, three-dimensional (3D) full-wave computational electromagnetics tools are increasingly being utilized to characterize these devices; however, standard approaches relying on eigenmode solvers are often computationally prohibitive and lack the scalability required to analyse devices beyond a few qubits. To overcome these limitations, this thesis introduces an efficient, alternative modeling framework for superconducting circuits, applied specifically to the evaluation of the qubit-qubit exchange coupling rate in transmon architectures. This coupling rate is a key design parameter that governs the entanglement speed for multi-qubit gates and influences error channels such as qubit crosstalk. Leveraging a field-based formalism within the broader context of macroscopic quantum electrodynamics, we establish that the exchange coupling rate is directly related to the electromagnetic dyadic Green’s function. We then map the imaginary part of the Green’s function to the system’s classical impedance response. Since the impedance response can be evaluated quickly and accurately in standard full-wave solvers, this new formulation provides a scalable way to characterize large-scale superconducting devices. We validate this framework by simulating four practical multi-qubit circuits, benchmarking the extracted coupling rates against both 3D numerical diagonalization methods and experimental data. We also demonstrate how the exchange coupling rate impacts qubit crosstalk, and characterize a multi-coupler device to find operational sweet spots, where the crosstalk is minimized while maintaining strong enough qubit-qubit coupling to enable fast multi-qubit gates. We then conclude this thesis and outline some plans for future work.
The spontaneous emission rate (SER) is an important figure of merit for any quantum bit (qubit), as it can play a significant role in the control and decoherence of the qubit. As a result, accurately characterizing the SER for practical devices is an important step in the design of quantum information processing devices. Here, we specifically focus on the experimentally popular platform of a transmon qubit, which is a kind of superconducting circuit qubit. Despite the importance of understanding the SER of these qubits, it is often determined using approximate circuit models or is inferred from measurements on a fabricated device. To improve the accuracy of predictions in the design process, it is better to use full-wave numerical methods that can make a minimal number of approximations in the description of practical systems. In this work, we show how this can be done with a recently developed field-based description of transmon qubits coupled to an electromagnetic environment. We validate our model by computing the SER for devices similar to those found in the literature that have been well-characterized experimentally. We further cross-validate our results by comparing them to simplified lumped element circuit and transmission line models as appropriate.
Understanding the spontaneous emission rate of transmon qubits is an important step in the design of many quantum electromagnetic devices, such as single photon sources and quantum computers. Despite its importance, the spontaneous emission rate is often determined using approximate circuit models or is inferred from measurements on a fabricated device. To improve the accuracy of predictions, it is better to use full-wave numerical methods for this problem. In this work, we show how this can be done with a recently developed field-based description of transmon qubits coupled to an electromagnetic environment. We demonstrate this process by computing the spontaneous emission rate of a single photon source and find qualitative agreement with a similar fabricated device.
Superconducting circuits consisting of a few low-anharmonic transmons coupled to readout and bus resonators can perform basic quantum computations. Since the number of qubits in such circuits is limited to not more than a few tens, the qubits can be designed to operate within the dispersive regime where frequency detunings are much stronger than coupling strengths. However, scaling up the number of qubits will bring the circuit out of the regime, and this invalidates current theories. We develop a formalism that allows to consistently diagonalize the superconducting circuit Hamiltonian beyond the dispersive regime. This will allow to study qubit-qubit interaction unperturbatively, therefore, our formalism remains valid and accurate at small or even negligible frequency detuning; thus, our formalism serves as a theoretical ground for designing qubit characteristics for scaling up the number of qubits in superconducting circuits. We study the most important circuits with single- and two-qubit gates, i.e., a single transmon coupled to a resonator and two transmons sharing a bus resonator. Surprisingly, our formalism allows to determine the circuit characteristics, such as dressed frequencies and Kerr couplings, in closed-form formulas that not only reproduce perturbative results, but also extrapolate beyond the dispersive regime and can ultimately reproduce (and even modify) the Jaynes-Cumming results at resonant frequencies.