This thesis discusses a series of experimental and theoretical studies on fast control and decoherence in superconducting quantum circuits. These two subjects are complementary aspects of the problem of improving the gate fidelities in quantum information processing devices based on superconducting circuits. \n \nIn the first part of this thesis, we present experiments on fast control of a superconducting flux qubit using strong driving. We explore driving with short pulses and a driving strength significantly exceeding the transition frequency. The observed dynamics is described in terms of quasienergies and quasienergy states, in agreement with Floquet theory. We observe the role of pulse shaping in the dynamics, as determined by non-adiabatic transitions between Floquet states. In addition, based on adiabatic perturbation theory in the Floquet picture, we propose a control scheme for fast and high-fidelity single-qubit operations with strong driving. These results pave the way to quantum control with a fidelity beyond the threshold of fault-tolerant quantum computation. \n \nIn the second part of this thesis, we discuss experimental methods and present the characterization of decoherence in superconducting resonators and superconducting qubits. Firstly, we present an analysis method on the measurements of photon loss in superconducting resonators. We use this method to characterize the microwave loss of aluminum oxide, an important material in superconducting circuits. The power and temperature dependence of the measured loss is consistent with loss due to two-level defects in amorphous materials. Secondly, we present experimental results on decoherence and noise spectroscopy of a superconducting flux qubit. Using the qubit as a noise spectrometer, we perform detailed measurements on the power spectral density of the flux noise in superconducting circuits over a wide range of frequencies, from 10^-3 to 10^8 Hz, and over a temperature range from 35 to 130 mK. We also present measurements of frequency and temperature dependence of the qubit energy relaxation.
This thesis discusses a series of experimental and theoretical studies on fast control and decoherence in superconducting quantum circuits. These two subjects are complementary aspects of the problem of improving the gate fidelities in quantum information processing devices based on superconducting circuits. \n \nIn the first part of this thesis, we present experiments on fast control of a superconducting flux qubit using strong driving. We explore driving with short pulses and a driving strength significantly exceeding the transition frequency. The observed dynamics is described in terms of quasienergies and quasienergy states, in agreement with Floquet theory. We observe the role of pulse shaping in the dynamics, as determined by non-adiabatic transitions between Floquet states. In addition, based on adiabatic perturbation theory in the Floquet picture, we propose a control scheme for fast and high-fidelity single-qubit operations with strong driving. These results pave the way to quantum control with a fidelity beyond the threshold of fault-tolerant quantum computation. \n \nIn the second part of this thesis, we discuss experimental methods and present the characterization of decoherence in superconducting resonators and superconducting qubits. Firstly, we present an analysis method on the measurements of photon loss in superconducting resonators. We use this method to characterize the microwave loss of aluminum oxide, an important material in superconducting circuits. The power and temperature dependence of the measured loss is consistent with loss due to two-level defects in amorphous materials. Secondly, we present experimental results on decoherence and noise spectroscopy of a superconducting flux qubit. Using the qubit as a noise spectrometer, we perform detailed measurements on the power spectral density of the flux noise in superconducting circuits over a wide range of frequencies, from 10^-3 to 10^8 Hz, and over a temperature range from 35 to 130 mK. We also present measurements of frequency and temperature dependence of the qubit energy relaxation.
This thesis presents circuit quantum electrodynamic (cQED) experiments done with artificial atoms coupled to a high quality factor superconducting microwave resonator. This work discusses the theoretical framework, design, fabrication, implementation and characterization of a multi-qubit system using superconducting persistent current qubits, often referred as flux qubits. \n \nWe developed a new type of cryogenic aid for fast turnaround microwave experiments. The minimum turnaround time to perform microwave experiments in dilution refrigerators is typically a few days, as required by cooling down and warming up the entire refrigerator. The cryogenic microwave payload and wiring needed for each qubit and resonator add a significant heat load to \nthe system which typically only has a few µW s of cooling power at millikelvin temperatures. We present a new experimental approach we developed in which a suitable sample holder is attached to a cold-insertable probe and brought in contact with transmission lines permanently mounted inside the cryostat. The total turnaround time is reduced to 8 hours if the target temperature is 80 mK. This new measurement method increased our ability to successfully identify good devices with fast design-characterization iterations, while allowing months-long measurements at 30 mK. \n \nOur device architecture uses several qubits, each of which is controlled via on-chip fast flux bias lines. Readout of the qubit’s quantum state is performed by homodyne detection of the resonator field at large driving power. We observe long coherence times at the optimal bias point of the flux qubits and present decoherence measurements around this optimal point, where the \nflux qubits couple mostly to magnetic flux noise. Dynamical decoupling techniques employing fast microwave pulses are used to protect the qubit’s coherence and allow calculating the magnetic flux noise power spectral density of the qubit’s environment. Photon-mediated interaction between distant qubits in the resonator allows the implementation of a two-qubit controlled-NOT gate \nusing the selective darkening technique. \n \nThis multi-qubit superconducting design is an interesting platform to study the decoherence properties of flux qubits, the properties of strong interaction of electromagnetic radiation with macroscopic quantum systems. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.
Superconducting devices as quantum bits (qubits) are one of the most promising candidates for implementing a practical quantum computer. Over the last decade, quantum coherence times of superconducting qubits have been continuously improved. Operating qubits in noise insensitive points, eliminating the participation of the lossy materials, and better isolation from environment increased the decoherence times from nanoseconds to tens of microseconds. One recent approach on the qubit design is to shunt the Josephson junctions with large capacitors to reduce the sensitivity to noise and to redistribute the stored electromagnetic energy in a well-engineered area. This approach was first applied on charge qubits and later adopted to other types of superconducting qubits. \n \nIn this thesis, we present the design and the characterization of a three-Josephson junction superconducting circuit with three large shunt capacitors. We present experiments on decoherence in the circuit used as a qubit. The qubit relaxation time T₁ is reaching as high as 47 μs and the spin-echo dephasing time T₂ = 9.4 μs. In addition, we present spectroscopy experiments and decoherence characterization in the qutrit subspace formed by the lowest three energy levels. The spectroscopy data is in excellent agreement with the complete circuit model based on the system capacitance matrix. At the flux symmetry point, the circuit has a large anharmonicity, defined as the difference between the 1-2 and 0-1 transition frequencies, approaching 2π × 3.7 GHz, which is enabling fast single qubit operations. We performed randomized benchmarking with the qubit gates with a duration of 1.62 ns and an average gate fidelity of 99.9%. \n \nMoreover, we present experiments with multi-level control in qutrit subspace, formed by the lowest three energy levels. We demonstrate a single step experimental implementation of a generalized Walsh-Hadamard gate. We use a decomposition of the quantum gate into two unitary operations, one implemented by an off-diagonal Hamiltonian and the other implemented by a diagonal Hamiltonian. The off-diagonal Hamiltonian is obtained by the simultaneous driving of the transitions between levels 0-1, 1-2, and 0-2, with the latter being a two-photon process. The diagonal Hamiltonian is effectively implemented by appropriately shifting the phases of adjacent pulses. The gate is characterized using tomography process and the average fidelity exceeds 90%, in good agreement with numerical simulations that take into account the multi-level structure of the system.
In the past few decades, quantum computation has become an active area of research due to prospects to execute high-efficiency quantum algorithms. Quantum algorithms promise to accelerate many practical computation problems, but they can not be managed on classical computers. While a vast amount of time and effort has been invested into this field, executing a quantum algorithm is still a difficult task. One major obstacle is decoherence, which can be induced by the environment. To better protect information against noises, designs for a quantum computer have quickly improved. However, many new designs lack systematic analysis. \n \nIn the first part of the thesis, we investigate a type of superconducting-circuit-based qubit design proposed recently. The qubit, inspired by the current-mirror effect, is called the "zero-pi qubit". In Chapter 2, we derived an approximated model through concrete mathematical calculations and simulations of the zero-pi qubit. The model is concise and accurately describes the qubit system. Based on the model, we further estimate the decoherence time of the qubit. The dephasing time due to the flux noise and the charge noise exceed 40 s, and the relaxation process is well-protected against flux noise and charge noise. \n \nIn the second part of the thesis, Chapter 3, we explore the qubit control in the strong driving regime. Driving a qubit with a strong signal is an approach to shorten the operation time, thus suppressing the influence of the decoherence. We achieve strong driving condition experimentally and observe complicated qubit dynamics, which is not predicted by any conventional model. We then propose a model based on Floquet theory which accurately describes the dynamics of the qubit.
Superconducting qubits are fabricated "loss-free" electrical circuits on a chip with size features of tens of nanometers. If cooled to cryogenic temperatures below -273 °C they behave as quantum elements, similar to atoms and molecules. Such a qubit can be manipulated by fast-oscillating magnetic fields and its resulting quantum state can be measured using a proper magnetic detector. These qubits could be the basic elements for a quantum processor. To that order, multiple qubits have to controllably interact, and to that purpose very high quality resonators are fabricated on the same chip. Effectively the interaction results from the phase-coherent exchange of a single photon between qubit and resonator. In this way it is highly analogous to the so-called cavity QED in atomic physics where single atoms couple to a single trapped photon, all inside a cavity resonator. In this thesis, a superconducting flux qubit has been coupled to an LC resonator and coherent exchange of a single photon between qubit and resonator has been achieved. A striking option in these superconducting circuits, not attainable in "common" atomic cavity-QED, is that the interaction between qubit and photon can be made very large, up to the limit that one no longer can talk about the qubit and resonator as such. This limit requires a fully different description and it leads to novel effects, e.g. in the energy spectrum of the qubit-resonator composite. The Bloch-Siegert spectral shift is experimentally demonstrated in this thesis. As a small but critical technical aside a new type of control line filter has been developed, which allows strong suppression of excess electrical noise that forms a threat for qubit coherence. Having two modes of operation, it shorts all low-frequency noise if control is idle while being transparent if control is active, where short or open is determined by the control line signal level itself.
Quantum information processing holds the promise of a paradigm shift in computational power for specific problem classes. While contemporary quantum processors operate in the Noisy Intermediate-Scale Quantum (NISQ) regime and remain highly susceptible to errors, ongoing experimental efforts on these near-term devices are crucial for advancing the underlying technology. Among the various hardware implementations, superconducting quantum circuits stand out as one of the most robust architectures for scalable quantum technologies. Nevertheless, the primary bottleneck in these systems is the fragility of quantum information: interactions with the environment inevitably perturb the quantum states, a phenomenon known as decoherence. This rapid loss of stored information imposes a fundamental limit on the time available to reliably execute computations on these devices. Within this context, this thesis focuses on the comprehensive experimental characterization of a planar chip featuring isolated, fixed-frequency superconducting transmon qubits. The measurements were conducted in a cryogenic environment, relying on a dilution refrigerator to maintain the device at millikelvin temperatures. To probe the quantum system under these conditions, custom measurement scripts were developed to operate state-of-the-art microwave control electronics (Keysight Quantum Control System), establishing full control over pulse generation, signal acquisition, and digital demodulation. The experimental workflow systematically mapped the physical properties of the device through a progressive series of characterization steps. Initial spectroscopic techniques probed the coupled resonator-qubit system, identifying its fundamental transition frequencies and relevant energy scales. Building upon these frequency-domain measurements, coherent control was achieved by calibrating resonant microwave pulses to drive single-qubit rotations, enabling the reliable preparation of the qubit in excited or coherent superposition states. Finally, specific time-domain protocols were implemented to evaluate the operational limits of the device. By executing standard pulse sequences, the characteristic decoherence metrics of the system, namely the longitudinal energy relaxation time (T1) and the inhomogeneous dephasing time (T2*), were successfully extracted, providing a quantitative assessment of the qubit's overall performance and establishing a baseline for future implementations of more advanced quantum control protocols.
A double SQUID qubit (Superconducting Quantum Interference Device) can be handled by applying microwave trains, but also by using fast flux pulses. In this second case the manipulation is based on the fast and radical modification of the qubit potential shape that induces non-adiabatic transitions between the computational states (the two lowest energy eigenstates), still avoiding transitions to upper levels. This modality is interesting because it allows faster operations with respect to other techniques, but also because it gives access to interesting nontrivial physical features, concerning in particular decoherence and adiabaticity. About decoherence, we observed experimentally the existence of an “optimal” bias region and the transition between two distinct decoherence regimes. These results can be explained by considering the effect of first and second order slow fluctuations which dominate on high frequency noise contributions. This allows a deep insight in the qubit decoherence mechanisms.