The field of quantum computation and simulation has its origins in the early 1980s, when the limitations of the existing paradigm of classical computing machines became apparent. The continuous miniaturization of circuit elements and the increase in their number per unit area will ultimately lead to practical difficulties and to the manifestation of quantum phenomena. This foresight triggered the research work for the invention of new principles of computation. The physical laws of our world are fundamentally quantum mechanical; that is why quantum systems have started to be considered as platforms for possible more powerful computing systems and simulators. Superconducting quantum circuits offer one of the most convenient and promising architectures in this field of research. They are macroscopic and, as a result, provide a better controllability. At the same time they can be produced with customary electronics fabrication methods and they are easily integrable with nowadays electronics. This dissertation contains experimental studies of fully coplanar superconducting quantum circuits comprising a transmon type artificial atom coupled to a quarter-wavelength waveguide resonator. This circuit represents the simplest simulator of light-matter interaction and acts as a testbed for the gate operations needed to control the quantum state in the field of quantum computation. The stimulated Raman adiabatic passage and the shortcut to its adiabaticity were experimentally studied as methods for the efficient population transfer between the ground and the second excited state of the transmon. The possibility of using a hybrid adiabatic-nonadiabatic pulse sequences for preparing an arbitrary quantum three-level state was shown theoretically. The operation of gates based on geometric phases was implemented on the same type of superconducting structure. Finally, the structure was used as a magnetic flux sensor. The magnetic flux resolution of this sensor is enhanced by the use of two properly modified phase estimation algorithms and it is potentially limited only by the Heisenberg uncertainty principle. The superiority of the realized sensor over the standard classical measurement done on the same sample is clearly demonstrated. This experiment indicates the utility of superconducting quantum circuits for the tasks of quantum metrology.
The field of quantum computation and simulation has its origins in the early 1980s, when the limitations of the existing paradigm of classical computing machines became apparent. The continuous miniaturization of circuit elements and the increase in their number per unit area will ultimately lead to practical difficulties and to the manifestation of quantum phenomena. This foresight triggered the research work for the invention of new principles of computation. The physical laws of our world are fundamentally quantum mechanical; that is why quantum systems have started to be considered as platforms for possible more powerful computing systems and simulators. Superconducting quantum circuits offer one of the most convenient and promising architectures in this field of research. They are macroscopic and, as a result, provide a better controllability. At the same time they can be produced with customary electronics fabrication methods and they are easily integrable with nowadays electronics. This dissertation contains experimental studies of fully coplanar superconducting quantum circuits comprising a transmon type artificial atom coupled to a quarter-wavelength waveguide resonator. This circuit represents the simplest simulator of light-matter interaction and acts as a testbed for the gate operations needed to control the quantum state in the field of quantum computation. The stimulated Raman adiabatic passage and the shortcut to its adiabaticity were experimentally studied as methods for the efficient population transfer between the ground and the second excited state of the transmon. The possibility of using a hybrid adiabatic-nonadiabatic pulse sequences for preparing an arbitrary quantum three-level state was shown theoretically. The operation of gates based on geometric phases was implemented on the same type of superconducting structure. Finally, the structure was used as a magnetic flux sensor. The magnetic flux resolution of this sensor is enhanced by the use of two properly modified phase estimation algorithms and it is potentially limited only by the Heisenberg uncertainty principle. The superiority of the realized sensor over the standard classical measurement done on the same sample is clearly demonstrated. This experiment indicates the utility of superconducting quantum circuits for the tasks of quantum metrology.
In circuit quantum electrodynamics the coherence of Cooper pairs in superconductors is employed to create macroscopic electric circuits with quantized energy levels. Such circuits can be coupled with each other and exploited as building blocks of a quantum computer. Accurate and robust control of the quantum state in the circuits is a central condition for the operation of the quantum computer and one of the prerequisites for the implementation of complex algorithms used in quantum information processing, such as quantum error correction. In this thesis adiabatic control in circuit quantum electrodynamics is investigated with the focus on manipulating three-level systems. \n \nIn adiabatic control the eigenstates of the system are slowly modified by changing the external control parameters, which govern the evolution of the system. If the changes in the parameters are slow enough, the state of the system follows the eigenstates in the adiabatic basis, thereby realizing the intended operation. The advantage of adiabatic control is its inherent robustness to small errors or noise in the control parameters; the result of the state manipulation only depends on the asymptotic values of the control parameters, but not on their exact values during the process. \n \nShortcuts to adiabaticity can be used to speed up the otherwise slow adiabatic control by introducing a correction pulse that compensates the diabatic losses during the state manipulation. This allows one to overcome the limitation on the speed of the protocol but simultaneously reduces the method's robustness to the variations in the control parameters. If the level of noise in the control parameters is known, using the shortcut it is possible to find the optimal level of robustness which is required to mitigate the noise. \n \nCircuit quantum electrodynamics offers a perfect experimental platform for investigating quantum control due to the possibility of realizing complicated control schemes using microwave electronics. With commercially available digital-to-analog converters the control signals can be digitally created, which enables accurate and coherent control of the quantum circuit. \n \nIn this thesis both theoretical and experimental results on adiabatic control applied to superconducting transmon circuits are presented. It is shown that stimulated Raman adiabatic passage can be used for population transfer in a three-level transmon, which can be further improved using shortcuts to adiabaticity. Furthermore, a scheme for implementing robust superadiabatic rotation gates in transmon is proposed. Finally, it is demonstrated that superconducting qubit can be used as an ultra-sensitive detector of magnetic flux.
The technology of quantum computing is believed to solve certain computational problems significantly faster than classical computers, enabling classically inaccessible problems. However, the technology is still in its infancy and it is still too early to conclude which physical system(s) will form the basis of tomorrow's quantum computer. Small-scale quantum processors, built on the superconducting transmon qubit, demonstrated already the anticipated quantum speed-up. Despite this tremendous milestone, scaling up to a full-fledged quantum computer is far from trivial: with these qubits, the flux-control causes crosstalk between qubits and heating, the room-temperature microwave control is hardly scalable and comes with high energetic radiation, and most importantly, the loss of quantum information in time leads to computational errors. Recent advances in various hybrid semiconductor materials enabled novel voltage-controlled transmons, gatemons, which are less susceptible to heating and crosstalk. Even more exciting, gatemons can be designed to host Majorana zero modes in a way that renders the qubit inherently protected against decoherence. In order to induce Majorana zero modes in such nanowire systems strong magnetic fields are required. Problematically, magnetic fields destroy the superconductivity that these microwave circuits rely on. In this thesis we integrate three types of hybrid Josephson junctions in magnetic field compatible microwave devices. Chapter 4 demonstrates the first graphene-based transmon. Due to the mono-atomic thickness of graphene in combination with magnetic field resilient coplanar waveguide resonators, we are able to operate the transmon circuit at an in-plane magnetic field of 1 Tesla. Chapter 5 embeds an InAs-Al semiconducting nanowire Josephson junction in a high quality factor superconducting transmission line resonator, demonstrating on-chip microwave generation. The gate-controllable semiconducting platform lends itself for fast pulse control, providing a perspective for the coherent on-chip manipulation of artificial two-level systems, in particular superconducting qubits such as transmons. Chapter 6 continues the development of InAs-Al nanowire transmons. The offset-charge-sensitive regime, additional plunger gates and magnetic field compatibility prepares the platform for the detection of coherent coupling between Majorana zero modes, a phenomena which unfortunately still remains unobserved. Additionally, we realise the first InSb-Al gatemon. The higher spin-orbit coupling makes InSb to be a preferred material in the search for Majorana signatures. Chapter 7 reports on the dynamics of quasiparticle tunneling events in real-time across the InAs-Al nanowire junction in a transmon architecture. The magnetic field compatibility of our device up to 1 Tesla together with in-situ voltage-controlled quasiparticle trap engineering, allows us to measure the survival of the charge-parity lifetime up to strong magnetic fields. A result which is extremely important in the research field of topological quantum computing, where the qubit space is defined in the charge-parity.
Quantum computers promise to solve certain problems such as quantum chemistry simulations much more efficiently than their classical counterparts. Although it is still unclear what material system will ultimately host large-scale quantum computers, solid-state systems are promising candidates due to their inherent scalability and advanced fabrication techniques that can be adapted from comparable technologies. Crucially, a future quantum computer will depend on the quality of its most fundamental building block, the quantum bit, or qubit. Qubits, although ideally insensitive to potential noise, are very susceptible to slight changes in their environment. Therefore, they do not only make the building block for quantum computers, but are also precise sensors. One of the most studied solid-state implementations of a qubit is the transmon, a weakly anharmonic oscillator based on superconducting capacitive and nonlinear inductive elements. Typically, Al-AlOx-Al superconductor-insulator-superconductor Josephson junctions are used for the latter. The interaction of the transmon with the control circuitry, typically superconducting resonators, is described by circuit quantum electrodynamics. In this PhD thesis, a more recently demonstrated type of qubit is further developed and studied in detail using circuit quantum electrodynamics. In these qubits, the Josephson element of the transmon is replaced with indium arsenide nanowires, forming a superconductor-normal metalsuperconductor junction. In addition to the standard flux tunability, these qubits can also be voltage tuned. Due to the compatibility of all the materials used with an applied magnetic field, this type of qubit is a good candidate to be used as a precise and accurate sensor in a magnetic field. The goal of this work is to introduce the in-plane magnetic field as a new tuning knob to the toolbox of circuit quantum electrodynamics. Advances in material science, especially the epitaxial growth of an aluminum shell directly on the indium arsenide nanowire, have enabled the fabrication of nanowire transmons with state-of-the-art coherence. An understanding of their workings in a zero-field environment is important before applying a magnetic field. Thus, we characterize the noise these qubits are subject to (Chapter 4) and find a strong coupling of charge two-level systems to their Josephson energy next to the expected weakly coupled flux and voltage noise. Applying a magnetic field reveals that coherence in these qubits can be observed up to 70 mT, substantially above the superconducting gap of bulk aluminum (Chapter 5). Effects limiting the performance include the thick and fully covering aluminum shell, and the alignment and stability of the magnetic field. The use of different nanowires, the installation of a persistent-current vector solenoid and additional magnetic shielding then enables the operation of voltage- and flux-tunable devices in a magnetic field (Chapter 6). This constitutes a good starting point for circuit quantum electrodynamics experiments in a magnetic field, such as the investigation of the microscopic origin of flux-noise.
Nonlinear superconducting circuits are a leading candidate to implement quantum information tasks beyond the reach of classical processors. One such task, quantum simulation, involves using a controllable quantum system to study the dynamics of another. In this thesis, we present a series of experiments using small systems of superconducting circuits to perform various quantum tasks relevant to quantum simulation.In the first experiment, we design and build a circuit comprising three transmon qubits whose dynamics mirror those of interacting bosonic particles on a lattice, described by the Bose-Hubbard Hamiltonian. We verify the predictions of the Bose-Hubbard model for this system spectroscopically and then use time-domain measurements to study the decoherence processes affecting the qubits. Using a Raman process, we engineer artificial decay dynamics in the system which allow us to prepare and stabilize otherwise inaccessible states of the transmon array.The second experimental demonstration concerns topological quantum matter. Certain quantum systems are characterized by quantities known as topological invariants, which are robust to local perturbations. These topological invariants are challenging to measure in naturally-occurring systems. We engineer an artificial system, comprising a transmon qubit coupled to a high-Q cavity, capable of undergoing a protocol known as the quantum walk, which also exhibits topological invariants. By using particular non-classical state of the cavity, we modify the quantum walk protocol to make the topological invariant directly accessible.Finally, we implement a hybrid quantum-classical algorithm—known as the variational quantum eigensolver—in a two-transmon quantum processor. As a proof of principle demonstration, we compute the ground-state and low-lying excited state energies of the hydrogen molecule as a function of nuclear separation. We show that an extension of the basic algorithm, known as the quantum subspace expansion, allows for the mitigation of errors caused by decoherence processes affecting the quantum processor.
Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer.In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.
Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer.In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.