Interactions between few-photon microwave fields and superconducting artificial atoms provide a method to study photon statistics and manipulate quantum states for photonic quantum information processing. Superconducting transmons couple to microwave fields through an anharmonic ladder of energy levels, producing a response beyond the two-level description. Here, we propose and analyze a detector based on a flux-tunable transmon qudit in circuit quantum electrodynamics. Within a Kerr-type model, we calculate its population response to classical-field excitation and quantized Fock and coherent states. In the four-photon example, a selected detuning produces periodic, nearly complete transfer to the fourth transmon level for a Fock input, while a coherent input of equal mean energy gives a population of approximately 0.2 at the same interaction time. In the linear regime, we derive a target-population bound for classical inputs. From single-tone and two-tone spectroscopy, analyzed using the cosine transmon Hamiltonian, we extract circuit parameters and resolve higher transitions. Ramsey interferometry characterizes dephasing on the fundamental transition. We propose a detection protocol combining flux-selected interaction with dispersive population readout. Our results establish a population-based framework for microwave-state discrimination using a superconducting artificial atom.
Increasing the efficiency of quantum processors is possible by moving from two-level qubits to elements with a larger computational base. An example would be a transmon-based superconducting atom, but the new basic elements require new approaches to control. To solve the control problem, we propose the use of nonclassical fields in which the number of photons is comparable to the number of levels in the computational basis. Using theoretical analysis, we have shown that (i) our approach makes it possible to efficiently populate on demand even relatively high energy levels of the qudit starting from the ground state; (ii) by changing the difference between the characteristic frequencies of the superconducting atom and a single field mode, we can choose which level to populate; and (iii) even the highest levels can be effectively populated on a sub-nanosecond time scale. We also propose the quantum circuit design of a real superconducting system in which the predicted rapid control of the transmon-based qudit can be demonstrated.
We study the dynamical and spectral characteristics of a quantum three-level ladder system, interacting with a continuous electromagnetic field in one-dimensional open waveguide. Common realization of such systems is a waveguide QED setup - a superconducting artificial atom (transmon), coupled to an open microwave transmission line. We derive an analytical solution for spontaneous emission of initially excited atom, and use it to study the probability of state detection and spectral density of output photon states. We find that for strong coupling of transmon to a waveguide emitted photons show correlation in frequency and can have the same energies, even if the three-level system is anharmonic.
Artificially engineered atoms, built using superconducting electrical circuits, have had a broad impact on the field of quantum information and quantum computing. Based on the Josephson effect, superconducting qubits have provided a robust platform for engineering light-matter interactions at the single-photon level. The ability to precisely control and manipulate single photons using superconducting qubits and cavities, a field now popularly known as circuit quantum electrodynamics (circuit QED), has enabled new and novel regimes in quantum physics, which previously remained inaccessible. For instance, the coupling between individual photons and artificial atoms have been shown to reach the ultrastrong and deep-strong regimes, a feat which is difficult to achieve with natural atoms. The superconducting circuit platform is now a promising contender for building large-scale quantum processors, attracting large investments from academic, industry and government players. \n \nThis thesis uses superconducting circuits to engineer photon interactions in two separate studies. The first is aimed at studying the physics of ``giant" artificial-atoms. The second study explores the route towards building a quantum heat engine using two parametrically-coupled, superconducting microwave cavities. We review the theoretical ideas and concepts which motivate our work, along with discussions of the design methodology, simulations, fabrication, measurement setup and the experimental findings. \n \nIn the first study, we explore a giant artificial atom, formed from a transmon qubit, which is coupled to propagating microwaves at multiple points along an open transmission line. The multipoint coupling nature of the transmon allows its radiated field to interfere with itself leading to some striking ``giant" atom effects. For instance, we observe strong frequency dependent couplings of the transmon's transition levels to its electromagnetic environment, a feature which is not observed with ordinary artificial atoms. We measure large on/off ratios, as high as
380, for the coupling rate of the
ket0−ket1 transition. Furthermore, we show that we can enhance or suppress the coupling rate of the
ket1−ket2 transition relative to the
ket0−ket1 transition, by more than a factor of
200. The relative modulation of the coupling rates was exploited to engineer a metastable state in the giant transmon and demonstrate electromagnetically-induced transparency (EIT), a typical signature of a lambda system. Our results show that we can transform the ladder structure of an ordinary transmon into a more interesting lambda system using a giant transmon, thereby paving the way for exploring new possibilities to study three-level physics in a waveguide-QED setting. \n \nExtending giant atom physics to multiple giant atoms, we then explore a device with two giant artificial atoms connected in a braided configuration to a transmission line. The braided topology of the qubits, offers an interesting regime where the qubits can interact with each other in a decoherence-free environment, where the interaction is mediated by virtual photons in the transmission line. We probe the resonant behavior of the qubits at two different frequency bias points, where we observe qualitatively different scattering behavior. Furthermore, when probing for the Autler-Townes Splitting (ATS), multiple resonances are observed for both resonant and off-resonant cases instead of the familiar doublet in the ATS spectroscopy. This comes as a surprise as the frequency-level spacings in both qubits are nominally identical. We believe these features could be an indication of a novel resonant interaction between the qubits facilitated by the braided topology. An effort to understand this theoretically is underway. \n \nFor our second study, we explore a system with two parametrically-coupled superconducting resonators, which implements an optomechanical-like interaction in an all-electrical network. The nonlinear nature of this interaction is mediated by a superconducting quantum interference device (SQUID), where the current in one resonator couples to the photon number in the other resonator. We propose to use this system to build a ``photonic piston" engine in the quantum regime. We motivate the feasibility of the proposal by reviewing key theoretical results which demonstrate an Otto-cycle by appropriately driving the system with noise. Our experimental findings demonstrate the crucial nonlinear coupling that is required for the engine to work. We also show that we can increase the coupling strength between the resonators depending on the chosen flux operating point.
The realization of a quantum network composed of quantum nodes which process quantum fields and quantum channels to transfer quantum information has recently been proposed. In recent years, fundamental experiments produced by superconducting circuits suggest that they are promising candidates for realization of a quantum network. Superconducting circuit QED can act as quantum nodes, which can be linked by quantum channels, to transfer flying microwave photons (quantum information) from site to site on chip with high fidelity. Therefore, controlling the propagating microwave photons on chip is an essential step towards realization of a quantum network.In this thesis, we demonstrate a specific quantum node, namely the single photon router. The active element of the router is a single three-level artificial atom, a superconducting transmon type qubit, strongly coupled to a superconducting 1D transmission line. Strong coupling between the artificial atom and propagating microwave photons, revealed in high degree scattering of the resonance waves, has been observed. By exploiting the phenomenon of electromagnetically induced transparency (EIT), we can route a single photon signal from an input port to either of two output ports with an on-off ratio of approximately 90%. The switching time of the router is shown to be a few nanoseconds, consistent with theoretical expectations and the device parameters. Besides the router, we also observed some fundamental phenomenon of single atom, such as strong nonlinearity, anomalous dispersion and the Mollow Triplet.This thesis describes the motivation, theoretical background, design, implementation and the measurement results.
Abstract We propose a theoretical scheme to implement non-leaky population transfer in a transmon-regime artificial atom. Due to the weak level anharmonicity of the transmon system, resonant driving leads to considerable quantum leakage. Facilitated by the level-transition rule, we develop a Λ-type interaction between the first three levels and the driving fields. Under two-photon resonance with large detuning, a reduced two-level system is obtained, with which we perform non-leaky population transfer by adjusting the microwave driving. With the accessible parameters, we further analyse the flexible reversibility and high robustness of the protocol. The present scheme provides a potential approach to optimizing population transfer with transmon artificial atoms.