The latest breakthroughs in microwave quantum photonics in superconducting circuits are presented. Advancing technologies of Josephson junction quantum bits (qubits) have been applied to control and measurement of itinerant microwave fields. Methods to generate nonclassical microwave states such as single-photon states and squeezed states have been developed. Techniques to characterize those nonclassical itinerant states have also been established. They are ready to be combined to demonstrate various quantum protocols, which extend the possibilities of quantum information processing, quantum metrology, and quantum communications using superconducting circuits.
The latest breakthroughs in microwave quantum photonics in superconducting circuits are presented. Advancing technologies of Josephson junction quantum bits (qubits) have been applied to control and measurement of itinerant microwave fields. Methods to generate nonclassical microwave states such as single-photon states and squeezed states have been developed. Techniques to characterize those nonclassical itinerant states have also been established. They are ready to be combined to demonstrate various quantum protocols, which extend the possibilities of quantum information processing, quantum metrology, and quantum communications using superconducting circuits.
Quantum information processing using superconducting qubit based on Josephson junction has become one of the most promising candidates for possible realization of a quantum computer. In the heart of the qubit circuits, the superconducting microwave resonator plays a key role in quantum operations and measurements, which enables single-photon level microwave quantum optics. During last decade, the coherence time, or the lifetime of the quantum state, of the superconducting qubit has been dramatically improved. Among several technological innovations, the improvement of superconducting microwave resonator's quality has been the main driving force in getting the qubit performance almost ready for elementary quantum computing architecture. In this paper, I will briefly review very recent progresses of the superconducting microwave resonators especially aimed for quantum device applications during the last decade. The progresses have been driven by ingenious circuit design, material improvement, and new measurement techniques. Even a rather radical idea of three-dimensional large resonators have been successfully implemented in a qubit circuit. All those efforts contributed to our understanding of the qubit decoherence mechanism and as a result to the improvement of qubit performance.
Over the last decade and a half, superconducting circuits have advanced to the point where we can generate and detect highly-entangled states, and perform universal quantum gates. Meanwhile, the coherence properties of these systems have improved more than 10,000-fold. I will describe recent experiments, such as the latest advance in coherence using a three-dimensional implementation of qubits interacting with microwave cavities, called “3D circuit QED.” The control and strong interactions possible in superconducting circuits make it possible to generate non-classical states of light, including large superpositions known as “Schrodinger cat” states. This field has many interesting prospects both for applications in quantum information processing, and fundamental investigations of the boundary between the macroscopic classical world and the microscopic world of the quantum.
Quantum optics in superconducting circuits, known as circuit QED, studies the interaction of photons at microwave frequencies with artificial atoms made of Josephson junctions. Although quite young, remarkable progress has been made in this field over the past decade, especially given the interest in building a quantum computer using superconducting circuits. In this thesis based on the appended papers, we look at generation, engineering and detection of microwave photons using superconducting circuits. We do this by taking advantage of the strong coupling, on-chip tunability and huge nonlinearity available in superconducting circuits. First, we present the strong photon-photon interaction measured experimentally, as shown in the giant cross-Kerr effect. In this work, conditional phase shift of about 20 degrees per photon was measured between two coherent fields at single photon level. Given this strong interaction, we propose and analyze a cascaded setup based on the cross-Kerr effect to detect itinerant microwave photons, a long outstanding problem with only recent experimental realizations. We show that a nondestructive detection of microwave photons is possible with few cascaded transmons. The on-chip tunability of Superconducting Quantum Interference Device (SQUID) is exploited to create a tunable superconducting resonator in the next presented experimental work. Finally, we show that by placing the atom at the end of a transmission line, microwave photons can be generated efficiently and on-demand. We also present a setup that can generate the photons in arbitrary wave packets.
A classical switch or router determines which of several inputs to the device passes through the output. In analogy, a quantum switch that coherently links the input to a superposition of several outputs plays an important role in quantum information processing, especially when operating at the fundamental limit where a single photon can be controlled. The controllable path superposition of propagating photonic qubits enabled by the quantum switch provides the basis for numerous quantum applications, including entanglement generation and distribution, enhanced quantum communication, and quantum random access memory. In this work, we realize such a quantum switch for microwave single photons with superconducting quantum circuits. We use a superconducting qubit to coherently switch the output path of single microwave photons and confirm quantum entanglement between the control qubit and the photons' propagation path. We further showcase the generation of quantum entanglement between a time-bin-encoded propagating photonic qubit and a superconducting qubit with the quantum switch, thus providing a versatile approach to generate reconfigurable multinode entanglement in one step. As such, our work provides a prime building block for microwave quantum networks and modular superconducting quantum computing.
Superconducting (SC) circuits can behave like atoms making transitions between a few energy levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip. This talk overviews a few of our theoretical studies on SC circuits and quantum information processing (QIP) including: SC qubits for single photon generation and for lasing; controllable couplings among qubits; how to increase the coherence time of qubits using a capacitor in parallel to one of the qubit junctions; hybrid circuits involving both charge and flux qubits; testing Bell’s inequality in SC circuits; generation of GHZ states; quantum tomography in SC circuits; preparation of macroscopic quantum superposition states of a cavity field via coupling to a SC qubit; generation of nonclassical photon states using a SC qubit in a microcavity; scalable quantum computing with SC qubits; and information processing with SC qubits in a microwave field. Controllable couplings between qubits can be achieved either directly or indirectly. This can be done with and without coupler circuits, and with and without data-buses like EM fields in cavities (e.g., we will describe both the variable-frequency magnetic flux approach and also a generalized double-resonance approach that we introduced). It is also possible to “turn a quantum bug into a feature” by using microscopic defects as qubits, and the macroscopic junction as a controller of it. We have also studied ways to implement radically different approaches to QIP by using “cluster states” in SC circuits.
In this thesis, I demonstrate and evaluate an on-demand source of single propagating microwaves photons. Working in the context of a quantum network, nodes are connected via propagating, nonclassical states of the electromagnetic field. As such, preparing and detecting propagating quantum states is an essential task. I work with one particular node consisting of a microfabricated, effective two level system coupled to a microwave resonator and study its ability to produce propagating nonclassical states, such as single photon states. In principle, states generated by this node could be sent to other such nodes. However, I send them into a Josephson parametric amplifier (JPA) to characterize the source. In particular, I discuss how to design and couple the two components that form my source: a fixed frequency transmon qubit and a 3D superconducting waveguide cavity. I demonstrate the ability to control of the dynamics of this combined system and implement a single photon generation protocol, which utilizes a single microwave control field that is far detuned from the photon emission frequency. To characterize the generation, I perform tomography on the propagating photon state to determine its density matrix ρ. I perform repeated JPA-backed, linear measurements of the propagating state. Based on the histograms of my measurements, I infer a maximum single photon component ρ₁₁ = 0.36 ± 0.01. I characterize the imperfections of the photon generation and detection, including detection inefficiency and measurement backaction. I find that within uncertainty my measurements match my expectation.