Abstract A basic element of a quantum network based on two single‐mode waveguides is proposed with different frequencies connected by a solid‐state qubit. Using a simple example of a possible superconducting implementation, the usefulness of the simplifications used in the general theoretical consideration has been justified. The non‐classical field in a single‐mode with a frequency of is fed to the input of a qubit controller and transformed into a non‐classical field in an output single‐mode with a frequency of . The interface can establish a quantum connection between solid‐state and photonic flying qubits with adjustable pulse shapes and carrier frequencies. This allows quantum information to be transferred to other superconducting or atomic‐based quantum registers or chips. The peculiarities of the wave‐qubit interactions are described, showing how they help to control the quantum state of the non‐classical field. On this basis, the operating principles of solid‐state and flying qubits for the future quantum information platforms are considered.
Abstract A basic element of a quantum network based on two single‐mode waveguides is proposed with different frequencies connected by a solid‐state qubit. Using a simple example of a possible superconducting implementation, the usefulness of the simplifications used in the general theoretical consideration has been justified. The non‐classical field in a single‐mode with a frequency of is fed to the input of a qubit controller and transformed into a non‐classical field in an output single‐mode with a frequency of . The interface can establish a quantum connection between solid‐state and photonic flying qubits with adjustable pulse shapes and carrier frequencies. This allows quantum information to be transferred to other superconducting or atomic‐based quantum registers or chips. The peculiarities of the wave‐qubit interactions are described, showing how they help to control the quantum state of the non‐classical field. On this basis, the operating principles of solid‐state and flying qubits for the future quantum information platforms are considered.
Flying qubit is one type of the basic elements for transmitting quantum information in quantum networks. It can serve as a communication link between quantum computer nodes and represents a vital part of secure data transmission. For high-fidelity information transmission, precise control of flying qubit is highly demanded. Here, based on quantized photons or phonons as the carriers of the flying qubit, we review the basics and the progress of the flying-qubit control in both theoretical and experimental aspects. The flying-qubit control system can be regarded as an open quantum system with N energy levels coupled with multiple quantum channels. Its input includes classical components, such as coherent or incoherent control functions, as well as quantum components, but its output only includes quantum components, and the system is affected by environmental noise. According to the characteristics of quantum input and quantum output fields, the flying-qubit control problems can be divided into 3 categories: generation, reception, and conversion. From a theoretical perspective, the different modeling approaches of the flying-qubit control are discussed and compared, including the master equation, the quantum trajectory, the scattering matrix, the quantum stochastic differential equation, and the quantum Langevin equation. Considering both control design schemes and experimental perspectives, various control strategies and experimental progress have been thoroughly discussed and analyzed. The applications and future directions of flying-qubit control are discussed. The realization of flying qubits plays a vital role in application scenarios such as quantum coherent interconnection, the construction of large-scale quantum chips with a large number of bits, and the link between computers and communication networks.
We propose a scheme to realize quantum networking of superconducting qubits based on the optomechanical interface. The superconducting qubits interact with the microwave photons, which then couple to the optical photons through the optomechanical interface. The interface generates a quantum link between superconducting qubits and optical flying qubits with tunable pulse shapes and carrier frequencies, enabling transmission of quantum information to other superconducting or atomic qubits. We show that the scheme works under realistic experimental conditions and it also provides a way for fast initialization of the superconducting qubits under 1 K instead of an operation temperature of 20 mK.
Generating strong interactions between single quanta of light and matter is central to quantum information science, and a key component of quantum computers and long-distance quantum networks. In quantum information processing, these interactions are required to create elementary logic operations (quantum gates) between stationary matter quantum bits (qubits) and photonic qubits that can be transmitted over long distances. Efficient quantum gates between photonic and matter qubits are a crucial enabler for a broad range of applications that include robust quantum networks, nondestructive quantum measurements, and strong photon-photon interactions. So far these qubit-photon gates have been achieved using single atoms and at microwave frequencies in circuit QED systems. Their implementation with solidstate quantum emitters, however, has remained a difficult challenge. We demonstrate that the qubit state of a photon can be controlled by a single solid-state qubit composed of a quantum dot (QD) strongly coupled to an optical nanocavity. We show that the QD acts as a coherently controllable qubit system that conditionally flips the polarization of a photon reflected from the cavity mode on picosecond timescales. This operation implements a controlled NOT (cNOT) logic gate between the QD and the incident photon, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and quantum computations. Our results represent an important step towards an all solid-state implementation of quantum networks and quantum computers, and provide a versatile approach for controlling and probing interactions between a photon and a single quantum emitter on ultra-fast timescales.
Abstract Spanning a range of hardware platforms, the building-blocks of quantum processors are today sufficiently advanced to begin work on scaling-up these systems into complex quantum machines. A key subsystem of all quantum machinery is the interface between the isolated qubits that encode quantum information and the classical control and readout technology needed to operate them. As few-qubit devices are combined to construct larger, fault-tolerant quantum systems in the near future, the quantum-classical interface will pose new challenges that increasingly require approaches from the engineering disciplines in combination with continued fundamental advances in physics, materials and mathematics. This review describes the subsystems comprising the quantum-classical interface from the viewpoint of an engineer, experimental physicist or student wanting to enter the field of solid-state quantum information technology. The fundamental signalling operations of readout and control are reviewed for a variety of qubit platforms, including spin systems, superconducting implementations and future devices based on topological degrees-of-freedom. New engineering opportunities for technology development at the boundary between qubits and their control hardware are identified, transversing electronics to cryogenics.
Quantum communication and quantum computation are anticipated quantum technologies which enable us to tackle problems which are hard or even impossible to solve by classical means. A major challenge in implementing real life applications for quantum communication and quantum computation is to implement efficient interfaces between different kinds of quantum systems which serve as carriers of quantum information, such as single photons and single material quantum systems (i.e. atoms, ions ...). These different kinds of quantum systems have different advantages and disadvantages if it comes to practical applications. Single photons, which may serve as flying qubits, can be transmitted over larger distances without suffering from too much decoherence while single material quantum systems may be used as matter qubits to store and process quantum information locally. For many applications, the particular advantages of both kinds of systems are needed. Hence, we require interfaces between flying qubits and matter qubits. In the first part of this thesis, we develop suitable protocols for implementing interfaces between these flying qubits and matter qubits. The key challenge is to design the interactions to allow the efficient coupling of single photons to single material quantum systems, such as atoms and ions. This is not only of interest for possible practical applications of quantum mechanics but also drives fundamental research, as it is directly connected to a precise control and enhancement of matter-field interactions. The second part of this thesis is concerned with the applications of quantum technologies for simulating complex quantum systems, which cannot be treated by classical means. In many cases, the simulation of complex quantum systems turns out to be a difficult task due to the basic structure of quantum mechanics. The same aspects of quantum mechanics, which allow us to tackle computationally hard problems by using quantum algorithms, create difficulties in understanding the behavior of complex quantum systems. However, the understanding of complex quantum systems is not only of fundamental importance but might also enable us to give answers to problems of practical significance, such as an explanation for high-temperature superconductivity. Here, we develop protocols for implementing such quantum simulations based on ions trapped in state of the art ion traps. The applications of these kinds of quantum simulations are ranging from fundamental research to process design, control, and optimization.
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.