The hybrid system of electron spins and resonator photons is an attractive architecture for quantum computing owing to the long coherence times of spins and the promise of long-distance coupling between arbitrary pairs of qubits via photons. For the device to serve as a building block for a quantum processer, it is also necessary to readout the spin qubit state. Here we analyze in detail the measurement process of an electron spin singlet-triplet qubit in quantum dots using a coupled superconducting resonator. We show that the states of the spin singlet-triplet qubit lead to readily observable features in the spectrum of a microwave field through the resonator. These features provide useful information on the hybrid system. Moreover, we discuss the working points which can be implemented with high performance in the current state-of-the-art devices. These results can be used to construct the high fidelity measurement toolbox in the spin-circuit QED system.
The hybrid system of electron spins and resonator photons is an attractive architecture for quantum computing owing to the long coherence times of spins and the promise of long-distance coupling between arbitrary pairs of qubits via photons. For the device to serve as a building block for a quantum processer, it is also necessary to readout the spin qubit state. Here we analyze in detail the measurement process of an electron spin singlet-triplet qubit in quantum dots using a coupled superconducting resonator. We show that the states of the spin singlet-triplet qubit lead to readily observable features in the spectrum of a microwave field through the resonator. These features provide useful information on the hybrid system. Moreover, we discuss the working points which can be implemented with high performance in the current state-of-the-art devices. These results can be used to construct the high fidelity measurement toolbox in the spin-circuit QED system.
Electron spins in semiconductor quantum dots are a promising platform for qubits. An essential step toward quantum error correction with spin qubits requires the ability to read out states and identify errors in the two-spin logical basis in a fast and high-fidelity way. Here we propose a scheme for spin singlet-triplet readout in a double quantum dot, with a superconducting resonator serving as the detector. Making use of a spin driving pulse sequence, this procedure can read out the spin states with a fidelity of 99.9 in a time 100 ns. Interestingly, this method makes use of the spin driving pulse to adjust the resonator signal field, leading to significant improvement of the spin readout. In addition, this method can be directly applied to multiplexed readout of qubits and repeated readout to identify logical errors. These results pave the way for the implementation of quantum error correction and scalable quantum processor with spin qubits.
Quantum computing is a powerful way of improving computing efficiency. For some specific problems, quantum processor can exponentially reduce computation time compared to the classical computer. Cavity quantum electrodynamics, which studies the coherent interaction between photons in an optical cavity and atoms or other two-level systems, is of enormous significance to quantum computing and quantum information processing. The strong coupling regime where a cavity photon mode and an atom exchange a single quantum energy coherently has attracted extensive interest in varied experimental platforms and systems such as alkali atoms, Rydberg atoms, large spin ensembles, magnons, etc. The semiconductor gate-defined double quantum dot (DQD) system is one of the most promising platforms that possesses advantages of advanced micro-nano processing technology and outstanding scalability. Introducing the concept of cavity QED into superconducting circuits (circuit QED, cQED), provides an opportunity to study interaction between electrons confined in semiconductor DQD and photons trapped in a superconducting resonator. Reaching strong coupling regime in this hybrid system paves the way to large-scale network of quantum computing. Electrons confined in DQD have large electrical dipoles, thus are able to couple to photons with relatively large coupling rate. Whereas, due to the charge noise and other dissipation channels in the hybrid system, the decoherence rate of the system is larger than the coupling rate, leaving the strong coupling a great challenge. In recent years, different methods have been proposed to access the strong coupling regime. Taking advantage of a high-quality heterojunction structure, an overlapping gate architecture and a low-pass LC filter on the DC bias line, the decoherence of the charge qubit and microwave leakage of the resonator can be effectively suppressed. The coupling strength of the hybrid system can be improved utilizing a high impedance resonator which is comprised of high kinetic inductance materials. Spin, which does not interact directly with the electric field, is insensitive to charge noise and has much longer decoherence time than charge qubit. However, the weak interaction results in a small coupling rate between the spin and the photon. To solve this problem, the method of coherent hybridization between the spin and the charge state is proposed to improve the coupling strength. This hybridization can be achieved by applying a non-uniform magnetic field, utilizing spin-orbit interaction or spin exchange interaction. At present, both charge qubits and spin qubits have achieved strong coupling with the cavity and the coupling distance between long-range qubits is greatly expanded as long as millimeters. This review introduces basic ideas about cQED and fundamental elements in semiconductor cQED systems. Recent progress in strong coupling hybrid system and methods for improving coupling structures are discussed. In future research, more efforts will be devoted into long-range transfer of qubit states and photon-mediated two or multi-qubit gates.
Coupling qubits to a superconducting resonator provides a mechanism to enable long-distance entangling operations in a quantum computer based on spins in semiconducting materials. Here, we demonstrate a controllable spin-photon coupling based on a longitudinal interaction between a spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a high-impedance superconducting resonator can produce the desired longitudinal coupling when the qubit is driven near the resonator's frequency. We measure the energy splitting of the qubit as a function of the drive amplitude and frequency of a microwave signal applied near the resonator antinode, revealing pronounced effects close to the resonator frequency due to longitudinal coupling. By tuning the amplitude of the drive, we reach a regime with longitudinal coupling exceeding 1 MHz. This mechanism for qubit-resonator coupling represents a stepping stone towards producing high-fidelity two-qubit gates mediated by a superconducting resonator.
One of the key elements in circuit quantum electrodynamics (cQED) is the superconducting microwave resonator, which can enable strong coupling to and between nanoscale quantum systems, and as a result has emerged as an essential tool for both quantum information and quantum sensing. In this thesis, I will discuss new applications of superconducting resonators in both of these fields, which illustrates how quantum information and quantum sensing can be strongly tied together. As the field of quantum information continues to grow, various platforms for realizing a quantum computer have been proposed. One attractive platform is to control and manipulate the spin of an electron to form spin qubits, which have demonstrated a long coherence time, weak coupling to the environment, and high fidelity single- and two-qubit gates. However, scaling beyond two qubits has proven challenging, as has incorporating long-range coupling, although both are crucial for making spin qubits a scalable quantum computing platform. The confinement of electromagnetic fields in superconducting resonators enables strong coupling to small systems, such as the single electrons of a spin qubit. In the first part of this thesis, I will describe how a singlet-triplet spin qubit coupled to a high-impedance superconducting resonator constitutes a promising architecture for mediating long-ranged interactions based on a longitudinal coupling between the resonator and qubit. I will discuss the technical developments needed to successfully implement this novel type of interaction in a hybrid spin qubit-resonator device. This type of interaction has several advantages over the more commonly studied transverse coupling scheme: it does not require careful tuning of the qubit frequency to match that of the resonator, which facilitates scaling to a large number of qubits, and also avoids the loss of coherence, which is inevitable in resonant coupling. This type of interaction therefore opens promising paths for generating high-fidelity long-ranged two-qubit coupling by employing superconducting microwave resonators. By adapting techniques used in quantum information, we have also developed a new technique in quantum sensing, where we employ superconducting resonators as a probe of quantum materials such as mesoscopic systems, two-dimensional materials, and heterostructures, which are otherwise challenging to experimentally access with conventional techniques. In the second part of my thesis, I will discuss how implementing cQED techniques from quantum information allows us to probe the symmetry of the order parameter in unconventional superconductors. Unconventional superconductors are not readily available, and we therefore study an engineered system that comprises of interfacing a ferromagnet and superconductor, in which unconventional pairing can be induced into the ferromagnet. This platform allows us to simultaneously excite spin waves, a collective excitation of spins, in the ferromagnet, which opens new directions for studying interactions between coherently coupled magnons (quanta of spin waves) and unconventional superconducting states. Finally, by applying the same technique to a two-dimensional (2D) superconductor, tungsten ditelluride, we leverage this platform in an attempt to study the order parameter of its intrinsic superconducting phase. The nature of the order parameter in this material remains an open question, as both topological and superconducting phases can exist, possibly simultaneously. This work thus demonstrates a unique use of superconducting resonators beyond quantum information applications.
The spin qubit in quantum dots is one of the leading platforms for quantum computation. A crucial requirement for scalable quantum information processing is the high efficient measurement. Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator. Especially, the phase shift of the resonator is sensitive to the spin states and the gate operations. The response of the resonator can be used to measure the spin qubit efficiently, which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.
Electron spin qubits in silicon quantum dots are an attractive candidate for large-scalable quantum computation. An essential step for quantum information processing based on spin qubits is to realize the spatially separated two-qubit gate and entanglement with high fidelity. Here, we consider two spin qubits coupled to a common superconducting resonator in circuit quantum electrodynamics. We investigate the long-range two-qubit iSWAP gate mediated by virtual microwave photons using a Gaussian smoothing pulse. We show that the entangling gate fidelity can reach [Formula: see text] under realistic experimental conditions and analyze the factors limiting gate fidelity. Moreover, we numerically demonstrate the generation of remote Bell entangled states of spin qubits with high fidelity. In addition, this spin–resonator architecture can be used to implement quantum algorithms using our scheme. These results pave the way for quantum information processing with spin qubits.