Electrically controlling single-spin qubits in a continuous microwave field
Arne LauchtJuha T. MuhonenFahd A. MohiyaddinRachpon KalraJuan Pablo DehollainSolomon FreerFay E. HudsonMenno VeldhorstRajib RahmanGerhard KlimeckKohei M. ItohDavid N. JamiesonJ. C. McCallumAndrew S. DzurakAndrea Morello
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Large-scale quantum computers must be built upon quantum bits that are both highly coherent and locally controllable. We demonstrate the quantum control of the electron and the nuclear spin of a single (31)P atom in silicon, using a continuous microwave magnetic field together with nanoscale electrostatic gates. The qubits are tuned into resonance with the microwave field by a local change in electric field, which induces a Stark shift of the qubit energies. This method, known as A-gate control, preserves the excellent coherence times and gate fidelities of isolated spins, and can be extended to arbitrarily many qubits without requiring multiple microwave sources.Keywords:
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Flux qubit
Flux qubits are among the first qubits that were ever demonstrated. They have some advantages when compared to capacitively shunted charge qubits, which are now commonly used for building prototypes of quantum processors. Specifically, flux qubits are intrinsically nonlinear systems and they remain so even with low charging energies, which is important for the suppression of large charge noise in solids. In spite of the clear advantages of flux qubits, their applications in multi-qubit devices—prototypes of quantum computers and simulators—are still limited. Flux qubits are also a very powerful tool for fundamental research. In this paper, we discuss the basic properties of flux qubits using the radio frequency superconducting quantum interference device geometry—the most fundamental realization of flux qubits. We also compare and analyze experimental realizations of flux qubits and propose further directions for research.
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Explosive increase of interest in quantum computing has resulted in various proposals for generation of quantum bits or qubits, the basic quantum computing unit. The superconducting qubits of Josephson Junction are the most widely accepted and currently used, while the Ion-trap and a similar molecule-based qubits have been proposed more recently. In these methods, each qubit is generated individually with great effort. Here we proposed a new technique using magnetic resonance imaging (MRI)-based qubit generation, by which multiple qubits can be generated. Simultaneously this provides a complete qubit platform for quantum computing. Central to the proposed method is the simultaneous generation of multiple qubits using the 'gradient' concept together with multiple radiofrequency coils, one for all qubits, and others for individual qubits with each small Q-coil. Another key concept is the time-encoded probability amplitude (TEPA) technique, using individual Q-coils together with the spin-echo series in each qubit incorporating readout gating for time-encoding. This MRI-based qubit-generation and qubit-encoding technique allowed us to develop an entirely new class of quantum computing platform. Our newly proposed MRI-based qubits are well-suited to currently available electronics, superconducting and semiconductor technologies, as well as nuclear magnetic resonance and MRI physics and technology.
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We use the density matrix formalism to analyze the interaction of interferometer-type superconducting qubits with a high quality tank circuit, which frequency is well below the gap frequency of a qubit. We start with the ground state characterization of the superconducting flux and charge qubits. Then, by making use of a dressed state approach, we describe the qubits' spectroscopy when the qubit is irradiated by a microwave field which is tuned to the gap frequency. The last section of the paper is devoted to continuous monitoring of qubit states by using a dc superconducting quantum interference device in the inductive mode.
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We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.
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Ten years ago the first superconducting qubit was demonstrated experimentally [1]. By now quantum computing with superconducting qubits has become a subject of intensive experimental and theoretical research in dozens of groups around the world. The idea of this Special Issue of the journal is to show the status of experimental research in this area after the first decade of work. Most of the best experimental groups working with superconducting qubits (with a few regrettable exceptions) are represented in this Special Issue. We hope that it gives a useful snapshot in time, demonstrating the main experimental achievements and directions of research in superconducting quantum computing. There are many possible physical realizations of qubits [2,3]. Among the candidate systems, the obvious advantages of quantum computing with Josephson junctions are the efficient control of a quantum circuit with voltage/current/microwave pulses and use of a well-developed technology suitable for large scale integration. The fast experimental progress in experiments with superconducting qubits in the last decade confirms the importance of these advantages. Superconducting qubits come in a variety of types, which are often separated into three categories: charge, flux, and phase qubits (though not all groups use this terminology). Single Cooper pair charge of an island carries the quantum information in the charge qubit (e.g., [1,4–16]), while the superconducting phase is the relevant degree of freedom for flux and phase qubits, which differ by the logic state encoding: two quantum levels in different wells of a potential profile are used in the flux qubit (e.g., [17–32]), and two levels in the same well are used in the phase qubit (e.g., [33–42]).
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This thesis presents results of theoretical and experimental work on superconducting persistent-current flux quantum bits. These qubits are promising candidates for the implementation of scalable quantum information processing. This work focuses on the study of one dimensional chains of inductively coupled flux qubits and on qubits with a tunable energy gap. Chains of flux qubits can be used as models of quantum spin chains, one of the most basic systems in many-body physics that has been extensively studied theoretically. The ability to design and tune the qubit and coupling parameters enables exploration of different phase regimes during measurements, in parameter regimes that are not accessible with magnetic materials. The study of the dynamics of quantum waves in an artificial spin chain can also be used to explore novel quantum phenomena with possible applications in quantum computing. Tunability of the minimal energy splitting (the gap) enables one to rapidly bring the flux qubit in and out of resonance with other quantum systems, including a harmonic oscillator. With tunable qubits it also becomes possible to create inter-qubit couplings of different vector nature, using magnetic fluxes. This permits the design of various interaction Hamiltonians for multiple qubit systems. These operations can be performed at the degeneracy point of the qubit, where coherence properties are optimal. Therefore the tunable flux qubit provides an attractive component for the implementation of scalable quantum computation.
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After a brief review on superconducting qubits, we overview our recent studies on controllable couplings in superconducting flux qubits via a variable frequency (ac) magnetic flux. In particular: (i) we describe how to use a variable frequency magnetic flux to control the coupling between two inductively coupled flux qubits, (ii) Based on this approach, using a variable frequency controlled coupling, we present a proposal on how to achieve scalable quantum computing circuits with flux qubits. We also explain why superconducting qubits can be coupled and addressed as trapped ions, as well as describe how to utilize dressed states to couple and decouple qubits and the data bus.
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Abstract : This project has experimentally characterized the coherent quantum nature of the superconducting persistent current qubits which were fabricated in the trilayer niobium technology. The quantum levels of these qubits have been mapped out using both standard microwave frequency spectroscopy as well as a new technique of amplitude spectroscopy. Important to the future implementation of these qubits for quantum computing applications is the demonstration of microwave sideband cooling of the qubits as well as a resonant read-out scheme. In addition to characterizing the quantum nature of a single qubit, this work has also explored the use of Rapid-Single-Flux superconducting circuits to rapidly control the qubit system.
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Thus far, we have examined quantum computing based on single particle states in atoms, ions and semiconductor structures. In this chapter, we will examine quantum states in superconductors and their application as qubits. This chapter is particularly extensive due to the large variety of possible superconducting quantum circuits. We introduce superconductivity and examine the three main types of superconducting qubit: the flux qubit, the charge qubit and the phase qubit. We will also examine the transmon qubit and circuit quantum electrodynamics.
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