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arXiv:quant-ph/0703147 · Physical Review A

Variable-frequency-controlled coupling in charge qubit circuits: Effects of microwave field on qubit-state readout

Xiao-Ling He, Yu-xi Liu, J. Q. You, Franco Nori

Published Aug 17, 2007• 22 citations
arXivHTMLPDF

Abstract

To implement quantum-information processing, microwave fields are often used to manipulate superconducting qubits. We study how the coupling between superconducting charge qubits can be controlled by variable-frequency magnetic fields. We also study the effects of the microwave fields on the readout of the charge-qubit states. The measurement of the charge-qubit states can be used to demonstrate the statistical properties of photons.

DOI
10.1103/physreva.76.022317

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To implement quantum-information processing, microwave fields are often used to manipulate superconducting qubits. We study how the coupling between superconducting charge qubits can be controlled by variable-frequency magnetic fields. We also study the effects of the microwave fields on the readout of the charge-qubit states. The measurement of the charge-qubit states can be used to demonstrate the statistical properties of photons.
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.
The Josephson qubit based on a superconducting single charge transistor inserted in a low-inductance superconducting loop is considered. The loop is inductively coupled to a radio-frequency driven tank circuit enabling the readout of the qubit states by measuring the effective Josephson inductance of the transistor. The effect of qubit dephasing and relaxation due to electric and magnetic control lines, as well as the measuring system, is evaluated. Recommendations for qubit operation with minimum decoherence are given.
Microwave drives play a central role in the control of superconducting quantum circuits, enabling qubit gates, readout, and parametric interactions. As the drive frequencies are typically an order of magnitude smaller than (twice) the superconducting gap, it is generally assumed that such drives do not disturb the BCS ground state. However, sufficiently strong drives can activate multiphoton pair-breaking processes that generate quasiparticles (QPs) and result in qubit errors. In this work, we present a theoretical framework for calculating the rates of multiphoton-assisted pair-breaking transitions induced by charge- or flux-coupled microwave drives. Through illustrative examples, we show that photon-assisted QP generation may affect novel high-frequency dispersive readout architectures, as well as Floquet-engineered superconducting circuits operating under strong driving.
In this thesis work, the quantum behavior of a superconducting circuit implementing a quantum bit or qubit is studied. With the use of resonant microwave pulses the full manipulation of the quantum state of this system is demonstrated: any rotation of the state vector around the Bloch sphere can be achieved and by using particular NMR inspired sequences of microwave pulses, the robustness of an operation against particular types of errors, like slow fluctuations of the transition frequency of the qubit, can be improved. The phenomenon of decoherence which drives the system from a quantum behavior to a classical one, has been characterized in the free evolution regime. By properly tuning the working point in the external parameter space, the qubit can be decoupled to first order from the fluctuations of its environment, thus enhancing coherence. In these optimal conditions, low frequency charge noise probably of microscopic origin appears to be the limiting source of decoherence for our system. The study of decoherence during driven evolution demonstrated that by suppressing the effect of those fluctuations of the environment slower than that of the driving field, the coherence of the qubit can be improved. Finally a new non-dissipative readout method based on microwave reflectometry has been set up, which improves the sensitivity and could in principle lead to a non-demolition measurement of the qubit state.
In this thesis work, the quantum behavior of a superconducting circuit implementing a quantum bit or qubit is studied. With the use of resonant microwave pulses the full manipulation of the quantum state of this system is demonstrated: any rotation of the state vector around the Bloch sphere can be achieved and by using particular NMR inspired sequences of microwave pulses, the robustness of an operation against particular types of errors, like slow fluctuations of the transition frequency of the qubit, can be improved. The phenomenon of decoherence which drives the system from a quantum behavior to a classical one, has been characterized in the free evolution regime. By properly tuning the working point in the external parameter space, the qubit can be decoupled to first order from the fluctuations of its environment, thus enhancing coherence. In these optimal conditions, low frequency charge noise probably of microscopic origin appears to be the limiting source of decoherence for our system. The study of decoherence during driven evolution demonstrated that by suppressing the effect of those fluctuations of the environment slower than that of the driving field, the coherence of the qubit can be improved. Finally a new non-dissipative readout method based on microwave reflectometry has been set up, which improves the sensitivity and could in principle lead to a non-demolition measurement of the qubit state.
In the circuit quantum electrodynamics architecture, both the resonance frequency and the coupling of superconducting qubits to microwave field modes can be controlled via external electric and magnetic fields to explore qubit-photon dynamics in a wide parameter range. Here, we experimentally demonstrate and analyze a scheme for tuning the coupling between a transmon qubit and a microwave resonator using a single coherent drive tone. We treat the transmon as a three-level system, with the qubit subspace defined by the ground and the second excited states. If the drive frequency matches the difference between the resonator and the qubit frequencies, a Jaynes-Cummings-type interaction is induced, which is tunable in both amplitude and phase. We show that coupling strengths of about 10 MHz can be achieved in our setup, limited only by the anharmonicity of the transmon qubit. This scheme has been successfully used to generate microwave photons with a controlled temporal shape [M. Pechal et al., Phys. Rev. X 4, 041010 (2014)] and can be directly implemented with superconducting quantum devices featuring larger anharmonicity for higher coupling strengths.