Coupling superconducting flux qubits at optimal point via dynamic decoupling with the quantum bus

Wang, Ying-Dan; Kemp, Alexander; Semba, Kouichi

doi:10.1103/physrevb.79.024502

Cited by 41 publications

(35 citation statements)

References 69 publications

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“…In this case, the light-matter coupling strength is comparable to the cavity and the qubit frequencies [25], and in the dipolar approximation, it is described by the quantum Rabi model (QRM) [26,27]. Apart from the fundamental interest of the USC regime, it has been intensively studied for demonstrating novel quantum optics phenomena [28][29][30][31][32], implementing quantum information tasks [33,34], as well as fast quantum computation [35][36][37][38][39] within circuit quantum electrodynamics (QED) [40,41]. The latter provides a promising solid-state architecture for performing quantum computation due to the desirable properties of superconducting qubits, such as long coherence times, and most importantly, its controllability and scalability [42].…”

Section: Introductionmentioning

confidence: 99%

Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED

Wang

Zhang

et al. 2016

Phys. Rev. A

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We present an experimentally feasible scheme to implement holonomic quantum computation in the ultrastrong-coupling regime of light-matter interaction. The large anharmonicity and the Z 2 symmetry of the quantum Rabi model allow us to build an effective three-level Λ-structured artificial atom for quantum computation. The proposed physical implementation includes two gradiometric flux qubits and two microwave resonators where single-qubit gates are realized by a two-tone driving on one physical qubit, and a two-qubit gate is achieved with a time-dependent coupling between the field quadratures of both resonators. Our work paves the way for scalable holonomic quantum computation in ultrastrongly coupled systems.

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Section: Introductionmentioning

confidence: 99%

Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED

Wang

Zhang

et al. 2016

Phys. Rev. A

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“…In this sense, the design of these novel gates becomes a challenge as the rotating-wave approximation (RWA) breaks down and the complexity of the quantum Rabi Hamiltonian emerges [24,25]. Preliminary efforts have been done in this direction involving different configurations of superconducting circuits [26][27][28]. Likewise, in a recent contribution, it has been discussed the possibility of performing protected quantum computing [29].…”

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confidence: 99%

Ultrafast Quantum Gates in Circuit QED

et al. 2012

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We present a method to implement fast two-qubit gates valid for the ultrastrong coupling (USC) and deep strong coupling (DSC) regimes of light-matter interaction, considering state-of-the-art circuit quantum electrodynamics (QED) technology. Our proposal includes a suitable qubit architecture and is based on a four-step sequential displacement of the intracavity field, operating at a time proportional to the inverse of the resonator frequency. Through ab initio calculations, we show that these quantum gates can be performed at subnanosecond time scales, while keeping a fidelity above 99%.Introduction.-With the advent of quantum information science [1], there have been enormous efforts in the design of devices with high level of quantum control and coherence [2]. Circuit QED [3][4][5] has become a leading technology for solidstate based quantum computation and its performance is approaching that of trapped ions [6] and all-optical implementations [7]. Considerable progress has been made in recent circuit QED experiments involving ultrastrong coupling [8,9], two-qubit gate and algorithms [11][12][13][14][15][16], three-qubit gate and entanglement [17][18][19]. Most of the proposed gates are based on slow dispersive interactions or faster resonant gates, and would require operation times of about tens of nanoseconds.To speed up gate operations, the latest circuit-QED technology offers the USC regime of light-matter interactions [8-10, 20, 21], where the coupling strength g is comparable to the resonator frequency ω r (0.1 < ∼ g/ω r < ∼ 1). This should open the possibility to achieve fast gates operating at subnanosecond time scales [22,23] . In this sense, the design of these novel gates becomes a challenge as the rotating-wave approximation (RWA) breaks down and the complexity of the quantum Rabi Hamiltonian emerges [24,25]. Preliminary efforts have been done in this direction involving different configurations of superconducting circuits [26][27][28]. Likewise, in a recent contribution, it has been discussed the possibility of performing protected quantum computing [29].In this Letter, we propose a realistic scheme to realize fast two-qubit controlled phase (CPHASE) gates between two newly designed flux qubits [30], coupled galvanically to a single-mode transmission line resonator (Fig. 1). Our proposal includes: (i) a CPHASE gate protocol operating at times proportional to the inverse of the resonator frequency; (ii) the design of the qubit-resonator system, allowing for high controllability on both the qubit transition frequency and the qubit-resonator coupling, in USC [8,9] and potentially the DSC regime [31] of light-matter interaction. Through ab initio numerical analysis, we discuss the main features of this scheme in detail and show that the fidelity could reach 99.6%. This is an important step in the reduction of resources requirement for fault-tolerant quantum computation [32].Design of a versatile flux qubit.-The junction array is schematically depicted in Fig. 1. It consists of a six-

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“…Using this tuning parameter we can control the coupling to a qubus by tuning into or out of the qubus frequency while operating at the optimal point. Moreover, we can implement strong transverse coupling to another qubit or to a resonator 11,12 …”

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confidence: 99%

Coherent operation of a gap-tunable flux qubit

Zhu

Kemp

Saito

et al. 2010

Applied Physics Letters

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We replace the Josephson junction defining a three-junction flux qubit's properties with a tunable direct current superconducting quantum interference devices (DC-SQUID) in order to tune the qubit gap during the experiment. We observe different gaps as a function of the external magnetic pre-biasing field and the local magnetic field through the DC-SQUID controlled by high-bandwidth on chip control lines. The persistent current and gap behavior correspond to numerical simulation results. We set the sensitivity of the gap on the control lines during the sample design stage. With a tuning range of several GHz on a qubit dynamics timescale, we observe coherent system dynamics at the degeneracy point.PACS numbers: 03.67. Lx,85.25.Hv,85.25.Cp Superconducting flux qubits consisting of three Josephson junctions embedded in a low inductance superconducting loop 1,2 are promising candidates for quantum information processing 3 . With the standard design, the magnetic field inducing an energy difference between the ground state and the first excited state is the only parameter that is adjustable during the experiment. At the degeneracy point, it reaches its minimum value (called the gap ∆), which is related to the quantum mechanical tunnel rate. However, the gap is determined at the time of production by the inherent parameters of the three Josephson junctions. One of the main problems in realizing quantum computation schemes using flux qubits is 1/f flux noise. At the degeneracy point the qubit is decoupled from low frequency flux noise 4,5 , and achieves maximal coherence times of several µs rather than ns at off-degeneracy point, wherefore this is the optimal operation point. Implementing a quantum processor based on flux qubits requires the operation at this optimal point at all times, especially when coupling several qubits. Recently, circuit quantum electrodynamics (QED) (quantum bus, qubus) schemes have been developed for many superconducting qubits 6-10 , however standard flux qubits suffer from the fact that bringing them in resonance with such a quantum bus requires to operate them at the offdegeneracy point.With the flux qubit we present here we aim to overcome these limitations by replacing the smallest junction of the flux qubit with a low inductance DC-SQUID loop 1,2 . Varying the magnetic flux penetrating this loop is equivalent to changing the critical current of the smallest junction, thus making the gap tunable during the experiment. Using this tuning parameter we can control the coupling to a qubus by tuning into or out of the qubus frequency while operating at the optimal point. Moreover, we can implement strong transverse coupling to another qubit or to a resonator 11,12 . a) xbzhu@will.brl.ntt.co.jp b) semba@will.brl.ntt.co.jp Our qubit is shown schematically in Fig. 1. Two identical junctions with Josephson energy E J in series with a symmetric DC-SQUID, in which each junction has a Josephson energy of α 0 E J are enclosed in a superconducting loop. The effective Josephson energy of the DC-SQUID ...

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Coupling superconducting flux qubits at optimal point via dynamic decoupling with the quantum bus

Cited by 41 publications

References 69 publications

Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED

Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED

Ultrafast Quantum Gates in Circuit QED

Coherent operation of a gap-tunable flux qubit

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