Weakly Flux-Tunable Superconducting Qubit
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Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.Keywords:
Transmon
Flux qubit
The flux qubit is often considered as a major design for the future of quantum integrated circuits and its properties have triggered intense interest in the last decade [1-2]. This superconducting circuit behaves as a two-level system, each level being characterized by the direction of a macroscopic permanent current flowing in the loop of the qubit. The permanent current, typically of the order of several hundreds of nAs, generates a large magnetic dipole, which offers interesting prospects for hybrid quantum circuits [3]. However, the flux qubit suffers from limited and irreproducible lifetimes which partially prevent these potential applications. Recently, a novel architecture where qubits are placed in a three dimensional cavity was introduced for transmon qubit [4]. It was shown that coherence properties can be greatly improved.
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Flux qubit
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Longitudinal coupling, which generates entanglement without energy exchange, has extensive applications in quantum computing and quantum simulation. However, achieving available direct and flexible longitudinal couplings between highly coherent superconducting qubits is challenging. In this study, a method is developed to achieve direct and flexible longitudinal couplings between superconducting qubits, including the direct longitudinal coupling between capacitively shunted flux qubits (C‐shunt flux qubits) and that between transmon qubits. Herein, first, a variant of the prototype C‐shunt flux qubit, the concentric C‐shunt flux qubit, is introduced. It is demonstrated that the large mutual inductance between concentric C‐shunt flux qubits produces a longitudinal coupling strength up to 21 MHz. It is also demonstrated that the method can be used to realize a strong longitudinal coupling between two transmon qubits. In the findings, it is demonstrated that it is possible to achieve a flexible and efficient direct longitudinal coupling between superconducting qubits, and open up new possibilities for the development of quantum gate operation methods as well as quantum simulation methods.
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Flux qubit
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With the long coherence time, fixed-frequency transmon qubits are a promising qubit modality for quantum computing. Currently, diverse qubit architectures that utilize fixed-frequency transmon qubits have been demonstrated with high-fidelity gate performance. Nevertheless, the relaxation times of transmon qubits can have large temporal fluctuations, causing instabilities in gate performance. The fluctuations are often believed to be caused by nearly on-resonance couplings with sparse two-level-system (TLS) defects. To mitigate their impact on qubit coherence and gate performance, one direct approach is to tune the qubits away from these TLSs. In this work, to combat the potential TLS-induced performance fluctuations in a tunable-bus architecture unitizing fixed-frequency transmon qubits, we explore the possibility of using an off-resonance microwave drive to effectively tune the qubit frequency through the ac Stark shift while implementing universal gate operations on the microwave-dressed qubit. We show that the qubit frequency can be tuned up to 20 MHz through the ac Stark shift while keeping minimal impacts on the qubit control. Besides passive approaches that aim to remove these TLSs through more careful treatments of device fabrications, this work may offer an active approach towards mitigating the TLS-induced performance fluctuations in fixed-frequency transmon qubit devices.
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We introduce the notion of qubit as unit of quantum information, illustrating how this notion can be implemented in nonlinear superconducting circuits via the charge and current degrees of freedom. Within these two types of qubits, we discuss the charge qubit, the transmon, and the flux qubit, illustrating the nature of the states that implement the qubit subspace and how they can be controlled and measured. We discuss how qubits can interact with each other directly or through mediators, illustrating different limits of interaction, introducing the notion of dipolar electric and magnetic moments, and demonstrating the tunability of interactions by different means. The chapter closes with a brief study of qubit coherence along the history of this field, with an outlook to potential near-term improvements.
Transmon
Flux qubit
Charge qubit
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The flux qubit is often considered as a major design for the future of quantum integrated circuits and its properties have triggered intense interest in the last decade [1-2]. This superconducting circuit behaves as a two-level system, each level being characterized by the direction of a macroscopic permanent current flowing in the loop of the qubit. The permanent current, typically of the order of several hundreds of nAs, generates a large magnetic dipole, which offers interesting prospects for hybrid quantum circuits [3]. However, the flux qubit suffers from limited and irreproducible lifetimes which partially prevent these potential applications. Recently, a novel architecture where qubits are placed in a three dimensional cavity was introduced for transmon qubit [4]. It was shown that coherence properties can be greatly improved.
Transmon
Flux qubit
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We demonstrate rapid, first-order sideband transitions between a superconducting resonator and a frequency-modulated transmon qubit. The qubit contains a substantial asymmetry between its Josephson junctions leading to a linear portion of the energy band near the resonator frequency. The sideband transitions are driven with a magnetic flux signal of a few hundred MHz coupled to the qubit. This modulates the qubit splitting at a frequency near the detuning between the dressed qubit and resonator frequencies, leading to rates up to 85 MHz for exchanging quanta between the qubit and resonator.
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Flux qubit
Charge qubit
Compatible sideband transmission
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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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Flux qubit
Charge qubit
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Superconducting transmon qubits with fixed frequencies are widely used in many applications due to their advantages of better coherence and less control lines compared to the frequency tunable qubits. However, any uncontrolled interactions with the qubits such as the two-level systems could lead to adverse impacts, degrading the qubit coherence and inducing crosstalk. To mitigate the detrimental effect from uncontrolled interactions between qubits and defect modes in fixed-frequency transmon qubits, we propose and demonstrate an active approach using an off-resonance microwave drive to dress the qubit and to induce the ac-Stark shift on the qubit frequency. We show experimentally that the qubit frequency can be tuned well away from the defect mode so that the impact on qubit coherence is greatly reduced while maintaining the universal controls of the qubit initialization, readout, and single-qubit gate operations. Our approach provides an effective way for tuning the qubit frequency and suppressing the detrimental effect from the defect modes that happen to be located close to the qubit frequency.
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Coherence time
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