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Quantum 2.0 Conference and Exhibition

Tuning Inter-qubit Coupling Strengths by Sideband Driving

Wei-Chen Chien, C. T. Lin, Tse-Yu Lai, Watson Kuo, Yen-Chun Chen, Chii‐Dong Chen

Published Jan 1, 2024• 0 citations
Publisher

Abstract

The iSWAP operation between two qubits serves as a means to quantify the inter-qubit coupling strength. We examine the inter-qubit coupling strength when the qubit frequency is modulated at 80 MHz. The coupling strength between qubits, mediated by various sidebands generated from the frequency modulation, can be dynamically tuned, following the Bessel dependencies. This dynamically tunable coupling between qubits provides a straightforward approach to create a testing ground for versatile quantum models.

DOI
10.1364/quantum.2024.qtu3a.11

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The iSWAP operation between two qubits serves as a means to quantify the inter-qubit coupling strength. We examine the inter-qubit coupling strength when the qubit frequency is modulated at 80 MHz. The coupling strength between qubits, mediated by various sidebands generated from the frequency modulation, can be dynamically tuned, following the Bessel dependencies. This dynamically tunable coupling between qubits provides a straightforward approach to create a testing ground for versatile quantum models.
We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved with a tunable coupling design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasistatic tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single-photon exchange in 6 ns. Qubit coherence times exceeding 20 μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon nonconserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80%.
Parametric modulation, valued for its versatility, is widely employed in superconducting circuits for quantum simulations and high-fidelity two-qubit gates. Conventionally, the qubit coupling strength is determined by the amplitude of the parametric flux pulse, which affects the qubit parameters dramatically. In this paper, we propose and implement a phase-modulation scheme to tune the interaction strength via adjustment of the relative phase between the parametric flux pulses applied to two coupled qubits. We characterize this modulation for sideband couplings, at both sweet and off-sweet spots, achieving a broad range of coupling strengths, as confirmed by both population dynamics and spectroscopy methods. This approach enables phase-controlled modulation of coupling strength, providing a promising candidate for parametrically driven quantum simulations and gate operations.
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.
Abstract Fabrication-friendly superconducting qubits continue to be a leading candidate for scalable quantum computing. Recent developments in tunable couplers have significantly advanced the progress toward practical quantum processors. However, high-performance quantum control, particularly two-qubit gates, depends on the delicate tuning of the coupler spectrum, as misalignment can lead to undesirable phenomena such as frequency crowding, which may cause errors including state leakage. Here, we propose an efficient method for characterizing the coupler spectrum through sideband drivings, obviating the need for additional components in current quantum processors. We demonstrate this technique experimentally by employing both continuous-wave and pulsed measurement protocols, successfully extracting the coupler spectrum. Furthermore, by utilizing the measured coupler spectrum, we calibrate the frequency dependence of the effective coupling strength between two qubits linked by the coupler. The proposed approach offers significant practical benefits, enabling the efficient characterization of the coupler spectrum in existing quantum architectures, thus paving the way for enhanced quantum control and scalability.
Sideband transitions have been shown to generate controllable interaction between superconducting qubits and microwave resonators. Up to now, these transitions have been implemented with voltage drives on the qubit or the resonator, with the significant disadvantage that such implementations only lead to second-order sideband transitions. Here we propose an approach to achieve first-order sideband transitions by relying on controlled oscillations of the qubit frequency using a flux-bias line. Not only can first-order transitions be significantly faster, but the same technique can be employed to implement other tunable qubit-resonator and qubit-qubit interactions. We discuss in detail how such first-order sideband transitions can be used to implement a high fidelity controlled-not operation between two transmons coupled to the same resonator.
We consider a pair of quantum dot-based spin qubits that interact via microwave photons in a superconducting cavity and that are also parametrically driven by separate external electric fields. For this system, we formulate a model for spin qubit entanglement in the presence of mutually off-resonant qubit and cavity frequencies. We show that the sidebands generated via the driving fields enable highly tunable qubit-qubit entanglement using only ac control and without requiring the qubit and cavity frequencies to be tuned into simultaneous resonance. The model we derive can be mapped to a variety of qubit types, including detuning-driven one-electron spin qubits in double quantum dots and three-electron resonant exchange qubits in triple quantum dots. The high degree of nonlinearity inherent in spin qubits renders these systems particularly favorable for parametric drive-activated entanglement. We determine multiple common resonance conditions for the two driven qubits and the cavity and identify experimentally relevant parameter regimes that enable the implementation of entangling gates with suppressed sensitivity to cavity photon occupation and decay. The parametrically driven sideband resonance approach that we describe provides a promising route toward scalability and modularity in spin-based quantum information processing through drive-enabled tunability that can also be implemented in micromagnet-free electron and hole systems for spin-photon coupling. Published by the American Physical Society 2024