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arXiv:2409.08915 · Physical Review Applied

Remote entangling gates for spin qubits in quantum dots using a charge-sensitive superconducting coupler

Harry Hanlim Kang, Ilan T. Rosen, Max Hays, Jeffrey A. Grover, William D. Oliver

Published Apr 24, 2025• 3 citations
arXivHTMLPDF

Abstract

We propose a method to realize microwave-activated cz gates between two remote spin qubits in quantum dots using a charge-sensitive superconducting coupler. The qubits are longitudinally coupled to the coupler, such that the transition frequency of the coupler depends on the logical qubit states; a capacitive network model using first-quantized charge operators is developed to illustrate this. Driving the coupler transition then implements a conditional phase shift on the qubits. Two pulsing schemes are investigated: a rapid, off-resonant pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. We develop non-Markovian time-domain simulations to accurately model gate performance in the presence of $1/{f}^{\ensuremath{\beta}}$ charge noise. Simulation results indicate that a cz gate fidelity exceeding 90% is possible with realistic parameters and noise models.

DOI
10.1103/physrevapplied.23.044055

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We propose a method to realize microwave-activated cz gates between two remote spin qubits in quantum dots using a charge-sensitive superconducting coupler. The qubits are longitudinally coupled to the coupler, such that the transition frequency of the coupler depends on the logical qubit states; a capacitive network model using first-quantized charge operators is developed to illustrate this. Driving the coupler transition then implements a conditional phase shift on the qubits. Two pulsing schemes are investigated: a rapid, off-resonant pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. We develop non-Markovian time-domain simulations to accurately model gate performance in the presence of 1/f\ensuremathβ1/{f}^{\ensuremath{\beta}}1/f\ensuremathβ charge noise. Simulation results indicate that a cz gate fidelity exceeding 90% is possible with realistic parameters and noise models.
Circuit QED elements can facilitate entanglement generation between distant spin qubits, acting as a quantum bus to connect different parts of future spin-based quantum processors. Here, microwave-activated phase gates between two remote, gate-defined spin qubits, where an offset-charge-sensitive (OCS) transmon coupler is driven, are proposed. The spin qubits, specifically resonant-exchange (RX) qubits, are longitudinally coupled to the coupler such that the coupler’s frequency depends on the logical states of the qubits. The capacitance network model is used to describe the coupling between the quantum dots and the coupler, using first-quantized charge operators instead of second quantized field operators. Two pulse sequences are investigated: a faster, off-resonant drive pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. The main source of decoherence for the system is the 1/f ᵝ charge noise, with β ∼ 1. The non-Markovian time-domain simulations in the presence of charge noise are employed to accurately predict the gate performance. Simulation results suggest that a CZ gate fidelity exceeding 90% is possible with realistic parameters and noise models.
Semiconductor double quantum dot (DQD) qubits coupled via superconducting microwave resonators provide a powerful means of long-range manipulation of the qubits' spin and charge degrees of freedom. Quantum gates can be implemented by parametrically driving the qubits while their transition frequencies are detuned from the resonator frequency. Long-range two-qubit CZ gates have been proposed for the DQD spin qubit within the rotating-wave approximation (RWA). Rapid gates demand strong coupling, but RWA breaks down when coupling strengths become significant relative to system frequencies. Therefore, understanding the detrimental impact of time-dependent terms ignored by RWA is critical for high-fidelity operation. Here, we go beyond RWA to study CZ gate fidelity for both DQD spin and charge qubits. We propose a novel parametric drive on the charge qubit that produces fewer time-dependent terms and show that it outperforms its spin counterpart. We find that drive amplitude - a parameter dropped in RWA - is critical for optimizing fidelity and map out high-fidelity regimes. Our results demonstrate the necessity of going beyond RWA in understanding how long-range gates can be realized in DQD qubits, with charge qubits offering considerable advantages in high-fidelity operation.
Semiconductor double quantum dot (DQD) qubits coupled via superconducting microwave resonators provide a powerful means of long-range manipulation of the qubits' spin and charge degrees of freedom. Quantum gates can be implemented by parametrically driving the qubits while their transition frequencies are detuned from the resonator frequency. Long-range two-qubit controlled-Z (CZ) gates have been proposed for the DQD spin qubit within the rotating-wave approximation (RWA). Rapid gates demand strong coupling, but RWA breaks down when coupling strengths become significant relative to system frequencies. Therefore, understanding the errors arising from approximations used is critical for high-fidelity operation. Here, we go beyond RWA to study CZ gate fidelity for both DQD spin and charge qubits. We propose a novel parametric drive on the charge qubit that produces smaller errors and show that the fidelity of the CZ gate outperforms its spin counterpart, resulting in a much smaller fidelity loss of 0.05%0.05\%0.05% compared to 0.80%0.80\%0.80% for the spin qubit, and greater robustness against qubit dephasing and photon loss. We find that drive amplitude -- a parameter dropped in RWA -- is critical for optimizing fidelity, with the charge qubit exhibiting better tolerance to drive amplitude variations. Our results demonstrate the necessity of going beyond RWA in understanding how long-range gates can be realized in DQD qubits, with charge qubits offering considerable advantages in high-fidelity operation.
We present an approach for entangling spin qubits via capacitive coupling mediated by an ac electric field-driven multielectron mediator quantum dot. To illustrate this method, we consider the case of a driven two-electron dot that mediates entanglement between resonant exchange qubits defined in three-electron triple quantum dots, which enable direct capacitive coupling and interaction with microwave fields via intrinsic spin-charge mixing. The method can also be applied to other types of spin qubits that can be coupled capacitively. We show that this approach leads to rapid, single-pulse universal entangling gates for resonant exchange qubits that are activated via the drive on the mediator dot. Unlike conventional tunneling-based two-qubit gates between exchange-only qubits, the capacitive interaction-based gates we describe do not require an extensive sequence of pulses to mitigate leakage. We describe how this drive-activated local entangling approach can be integrated with the driven sideband-based long-range approach for cavity-mediated entangling gates developed in our previous work in order to enable modularity for spin-based quantum information processing.
We propose entangling operations based on the energy curvature couplings of encoded spin qubits to a superconducting cavity, exploring the nonlinear qubit response to a gate voltage variation. For a two-qubit (nnn-qubit) entangling gate we explore acquired geometric phases via a time-modulated longitudinal ${\ensuremath{\sigma}}_{z}$ coupling, offering gate times of tens of nanoseconds even when the qubits and the cavity are far detuned. No dipole moment is necessary: the qubit transverse ${\ensuremath{\sigma}}_{x}$ coupling to the resonator is zero at the full sweet spot of the encoded spin qubit of interest (a triple quantum dot three-electron exchange-only qubit or a double quantum dot singlet-triplet qubit). This approach allows always-on, exchange-only qubits, for example, to stay on their ``sweet spots'' during gate operations, minimizing the charge noise and eliminating an always-on static longitudinal qubit-qubit coupling. We calculate the main gate errors due to the (1) diffusion (Johnson) noise and (2) damping of the resonator, the (3) 1/f1/f1/f-charge noise qubit gate dephasing and 1/f1/f1/f noise on the longitudinal coupling, (4) qubit dephasing and AC-Stark frequency shifts via photon fluctuations in the resonator, and (5) spin-dependent resonator frequency shifts (via a ``dispersivelike'' static curvature coupling), most of them associated with the nonzero qubit energy curvature (quantum capacitance). Using spin-echo-like error suppression at optimal regimes, gate infidelities of ${10}^{\ensuremath{-}2}--{10}^{\ensuremath{-}3}$ can be achieved with experimentally existing parameters. The proposed schemes seem suitable for remote spin-to-spin entanglement of two spin qubits or a cluster of spin qubits: an important resource of quantum computing.
Abstract High-quality entangling gates are crucial for scalable quantum information processing. Implementing all-microwave two-qubit gates on fixed-frequency transmons offers advantages in reducing wiring complexity, but the gate performance is often limited due to the residual ZZ interaction and the frequency crowding problem. Here, we introduce a novel scheme that enables a microwave drive-activated CZ gate compatible with the coupler structure to suppress the residual ZZ interaction. The microwave drive is applied to the coupler and the microwave drive frequency remains far detuned from the system’s transition frequency to alleviate the frequency crowding problem. We model the gate process analytically and demonstrate a theoretical gate fidelity up to 99.9% numerically. Our scheme is compatible with current coupler-structure-based circuits, and insensitive to microwave crosstalk, showing a possible path for all-microwave quantum operations at scale.