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arXiv:2607.23959

Scalable Photon-Mediated Two-Qubit Gates with Spectrally Noisy Quantum Emitters

Shreekanth S. Yuvarajan, W. A. Coish, David Hucul, Herbert F. Fotso

quant-phphysics.opticsSubmitted Jul 27, 2026• Announced Jul 28, 2026• 0 citations

Abstract

When two quantum bits are coupled through a cavity, a two-qubit gate can be realized between them either in the near-resonant regime over a timescale established by the coupling strength of the qubits with the cavity, or in the dispersive regime, over a longer timescale established by the combination of the coupling strength and the frequency detuning between the qubits and the cavity. When the qubits are spectrally noisy or differently detuned from the cavity, the fidelity for the operation can be drastically reduced in either case, precluding scalable realizations. We introduce the protocol for optimal cavity-enabled gates (POCEG) that is shown, through reliable numerical and analytical solutions, to overcome spectral differences between quantum bits and to achieve high fidelity between disparate/noisy quantum emitters. Namely, for a cavity with low damping rate, we apply a sequence of pulses to the qubits at the frequency of the cavity while periodically modulating the coupling of the qubits to the cavity. Alternatively, in the case of a large damping rate, we operate in the dispersive regime and overcome spectral disparities by applying the pulses at a frequency far-detuned from the cavity. In both instances, we find for the quantum state transfer between the two qubits that, with a modest inter-pulse delay, the fidelity that would otherwise be strongly suppressed by the spectral mismatch of the qubits can be increased beyond 99.9%. These protocols have the capacity to bring two-qubit gates between solid state systems across the threshold required for fault-tolerant quantum computing.

Comments: 15 pages, 7 figures

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When two quantum bits are coupled through a cavity, a two-qubit gate can be realized between them either in the near-resonant regime over a timescale established by the coupling strength of the qubits with the cavity, or in the dispersive regime, over a longer timescale established by the combination of the coupling strength and the frequency detuning between the qubits and the cavity. When the qubits are spectrally noisy or differently detuned from the cavity, the fidelity for the operation can be drastically reduced in either case, precluding scalable realizations. We introduce the protocol for optimal cavity-enabled gates (POCEG) that is shown, through reliable numerical and analytical solutions, to overcome spectral differences between quantum bits and to achieve high fidelity between disparate/noisy quantum emitters. Namely, for a cavity with low damping rate, we apply a sequence of pulses to the qubits at the frequency of the cavity while periodically modulating the coupling of the qubits to the cavity. Alternatively, in the case of a large damping rate, we operate in the dispersive regime and overcome spectral disparities by applying the pulses at a frequency far-detuned from the cavity. In both instances, we find for the quantum state transfer between the two qubits that, with a modest inter-pulse delay, the fidelity that would otherwise be strongly suppressed by the spectral mismatch of the qubits can be increased beyond 99.9%. These protocols have the capacity to bring two-qubit gates between solid state systems across the threshold required for fault-tolerant quantum computing.

A variety of photon-mediated operations are critical to the realization of scalable quantum information processing platforms and their accurate characterization is essential for the identification of optimal regimes and their experimental realizations. Such light-matter interactions are often studied with a broad variety of analytical and computational methods that are constrained by approximation techniques or by computational scaling. Quantum processors present a new avenue to address these challenges. We consider the case of cavity mediated two-qubit gates. To investigate quantum state transfer between the qubits, we implement simulations with quantum circuits that are able to reliably track the dynamics of the system. Our quantum algorithm, compatible with NISQ (Noisy Intermediate Scale Quantum) era systems, allows us to map out the fidelity of the state transfer operation between qubits as a function of a broad range of system parameters including the respective detunings between the qubits and the cavity, the damping factor of the cavity, and the respective couplings between the qubits and the cavity. The algorithm provides a robust and intuitive solution, alongside a satisfactory agreement with analytical solutions or classical simulation algorithms in their respective regimes of validity. It allows us to identify under-explored regimes of optimal performance, relevant for heterogeneous quantum platforms, where the two-qubit gate can be rather effective between far-detuned qubits that are neither resonant with each other nor with the cavity. Besides its present application, the method introduced in the current paper can be efficiently used in otherwise untractable variations of the model and in various efforts to simulate and optimize photon-mediated two-qubit gates and other relevant operations in quantum information processing.

In many quantum architectures the solid-state qubits, such as quantum dots or color centers, are interfaced via emitted photons. However, the frequency of photons emitted by solid-state systems exhibits slow uncontrollable fluctuations over time (spectral diffusion), creating a serious problem for implementation of the photon-mediated protocols. Here we show that a sequence of optical pulses applied to the solid-state emitter can stabilize the emission line at the desired frequency. We demonstrate efficiency, robustness, and feasibility of the method analytically and numerically. Taking nitrogen-vacancy center in diamond as an example, we show that only several pulses, with the width of 1 ns, separated by few ns (which is not difficult to achieve) can suppress spectral diffusion. Our method provides a simple and robust way to greatly improve the efficiency of photon-mediated entanglement and/or coupling to photonic cavities for solid-state qubits.

The ability to entangle distant quantum nodes is essential for the construction of quantum networks and for quantum information processing. For solid-state quantum emitters used as qubits, it can be achieved by photon interference. When the emitter is subject to spectral diffusion, this process can become highly inefficient, impeding the achievement of scalable quantum technologies. We study two-photon interference in the context of a Hong-Ou-Mandel (HOM)-type experiment for two separate quantum emitters, with different detunings with respect to a specific target frequency. We evaluate the second order coherences that characterize photon indistinguishability between the two emitters. We find that the two-photon interference operation that is inefficient in the absence of a control protocol, when the two detunings are different and spectral overlap is lessened, can be highly improved by a periodic sequence of $\ensuremath{\pi}$ pulses at a set target frequency. Photon indistinguishability in solid state and other quantum emitters subject to spectral diffusion can thus be enhanced by the proposed pulse sequence and similar external control protocols despite the fluctuations in the environment.

Implementing high-fidelity controlled two-qubit gates in dipole-dipole interacting systems, such as rare-earth-ion crystals, in hindered by spectral inhomogeneity and weak coupling. Existing method often rely on detuned pulses, making them susceptible to frequency errors and AC Stark shifts. We propose a robust resonant scheme for arbitrary controlled two-qubit gates that utilizes asymmetric excitation and pulse engineering to achieve decoupled, parallel qubit control. Simulations on rare-earth-ion ensemble qubits demonstrate gate fidelities exceeding 99% within a 170 kHz detuning range with off-resonant excitation below 0.2%. This approach offers a robust, scalable route for quantum computing in spectrally crowded systems.

Accurate and efficient implementation of parallel quantum gates is crucial for scalable quantum information processing. However, the unavoidable crosstalk between qubits in current noisy processors impedes the achievement of high gate fidelities and renders full Hilbert-space control optimization prohibitively difficult. Here, we overcome this challenge by reducing the full-system optimization to crosstalk-robust control over constant-sized subsystems, which dramatically reduces the computational cost. Our method effectively eliminates the leading-order gate operation deviations induced by crosstalk, thereby suppressing error rates. Within this framework, we construct analytical pulse solutions for parallel single-qubit gates and numerical pulses for parallel multi-qubit operations. We validate the proposed approach numerically across multiple platforms, including coupled nitrogen-vacancy centers, a nuclear-spin processor, and superconducting-qubit arrays with up to 200 qubits. As a result, the noise scaling is reduced from exponential to linear for parallel single-qubit gates, and an order-of-magnitude reduction is achieved for parallel multi-qubit gates. Moreover, our method does not require precise knowledge of crosstalk strengths and makes no assumption about the underlying qubit connectivity or lattice geometry, thereby establishing a scalable framework for parallel quantum control in large-scale quantum architectures.

The recent realization of a coherent interface between a single electron in a silicon quantum dot and a single photon trapped in a superconducting cavity opens the way for implementing photon-mediated two-qubit entangling gates. In order to couple a spin to the cavity electric field, some type of spin-charge hybridization is needed, which impacts spin control and coherence. In this work we propose a cavity-mediated two-qubit gate and calculate cavity-mediated entangling gate fidelities in the dispersive regime, accounting for errors due to the spin-charge hybridization, as well as photon- and phonon-induced decays. By optimizing the degree of spin-charge hybridization, we show that two-qubit gates mediated by cavity photons are capable of reaching fidelities exceeding

90%

in present-day device architectures. High iswap gate fidelities are achievable even in the presence of charge noise at the level of $2\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{eV}$.