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arXiv:2511.22214

Controlled-SWAP gates by tuning of interfering transition pathways in neutral atom arrays

Mohammadsadegh Khazali, Klaus Mølmer

quant-phSubmitted Nov 27, 2025• Announced Nov 28, 2025• 0 citations
v1 · Nov 27, 2025v2 · May 21, 2026
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Abstract

Neutral-atom quantum processors employ Rydberg blockade for multiqubit phase operations but lack similar native exchange and conditional exchange gates, which are essential primitives for state verification, fermionic and XY-model simulation, and efficient routing in large qubit arrays. We demonstrate that by lifting the degeneracy between interfering transition pathways, a single Rydberg-excited atom can control state exchange between pairs of atoms. Using this mechanism, we realize a direct controlled-SWAP (Fredkin) operation with more than 99\% fidelity and an order-of-magnitude reduction in circuit depth and reduced exposure to decay and decoherence of Rydberg state components compared with decomposed implementations. The mechanism operates robustly under Doppler broadening at ~150 $μ$K and realistic laser-intensity noise and extends naturally to an entire family of useful gates, including multi-control conditional exchanges (C$_k$-SWAP) and conditional multiplexed SWAP gates. By incorporating controlled exchange operations as native physical operations on neutral atoms, our work provides multiqubit gates that enable higher-order state-verification protocols, occupation-dependent simulations, and conditional routing across optical lattices.

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Neutral-atom quantum processors employ Rydberg blockade for multiqubit phase operations but lack similar native exchange and conditional exchange gates, which are essential primitives for state verification, fermionic and XY-model simulation, and efficient routing in large qubit arrays. We demonstrate that by lifting the degeneracy between interfering transition pathways, a single Rydberg-excited atom can control state exchange between pairs of atoms. Using this mechanism, we realize a direct controlled-SWAP (Fredkin) operation with more than 99% fidelity and an order-of-magnitude reduction in circuit depth and reduced exposure to decay and decoherence of Rydberg state components compared with decomposed implementations. The mechanism operates robustly under Doppler broadening at ~150 μμμK and realistic laser-intensity noise and extends naturally to an entire family of useful gates, including multi-control conditional exchanges (Ck_kk​-SWAP) and conditional multiplexed SWAP gates. By incorporating controlled exchange operations as native physical operations on neutral atoms, our work provides multiqubit gates that enable higher-order state-verification protocols, occupation-dependent simulations, and conditional routing across optical lattices.
Arrays of qubits encoded in the ground-state manifold of neutral atoms trapped in optical (or magnetic) lattices appear to be a promising platform for the realization of a scalable quantum computer. Two-qubit conditional gates between nearest-neighbor qubits in the array can be implemented by exploiting the Rydberg blockade mechanism, as was shown by D. Jaksch et al. [Phys. Rev. Lett. 85, 2208 (2000)]. However, the energy shift due to dipole-dipole interactions causing the blockade falls off rapidly with the interatomic distance, and protocols based on direct Rydberg blockade typically fail to operate between atoms separated by more than one lattice site. In this work, we propose an extension of the protocol of Jaksch et al. for controlled-ZZZ and controlled-not gates which works in the general case where the qubits are not nearest neighbors in the array. Our proposal relies on the Rydberg excitation hopping along a chain of ancilla noncoding atoms connecting the qubits on which the gate is to be applied. The dependence of the gate fidelity on the number of ancilla atoms, the blockade strength, and the decay rates of the Rydberg states is investigated. A comparison between our implementation of a distant controlled-not gate and one based on a sequence of nearest-neighbor two-qubit gates is also provided.
Abstract Neutral atom arrays are particularly promising for large-scale quantum computing because it is possible to prepare large-scale qubit arrays. An unsolved issue is how to selectively excite one qubit deep in a 3D atomic array to Rydberg states. In this work, we show two methods for this purpose. The first method relies on a well-known result: in a dipole transition between two quantum states driven by two off-resonant fields of equal strength but opposite detunings ±Δ, the transition is characterized by two counter-rotating Rabi frequencies Ω e ± i Δ t (or ± Ω e ± i Δ t if the two fields have a π -phase difference). This pair of detuned fields lead to a time-dependent Rabi frequency 2 Ω cos ( Δ t ) (or 2 i Ω sin ( Δ t ) ), so that a full transition between the two levels is recovered. We show that when the two detuned fields are sent in different directions, one atom in a 3D optical lattice can be selectively addressed for Rydberg excitation, and when its state is restored, the state of any nontarget atoms irradiated in the light path is also restored. Moreover, we find that the Rydberg excitation by this method can significantly suppress the fundamental blockade error of a Rydberg gate, paving the way for a high-fidelity entangling gate with a commonly used quasi-rectangular pulse that is easily obtained by pulse pickers. Along the way, we find a second method for single-site Rydberg addressing in 3D, where a selected target atom can be excited to the Rydberg state while preserving the state of any nontarget atom due to a spin-echo sequence. The capability to selectively address a target atom in 3D atomic arrays for Rydberg excitation makes it possible to design a large-scale neutral-atom information processor based on the Rydberg blockade.
Neutral-atom arrays serve as an ideal platform to study quantum simulation and quantum logic gates, where intense efforts have been devoted to improving the fidelity of two-qubit gates. We report our recent findings in constructing a different type of two-qubit controlled phase gate protocol with neutral atoms enabled by the Rydberg blockade, which aims at both robustness and high fidelity. It relies upon a modulated driving pulse with a specially tailored smooth waveform to gain appropriate phase accumulations for quantum gates. The major features include finishing a gate operation within a single pulse, not necessarily requiring individual site addressing, not being sensitive to the exact value of the blockade shift, and suppressing the population leakage error and rotation error. Building upon this progress, we further develop an upgrade in the form of dual-pulse off-resonant modulated driving with the major distinct feature of Doppler insensitivity, in order to address the challenge to the fidelity of two-qubit gates caused by residual thermal motion of the cold atoms. In principle, the gate fidelity remains reasonably high over a relatively significant velocity range of the qubit atoms. Moreover, we anticipate that this will inspire future improvements in gate protocols for other types of qubit platforms, and the strategies used here may find applications in the area of quantum optimal control.
Diagonal gates are ubiquitous primitives in quantum algorithms, from phase oracles, hypergraph-state preparation, and multi-control logic to Hamiltonian simulation of spin models and digitized lattice field theories, where Ising interactions and local potential terms are diagonal in the encoded basis. Standard compilers, however, often lower diagonal structure into one- and two-qubit gates before neutral-atom hardware can exploit native Rydberg-mediated multiqubit controlled-phase operations. We propose a Möbius-guided compiler that maps a diagonal phase function to a phase hypergraph via subset-lattice Möbius inversion. The hypergraph retains the support and angle of each many-body phase term, allowing sparse or local high-order structure to be routed as native multiqubit controlled-phase candidates when feasible and decomposed otherwise. The neutral-atom scheduler accounts for atom motion, interaction-zone constraints, blockade feasibility, and error costs, enabling a direct comparison between native high-order execution and decomposed alternatives. Benchmarks against routed ZAP and ZX-calculus baselines show improved estimated success for algorithmic instances with exploitable three- and four-body phase terms, and comparable performance on predominantly two-body instances. These results provide a feasible compilation strategy for more fully exploiting the native capabilities of neutral-atom hardware, using atom reconfigurability and Rydberg-mediated multiqubit phase operations as practical resources for more efficient quantum computation.
Neutral atom arrays have rapidly emerged as a leading platform for quantum computing, boasting scalable, configurable arrays of single atoms trapped in optical tweezers, fast, high-fidelity entangling gates through Rydberg interactions, and programmable, parallelized control of qubit operations. Coupling an atom array to an optical cavity opens a new frontier. Leveraging enhanced light-atom interactions in cavity quantum electrodynamics, cavity- coupled atom arrays acquire capabilities that can further expand the neutral atom toolbox, including cavity-enhanced atom readouts, atom-photon entanglement, and photon-mediated interactions between distant atoms. This thesis presents a quantum hardware platform that integrates an array of neutral atoms with a high-finesse optical cavity. After describing the design and development of the experimental apparatus, I demonstrate high-fidelity atom state readout through the cavity, achieving improved speed and atom survival compared to conventional free-space imaging methods. I then introduce a new technique for selectively controlling atom-cavity coupling on arbitrary subsets of the array, using local AC Stark shifts on the excited states of the atoms. Building on these tools, I demonstrate fast, non-destructive cavity-based readout of atom arrays, a crucial bottleneck of atom array platforms. I also showcase real-time measurement and feedback capabilities with a demonstration of classical error correction, using a register of atomic bits. Finally, I describe progress toward implementing single- and two-qubit gates within the cavity-coupled system. By combining coherent control, tunable interactions, and high-fidelity, non-destructive readout integrated and real-time feedback, the cavity-coupled Rydberg atom array offers a promising path toward fault-tolerant quantum computing.
We present a theoretical scheme for a family of fast and high-fidelity two-qubit iSWAP gates between neutral atoms separated by more than 20 um, enabled by resonant dipole-dipole spin-exchange interactions between Rydberg states. The protocol harnesses coherent excitation-exchange-deexcitation dynamics between the qubit and the Rydberg states within a single and smooth laser pulse, in the presence of strong dipole-dipole interactions. We utilize optimal control methods to achieve theoretical gate fidelities and durations comparable to blockade-based gates in the presence of relevant noise, while extending the effective interaction range by an order of magnitude. This enables entanglement well beyond the blockade radius, offering a route toward fast, high-connectivity quantum processors.