Fock State Generation and SWAP using a Rabi-Driven Qubit
Natan Karaev, Eliya Blumenthal, Shay Hacohen-Gourgy
Abstract
The deterministic generation and SWAP of Fock states in isolated high-Q modes form a core foundation for architectures in bosonic quantum computing. Conventionally, these operations necessitate strong coupling to a qubit, which inherently compromises the required cavity isolation. To address this trade-off, we introduce a tunable mechanism wherein a weakly coupled qubit, which preserves mode isolation, is driven to induce a strong interaction on demand. By leveraging a Rabi-driven, qubit-mediated sideband interaction, we realize on-demand Jaynes-Cummings coupling between a transmon and a long-lived cavity mode. Using a superconducting flute cavity with two high-Q modes, we deterministically demonstrate Fock state preparation up to n=5 at operation times of less than 2 microseconds per photon. We also demonstrate and characterize single-photon SWAP in approximately 2 microseconds. Finally, we adapt our SWAP method to generate a dual-rail Bell state. While current performance is constrained by baseline coherence rather than fundamental methodological limits, the protocol scales inherently to accommodate higher photon numbers and faster operational regimes. By enabling complex operations on modes that remain strictly weakly coupled to qubits, this approach establishes a robust pathway for advancing scalable bosonic quantum computing.
AI Impact Assessments
(3 models)Scientific Impact Assessment
Core Contribution
This paper introduces a method for deterministic Fock state generation (up to |n=5⟩) and single-photon SWAP between two high-Q cavity modes, mediated by a Rabi-driven transmon qubit. The key innovation is using a sideband interaction on a weakly coupled qubit to create effective on-demand Jaynes-Cummings coupling, thereby circumventing the fundamental tension in bosonic quantum computing between strong qubit-cavity coupling (needed for control) and cavity isolation (needed for long coherence). The protocol uses analytical waveforms rather than optimal control pulses, and the authors extend the SWAP mechanism to generate a dual-rail Bell state (|1,0⟩+|0,1⟩)/√2.
The approach builds directly on prior Rabi-driven cavity reset methods [20, 21] and the cooling technique of [15], extending them to constructive state preparation and inter-mode transfer. The intellectual progression is clear: if Rabi-driven sideband interactions can drain a cavity, they can also fill one and swap excitations between two.
Methodological Rigor
The theoretical framework is cleanly presented. The derivation from the full dispersive Hamiltonian through rotating wave approximation to the effective Jaynes-Cummings form (Eq. 3) is standard but appropriately executed. The bright/dark mode decomposition for the SWAP analysis (Appendix B) provides useful analytic insight for the n=1 case, showing τ'₁ = √2·τ₁ and the phase structure of the gate.
However, several aspects raise concerns about rigor:
1. Fidelity characterization is incomplete. Fock state fidelities decrease from 91.6% (|1⟩) to 63.0% (|5⟩), extracted via maximum likelihood estimation from Wigner characteristic functions. No error bars or confidence intervals are provided. The Bell state is characterized only qualitatively ("due to the similarity between the theoretical prediction and our experimental results we conclude that the Bell state was successfully generated") without reporting a fidelity or entanglement witness value.
2. SWAP fidelity is modest. The single-photon SWAP achieves only 68.4% fidelity, which is far below thresholds relevant for quantum computing applications. The authors acknowledge this is limited by coherence times (T₁q = 22.8 μs, T₂q = 21.8 μs) rather than fundamental limits, but no concrete roadmap with quantitative projections for achievable fidelities in improved systems is provided beyond the generation simulations in Appendix E.
3. Higher-photon SWAP is only simulated, not demonstrated, and the authors note the Hilbert space is "computationally prohibitive to simulate" with decoherence — yet this is precisely where the non-trivial dynamics (qubit nonlinearity creating non-ideal transfer) would need careful characterization.
4. The 200 ns ramp times introduce significant dephasing per photon addition step, which the authors identify as the dominant discrete fidelity-loss mechanism. The simulations in Appendix E showing improvement with 20 ns ramps and 10× better coherence are encouraging but represent aspirational rather than demonstrated performance.
Potential Impact
The work addresses a genuine architectural challenge in bosonic quantum computing: how to perform fast, deterministic operations on high-Q modes without degrading their coherence through strong static coupling. The weak-coupling regime preserved by this protocol is attractive for scalable multi-mode systems, particularly dual-rail encodings.
However, the practical impact is currently limited by the modest fidelities achieved. The 68.4% SWAP fidelity and 63% fidelity for |5⟩ are proof-of-concept level. Competing approaches using strong dispersive coupling with optimal control (e.g., Eickbusch et al., Ref [3]) or parametric methods achieve substantially higher fidelities, albeit with the coupling-isolation tradeoff the authors seek to avoid.
The dual-rail Bell state generation is a nice demonstration of versatility, but without quantitative fidelity metrics, it's difficult to assess competitiveness. The approach could become more impactful if demonstrated in systems specifically designed to minimize TLS coupling and enable shorter ramps.
Timeliness & Relevance
The paper is well-timed. Bosonic quantum computing with superconducting cavities is a rapidly advancing field, with dual-rail encodings (Ref [12], Teoh et al.) and Fock-state-based error correction codes receiving significant attention. The tension between cavity Q and qubit coupling strength is widely recognized. The idea of using driven interactions to create effective coupling on demand is not new in circuit QED but its systematic application to Fock state preparation and SWAP is a useful contribution.
Strengths
Limitations
Overall Assessment
This paper presents a physically well-motivated protocol that addresses a real architectural challenge in bosonic quantum computing. The theoretical framework is sound and the demonstrations are clearly proof-of-concept. However, the modest fidelities, limited experimental scope (particularly for SWAP), and absence of quantitative entanglement metrics for the Bell state weaken the immediate impact. The work is best viewed as an early demonstration of a promising technique that requires significant system-level improvements to become competitive. It represents incremental progress building on the group's prior cavity reset work rather than a transformative advance.
Generated Apr 9, 2026
Comparison History (50)
Paper 2 addresses a fundamental hardware bottleneck in bosonic quantum computing—the trade-off between qubit coupling and cavity isolation. By providing a scalable, experimentally demonstrated solution (Fock states up to n=5 and SWAP operations), it offers immediate, practical advancements for near-term quantum architectures, leading to high potential impact.
Paper 2 has higher likely impact: it introduces a broadly applicable experimental paradigm (“molecular attoscope”) enabling direct, coherent tracking of coupled electronic–nuclear dynamics in neutral molecules on attosecond-to-femtosecond timescales—an important, timely capability for photochemistry, ultrafast spectroscopy, and quantum control. The methodological innovation (DUV pulse shaping + holographic readout of entangled wave packets) generalizes beyond benzene and could reshape measurements across chemistry and AMO physics. Paper 1 is strong and relevant to bosonic quantum computing, but is more specialized and incremental relative to existing cavity-QED control toolchains.
Paper 1 introduces a highly novel theoretical advancement to the Linear Combination of Unitaries (LCU), a core primitive in many quantum algorithms. By utilizing discarded ancilla outcomes for low-rank recovery and quantum cryptography, it offers broad cross-domain applications. While Paper 2 provides a strong experimental hardware demonstration for bosonic systems, Paper 1's foundational algorithmic improvements have the potential to fundamentally enhance the efficiency and application scope of a wider array of quantum computing algorithms.
Paper 2 likely has higher impact due to its methodological novelty (a “molecular attoscope” enabling coherent, shaped-pulse holographic readout of coupled electronic–nuclear dynamics) and broad relevance across attosecond science, photochemistry, spectroscopy, and quantum dynamics. Directly measuring electronic coherence in a neutral polyatomic molecule while simultaneously resolving nuclear motion addresses a central, timely challenge and could generalize to many chemical systems, influencing both fundamental understanding and control of photochemical processes. Paper 1 is strong for bosonic quantum computing architectures, but its impact is more specialized to superconducting hardware platforms.
Paper 2 has higher estimated impact: it introduces a broadly applicable conceptual shift—interference governed by the relative phase between state preparation and measurement basis—experimentally validated with near-unity visibility and a “three-scan equivalence” not possible in standard interferometers. It also unifies phenomena across quantum optics and atomic physics (CPT/EIT, photonic interference, diffraction) under a common measurement-defined mode framework, suggesting wide cross-field relevance and new device paradigms in quantum photonics. Paper 1 is technically strong and application-driven for bosonic quantum computing, but its impact is more domain-specific.
Paper 2 demonstrates a novel experimental technique for Fock state generation and SWAP using Rabi-driven qubits that addresses a fundamental trade-off in bosonic quantum computing (strong coupling vs. cavity isolation). It presents concrete experimental results (Fock states up to n=5, Bell state generation) with a scalable protocol applicable to superconducting quantum computing architectures. Paper 1 presents an incremental improvement to fault-tolerant syndrome extraction for CSS codes. While both are valuable, Paper 2's experimental demonstration of a new paradigm for bosonic quantum computing has broader impact potential and more immediate practical relevance.
Paper 2 likely has higher impact due to a clear experimental advance with near-term architectural relevance: on-demand strong interaction from a weakly coupled qubit, enabling deterministic Fock-state generation and SWAP in high-Q cavities—core primitives for bosonic quantum computing. The work is methodologically rigorous (hardware demonstration, timing, fidelity characterization) and directly applicable to scalable superconducting platforms. Paper 1 is conceptually novel and potentially broad, but appears more theoretical/algorithmic with impact contingent on wider validation across VQA settings and practical adoption.
Paper 2 presents a unified theoretical framework with broad applicability across quantum simulation of nonlinear spectroscopy—a fundamentally hard classical problem. It introduces a novel algorithmic approach (generalized parameter shift rule) that bypasses costly commutator evaluations, demonstrates practical results on real quantum hardware (12-qubit systems), and applies across diverse physical systems (spin chains, spin liquids, atomic systems). Its breadth of impact spans quantum computing, condensed matter, chemistry, and spectroscopy. Paper 1, while technically solid, addresses a more incremental advance in bosonic quantum computing hardware with performance currently limited by coherence constraints.
Paper 2 likely has higher impact: it provides rigorous, general results for Gibbs-state preparation under engineered system–bath interactions, showing KMS detailed balance can overcome Lamb-shift complications and yielding explicit complexity/mixing-time guarantees. This is broadly applicable across quantum algorithms, open quantum systems, and quantum simulation, with strong methodological rigor and timeliness for thermalization and near-term quantum advantage. Paper 1 is a valuable experimental advance for bosonic quantum computing, but its impact is more platform-specific and incremental relative to existing cavity–qubit control techniques.
Paper 2 addresses a critical hardware trade-off in bosonic quantum computing by experimentally demonstrating on-demand strong interaction with weakly coupled qubits. Its concrete experimental results, including deterministic Fock state generation and SWAP operations, offer immediate and highly relevant applications for scalable quantum computing architectures. In contrast, Paper 1 provides a theoretical understanding of thermalization breakdown, which, while fundamentally important, has a narrower immediate practical impact compared to the rapidly advancing field of experimental quantum computing.