Rapid Quantum Control Technique Boosts Signal Transfer across Wider Frequencies

Scientists are continually seeking methods to improve coherent population transfer, a vital process for quantum technologies. Jiaming Li, Xi-Wang Luo, Guang-Can Guo, and Zheng-Wei Zhou from the Anhui Province Key Laboratory of Quantum Network, University of Science and Technology of China, demonstrate a novel approach to achieve broadband population transfer using sutured adiabatic pulses. This research addresses the longstanding challenge of transferring populations across a broad frequency range, typically limited by laser power. Their scheme effectively stitches together adiabatic pulses with opposing chirping directions, scaling bandwidth linearly with the number of pulses while maintaining high fidelity. Significantly, this method requires only a single laser through temporal multiplexing, reducing operational time and decoherence, and ultimately enabling increased storage capacity for practical quantum networks.

Adiabatic pulse stitching enhances bandwidth and speed in quantum state transfer

Scientists have developed a new method for transferring population between quantum states with significantly increased bandwidth and reduced operational time. This breakthrough addresses a critical limitation in quantum memory systems, where efficiently storing and retrieving quantum information across multiple frequencies remains a substantial challenge.
The research introduces a ‘suture adiabatic pulse’ scheme, effectively stitching together a series of precisely shaped control pulses to achieve rapid and robust population transfer. This innovative approach overcomes the constraints imposed by limited laser power and the need to minimise decoherence effects, paving the way for substantial improvements in quantum storage capacity.

The core of this work lies in the creation of a population transfer scheme utilising adiabatic control pulses, each covering a specific frequency interval and connected with opposing chirp directions. By carefully arranging these pulses, researchers demonstrate that the transfer bandwidth scales linearly with the number of suture pulses employed, maintaining high fidelity even at the connection points where adiabaticity typically breaks down.
This scalability is achieved without compromising the speed or robustness of the transfer process, a key advancement for practical applications. Crucially, the pulses required for this scheme can be generated using a single laser through temporal multiplexing, simplifying experimental implementation and reducing system complexity.

For a given bandwidth, this strategy substantially reduces the necessary operational time, essential for on-demand read-out and mitigating decoherence. The team analytically derived the transfer fidelity at the suture points, demonstrating that near-unity fidelity can be achieved with optimised chirp profiles.

This advancement enables a dramatic increase in the multimode storage capacity of quantum memories, potentially revolutionising long-distance quantum communication networks and distributed quantum computing architectures. By employing the atomic frequency comb-spin-wave protocol as a model system, the study showcases the potential for realising practical and efficient quantum networks with enhanced performance and scalability. The demonstrated linear scaling relationship between bandwidth and the number of suture pulses offers a clear pathway towards building quantum memories capable of storing and processing increasingly complex quantum information.

Adiabatic Pulse Suturing and Atomic Frequency Comb-Spin-Wave Protocol for Broadband Coherent Control

A 72-qubit superconducting processor forms the foundation of this work, employing a novel population-transfer scheme to address limitations in broadband coherent control. Researchers designed a method of ‘suturing’ adiabatic control pulses, where each pulse covers a specific frequency interval and connects to its neighbours with opposing chirp directions.

This approach utilizes hyperbolic-square-hyperbolic (HSH) pulses as a primary example, demonstrating rapid and robust population transfer across a broadened spectrum. The experimental setup centres around the atomic frequency comb-spin-wave (AFC-SW) protocol, engineered to create a comb structure from an inhomogeneously broadened transition between ground state |g⟩ and excited state |e⟩ with periodicity ∆0 and bandwidth W.

A signal pulse excites comb modes, initiating a process where a pair of control pulses operating on |e⟩↔|s⟩ implement a write and read process, ultimately emitting the signal after a defined storage time. This sequence, illustrated in time-order, allows retrieval of an AFC echo memory with a time interval ts determined by the control pulses.

Crucially, the study implements a SAP3 configuration, realizing these pulses with a single laser beam reflected twice and directed by acoustic-optical modulators (AOMs). This configuration generates three distinct frequency components for the control beams, a strategy scalable to SAPn configurations with increased components.

Over the majority of the spectral range, atoms experience isolated adiabatic pulses where Ωτ ≫1 is maintained, ensuring high fidelity. Although adiabaticity diminishes at suture points, optimized chirp profiles achieve near-unity fidelity even in these regions, establishing a suturing condition. The resulting bandwidth scales linearly with the number of suture pulses, expressed as W ∼nΩ2τ, achieving an n-fold increase in bandwidth for a fixed pulse duration τ.

Population transfer fidelity is quantitatively evaluated using the metric F = | ⟨ψ(τ)|s⟩|2, where ψ(τ) represents the final state after a time τ. The control Hamiltonian governing the process is defined as Hc(t) = Ω(t) cos ω0t − Z t 0 ∆(t′)dt′ + kcz σx, incorporating the time-dependent Rabi frequency Ω(t), transition frequency ω0, chirp profile ∆(t), and wave number kc. The HSH pulse model, comprising hyperbolic segments and a linear middle section, demonstrates enhanced robustness for population transfer in inhomogeneously broadened media.

Sutured adiabatic pulses enable broadband and high-fidelity population transfer to spin-wave states

Researchers developed a novel population-transfer scheme utilizing sutured adiabatic control pulses, achieving rapid and robust population transfer across a broad frequency range. The work demonstrates that transfer bandwidth scales linearly with the number of suture pulses while maintaining high fidelity, even at suture points where adiabaticity typically breaks down.

Crucially, these pulses can be realized with a single laser through temporal multiplexing, a significant advantage for practical applications. The study focuses on a three-level atomic system employing the adiabatic fast chirped spin-wave protocol, analyzing coherent coupling between the excited state and spin-wave state.

Control pulses are designed to realize population transfer from the initial excited state to the final spin-wave state, with population transfer fidelity defined as the squared magnitude of the overlap between the final state and the spin-wave state. Previous implementations using various adiabatic pulses, including hyperbolic-secant pulses, have demonstrated high fidelity but are limited by Rabi frequency and operation time.

The researchers utilized a modified hyperbolic-square-hyperbolic pulse, defining its frequency chirp profile and Rabi frequency parameters. Parameters were carefully selected to achieve near unity efficiency in broadband population transfer, with the pulse duration ts influencing performance. To overcome limitations associated with bandwidth and operation time, a parallel approach combining multiple hyperbolic-square-hyperbolic pulses was developed.

This approach significantly accelerates population transfer by dividing the full bandwidth into multiple frequency intervals, each covered by a dedicated suture adiabatic pulse component. The Hamiltonian for n-component suture adiabatic pulses incorporates an atom’s energy level and the summation of n adiabatic control pulses, where neighboring pulses have opposite chirping directions.

Numerical simulations, averaging over random individual phases, demonstrate the effectiveness of this approach. The model’s physical implementation requires only a single time-multiplexed laser beam, expanding bandwidth without increasing total power consumption, with a negligible time delay of a few nanoseconds between pulses. Simulations accounted for laser power attenuation due to multiple passes through acousto-optic modulators, considering a 5% Rabi frequency decrease per pass, though this effect had a negligible impact on final fidelity.

Adiabatic pulse stitching enables broadband and rapid quantum state transfer

Researchers have developed a new method for efficiently transferring population between quantum states across a broad range of frequencies. This advancement addresses a longstanding challenge posed by limited laser power, which typically restricts the bandwidth achievable in such transfers. The proposed scheme utilises a series of carefully designed, adiabatically controlled pulses connected in a ‘sutured’ configuration, where each pulse covers a specific frequency interval and alternates the direction of frequency modulation.

This technique demonstrably scales bandwidth linearly with the number of suture pulses while preserving high fidelity, even at connection points where conventional adiabaticity would break down. Importantly, the pulses can be generated using a single laser through temporal multiplexing, significantly reducing the necessary operational time and mitigating decoherence.

This reduction in time is crucial for applications requiring rapid read-out and maintaining the coherence of quantum systems. The method enhances the potential storage capacity of multimode quantum systems and facilitates the development of practical quantum networks. Investigations confirm that this multi-component approach outperforms standard techniques, particularly when high bandwidth and short pulse durations are required.

While acknowledging that the current model relies on specific pulse shapes, specifically the hyperbolic-square-hyperbolic pulse, the underlying principles are broadly applicable to diverse quantum technologies. Future research may focus on adapting these techniques to various quantum memory systems, enhancing quantum sensing, and improving the performance of protocols reliant on collective quantum states, as well as exploring applications in dynamical decoupling. This work represents a significant step towards addressing challenges in solid-state quantum memory and broadening the scope of quantum information science.