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arXiv:2202.09709 · PRX Quantum

Spinor Matter-Wave Control with Nanosecond Spin-Dependent Kicks

Liyang Qiu, Lingjing Ji, Jiangyong Hu, Yizun He, Yuzhuo Wang, Saijun Wu

Published Oct 3, 2022• 7 citations
arXivHTMLPDF

Abstract

Significant aspects of advanced quantum technology today rely on rapid control of atomic matterwaves with hyperfine Raman transitions.Unfortunately, efficient Raman excitations are usually accompanied by uncompensated dynamic phases and coherent spin leakages, preventing accurate and repetitive transfer of recoil momentum to large samples.We provide a systematic study to demonstrate that the limitations can be substantially overcome by dynamically programming an adiabatic pulse sequence.Experimentally, counterpropagating frequency-chirped pulses are programmed on an optical delay line to parallelly drive five m = 0 hyperfine Raman transitions of 85 Rb atoms for spin-dependent kick (SDK) within τ = 40 ns, with an f SDK ≈ 97.6% inferred fidelity.Aided by numerical modeling, we demonstrate that by alternating the chirps of successive pulses in a balanced fashion, accumulation of nonadiabatic errors including the spin leakages can be managed, while the dynamic phases can be robustly cancelled.Operating on a phasestable delay line, the method supports precise, fast, and flexible control of spinor matterwave with efficient Raman excitations.

DOI
10.1103/prxquantum.3.040301

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Significant aspects of advanced quantum technology today rely on rapid control of atomic matterwaves with hyperfine Raman transitions.Unfortunately, efficient Raman excitations are usually accompanied by uncompensated dynamic phases and coherent spin leakages, preventing accurate and repetitive transfer of recoil momentum to large samples.We provide a systematic study to demonstrate that the limitations can be substantially overcome by dynamically programming an adiabatic pulse sequence.Experimentally, counterpropagating frequency-chirped pulses are programmed on an optical delay line to parallelly drive five m = 0 hyperfine Raman transitions of 85 Rb atoms for spin-dependent kick (SDK) within τ = 40 ns, with an f SDK ≈ 97.6% inferred fidelity.Aided by numerical modeling, we demonstrate that by alternating the chirps of successive pulses in a balanced fashion, accumulation of nonadiabatic errors including the spin leakages can be managed, while the dynamic phases can be robustly cancelled.Operating on a phasestable delay line, the method supports precise, fast, and flexible control of spinor matterwave with efficient Raman excitations.
The absorption of traveling photons resonant with electric dipole transitions of an atomic gas naturally leads to electric dipole spin-wave excitations.For a number of applications, it would be highly desirable to shape and coherently control the spatial waveform of the spin waves before spontaneous emission can occur.This paper details a recently developed optical control technique to achieve this goal, where counterpropagating, shaped subnanosecond pulses impart subwavelength geometric phases to the spin waves by cyclically driving an auxiliary transition.In particular, we apply this technique to reversibly shift the wave vector of a spin wave on the D2 line of laser-cooled 87 Rb atoms by driving an auxiliary D1 transition with shape-optimized pulses, so as to shut off and recall superradiance on demand.We investigate a spin-dependent momentum transfer during the spin-wave control process, which leads to a transient optical force as large as ∼1hk/ns, and study the limitations to the achieved 70 ∼ 75% spin-wave control efficiency by jointly characterizing the spin-wave control and matter-wave acceleration.Aided by numerical modeling, we project potential future improvements of the control fidelity to the 99% level when the atomic states are better prepared and by equipping a faster and more powerful pulse shaper.Our technique also enables a background-free measurement of the superradiant emission to unveil the precise scaling of the emission intensity and decay rate with optical depth.
Excitation of trapped-ion hyperfine qubits with fast optical Raman pulses enables faster-than-trap-period entangling gates with qubits of long coherence time for practical quantum computation. Achieving high-fidelity fast two-qubit gates requires high-quality spin-dependent kicks (SDKs), which form their fundamental building blocks. Here, we characterize the control parameters (including Raman frequency difference, pulse arrival times, Lamb--Dicke parameter, temperature, pulse width, and SDK time) that maximize the performance of single-ion SDKs for protocols compatible with performed experiments involving a small number of fast pulses. We demonstrate through analytical methods and numerical simulations that, within the model commonly used for infidelity optimization, finite pulse duration is the dominant source of error, exceeding the contribution of secular motion by orders of magnitude for nanosecond-scale SDKs. Low infidelities -- below 10−310^{-3}10−3 for schemes with ≳10\gtrsim10≳10 fixed-amplitude, equispaced, picosecond pulses -- are achievable in SDK times on the order of nanoseconds. These results provide quantitative design rules for achieving competitive SDK fidelities with current pulsed-laser technology, laying the foundation for sub-microsecond trapped-ion quantum entangling operations.
Precise control of hyperfine matterwaves via Raman excitations is instrumental to a class of atom-based quantum technology. We investigate the Raman matterwave control technique for alkaline-like atoms in an intermediate regime characterized by the single-photon detuning where a choice can be made to balance the Raman excitation power efficiency with the control speed, excited-state adiabatic elimination, and spontaneous-emission suppression requirements. Within the regime, rotations of atomic spinors by the Raman coupling are biased by substantial light shifts. Taking advantage of the fixed bias angle, we show that composite biased rotations can be optimized to enable precise ensemble spinor matterwave control within nanoseconds, even for multiple Zeeman pseudospins defined on the hyperfine ground states and when the laser illumination is strongly inhomogeneous. Our scheme fills a technical gap in light pulse atom interferometry for achieving high-speed Raman spinor matterwave control with moderate laser power.
We propose high-fidelity single-qubit spin-dependent kicks (SDKs) for trapped ions using nanosecond Raman pulses via amplitude modulation of a continuous-wave laser with a tunable beat frequency. We develop a general method for maintaining SDK performance in the presence of micromotion by identifying optimal choices of the RF phase and frequency that suppress unwanted backward kicks. The proposed scheme enables SDK infidelities as low as 10−910^{-9}10−9 in the absence of micromotion, and below 10−510^{-5}10−5 with micromotion. This study lays the foundation for the realization of sub-trap-period and high-fidelity two-qubit gates based on SDKs.
Precise control of hyperfine matterwaves via Raman excitations is instrumental to a class of atom-based quantum technology. We investigate the Raman spinor control technique for alkaline atoms in an intermediate regime of single-photon detuning where a choice can be made to balance the Raman excitation power efficiency with the control speed, excited-state adiabatic elimination, and spontaneous emission suppression requirements. Within the regime, rotations of atomic spinors by the Raman coupling are biased by substantial light shifts. Taking advantage of the fixed bias angle, we show that composite biased rotations can be optimized to enable precise ensemble spinor matterwave control within nanoseconds, even for multiple Zeeman pseudo-spins defined on the hyperfine ground states and when the laser illumination is strongly inhomogeneous. Our scheme fills a technical gap in light pulse atom interferometry, for achieving high speed Raman spinor matterwave control with moderate laser power.
Scalable quantum gates remain a central challenge for trapped-ion quantum computing. Ultrafast gates driven by spin-dependent kicks (SDKs) provide a promising approach. However, current protocols rely on mode-locked lasers, suffering from inflexible timing control and limited single-SDK fidelity. To overcome this, we propose a scheme using rapid adiabatic passage (RAP) pulses modulated from a continuous-wave laser. We demonstrate that this RAP-based approach suppresses the sensitivity of SDKs to fluctuations in optical intensity, thereby enabling the construction of robust entangling gates. Furthermore, the programmable nature of these modulated pulses allows for precise control over pulse sequences, further optimizing gate performance.