Spectral-Domain Coherent Control of Broadband Raman Coupling in Atom Interferometry

Abstract: The performance of atom interferometers is commonly limited by the finite spectral acceptance of atomic beam splitters and mirrors, which restricts efficient coupling to atoms with large Doppler shifts and reduces the usable atomic flux. Here, we demonstrate spectral-domain coherent control of Raman coupling by engineering its effective two-photon spectrum. By synthesizing multiple frequency components, the Raman interaction simultaneously addresses a broad range of atomic velocities, effectively overcoming the conventional transit-time-limited linewidth. Implemented in a continuous atomic-beam Mach-Zehnder interferometer, where the transverse Doppler broadening is 17 times larger than the intrinsic Raman linewidth, this approach enhances the fringe contrast from 5.9(2)% to 15.1(2)%, indicating a substantial increase in effective atomic participation. Our results establish spectral-domain coherent control as a general strategy for achieving spectrally robust atom interferometry and open new opportunities for quantum sensing in systems with strong inhomogeneous broadening.

• The paper presents a novel approach that replaces a single narrow Raman resonance with a comb of frequency components to match Doppler-broadened atomic distributions.
• The methodology boosts population transfer efficiency and fringe contrast in a thermal 87Rb beam interferometer without increasing system complexity.
• Experimental results demonstrate increased atomic participation and robustness to beam geometry, paving the way for enhanced sensitivity in quantum sensors.

Spectral-Domain Coherent Control of Broadband Raman Coupling in Atom Interferometry

Introduction

This work addresses the spectral acceptance limitations of coherent atom-light interactions in Mach-Zehnder atom interferometers, specifically the challenge of efficiently coupling a Doppler-broadened atomic ensemble when the two-photon Raman linewidth is much narrower than the velocity-induced detuning spread. Conventional mitigation strategies—such as laser cooling, beam narrowing, or pulse shortening—face trade-offs related to system complexity, dephasing, and transient power requirements. Coherent control methodologies based on temporal-domain modulation similarly contend with increased spontaneous emission and are less viable in continuous-beam architectures.

Principle of Spectral-Domain Coherent Control

The authors introduce a spectral-domain coherent control paradigm for engineering the Raman two-photon spectrum. By generating multiple phase-coherent frequency components via electro-optic and phase modulation, the system replaces the single narrow resonance with a comb of discrete resonances. This spectral response directly matches the distribution of Doppler shifts in the ensemble, maximizing the overlap in velocity space and therefore increasing the number of actively participating atoms.

The theoretical underpinning involves constructing an effective Raman coupling Hamiltonian where multiple two-photon detunings are simultaneously addressed. The comb spacings and amplitudes are tunable, enabling bandwidth extension without modifying interaction geometry or temporal pulse shapes. The approach establishes a duality with temporal-domain control, but shifts the handle entirely into the spectral domain, circumventing the limitations of temporal modulation particularly pertinent for continuous atomic beams.

Experimental Implementation and Results

The broadband Raman coupling scheme is realized in a thermal $^{87}\mathrm{Rb}$ atomic beam interferometer. The setup employs a collimated 1 mm Raman beam and a mean atomic velocity of 175 m/s, yielding a 175 kHz transit-time-limited linewidth. The Doppler broadening is measured at 3.0 MHz, a factor of 17 above the intrinsic Raman width, reflecting a substantial velocity-induced detuning spread.

By imprinting sidebands using an EOM and a phase modulator, a set of frequency components is synthesized on the Raman field. Spectral characterization confirms the production of evenly spaced resonances, with the central three achieving nearly uniform coupling strengths. This spectral structure significantly enhances the total transfer efficiency: the broadband scheme realizes a population transfer of 0.39(6) compared to 0.14(2) for the conventional single-frequency transition, commensurate with the number of addressed velocity classes.

Most notably, atom interferometric fringe contrast is increased from 5.9(2)% to 15.1(2)%—a substantial enhancement in the effective atomic participation without sacrificing the scale factor of the interferometer. Notably, the performance gain is achieved without resorting to focused beams or shortened pulse durations, demonstrating that the scheme fundamentally relaxes the traditional bandwidth/geometry trade-off in atom interferometry.

Practical and Theoretical Implications

The implementation of spectral-domain coherent control establishes a new methodology for broadband, high-fidelity coherent manipulation in quantum sensors subject to inhomogeneous detunings. Key implications include:

• Enhanced Sensitivity and Flux Utilization: By tripling the number of participating atoms, the method provides a direct route to increased interferometric sensitivity for inertial sensing, gravimetry, and related metrological applications.
• Flexibility in System Engineering: The spectral profile can be tailored by adjusting modulation depth and frequency, offering adaptability to different atomic ensembles and thermal spreads without major hardware changes.
• Robustness to Beam Geometry: The scheme achieves broadband response with well-collimated beams, sidestepping issues of wavefront inhomogeneity and residual field gradients endemic to tightly focused pulses.
• Complementarity to Temporal-Domain Control: Spectral control provides a parallel or hybrid approach to optimal control—in principle allowing joint optimization in both spectral and temporal domains.

Outlook and Future Directions

The presented approach is broadly extensible to other quantum platforms subjected to inhomogeneous broadening, including Bragg transitions and systems employing coherent population trapping. The flexibility to engineer the amplitudes and phases of individual frequency components suggests further integration with quantum optimal control, potentially leveraging arbitrary waveform generation for even broader or custom spectral profiles.

Future work may focus on:

• Scaling the number of resonances to address broader or multimodal velocity distributions,
• Implementing Doppler-resolved or velocity-selective sensing analogous to spatiotemporal encoding in MRI,
• Extending the modulation techniques to new quantum systems and platforms,
• Systematic investigation of limits imposed by higher temperatures, field noise, and residual power inhomogeneity.

Conclusion

This study demonstrates that spectral-domain coherent control is a powerful strategy for overcoming intrinsic velocity-broadening challenges in atom interferometry. By engineering the spectral response of the Raman coupling to match the Doppler distribution, the approach achieves order-of-magnitude gains in effective atomic participation and measurement contrast, preserving the interferometer’s scale factor. The results position spectral-domain engineering as a versatile and robust tool for broadband quantum control in precision measurement and quantum sensing applications (2604.03629).