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Unidirectional Raman emissions of Stokes photons via chiral atom-photon coupling in a ring cavity

Hai Wang, Haole Jiao, Minjie Wang, Jiajin Lu, Can Sun, Zhifang Yang, Mengqi Xi, Shujing Li

Published Mar 10, 2025• 0 citations
Publisher

Abstract

Abstract In the presented experiment, we demonstrated unidirectional Raman emission from 87Rb atoms in a ring cavity. A bias magnetic field B0 is applied along z-direction to define the quantum axis. By transversely applying write laser pulses to drive a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from a chiral (σ+) transition. The emissions are coupled into the clock-wise (z-direction) and counter-clock-wise modes (-z-direction) of a running-wave cavity, respectively. We reveal spin-momentum correlation in the Stokes emissions. The Stokes emissions constrained to the spin-momentum correlation are found to be a violation of the Kirchhoff’s law of thermal radiation. Thus, we demonstrated that the Stokes emissions can propagate along the clock-wise or counter-clock-wise mode via polarization dissipation based on the spin-momentum correlation. The directional factor is up to 1500:1.

DOI
10.21203/rs.3.rs-5828047/v1

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Abstract In the presented experiment, we demonstrated unidirectional Raman emission from 87Rb atoms in a ring cavity. A bias magnetic field B0 is applied along z-direction to define the quantum axis. By transversely applying write laser pulses to drive a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from a chiral (σ+) transition. The emissions are coupled into the clock-wise (z-direction) and counter-clock-wise modes (-z-direction) of a running-wave cavity, respectively. We reveal spin-momentum correlation in the Stokes emissions. The Stokes emissions constrained to the spin-momentum correlation are found to be a violation of the Kirchhoff’s law of thermal radiation. Thus, we demonstrated that the Stokes emissions can propagate along the clock-wise or counter-clock-wise mode via polarization dissipation based on the spin-momentum correlation. The directional factor is up to 1500:1.
The non-reciprocal (unidirectional) atom-photon couplings are crucial for modern photonics ranging from chiral quantum networks to cold-atom many-body physics. In the presented experiment, we demonstrated unidirectional Raman emission of Stokes photons from 87Rb atoms in a ring cavity. A bias magnetic field B0 is applied along z-direction on the atoms to define the quantum axis, which breaks the time inverse symmetry. By transversely applying write laser pulses to drive a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from a chiral (σ+) transition. The emissions are coupled into the clock-wise (z-direction) and counter-clock-wise (-z-direction) modes of a running-wave cavity, respectively. According to the mirror (parity) symmetry of the atom-light coupling, we demonstrated that spins (polarizations) of the Stokes fields are correlated with their propagation directions along z and -z -axis. The Stokes emissions constrained to the spin-momentum correlation are found to be violation of Kirchhoff's law of thermal radiation. Based on the correlation, we demonstrated that the Stokes emissions propagate along the clock-wise or counter-clock-wise via polarization dissipation. The directional factor is up to 1500:1.
In this study, we demonstrated unidirectional Raman emission from cold 87 Rb atoms with a ring cavity. A bias magnetic field B 0 is applied along the z direction to define the quantization axis. By transversely applying pumping laser pulses on a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from an individual circularly polarized ( σ + ) transition. The emissions are coupled into the running-wave cavity where two quarter-wave plates are inserted. We observed that the Stokes field is right-hand-circularly (left-hand-circularly) polarized when propagating along the z (–z) direction. The measured polarization extinction of the Stokes field escaped from the ring cavity is beyond 1400:1. A polarization interferometer is inserted in the cavity and the two orthogonal polarization modes are spatially split when propagating in the interferometer. By blocking one arm of the interferometer, we demonstrated a chiral cavity system with the atoms in the presence of the magnetic field. On this basis, we demonstrated unidirectional Raman emissions of Stokes photons in the ring cavity. The unidirectional factor reaches ∼99.8%.
The enhancement of atom-photon interactions with F-P cavities provides a suitable platform for studying quantum optics and atomic physics. However, the emission fields in linear F-P cavities are in the standing-wave mode, which leads to non-uniform atom-photon coupling and a short storage lifetime of cavity-enhanced spin-wave quantum storages. This study experimentally demonstrates traveling-wave atom-light interactions in an F-P cavity that can preserve light helicity. First, a bias magnetic field is applied along the z-axis to define the quantization axis, which lifts the Zeeman degeneracy and breaks the time reversal symmetry. Next, non-classically correlated pairs of Stokes photons and spin waves are produced based on the Duan-Lukin-Cirac-Zoller scheme. The Stokes photons initially emitted from a single circularly polarized atomic transition have left-and right-hand circular polarizations when propagating along the +z (forward) and -z (backward) directions, which are preserved in the chiral cavity within the atom-photon interaction region. Thus, when the forward and backward Stokes fields resonate with the cavity, they may interact with the atoms in a traveling-wave manner. This is confirmed by measuring the time-dependent retrieval efficiencies of spin waves correlated with the forward and backward Stokes fields. This work paves the way for demonstrating traveling-wave atom-photon interactions in F-P cavities.
We propose a scheme of heralded storages of single photons via spontaneous Raman scattering (SRS) enhanced by a chiral standing-wave cavity, which creates a non-classically correlated pair of a spin wave and a Stokes photon (heralded photon) when a single-photon “write” pulse is applied to an atomic ensemble. In this scheme, to decrease the power of the write pulse for SRS, the write laser and the Stokes fields are aligned to propagate near-collinearly through the atoms. The short storage lifetime remains a challenge in standing-wave cavity-enhanced SRS. To prolong the storage lifetime, we achieve a long-wavelength spin-wave memory by removing the backward-scattering Stokes field from the cavity, which is realized by using non-reciprocal couplings between the Stokes fields and the atoms. In the experiment, the memory lifetime is demonstrated up to ∼100 μs, which is beyond the limitation (∼350 ns) of the storage in the atomic ensembles coupled to standing-wave cavities. We achieve a spin-wave memory for the write pulse containing a mean photon number of ∼4. The quantum correlation of the Stokes-spin-wave pairs is beyond 2, which gives strong evidence of non-classical behavior.
Unidirectional photon emission is crucial for constructing quantum networks and realizing scalable quantum information processing. In the present work an efficient scheme is developed for the unidirectional emission of a tunable squeezed microwave field. Our scheme is based on a chiral cavity magnonic system, where a magnon mode in a single-crystalline yttrium iron garnet (YIG) sphere is selectively coupled to one of the two degenerate rotating microwave modes in a torus-shaped cavity with the same chirality. With the YIG sphere driven by a two-color Floquet field to induce sidebands in the magnon-photon coupling, we show that the unidirectional emission of a tunable squeezed microwave field can be generated via the assistance of the dissipative magnon mode and a waveguide. Moreover, the direction of the proposed one-way emitter can be controlled on demand by reversing the biased magnetic field. Our work opens up an avenue to create and manipulate one-way nonclassical microwave radiation field and could find potential quantum technological applications.
We propose an scheme for directional single-photon source based on a chiral cavity-magnon system. In this system, the magnon mode in a single-crystal yttrium iron garnet (YIG) sphere is coupled to only one of two rotating microwave modes in the torus-shaped microwave cavity. When two-photon drives are applied to both ports of the waveguide, the chiral cavity-magnon coupling leads to an unconventional photon blockade in one propagation direction, resulting in directional single-photon emission. The emission direction of the single-photon source can be controlled by reversing the biased magnetic field. Furthermore, we further examine the effects of imperfect chiral cavity-magnon coupling and the coupling between the two cavity modes on the photon blockade behavior. The results show that the system retains robustness in the presence of these nonideal factors, and the unidirectional photon blockade effect remains clearly preserved.