Superradiant parametric conversion of spin waves

Abstract

Atomic-ensemble spin waves carrying single-photon Fock states exhibit nonclassical many-body correlations in-between atoms. The same correlations are inherently associated with single-photon superradiance, forming the basis of a plethora of quantum light-matter interfaces. We devise a scheme allowing the preparation of spatially-structured superradiant states in the atomic two-photon cascade using spin-wave light storage. We thus show that long-lived atomic ground-state spin waves can be converted to photon pairs opening the way towards non-linear optics of spin waves via multi-wave mixing processes.Spatially extended atomic ensembles are a particularly versatile medium allowing generation and control of light with widely varying properties. Fulfilment of the phasematching (PM) condition facilitates efficient generation, storage and retrieval of single photons. Superradiance is inherently linked to the phase-matched emission and, in particular, spin waves (SW) that store information about light in the atomic coherence are superradiant Dicke states N −1/2 j e iKrj |g 1 . . . h j . . . g N [1]. In practice, the Λ scheme of atomic levels, which forms the basis of the Duan-Lukin-Cirac-Zoller quantum entanglement distribution protocol [2], is well-known for its capabilities to generate photon pairs with both non-trivial temporal [3] and spatially-multimode structure [4]. There, light is interfaced with a coherence between two metastable ground-state sublevels.An alternative ladder or diamond schemes allow generation of two-color photon pairs, and attract much attention [5-8] also for single-photon storage [9, 10] and in Rydberg-blockaded media [11,12]. In those cases the associated SWs lie between a ground state and an excited atomic state and are intermediate steps in the generation of a photon pair. More complex manipulations of those SWs, such as temporal [13][14][15][16][17] and spatial [18] mutli-SW beamsplitters demonstrated for ground-state SWs, remain elusive. Such control would allow SW-based enginieering of photon pair emission, possibly also extensible to deterministic quantum nonlinear optics based on Rydberg atoms [19].Here we show that the properties of a phase-matched cascaded atomic decay can be engineered via proper preparation of the atomic SW state. In particular, we treat the SW prepared in the Raman process as one of the fields participating in the parametric conversion, similarly as a pump field in a spontaneous parametric downconversion (SPDC) in nonlinear crystals [20]. Hitherto schemes necessarily required that the atom starts and ends the wave-mixing processes in the same ground state, as dictated by the principles of energy and momentum conservation. This requirement can be leveraged if the ensemble is first prepared in a superposition state, or in other words some coherence is present.We generate a strong atomic coherence (SW) between |g and |h via Raman interaction in the Λ scheme, as depicted in Fig. 1(a). The two pump fields (P1 and P2), as in Fig. 1(b), then transfer the coherence |g h|...

Massively-multiplexed generation of Bell-type entanglement using a quantum memory

“…Importantly, WV-MUX-QM platform offers versatility beyond entanglement generation, such as intramemory processing of stored spin-wave states 69 -including operations on non-classical single-excitation states 50 , continuous variable and temporal processing 70,71 or ultra-narrow band temporal imaging 72 . Furthermore, the rich atomic structure of alkali atoms may be employed to design various light-matter interfaces suited for a particular application 73 .…”