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arXiv:1111.2110 · Physical Review Letters

Demonstration of the Interaction between Two Stopped Light Pulses

Yi‐Hsin Chen, Meng-Jung Lee, Wei-Lun Hung, Ying-Cheng Chen, Yong‐Fan Chen, Ite A. Yu

Published Apr 23, 2012• 78 citations
arXivHTMLPDF

Abstract

This study reports the first experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. The scheme with motionless light pulses maximizes the interaction time and can achieve a considerable efficiency even below single-photon level. To demonstrate the enhancement of optical nonlinear efficiency, the experiment in this study used the process of one optical pulse switched by another based on the effect of electromagnetically induced transparency. Moving light pulses activate switching at an energy per area of 2 photons per atomic absorption cross section as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. This study demonstrates that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section, and that the light level can be further reduced by increasing the optical density of the medium. The result of this work enters a new regime of low-light physics.

DOI
10.1103/physrevlett.108.173603

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This study reports the first experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. The scheme with motionless light pulses maximizes the interaction time and can achieve a considerable efficiency even below single-photon level. To demonstrate the enhancement of optical nonlinear efficiency, the experiment in this study used the process of one optical pulse switched by another based on the effect of electromagnetically induced transparency. Moving light pulses activate switching at an energy per area of 2 photons per atomic absorption cross section as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. This study demonstrates that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section, and that the light level can be further reduced by increasing the optical density of the medium. The result of this work enters a new regime of low-light physics.
We experimentally demonstrate an all-optical switching (AOS) scheme based on double slow light (DSL) pulses, in which one pulse is switched by another due to the cross-Kerr nonlinearity. The interaction time is prolonged by optically dense atomic media and matched group velocities. The interaction strength is maintained at a high level by keeping both fields at their electromagnetically-induced-transparency resonances to minimize the linear loss. In the AOS without the DSL scheme, the group velocity mismatch sets an upper limit on the switching efficiency of two photons per atomic cross section as discussed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)]. Compared to that limit, we have obtained an enhanced switching efficiency by a factor of 3 with our DSL scheme. The nonlinear efficiency can be further improved by increasing the optical depth of the medium. Our work advances low-light-level nonlinear optics and provides essential ingredients for quantum many-body physics using strongly interacting photons.
Electromagnetically induced transparency in an optically thick, cold medium creates a unique system where pulse-propagation velocities may be orders of magnitude less than c and optical nonlinearities become exceedingly large. As a result, nonlinear processes may be efficient at low-light levels. Using an atomic system with three, independent channels, we demonstrate a quantum interference switch where a laser pulse with an energy per area of $\ensuremath{\sim}23$ photons per ${\ensuremath{\lambda}}^{2}/(2\ensuremath{\pi})$ causes a 1/e1/e1/e absorption of a second pulse.
In this thesis I report the first experimental observation of an interaction between a postselected single photon and an electromagnetic field containing a few thousand photons.This is the first observation of the nonlinear interactions due to a single freely propagatingphoton. To achieve this pioneering result, we have theoretically and experimentally studied nonlinear optics at the few-photon level. To the best of our knowledge, the pulse energy levels used for our experiments are the lowest energies used for cross-phase modulation in free space.Photon-photon interaction is vanishingly weak; the strength of this interaction can be enhanced in the presence of a material medium as mediator. In our experiment, we use an atomic coherence effect, namely Electromagnetically-induced Transparency (EIT), to produce a `giant' nonlinear interaction between two optical fields. We have observed a nonlinear phase shift of 18 ± 4 µrad per freely propagating single photon. This is done by sending very weak coherent state pulses into the medium and subsequently detecting single photons, inferring the existence of a photon in the interaction region.The optical setup, electronics and software that have been developed during the completion of this thesis provide the basis for several exciting future light-matter interaction experiments. We have produced phase-stable laser beams to create EIT in our cold Rubidium sample prepared in a Magneto-optical Trap (MOT). The laser system and the improved MOT setup have been built to be stable so that we can do very long data acquisition runs. We also have develop ed to ols and software with high precision and stability such that we can measure phase shifts down to micro-radian level.
We observed electromagnetically induced transparency-based four-wave mixing (FWM) in the pulsed regime at low light levels. The FWM conversion efficiency of 3.8(9)3.8(9)%3.8(9) was observed in a four-level system of cold 87Rb{}^{87}\mathrm{Rb}87Rb atoms using a driving laser pulse with a peak intensity of $\ensuremath{\approx}80$ $\ensuremath{\mu}\mathrm{W}/{\mathrm{cm}}^{2}$, corresponding to an energy of $\ensuremath{\approx}60$ photons per atomic cross section. Comparison between the experimental data and the theoretical predictions proposed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)] showed good agreement. Additionally, a high conversion efficiency of 46(2)46(2)%46(2) was demonstrated when applying this scheme using a driving laser intensity of $\ensuremath{\approx}1.8$ mW/cm2\mathrm{mW}/{\mathrm{cm}}^{2}mW/cm2. According to our theoretical predictions, this FWM scheme can achieve a conversion efficiency of nearly 100100%100 when using a dense medium with an optical depth of 500.
We have experimentally and theoretically studied the scheme of all-optical switching based on stored light pulses. In the scheme, a probe pulse is stored in the medium by the effect of electromagnetically induced transparency and a switching pulse is applied during the storage in order to turn off the probe transmission in the retrieval process. Our study takes into account the attenuation of the switching pulse induced by the stored probe pulse. The consistency between the experimental data and the theoretical predictions from the numerical calculation is satisfactory. We further derive the analytical expressions of energy losses of the probe and switching pulses. The results from the analytical expressions are in good agreement with those from the numerical calculation. Using the analytical expressions, we investigated the switching efficiency with the consideration of the Gaussian beam profile of the photon pulses and explored the feasibility of switching a single photon by another.
We report a systematic investigation of cross-phase modulation using techniques of light storage (LS) and stationary light pulse (SLP) based on the effect of electromagnetically induced transparency. Two light fields are stopped in the medium to prolong the interaction time, which enhances the efficiency of optical nonlinearity. We numerically study the properties of the signal SLP field such as lifetime, light pulse storage efficiency, and stored peak intensity as functions of optical density (OD) and coupling field intensity. The strong nonlinearity between LS and SLP photons leads to a phase shift of up to π at the single-photon level. Meanwhile, both the LS and SLP photons maintain energy transmission higher than 20%, as the OD of the SLP subsystem is larger than 700. The energy transmission and phase shift can be further improved by increasing the OD of the medium. The performance of the quantum phase gate between two single photons is expected by applying such strong photon interaction.