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arXiv:1412.0485 · Physical Review A

Estimation of temporal separation of slow light pulses in atomic vapors by weak measurement

Pardeep Kumar, Shubhrangshu Dasgupta

Published Apr 3, 2015• 4 citations
arXivHTMLPDF

Abstract

We show how two circular polarization components of a linearly polarized pulse, propagating through a coherently driven dilute atomic vapor, can be well resolved in a time domain by weak measurement. The slower group velocity of one of the components due to electromagnetically induced transparency leads to a differential group delay between the two components. For a low number density, this delay may not be large enough to temporally resolve the two components. We show how this can be enhanced in terms of the mean time of arrival of the output pulse through a postselected polarizer. We demonstrate the idea with all the analytical and numerical results, with a specific example of alkali atoms.

DOI
10.1103/physreva.91.043803

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We show how two circular polarization components of a linearly polarized pulse, propagating through a coherently driven dilute atomic vapor, can be well resolved in a time domain by weak measurement. The slower group velocity of one of the components due to electromagnetically induced transparency leads to a differential group delay between the two components. For a low number density, this delay may not be large enough to temporally resolve the two components. We show how this can be enhanced in terms of the mean time of arrival of the output pulse through a postselected polarizer. We demonstrate the idea with all the analytical and numerical results, with a specific example of alkali atoms.
The dynamics of resonant light propagation in rubidium vapor in a cell with antirelaxation wall coating are investigated. We change the polarization of the input light and measure the time dependence of the polarization after the cell. The observed dynamics are shown to be analogous to those in electromagnetically induced transparency. Spectral dependence of light pulse delays is found to be similar to that of nonlinear magneto-optic rotation. Delays up to $\ensuremath{\approx}13\mathrm{ms}$ are observed, corresponding to a 8m/s8\mathrm{m}/\mathrm{s}8m/s group velocity. Fields of a few microgauss are used to control the group velocity.
We propose a "channelization" architecture to achieve wide-band electromagnetically induced transparency (EIT) and ultra-slow light propagation in atomic Rb-87 vapors. EIT and slow light are achieved by shining a strong, resonant "pump" laser on the atomic medium, which allows slow and unattenuated propagation of a weaker "signal" beam, but only when a two-photon resonance condition is satisfied. Our wideband architecture is accomplished by dispersing a wideband signal spatially, transverse to the propagation direction, prior to entering the atomic cell. When particular Zeeman sub-levels are used in the EIT system, then one can introduce a magnetic field with a linear gradient such that the two-photon resonance condition is satisfied for each individual frequency component. Because slow light is a group velocity effect, utilizing differential phase shifts across the spectrum of a light pulse, one must then introduce a slight mismatch from perfect resonance to induce a delay. We present a model which accounts for diffusion of the atoms in the varying magnetic field as well as interaction with levels outside the ideal three-level system on which EIT is based. We find the maximum delay-bandwidth product decreases with bandwidth, and that delay-bandwidth product 1 should be achievable with bandwidth 50 MHz (5 ns delay). This is a large improvement over the 1 MHz bandwidths in conventional slow light systems and could be of use in signal processing applications.
We experimentally studied Zeeman electromagnetically induced transparency (EIT) resonances and slow light propagation in hot Rb vapor. Propagation of weak σ− polarized light pulses in the presence of stronger σ+ polarized background through Rb vapor was realized using a single laser beam and the Pockels cell. The dependences of slow light group velocity and fractional pulse delay on the overall laser beam intensity and temporal pulse length showed that lower optical power and longer light pulses lead to improved EIT and slow light features. The connection between EIT and slow light was also investigated showing that narrower and more contrasted EIT resonances are necessary for further decreasing a group velocity.
We carry out an analysis of an earlier proposed "channelization" architecture for wideband slow light propagation and pulse delays in atomic vapors using electromagnetically induced transparency (EIT). In the channelization architecture, a wideband input signal pulse is spatially dispersed in the transverse dimension, sent through an EIT medium consisting of an initially spin-polarized atomic vapor illuminated by a monochromatic, co-propagating pump laser, then spatially recombined. An inhomogenous magnetic field is used to Zeeman shift the atomic vapor into two-photon (Raman) resonance with the signal-pump transitions at all locations. Extending on previous analyses, we show in detail how the reconstructed pulse will be delayed only if a slight mis-match from the two-photon resonance is introduced. If the desired delay is taken as a constrained parameter, we find the bandwidth can be increased by large factor. We present an analytic treatment which optimizes the bandwidth given a desired delay and constraints on the pump power and focusing. We find bandwidth increases on the order of 5 times (100 MHz versus 20 MHz) should be possible for delays of interest (10 ns) to applications in telecommunications and radar. Interestingly, due to the mis-match requirement, we find the channelization can not increase the optimal delay-bandwidth product over conventional slow light.
We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.
We describe a Raman scheme where the group velocity of an optical pulse can be altered dramatically. With this none electromagnetically-induced-transparency scheme, we show that when on a two-photon resonance, a light pulse can propagate with extremely slow group velocity. Both pulse narrowing and broadening can occur depending upon the choice of two-photon detuning. When using a tuned far-off two-photon resonance, we show that the pulse propagates ``superluminally'' in the medium with pulse narrowing.