Showing papers by "Vivek Venkataraman published in 2022"

A comprehensive model for Doppler spectra in thermal atomic vapour

Abstract: We theoretically and experimentally investigate the Doppler-broadened absorption spectrum of the D2-line (52 S 1/2 → 52 P 3/2) in thermal (room-temperature) Rb vapour over a wide range of probe intensity, probe diameter, and vapour cell temperature to understand the effect of hyperfine pumping and transit-relaxation on the absorption lineshape. We present a relatively simple but comprehensive five-level rate equation model which incorporates optical pumping of the atomic population into the other ‘dark’ ground hyperfine level (leading to absorption saturation), including contributions of the three closely spaced (within one Doppler linewidth) hyperfine levels in the excited state 52 P 3/2 (allowed by the dipole transition selection rules). D2-line transmission spectra predicted by our model show excellent agreement (rms error <5% ) with the experimental data for a wide range of probe intensities (from ∼0.001Isat0 all the way up to ∼10Isat0 , where I sat0 is the usual two-level saturation intensity for the respective atomic transition), vapour cell temperatures (24.2°C–53.5 °C) and beam diameters (1/e 2 width ∼1−6 mm). Our model also takes into account the finite transit time of atoms across the probe beam cross-section, and correctly predicts the dependence of effective saturation intensity on probe diameter (up to ∼10× lower saturation intensity as compared to I sat0 for few-mm beam sizes). This ab initio five-level model can thus predict accurate Doppler lineshapes for any given experimental parameter set for a linearly polarized probe without any fitting parameters, and can be easily applied to any other atomic system by an appropriate change of the atomic constants in the rate equations.

Reduced rate equation models for accurate SAS of 87Rb D2 line

Abstract: We propose simplified reduced 5-level and 7-level rate equation models to calculate saturated absorption spectra of the Rb D2 line, and experimentally verify for transitions from upper and lower hyperfine ground state of 87Rb, respectively.

Broadband telecom photon pairs from a fiber-integrated PPLN ridge waveguide.

Abstract: We demonstrate a spectrally correlated photon-pair source at telecom wavelengths (spanning across the S-, C-, and L-bands), based on type-0 spontaneous parametric downconversion (SPDC) in a fiber-coupled Zn-indiffused MgO doped periodically poled lithium niobate (PPLN) ridge waveguide. Modal analysis of the waveguide performed through numerical finite element method (FEM) simulation indicates that device temperature can be used to dramatically vary and control the emission spectrum. Efficient photon-pair generation is measured over a broad wavelength range from ∼1520 - 1580 nm [full width at half maximum (FWHM) > 45 nm] with a coincidence-to-accidental ratio (CAR) as high as ∼668 and spectral brightness ∼2.5 × 107 pairs/s/mW/nm. Such sources can be employed in wavelength division multiplexed (WDM) quantum key distribution (QKD) over existing fiber-optic networks.

Telecom-Band Quantum Light Sources Using Dispersion-Engineered SOI Waveguides

Abstract: We theoretically demonstrate the generation of spectrally pure (purity ~ 95%) heralded single photons, and also broadband (~60-nm around 1550-nm) polarization-entangled photon pairs (concurrence>0.96), at telecom wavelengths via extensive dispersion engineering in silicon nano-waveguides.

Spectrally-Pure Integrated Telecom-Band Photon Sources in Silicon

Abstract: We demonstrate theoretically the direct (filter-free) generation of spectrally uncorrelated photon pairs, and consequently high-purity heralded single photons at telecommunication wavelengths, in SOI (silicon-on-insulator) nanowaveguides via spontaneous four-wave mixing (SFWM). The inherent strong waveguide dispersion in such high-index contrast structures is employed for discrete-band phase-matching in order to suppress the pair spectral correlations, and to produce photons in the fundamental spatial mode for easy in-out coupling to single-mode optical fibers. Additionally, by optimizing the source parameters like waveguide length, dispersion and pump bandwidth, the group-velocity of generated photons is also engineered to further eliminate spectral correlations. Extensive numerical mode simulations for various waveguide widths are used to design intrinsically pure photon sources via joint spectral amplitude (JSA) engineering. Purity $>$95% is shown to be achievable in a silica-clad SOI waveguide with a cross-section 220 × 600 nm$^{2}$ for typical values of length and pump bandwidth. The wavelength of these heralded single photons can be tuned in the range of 1266–1363 nm ($\sim$ O-band) by varying the pump wavelength. Considering the high accuracy provided by silicon nanofabrication techniques, the feasibility of generating single photons across the wavelength range 1333–1425 nm (E-band) and 1437–1510 nm (S-band) for slightly different waveguide widths (625 nm & 650 nm) is also demonstrated. These filter-free integrated photon sources could be important for large-scale linear optical quantum computation (LOQC) and long-distance fiber-optic quantum key distribution (QKD) networks.

Silicon-on-Insulator Architectures for Brillouin scattering

Abstract: We report Brillouin scattering gain in two novel Silicon-on-Insulator architectures – double slab and double-lobed waveguides. We show that the geometrical parameters influence the Brillouin gain and frequency shift, thereby offering flexibility to maximize gain.

Spectrally and spatially tunable source of correlated photons from a type-II ppKTP crystal

Abstract: We demonstrate the transition of photon-pair generation from the degenerate+collinear to non-degenerate+non-collinear regime by selective tuning of pumpwavelength and crystal temperature in a 3-cm long type-II ppKTP crystal.

On-chip Brillouin gain with Orbital Angular Momentum Modes in Silicon Photonic waveguides

Abstract: We present the excitation of forward Brillouin scattering using orbital angular momentum modes in silicon waveguides through numerical simulations. The highest gain coefficients (~540W-1m-1) are obtained for equal charges of the pump, and Stoke’s modes.

Silicon Photonic Wires for Broadband Polarization Entanglement at Telecommunication Wavelengths

Abstract: Compact and scalable sources of broadband polarization entanglement at telecommunication wavelengths will pave the way for multiuser long-distance quantum communication at enhanced data rates, but progress toward this goal has been hindered due to large birefringence in conventional silicon-on-insulator nanowaveguides. The authors theoretically demonstrate, via dispersion engineering, the successful generation of polarization-entangled photon pairs over a broad range of wavelengths. This work also provides a strategy to avoid entanglement degradation due to polarization-mode dispersion. The proposed devices will be useful building blocks for large-scale quantum communication networks.

5-Level Model for D2-Line Absorption Spectrum in Thermal Rb Vapor

Abstract: We present a 5-level rate equation model for the Doppler-broadened Rb D2-line absorption spectrum and experimentally verify that it correctly predicts upto 10× lower saturation intensity as compared to the simple 2-level system, induced by hyperfine pumping.

An efficient fiber-integrated broadband source of telecom photon-pairs

Abstract: We report an efficient source of correlated photons spanning the C- & L-bands of the telecom spectrum with a coincidence-to-accidental ratio (CAR) > 1300 and spectral brightness 2.5x107 pairs/s/mW/nm using a fiber-pigtailed Zn-indiffused MgO:PPLN ridgewaveguide.