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C. Raman

Publications and source records attributed to C. Raman.

At least 19 recordsLinked to original sources
arXiv2019

Near source fluorescence spectroscopy for miniaturized thermal atomic beams

Miniature atomic beams can provide new functionalities for atom based sensing instruments such as atomic clocks and interferometers. We recently demonstrated a planar silicon device for generating well-collimated thermal atomic beams [Nat Commun 10, 1831 (2019)]. Here, we present a near-source fluorescence spectroscopy (NSFS) technique that can fully characterize such miniature beams even when measured only a few millimeters from the nozzle exit. We also present a recipe for predicting the fluorescence spectrum, and therefore, the source angular distribution, even under conditions of strong laser saturation of the probing transition. Monte Carlo simulations together with multi-level master equation calculations fully account for the influence of optical pumping and spatial extension of the Gaussian laser beam. A notable consequence of this work is the agreement between theory and experimental data that has allowed fine details of the angular distribution of the collimator to be resolved over 3 decades of dynamic range of atomic beam output flux.

physics.atom-ph↗
arXiv2018

Hanbury Brown-Twiss correlations and multimode dynamics of quenched spinor Bose-Einstein condensates

We have studied the interaction between multiple, competing spatial modes that are excited by a quantum quench of an antiferromagnetic spinor Bose-Einstein condensate. We observed Hanbury Brown-Twiss correlations and associated super-Poissonian noise in the mode populations. The decay of these correlations was consistent with experimentally observed spin domain patterns. Data were compared with a real-space Bogoliubov theory as well as numerical solution of the coupled Gross-Pitaevskii equations that were seeded by quantum noise via the truncated Wigner approximation. The spatial modes that were both observed experimentally and deduced theoretically are intimately connected to the inhomogeneous density profile of the condensate, which imparts many rich features to the dynamics.

cond-mat.quant-gas↗
arXiv2016

Precise measurements on a quantum phase transition in antiferromagnetic spinor Bose-Einstein condensates

We have experimentally investigated the quench dynamics of antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero temperature quantum phase transition at zero quadratic Zeeman shift $q$. The rate of instability shows good agreement with predictions based upon solutions to the Bogoliubov de-Gennes equations. A key feature of this work was removal of magnetic field inhomogeneities, resulting in a steep change in behavior near the transition point. The quadratic Zeeman shift at the transition point was resolved to 250 milliHertz uncertainty, equivalent to an energy resolution of $k_B \times 12$ picoKelvin. To our knowledge, this is the first demonstration of sub-Hz precision measurement of a phase transition in quantum gases. Our results point to the use of dynamics, rather than equilibrium studies for high precision measurements of phase transitions in quantum gases.

cond-mat.quant-gas↗
arXiv2014

Precise measurements on a quantum phase transition in antiferromagnetic spinor Bose-Einstein condensates

We investigate, both experimentally and theoretically, the quench dynamics of antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero temperature quantum phase transition at zero quadratic Zeeman shift q. Both the rate of instability and the associated finite wavevector of the unstable modes - show good agreement with predictions based upon numerical solutions to the Bogoliubov de-Gennes equations. A key feature of this work is inclusion of magnetic field inhomogeneities that smooth the phase transition. Once these were removed, we observed a dramatic sharpening of the transition point, which could then be resolved within a quadratic Zeeman shift of only 1-2 Hz. Our results point to the use of dynamics, rather than equilibrium quantities for high precision measurements of phase transitions in quantum gases.

cond-mat.quant-gas↗
arXiv2013

Optical Control of a Quantum Rotor

The possibility to coherently control a quantum rotor is investigated theoretically. The rotor is realized by an antiferromagnetic spin-1 Bose-Einstein condensate, trapped in the optical field of a Fabry-Perot resonator. By tuning the pumping field of the resonator, coherent control over the rotor is achieved. The technique is illustrated by the numerical simulation of a protocol that transforms the rotor's ground state into a squeezed state. The detection of the squeezed state via measurement of intensity-correlations of the cavity field is proposed.

quant-ph↗
arXiv2012

Antiferromagnetic Spatial Ordering in a Quenched One-dimensional Spinor Gas

We have experimentally observed the emergence of spontaneous antiferromagnetic spatial order in a sodium spinor Bose-Einstein condensate that was quenched through a magnetic phase transition. For negative values of the quadratic Zeeman shift, a gas initially prepared in the F = 1, mF = 0 state collapsed into a dynamically evolving superposition of all 3 spin projections, mF = 0, +/-1. The quench gave rise to rich, nonequilibrium behavior where both nematic and magnetic spin waves were generated. We characterized the spatiotemporal evolution through two particle correlations between atoms in each pair of spin states. These revealed dramatic differences between the dynamics of the spin correlations and those of the spin populations.

cond-mat.quant-gas↗
arXiv2011

Quantum Phase Transition in an Antiferromagnetic Spinor Bose-Einstein Condensate

We have experimentally observed the dynamics of an antiferromagnetic sodium Bose-Einstein condensate (BEC) quenched through a quantum phase transition. Using an off-resonant microwave field coupling the F = 1 and F = 2 atomic hyperfine levels, we rapidly switched the quadratic energy shift q from positive to negative values. At q = 0 the system undergoes a transition from a polar to antiferromagnetic phase. We measured the dynamical evolution of the population in the F = 1, m_F = 0 state in the vicinity of this transition point and observed a mixed state of all 3 hyperfine components for q < 0. We also observed the coarsening dynamics of the instability for q<0, as it nucleated small domains that grew to the axial size of the cloud.

cond-mat.quant-gas↗
arXiv2009

Continuous vortex pumping into a spinor condensate with magnetic fields

We study the mechanisms and the limits of pumping vorticity into a spinor condensate through manipulations of magnetic (B-) fields. We discover a fundamental connection between the geometrical properties of the magnetic fields and the quantized circulation of magnetically trapped atoms, a result which generalizes several recent experimental and theoretical studies. The optimal procedures are devised that are capable of continuously increasing or decreasing a condensate's vorticity by repeating certain two step B-field manipulation protocols. We carry out detailed numerical simulations that support the claim that our protocols are highly efficient, stable, and robust against small imperfections of all types. Our protocols can be implemented experimentally within current technologies.

cond-mat.other↗
arXiv2006

Detecting level crossings without looking at the spectrum

In many physical systems it is important to be aware of the crossings and avoided crossings which occur when eigenvalues of a physical observable are varied using an external parameter. We have discovered a powerful algebraic method of finding such crossings via a mapping to the problem of locating the roots of a polynomial in that parameter. We demonstrate our method on atoms and molecules in a magnetic field, where it has implications in the search for Feshbach resonances. In the atomic case our method allows us to point out a new class of invariants of the Breit-Rabi Hamiltonian of magnetic resonance. In the case of molecules, it enables us to find curve crossings with practically no knowledge of the corresponding Born-Oppenheimer potentials.

physics.atom-ph↗
arXiv2006

Axicon Lens for Coherent Matter Waves

We have realized a conical matter wave lens. The repulsive potential of a focused laser beam was used to launch a Bose-Einstein condensate into a radially expanding wavepacket whose perfect ring shape was ensured by energy conservation. In spite of significant interactions between atoms, the spatial and velocity widths of the ring along its radial dimension remained extremely narrow, as also confirmed by numerical simulations. Our results open the possibility for cylindrical atom optics without the perturbing effect of mean-field interactions.

cond-mat.other↗
arXiv2006

Dynamics of rotating Bose-Einstein condensates probed by Bragg scattering

Gaseous Bose-Einstein condensates (BECs) have become an important test bed for studying the dynamics of quantized vortices. In this work we use two-photon Doppler sensitive Bragg scattering to study the rotation of sodium BECs. We analyze the microscopic flow field and present laboratory measurements of the coarse-grained velocity profile. Unlike time-of-flight imaging, Bragg scattering is sensitive to the direction of rotation and therefore to the phase of the condensate. In addition, we have non-destructively probed the vortex flow field using a sequence of two Bragg pulses.

cond-mat.other↗
arXiv2005

Bragg Spectroscopy of Vortex Lattices in Bose-Einstein condensates

We have measured the velocity field of a vortex lattice within a sodium Bose-Einstein condensate using Bragg scattering. The phase gradient of the macroscopic wavefunction was mapped into the spatial structure of the diffracted atom cloud, allowing for single shot measurement of the rotation parameters. A combination of spectral and spatial information yields a complete description of the superfluid flow, coarse-grained over the lattice structure, including direct and independent measurements of the rate and sense of rotation. Signatures of the microscopic quantum rotation have also been observed.

cond-mat.other↗
arXiv2005

Metastable Bose-Einstein Condensate in a Linear Potential

We have created a Bose-Einstein condensate whose spin orientation is metastable. Condensates were transferred into a quadrupole magnetic trap, where Majorana transitions limited the lifetime to a few hundred milliseconds, about 30 times the trapping period. Atoms held in the trap frequently displayed a ring-shaped time-of-flight distribution. We speculate that such a ring could be either a quantized vortex or a feature of the Majorana loss dynamics in the quantum regime.

cond-mat.other↗
arXiv2004

An Optically Plugged Quadrupole Trap for Bose-Einstein Condensates

We created sodium Bose-Einstein condensates in an optically plugged quadrupole magnetic trap (OPT). A focused, 532nm laser beam repelled atoms from the coil center where Majorana loss is significant. We produced condensates of up to $3 \times 10^7$ atoms, a factor of 60 improvement over previous work [1], a number comparable to the best all-magnetic traps, and transferred up to $9 \times 10^6$ atoms into a purely optical trap. Due to the tight axial confinement and azimuthal symmetry of the quadrupole coils, the OPT shows promise for creating Bose-Einstein condensates in a ring geometry.

cond-mat.other↗
arXiv2001

Formation and Decay of Vortex Lattices in Bose-Einstein Condensates at Finite Temperatures

The dynamics of vortex lattices in stirred Bose-Einstein condensates have been studied at finite temperatures. The decay of the vortex lattice was observed non-destructively by monitoring the centrifugal distortions of the rotating condensate. The formation of the vortex lattice could be deduced from the increasing contrast of the vortex cores observed in ballistic expansion. In contrast to the decay, the formation of the vortex lattice is insensitive to temperature change.

cond-mat.soft↗
arXiv2001

Vortex Nucleation in a Stirred Bose-Einstein Condensate

We studied the nucleation of vortices in a Bose-Einstein condensate stirred by a laser beam. We observed the vortex cores using time-of-flight absorption imaging. By varying the size of the stirrer, we observed either discrete resonances or a broad response as a function of the frequency of the stirrer's motion. Stirring beams small compared to the condensate size generated vortices below the critical rotation frequency for the nucleation of surface modes, suggesting a local mechanism of generation. In addition, we observed the centrifugal distortion of the condensate due to the rotating vortex lattice and found evidence for bent vortices.

cond-mat.soft↗
arXiv2001

Realization of Bose-Einstein condensates in lower dimensions

Bose-Einstein condensates of sodium atoms have been prepared in optical and magnetic traps in which the energy-level spacing in one or two dimensions exceeds the interaction energy between atoms, realizing condensates of lower dimensionality. The cross-over into two-dimensional and one-dimensional condensates was observed by a change in aspect ratio and saturation of the release energy when the number of trapped atoms was reduced.

cond-mat.soft↗