arXiv subjects

Thad Walker

Publications and source records attributed to Thad Walker.

3 recordsLinked to original sources

arXiv2024

A numerical study of the spatial coherence of light in collective spontaneous emission

We present a numerical study of the spatial coherence of light that is radiated from a dilute ensemble of atoms. The spatial coherence is established as a result of the collective (cooperative) coupling of the atoms to the light, and is qualitatively different from the coherence of a laser. Specifically, the coherence in collective spontaneous emission does not rely on population inversion and stimulated emission, is governed by anti-phasing of the dipoles (subradiance), and the key figure-of-merit for the observed coherence is N /(L/{\lambda}) . Here, N is the number of atoms in the ensemble, L is the size of the sample, and {\lambda} is the wavelength of the emitted light.

quant-ph↗

arXiv2019

Dual-axis pi-pulse spin-exchange relaxation-free magnetometer

We present a new spin-exchange relaxation-free vector magnetometer with suppressed 1/f probe noise, achieved by applying a small DC bias field and a comb of magnetic DC $π$ pulses along the pump direction. This results in a synchronous orthogonal AC response for each of its two sensitive axes. The new magnetometer is particularly well-suited to applications such as biomagnetism in which the signal to be measured carries a dominant component of its power at low frequencies. The magnetometer reaches a technical noise floor of 8.4 fT/sqrt(Hz) (x) and 11 fT/sqrt(Hz) (y) at 0.01 Hz. A single-axis DC SERF sharing the same experimental apparatus attains 61 fT/sqrt(Hz) at the same frequency. A noise minimum of 1.1 fT/sqrt(Hz) (x) and 2.0 fT/sqrt(Hz) (y) is reached by the new magnetometer at 10 Hz, compared to 0.7 fT/sqrt(Hz) at 25 Hz for a DC SERF.

physics.atom-ph↗

arXiv2015

Synchronous Spin-Exchange Optical Pumping

We describe a new approach to precision NMR with hyperpolarized gases designed to mitigate NMR frequency shifts due to the alkali spin exchange field. The electronic spin polarization of optically pumped alkali atoms is square-wave modulated at the noble-gas NMR frequency and oriented transverse to the DC Fourier component of the NMR bias field. Noble gas NMR is driven by spin-exchange collisions with the oscillating electron spins. On resonance, the time-average torque from the oscillating spin-exchange field produced by the alkali spins is zero. Implementing the NMR bias field as a sequence of alkali 2$ π$-pulses enables synchronization of the alkali and noble gas spins despite a 1000-fold discrepancy in gyromagnetic ratio. We demonstrate this method with Rb and Xe, and observe novel NMR broadening effects due to the transverse oscillating spin exchange field. When uncompensated, the spin-exchange field at high density broadens the NMR linewidth by an order of magnitude, with an even more dramatic suppression (up to 70x) of the phase shift between the precessing alkali and Xe polarizations. When we introduce a transverse compensation field, we are able to eliminate the spin-exchange broadening and restore the usual NMR phase sensitivity. The projected quantum-limited sensitivity is better than 1 nHz/$\sqrt{\rm Hz}$.

physics.atom-ph↗