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arXiv:1104.0236 · New Journal of Physics

Minimally destructive detection of magnetically trapped atoms using frequency-synthesized light

M Kohnen, P G Petrov, Robert A. Nyman, E A Hinds

Published Aug 9, 2011• 25 citations
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Abstract

We present a technique for atomic density measurements by the off-resonant phase shift induced on a two-frequency, coherently synthesized light beam. We have used this scheme to measure the column density of a magnetically trapped atom cloud and to monitor oscillations of the cloud in real time by making over a hundred non-destructive local density measurements. For measurements using pulses of 10 4 –10 5 photons lasting ∼10 μs, the precision is limited by statistics of the photons and the photodiode avalanche. We explore the relationship between measurement precision and the unwanted loss of atoms from the trap and introduce a figure of merit that characterizes it. This method can be used to probe the density of a Bose–Einstein condensate (BEC) with minimal disturbance of its phase.

DOI
10.1088/1367-2630/13/8/085006

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We present a technique for atomic density measurements by the off-resonant phase shift induced on a two-frequency, coherently synthesized light beam. We have used this scheme to measure the column density of a magnetically trapped atom cloud and to monitor oscillations of the cloud in real time by making over a hundred non-destructive local density measurements. For measurements using pulses of 10 4 –10 5 photons lasting ∼10 μs, the precision is limited by statistics of the photons and the photodiode avalanche. We explore the relationship between measurement precision and the unwanted loss of atoms from the trap and introduce a figure of merit that characterizes it. This method can be used to probe the density of a Bose–Einstein condensate (BEC) with minimal disturbance of its phase.
We present a minimally destructive interferometric measurement of the column density of cold atoms with a passively stabilized separated beam path interferometer. We achieve a heating rate of 4 (photons/sec)/atom. The interferometer employed in these measurements utilizes a photodiode rather than a charge-coupled device camera and is suitable for the measurement of fast dynamics in Bose-Einstein condensates and real-time feedback with atom lasers. The technique is compatible with modulation techniques employing squeezed light as the probe beam for sub-shot-noise-limited measurements.
We present the first detector that is capable of recording high-bandwidth atom number density measurements of a Bose-Einstein condensate, with a readout of the data in real time. It is based on a shot-noise-limited unbalanced Mach-Zehnder interferometer designed for a minimally destructive measurement of the atom column density of a Bose-Einstein condensate. The shot-noise limit is reached by phase modulating the laser in one arm and phase locking the interferometer with a second-color laser. The detector is characterized, and its sensitivity for a fractional change in the column density of a Bose-Einstein condensate is calculated. With this detection system it may be possible to implement feedback to stabilize a Bose-Einstein condensate or an atom laser.
This thesis reports on the development of novel techniques for the detection and manipulation of cold gases of neutral atoms. The research presented focusses on the implementation of a photonic waveguide chip into an atom optics experiment. Our photonic chip consists of 12 parallel waveguides with a 10 µm pitch and a 16 µm trench in the centre. A wire subchip underneath the photonic chip can create magnetic fields to guide atoms into the trench and hold them there. The electric field of the light mode propagating through the waveguides has a 1/e radius of 2:2 µm. This small light mode can readily be used for local measurements of the atomic density. This thesis describes the setup of the waveguide chip experiment and gives a detailed characterisation of the interaction between light and atoms in the trench. Additionally, I report on a scheme for detecting atoms while minimising the number of scattered photons for a given precision of the measurement. We use a light beam synthesized from two frequencies tuned to either side of the atomic resonance and detect the differential phase shift they acquire when passing through an atomic cloud by referencing the beat between the two frequencies to a local oscillator. Unlike most similar techniques our beam does not contain a carrier signal and can therefore be balanced around the atomic resonance in order to cancel the mean optical dipole force on the atoms.
All optical techniques used to probe the properties of Bose-Einstein condensates have been based on dispersion and absorption that can be described by a two-level atom. Both phenomena lead to spontaneous emission that is destructive at the low energies involved with ultracold atomic systems. Recently, both were shown to lead to the same limit to the signal to noise ratio for a given destruction. We develop a new method for calculating the phase shift of a laser beam and show that no single-pass optical technique using classical light and a three-level atom can exceed this limit. This puts significant restrictions on potential nondestructive measurement schemes.
The measurement of the optical phase of far-detuned laser light by phase modulation spectroscopy is promising as a non-destructive probe of the dynamics of Bose-Einstein condensates. We have developed the experimental technique on a room-temperature Cs vapour cell, and a Cs magneto-optical trap (MOT). We measure column densities with an accuracy better than 7%. A numerical model, including Doppler broadening and transitions between all relevant hyperfine levels, accurately describes the data from the Cs cell. The same model without Doppler broadening gives quantitative agreement with measurements from the Cs MOT. We show that the conditions needed for non-destructive measurements of BEC dynamics are achievable with conventional equipment and a simple, robust experimental setup.
We demonstrate a minimally perturbative detection method—Raman driven spin noise spectroscopy (RDSNS)—to directly probe the local density of a cloud of cold 39K atoms in real time. The technique relies on detecting the polarization fluctuations of a far-off resonant focused probe light passing through the atoms, driven by two coherent Raman lasers connecting adjacent Zeeman spin states. This is an easily implementable scheme with a wide-ranging applicability in detecting cold atoms. Unlike traditional fluorescence and absorption imaging techniques, this approach does not require stochastic photon scattering from trapped atoms, thereby leaving the cold cloud unperturbed. The data can be collected in MHz refresh rate, allowing for near real-time detection. We validated this technique by monitoring the strength of the RDSNS signal by varying the key parameters of the three-dimensional magneto-optical trap (MOT) and the cold atom source (two-dimensional MOT). We performed experiments to probe local densities across the cold atom cloud using our technique and compared it with the inverse Abel-transformed density profiles extracted from the fluorescence of a spherically symmetric atomic cloud and found excellent agreement. This detection protocol is particularly advantageous for probing systems that lack inherent symmetry, where the Abel transformation is inapplicable. Moreover, RDSNS imaging is suitable for extracting the dynamical properties of systems in real time.