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Physical Review A

Rapid real-time detection of cold atoms with minimal destruction

Jessica Lye, J. J. Hope, J. D. Close

Published Feb 3, 2004• 10 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.69.023601

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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.
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 propose and analyze a series of nondestructive dynamic detectors for Bose-Einstein condensates based on photodetectors operating at the shot-noise limit. These detectors are compatible with real-time feedback to the condensate. The signal-to-noise ratio of different detection schemes are compared subject to the constraint of minimal heating due to photon absorption and spontaneous emission. This constraint leads to different optimal operating points for interference-based schemes. We find the somewhat counterintuitive result that without the presence of a cavity, interferometry causes as much destruction as absorption for optically thin clouds. For optically thick clouds, cavity-free interferometry is superior to absorption, but it still cannot be made arbitrarily nondestructive. We propose a cavity-based measurement of atomic density which can in principle be made arbitrarily nondestructive for a given signal-to-noise ratio.
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.
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.
Cold-atom interferometry is a powerful tool for high-precision measurements of the quantum properties of atoms, many-body interactions and gravity. Further enhancement of sensitivity and reduction of complexity of these devices are crucial conditions for success of their applications. Here we introduce a multi-path interferometric scheme that offers advances in both these aspects. It uses coherent coupling between Bose-Einstein condensates in different Zeeman states to generate an interferometric signal with sharp fringes. We realise such an interferometer as a compact easy-to-use atom-chip device and thus provide an alternative method for measurement of the light-atom and surface-atom interactions.