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Review of Scientific Instruments

Signal-to-noise enhancement in optical spectroscopy of single indium ions using amplitude modulation and synchronous detection

Eric A. Burt, Warren Nagourney

Published Feb 1, 1997• 3 citations
Publisher

Abstract

A technique for reducing background and improving the signal-to-noise ratio in optical spectroscopy of weak atomic transitions is described. In this technique an incident laser is square-wave amplitude modulated and the resonance fluorescence from the atom is then detected out of phase with the excitation when the laser is off. Due to the finite lifetime of the atom’s excited state, appreciable signal is observed when the laser is off and instrumental scattering background is virtually eliminated. Applying this technique to an experiment on single laser-cooled In+ ions, we have observed a factor of 500 reduction in background and a factor of 5 increase in the signal-to-noise ratio.

DOI
10.1063/1.1147869

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A technique for reducing background and improving the signal-to-noise ratio in optical spectroscopy of weak atomic transitions is described. In this technique an incident laser is square-wave amplitude modulated and the resonance fluorescence from the atom is then detected out of phase with the excitation when the laser is off. Due to the finite lifetime of the atom’s excited state, appreciable signal is observed when the laser is off and instrumental scattering background is virtually eliminated. Applying this technique to an experiment on single laser-cooled In+ ions, we have observed a factor of 500 reduction in background and a factor of 5 increase in the signal-to-noise ratio.
Photon counting has been widely used in weak light even to single photon detection.However,the Shot noise of photon-counting N for Poisson statistics seriously affects the sensitivity.A method using wavelength modulation spectroscopy technique to improve signal-to-noise ratio of photon-counting is presented.The light from an external cavity diode laser is wavelength modulated and then chopped to a series of laser pulses by using a acoustic-optical modulator,the counts of fluorescence photon are recorded and then converted to an analog signal.The analog signal is added to a lock-in amplifier for harmonic demodulation using lock-in method.The fluorescence spectrum is the 6S_1/2(F=4)→6P_3/2 transition of Cs D_2 line and the detected probability of photon is 35%.The experimental demonstration has achieved a 51 dB signal-to-noise improvement factor in 2f harmonic detection.The minimum detectable count here is 3×10-4 with the residual noise 7.54 μV.The results show a potential application in fluorescence detection of cold molecules or single molecule.
Laser spectroscopy is a versatile tool to investigate the internal structure of atoms, molecules and nuclei.High precision spectroscopy requires laser cooling to reduce Doppler broadening and Doppler shifts.Sympathetic cooling extends these advantages to species which do not have an appropriate transition for laser cooling and allows precision spectroscopy of previously inaccessible species using the quantum logic technique.The original implementation of quantum logic spectroscopy relies on the long lifetime of the excited spectroscopy state to realize a transfer sequence.Precision spectroscopy of transitions with a short-lived excited state is typically performed using laser induced uorescence or laser absorption spectroscopy.Neither of these methods has reached the fundamental quantum projection noise limit and are rather limited by photon shot noise.For these situations, we develop the technique of photon recoil spectroscopy as an extension of quantum logic spectroscopy to broad transitions.The common motion of a sideband-cooled two-ion crystal is excited by photon recoil during photon absorption on the spectroscopically investigated ion.This motional excitation is transferred to an internal state excitation on the co-trapped ion of a dierent species and detected with high eciency.We demonstrate the technique on the 2 S 1/2 ↔ 2 P 1/2 transition of the 40 Ca + ion and resolve the line center to 1/300 of its observed linewidth.The simplicity and versatility of the technique promises applications to other previously inaccessible species.A variation of the technique is proposed for rotational state preparation of molecular ions.A moving optical lattice is employed to exert a state-dependent force on the molecule.This would allow the detection of the rotational state of the molecular ion and therefore enable precision spectroscopy of specic rovibrational transitions in the molecule.
We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation spectroscopy signals. With this technique, which utilizes a traveling-wave acousto-optic modulator to simultaneously frequency-shift and modulate the saturating beam, we have routinely observed zero background, Doppler-free saturation resonances in the I2I_{2}I2​ at 514.5 nm. With a low-noise laser or high pump modulation frequency, we have achieved detection sensitivity approaching the fundamental quantum limit.
This thesis demonstrates the experimental implementation of a homodyne interference technique to detect weak atomic transitions. The demonstration of the technique uses two pathways to excite the cesium atom from its ground state to an excited state. One is a Stark-induced transition that is regulated by controlling a dc field across the cesium atoms. This is excited by a 411 nm laser field. A two-photon transition excited by an 822 nm laser field concurrently excites the same transition. The simultaneous excitation of the two pathways results in a quantum interference of the transition amplitudes. Phase modulating one of the beams gives rise to an amplitude modulation of the net excitation rate resulting in an amplitude modulated fluorescence signal. We observe the fluorescence at 794.6 nm due to the decay of the excited state to the ground state via an intermediate state. The strength of the amplitude modulation of the fluorescence signal is a measure of weak signal transition amplitude which in this demonstration is the controllable Stark-induced transition. Results of this work have shown improvements of two to three orders of magnitude in signal to noise over direct detection of the weak interaction. With possible improvements that are discussed it is projected to be much better.
Ion traps and cw dye lasers allow one to prepare small clouds of ions, or single ions. They can be detected even by absorptive techniques, such that the information is contained in one field mode only, the mode of the irradiating light. The noise can approach the limit. For excitation on weak and narrow lines, random breaks of the scattering on a competing resonance line can reveal the quantized signal absorption. These interruptions are the signature of quantum jumps which characterize the internal dynamics of a single atomic particle under quasi-continuous observation.