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Low Noise Simultaneous Fluorescence Detection of Two Atomic States (PREPRINT)

M. Cashen, Fixler, J. B., Grant W. Biedermann, Mark A. Kasevich

Published Apr 5, 2006• 0 citations
OpenAlex

Abstract

We have demonstrated a new technique for fluorescence detection of ultracold atoms. Fluorescence from two spatially separated clouds of ultracold atoms illuminated by a mutual probe laser was imaged onto opposite quadrants of a position-sensitive detector. The populations in the two separated atomic clouds were measured by integrating the quadrant detector photocurrents. Simultaneous detection of the populations of the two atomic clouds was used to reduce noise caused by fluctuations in detection laser amplitude and frequency. Using this technique we observed quantum projection noise limited detection signal-to-noise ratios exceeding 2000:1. To demonstrate the application of our highly Sensitive detection we compared two atomic clocks with interrogation times of only 80 ms to obtain a relative frequency stability of 1x 10(exp -13) Tau (exp -1/2) where TAU is the integration time in seconds.

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We have demonstrated a new technique for fluorescence detection of ultracold atoms. Fluorescence from two spatially separated clouds of ultracold atoms illuminated by a mutual probe laser was imaged onto opposite quadrants of a position-sensitive detector. The populations in the two separated atomic clouds were measured by integrating the quadrant detector photocurrents. Simultaneous detection of the populations of the two atomic clouds was used to reduce noise caused by fluctuations in detection laser amplitude and frequency. Using this technique we observed quantum projection noise limited detection signal-to-noise ratios exceeding 2000:1. To demonstrate the application of our highly Sensitive detection we compared two atomic clocks with interrogation times of only 80 ms to obtain a relative frequency stability of 1x 10(exp -13) Tau (exp -1/2) where TAU is the integration time in seconds.
Laser noise usually limits the stability of optical frequency ratio measurements, limiting the speed and precision one can compare two atomic frequency standards. In this talk I will describe two methods, correlation and differential spectroscopy, which utilize correlations in laser noise to increase the achievable interrogation time and thus increase the frequency comparison stability. Correlation spectroscopy is a technique which uses a parity measurement following a synchronized Ramsey interrogation to measure the relative frequency of two similar frequency atomic clocks. With this technique we achieve a measurement instability of (4×10^(-16))⁄√(τ⁄s) for a comparison of two single 27Al+ ion clocks. Differential spectroscopy uses an atomic clock with low projection noise, here a 171Yb lattice clock, to correct the phase noise of a second, higher frequency clock’s local oscillator thereby reducing the measurement instability to the level of the first. This can be further extended using two lattice clocks in a zero dead time configuration to correct the phase noise beyond the interrogation time reachable for a single Yb lattice clock. With these techniques we achieve measurement stabilities of (2.5×10^(-16))⁄√(τ⁄s) and (2×10^(-16))⁄√(τ⁄s) for a comparison between a single 27Al+ ion clock and a 171Yb lattice clock running as single clock and in a zero dead time configuration respectively. In addition to these techniques, I will also discuss recent progress towards characterizing the systematics of the NIST 40Ca+/27Al+ optical atomic clock.
We investigate theoretically and experimentally a nondestructive interferometric measurement of the state population of an ensemble of laser-cooled and trapped atoms. This study is a step toward generation of (pseudo)spin squeezing of cold atoms targeted at the improvement of the cesium clock performance beyond the limit set by the quantum projection noise of atoms. We calculate the phase shift and the quantum noise of a near-resonant optical probe pulse propagating through a cloud of cold 133Cs^{133}\mathrm{Cs}133Cs atoms. We analyze the figure of merit for a quantum nondemolition (QND) measurement of the collective pseudospin and show that it can be expressed simply as a product of the ensemble optical density and the pulse-integrated rate of the spontaneous emission caused by the off-resonant probe light. Based on this, we propose a protocol for the sequence of operations required to generate and utilize spin squeezing for the improved atomic clock performance via a QND measurement on the probe light. In the experimental part we demonstrate that the interferometric measurement of the atomic population can reach a sensitivity of the order of Nat\sqrt{{N}_{\mathrm{at}}}Nat​​ in a cloud of Nat{N}_{\mathrm{at}}Nat​ cold atoms, which is an important benchmark toward the experimental realization of the theoretically analyzed protocol.
We demonstrate how to realize an optical clock with neutral atoms that is competitive to the currently best single ion optical clocks in accuracy and superior in stability. Using ultracold atoms in a Ca optical frequency standard, we show how to reduce the relative uncertainty to below 10(-15). We observed atom interferences for stabilization of the laser to the clock transition with a visibility of 0.36, which is 70% of the ultimate limit achievable with atoms at rest. A novel scheme was applied to detect these atom interferences with the prospect to reach the quantum projection noise limit at an exceptional low instability of 4 x 10(-17) in 1 s.
Magnetic microtraps for ultracold atoms combine tight confinement, long storage times, high degree of control over the atomic position, and good optical access. Atom detection can be implemented via fluorescence, absorption, or dispersive measurements. All three types of measurements can be enhanced by an optical resonator, where the enhancement factor is proportional to the resonator finesseF. In the limit of negligible detector counts, in a setup where the detector registers n counts per atom in a fluorescence measurement, the atom number resolution is DeltaNf= (N/n)1/2for a fluorescence measurement, and DeltaNa= (1/4N)1/2for an absorption measurement. This means that absorption and fluorescence measurements perform similarly for one atom, but that the absorption measurement is superior for N >1. However, in absorption measurements the number of atoms must remain sufficiently small for the atomic absorption not to deteriorate the resonator finesse. Larger samples can be probed by dispersive measurements, where the tuning of the resonator due to the atomic index of refraction is measured. In the absence of technical noise, the corresponding atom number resolution is independent of detuning from the atomic transition, and given by DeltaNd= (1/n)1/2. Using fluorescence detection in a resonator of finesse F=8000, we detect two counts per atom in 250 mus, enabling single-atom detection with 75% efficiency (Teper et al., 2006). Absorption detection with 3.3% transmission attenuation enables us to measure small samples with up to 10 atoms with a resolution of about one atom. Absorptive and dispersive detection are interesting in that the resolution is not decreased with higher atom number. The reason is that both rely on the forward scattered signal, where the fields emitted by different atoms add coherently. These methods therefore allow non-destructive measurements below the atom-number shot noise, provided that the optical depth of the sample exceeds unity. We have realized an optical depth of several thousand inside the resonator, which should enable trapped-atom detection significantly below the shot-noise limit. If the detection is applied in a state-selective manner to a sample of two-level atoms (Geremia et al., 2004), conditional spin squeezing results. Such squeezing of the pseudo-spin for an atomic clock transition can be used to realize a hyperfine atomic clock on a microchip (Treutlin et al., 2004) that operates below the atom number shot noise limit (standard quantum limit). We report experimental progress towards pseudo-spin squeezing for87Rb atoms trapped on the microchip.