Quantum Projection Noise in an Atomic Fountain: A High Stability Cesium Frequency Standard

TLDR

The study presents a laser‑cooled cesium fountain clock operating in the quantum‑limited regime. The clock’s short‑term stability is measured with an ultrastable cryogenic sapphire oscillator while varying the detected atom number. The fountain achieves an Allan deviation following the N_at⁻¹⁄² projection‑noise law, reaching a relative stability of 4×10⁻¹⁴ τ⁻¹⁄² with 6×10⁵ atoms— the best short‑term stability reported for primary standards, a five‑fold improvement.

Abstract

We describe the operation of a laser cooled cesium fountain clock in the quantum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms ${N}_{\mathrm{at}}$. For ${N}_{\mathrm{at}}$ varying from $4\ifmmode\times\else\texttimes\fi{}{10}^{4}$ to $6\ifmmode\times\else\texttimes\fi{}{10}^{5}$ the Allan standard deviation of the frequency fluctuations is in excellent agreement with the ${N}_{\mathrm{at}}^{\ensuremath{-}1/2}$ law of atomic projection noise. With $6\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms, the relative frequency stability is $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$, where \ensuremath{\tau} is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results.