Photocurrent shot noise represents the fundamental quantum limit for amplitude, phase and timing measurements of optical signals. It is generally assumed that non-classical states of light must be employed to alter the standard, time-invariant shot noise detection limit. However, in the detection of periodic signals, correlations in the shot noise spectrum can impact the quantum limit of detection. Here, we show how these correlations can be exploited to improve shot noise-limited optical pulse timing measurements by several orders of magnitude. This has allowed us to realize a photodetected pulse train timing noise floor at an unprecedented 25 zs Hz~(-1/2) (corresponding phase noise of -179 dBc Hz~(-1) on a 10 GHz carrier), ～5 dB below the level predicted by the accepted time-invariant shot noise behaviour. This new understanding of the shot noise of time-varying signals can be used to greatly improve photonic systems, affecting a wide range of communication, navigation and precision measurement applications.
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