Quantum correlations in multimode optical fields

In ultrashort optical pulses, quantum correlations among photon numbers in different spectral components are essential for comprehending and characterizing the pulse's internal quantum structure and achieving amplitude squeezing through spectral filtering. Traditional tomographic and squeezing experiments only provide overall pulse statistics, averaging noise variances across all spectral and temporal modes, leaving the quantum characteristics of individual frequency components and the pulse's internal quantum structure unexplored. Attempts to observe multimode quantum correlations in fiber optics have been restricted to a limited number of wavelength modes, typically around 10, due to squared scaling with the required spectral filters and ill-conditioning of the inversion problem depending on the choice of filters (1). To address these limitations, we introduce a novel experimental method using arbitrarily reconfigurable filters to investigate the internal quantum correlation distribution within the multimode field with heightened resolution, allowing for the examination of covariances between approximately 100 modes. Building upon these insights into internal quantum correlations, we identify optimal spectral filters capable of reducing photon-number fluctuations to as much as 4 dB below the standard quantum limit.

1. Spälter, S., et al. Physical Review Letters 81.4 (1998): 786.