Researchers Achieve 3D Imaging of Biphoton Spatiotemporal Wave Packets Directly

Researchers are now able to fully characterise the complex spatiotemporal structure of light, a crucial step towards harnessing the full potential of photons for information transfer and computation. Yang Xue, Ze-Shan He, and Hao-Shu Tian, from the Laboratory of Quantum Information at the University of Science and Technology of China, alongside Qin-Qin Wang, Bin-Tong Yin, and Jun Zhong et al, have developed a novel technique called 3D imaging of photonic wave packets. This all-optical method allows for the comprehensive observation of biphoton correlations , spatial-spatial, spectral-spectral, and spatiotemporal , revealing previously hidden details within light fields and overcoming the limitations of existing techniques when dealing with weak photon signals. The ability to visualise these structures promises to deepen our understanding of nonlinear optical dynamics and unlock new possibilities for photonic technologies.

Experiments involved applying a constant spectral translation to signal or idler photons using cross-phase modulation in a photonic crystal fiber, followed by spectral shearing interferometry to fully characterise the joint spectral amplitude. By introducing spatial post-selection before temporal measurement, the team obtained spatially resolved spectral amplitude data, ultimately enabling a complete characterization of the spatial-spectral and spatiotemporal structure of the biphoton wavefunction. This method circumvents the need for external reference pulses and avoids low sampling efficiency, offering a substantial improvement over existing techniques. The ability to characterise the joint complex amplitudes and hidden correlations within photonic degrees of freedom is crucial for exploiting photons as quantum information carriers and enhancing the purity of quantum states.

Biphoton Wave Packet Reconstruction via 3D Imaging

This research addresses a significant challenge in quantum optics: observing spatiotemporal structures in weak photon flux quantum states, a feat previously limited by technical difficulties. The study pioneered a self-referenced, high-efficiency technique to overcome these limitations and reveal correlations within quantum light fields. Researchers generated biphotons via spontaneous parametric down-conversion and then employed a bespoke system to capture their spatiotemporal characteristics. The core of the method involves reconstructing the biphoton wavefunction, represented as |ψ⟩= Z dksdkiψ(ks, ki)|1:ks⟩|1:ki⟩, where ψ(ks, ki) defines the biphoton’s spatiotemporal wave packet.

Experiments utilized spatial and spectral modes, denoted by subscripts s and i for signal and idler photons respectively, to map the complex amplitude spanning the photonic Hilbert space. The team engineered a system capable of performing joint spatiotemporal amplitude (JSTA) measurements, facilitating three-dimensional imaging of the quantum states. By performing Fourier transformations to the frequency domain, scientists were able to visualise the biphoton wavefunction and identify instances where it could not be factorised into purely spatial and temporal components, indicating the presence of spatiotemporal correlations. Crucially, the developed method achieves direct experimental observation of spatiotemporal correlations in biphoton quantum states, something not previously demonstrated. The system delivers a significant advancement over existing techniques which primarily focused on measuring either spatial or spectral correlations in isolation.

Biphoton Wave Packet Structure Fully Characterised

This breakthrough delivers a new tool for understanding dynamics in nonlinear optics and potentially expanding photon applications in computing. The geometrical center of the signal photons’ spatial intensity distribution shifted in the direction opposite to the post-selected spatial position of the idler photons, originating from transverse momentum conservation in the SPDC process. Measurements confirm that the spatial phase distribution of the signal photons remained nearly uniform, independent of the post-selected spatial position of the idler photons, reflecting the uniform wavefront of the pump pulse. Researchers integrated over the spectrum to obtain the spatial intensity distribution and averaged the spectral phase to extract the wavefront distribution, providing detailed spatial information.

Tests prove the method’s ability to measure spectrally resolved joint spatial phase, utilising a photonic crystal fiber as a fundamental Gaussian spatial mode filter. When the classical control pulse was blocked, the transmitted signal photons provided a well-defined spatial phase reference for the original beam, enabling retrieval of the spectrally resolved joint spatial phase. Data shows that the joint spectral interference pattern exhibited tilted fringes, qualitatively indicating correlations in the joint spectral phase. Noticeable curvature in these fringes revealed the existence of higher-order joint spectral phase correlations, originating from the higher-order spectral dispersion of the pump pulse. This experimentally determined GDD value agrees well with the theoretical value calculated from the grating pairs parameters, validating the accuracy of the method. The research demonstrates a comprehensive characterization of the joint spatiotemporal correlations of biphotons, offering insights into fundamental light-matter interactions and paving the way for advanced photonic technologies.

3D Imaging Reveals Quantum Light Structure

The study highlights the significant, often overlooked, role of both the nonlinear crystal and the pump pulse in the spontaneous parametric down-conversion process. Authors acknowledge that a more detailed quantitative understanding of how the pump pulse’s spatial-spectral structure influences biphoton states requires further investigation. This approach is also directly applicable to high-gain regimes, potentially offering deeper insights into the dynamics of quantum nonlinear optics. While the current work establishes a powerful new tool, a complete quantitative understanding of the pump pulse’s influence remains an open question.