Generation and characterization of spatially structured few-photon states of light

Zavatta A., Valentina Parigi, Bellini M.

Liang Cui, Shengjie Zhu, Xiaoying Li

We investigate the spectro-temporal mode property of the photon pairs generated via inter-spatial-mode spontaneous four-wave mixing in a few-mode fiber (FMF) that supports two linearly polarized spatial modes in the 1550 nm telecom band. The two pulsed pumps for the process are in distinct spatial modes, while the created signal and idler photon pairs are also occupying different spatial modes. By exploiting the unique inter-mode phase matching condition in the FMF, we can tailor the spectro-temporal mode structure of the photon pairs by changing the pulse duration (spectral bandwidth) of the two pumps. Especially, photon pairs with a nearly round-shaped factorable joint spectrum are experimentally realized. The techniques are valuable for generating and manipulating quantum states in FMFs and exploiting the spatial degree of freedom in quantum information processing.

Yang Xue, Ze-Shan He, Hao-Shu Tian +6 more

Photons are among the most important carriers of quantum information owing to their rich degrees of freedom (DoFs), including various spatiotemporal structures. The ability to characterize these DoFs, as well as the hidden correlations among them, directly determines whether they can be exploited for quantum tasks. While various methods have been developed for measuring the spatiotemporal structure of classical light fields, owing to the technical challenges posed by weak photon flux, there have so far been no reports of observing such structures in their quantum counterparts, except for a few studies limited to correlations within individual DoFs. Here, we propose and experimentally demonstrate a self-referenced, high-efficiency, and all-optical method, termed 3D imaging of photonic wave packets, for comprehensive characterization of the spatiotemporal structure of a quantum light field, i.e., the biphoton spatiotemporal wave packet. Benefiting from this developed method, we successfully observe the spatial-spatial, spectral-spectral, and spatiotemporal correlations of biphotons generated via spontaneous parametric down-conversion, revealing rich local and nonlocal spatiotemporal structure in quantum light fields. This method will further advance the understanding of the dynamics in nonlinear quantum optics and expand the potential of photons for applications in quantum communication and quantum computing.

Bellini M., Zavatta A., Viciani S.

Zavatta A., Viciani S., Valentina Parigi +1 more

Alessio D’Errico, Felix Hufnagel, Filippo Miatto +2 more

Spontaneous parametric downconversion is the primary source to generate entangled photon pairs in quantum photonics laboratories. Depending on the experimental design, the generated photon pairs can be correlated in the frequency spectrum, polarization, position-momentum, and spatial modes. Exploring the spatial modes' correlation has hitherto been limited to the polar coordinates' azimuthal angle, and a few attempts to study Walsh mode's radial states. Here, we study the full-mode correlation, on a Laguerre-Gauss basis, between photon pairs generated in a type-I crystal. Furthermore, we explore the effect of a structured pump beam possessing different spatial modes onto bi-photon spatial correlation. Finally, we use the capability to project over arbitrary spatial mode superpositions to perform the bi-photon state's full quantum tomography in a 16-dimensional subspace.