Measuring azimuthal and radial modes of photons

Abstract

With the emergence of the field of quantum communications, the appropriate choice of photonic degrees of freedom used for encoding information is of paramount importance. Highly precise techniques for measuring the polarisation, frequency, and arrival time of a photon have been developed. However, the transverse spatial degree of freedom still lacks a measurement scheme that allows the reconstruction of its full transverse structure with a simple implementation and a high level of accuracy. Here we show a method to measure the azimuthal and radial modes of Laguerre-Gaussian beams with a greater than 99 % accuracy, using a single phase screen. We compare our technique with previous commonly used methods and demonstrate the significant improvements it presents for quantum key distribution and state tomography of high-dimensional quantum states of light. Moreover, our technique can be readily extended to any arbitrary family of spatial modes, such as mutually unbiased bases, Hermite-Gauss, and Ince-Gauss. Our scheme will significantly enhance existing quantum and classical communication protocols that use the spatial structure of light, as well as enable fundamental experiments on spatial-mode entanglement to reach their full potential.

Entanglement protection of high-dimensional states by adaptive optics

“…. Note that the final state read-out within the p=0 subspace was recently experimentally demonstrated in [44].…”

Entanglement certification from theory to experiment

“…This type of entanglement was first demonstrated with Schmidt number d ent = 3 in an experiment that measured a generalised Bell-type (Collins-Gisin-Linden-Massar-Popescu [149]) inequality with single-outcome, holographic projective filters that allowed the measurement of coherent superpositions of OAM at the single photon level [150]. In recent years, the development of computer-programmable wavefrontshaping devices such as spatial light modulators (SLMs) have allowed the measurement of OAM-entangled states with ever-increasing dimension [151,152]. Examples of such experiments include the certification of d ent = 100 spatial-mode entanglement with a visibility-based entanglement witness [153] and d ent = 11 OAM-entanglement with a generalised Bell-type test [154], both with certain assumptions on the state.…”

Overcoming Noise in Entanglement Distribution

“…ENTANGLEMENT (PATHWAY II) The second pathway to noise resilience takes advantage of the larger number of mutually unbiased bases in higher dimensions. Here, we explore this pathway using measurements of orbital angular momentum MUBs, for which precise measurements techniques have only recently been developed [54]. Mutually unbiased bases are an invaluable tool in many quantum information tasks, such as quantum state tomography, quantum cryptography, and entanglement certification.…”