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Utilizing the phase-matching conditions of inter-modal four-wave mixing in an elliptical-core few-mode fiber supporting three non-degenerate modes, we experimentally demonstrate schemes for generating orbital-angular-momentum (OAM)-entangled photon pairs with high mode purity and for achieving highly mode-selective frequency conversion of beams in OAM-compatible (LP11a, LP11b) mode basis. These techniques expand the toolbox for using OAM modes in both classical and quantum communications and information processing. 

Quantum communication links and networks are needed for secure information exchange and for interconnecting future quantum computers. However, their capacity decreases exponentially with distance due to the effect of fiber attenuation that cannot be undone by amplification (although quantum repeaters are an active area of research, they are almost as hard to build as quantum computers). Hence, the only way to increase the quantum communication capacity is by employing more degrees of freedom (optical modes) over which this information can be encoded and transmitted. While frequency (WDM), temporal, and polarization modes have already been exploited for this purpose, the use of many spatial modes has only recently become possible owing to the development of low-loss few-mode fibers (FMFs). This talk will present the work of Prof. M. Vasilyev’s research group on the development of two key enablers of quantum communication over spatial modes of FMFs: 1) generator of spatially-entangled photon pairs and 2) receiver sub-system that can perform projective measurements that alternate between two sets of mutually unbiased bases in a given spatial mode space (this sub-system can also perform dynamically reconfigurable de-multiplexing of spatial modes of the FMF). Both of the above devices / sub-systems are based on the spatial-mode-selective quantum frequency conversion process, implemented in a medium with either second-order nonlinearity (multimode lithium niobate waveguide) or third-order nonlinearity (custom-made FMF). The talk will introduce the principles of their operation, as well as the recent experimental results obtained in both media. 

We discuss three emerging applications of wavelength conversion: 1) hybrid amplification outside of EDFA band, based on a combination of two wavelength converters and an EDFA, 2) spatial-mode-selective wavelength conversion, and 3) generation of spatial-mode-entangled photon pairs. 

We discuss wavelength conversion of a selected signal spatial mode, which preserves its quantum state and does not disturb other signal spatial modes. We present the results for a lithium niobate waveguide and a few-mode-fiber. 

We describe OAM-compatible mode-selective frequency conversion in a few-mode fiber and experimentally demonstrate downconversion of various superpositions of signal modes LP11a and LP11b to the same LP11b mode with conversion efficiency differences <0.8 dB. 

We present a scheme for spatial-mode-selective frequency conversion in a few-mode fiber and experimentally demonstrate upconversion of arbitrary superpositions of two signal modes from C-band to the fundamental mode in S-band with conversion efficiencies within 1 dB range of one another. 

Aiming at producing spatial-mode-entangled photon pairs in a few-mode fiber, we experimentally demonstrate generation of idler beam from a seed signal in a superposition of two fiber modes. For every signal mode superposition, we observe the indication of idler mode orthogonality to the signal mode. 

We describe a novel scheme for generation of orbital-angular-momentum-entangled photons in a few-mode fiber. We experimentally verify the underlying inter-modal parametric processes with two-mode classical signal input, observing high mode purity of the generated idler.