Considerable progress in wireless power transfer has been made in the realm of non-radiative transfer, which employs magnetic-field coupling in the near field. A combination of circuit resonance and impedance transformation is often used to help to achieve efficient transfer of power over a predetermined distance of about the size of the resonators. The development of non-radiative wireless power transfer has paved the way towards real-world applications such as wireless powering of implantable medical devices and wireless charging of stationary electric vehicles. However, it remains a fundamental challenge to create a wireless power transfer system in which the transfer efficiency is robust against the variation of operating conditions. Here we propose theoretically and demonstrate experimentally that a parity-time-symmetric circuit incorporating a nonlinear gain saturation element provides robust wireless power transfer. Our results show that the transfer efficiency remains near unity over a distance variation of approximately one metre, without the need for any tuning. This is in contrast with conventional methods where high transfer efficiency can only be maintained by constantly tuning the frequency or the internal coupling parameters as the transfer distance or the relative orientation of the source and receiver units is varied. The use of a nonlinear parity-time-symmetric circuit should enable robust wireless power transfer to moving devices or vehicles.

We describe an encrypted communication principle that forms a secure link between two parties without electronically saving either of their keys. Instead, random cryptographic bits are kept safe within the unique mesoscopic randomness of two volumetric scattering materials. We demonstrate how a shared set of patterned optical probes can generate 10 gigabits of statistically verified randomness between a pair of unique 2 mm3 scattering objects. This shared randomness is used to facilitate information-theoretically secure communication following a modified one-time pad protocol. Benefits of volumetric physical storage over electronic memory include the inability to probe, duplicate or selectively reset any bits without fundamentally altering the entire key space. Our ability to securely couple the randomness contained within two unique physical objects can extend to strengthen hardware required by a variety of cryptographic protocols, which is currently a critically weak link in the security pipeline of our increasingly mobile communication culture.

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This paper presents an optical method of storing random cryptographic keys within a reconfigurable volume of polymer-dispersed liquid crystal (PDLC). We suggest a PDLCbased device that functions as an integrated optical physical unclonable function (PUF). Our device can selectively access a dense set (up to 10 Gb/mm 3 in theory) of non-electronically saved random bits. Furthermore, this optical PUF can fully erase and transform these bits into a new random configuration in less than one second, via a simple electrical signal. When a short voltage spike is applied across the PDLC film interface, its optical scattering potential completely decorrelates. We confirm this phenomenon with detailed experiments on a proof-of-concept device, thereby suggesting the security use of a new class of optical materials as (i) securely and efficiently reconfigurable PUFs, and (ii) an erasable storage medium for random cryptographic keys. Our work can eventually help address the challenge of quickly and completely erasing sensitive digital electronic memory and/or key material. It also establishes a new and hopefully fruitful connection between security questions and the material sciences.

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