Harry Potter's Invisibility Cloak and Quantum Spoofing

To observe, we capture the reflection of electromagnetic waves that come from light sources (emitter) on objects in our eyes (receiver). A magical garment such as Harry Potter's invisibility cloak should absorb that light and emit carefully crafted electromagnetic waves to fool us into believing that something isn't there. Of course, we are assuming that a scientific explanation like that makes sense in the Wizarding world. In the context of radar, one would call that magical item a spoofer, a third party that absorbs and emits those carefully crafted electromagnetic waves to false an object's position, shape, speed, etc. The difference between these situations is that, in radar, the receiver is also the emitter of the electromagnetic waves. That difference gives the emitter an edge as it can encode information (into the pulse shape and spectral content) to distinguish a truly reflected signal from the one generated by a spoofer. Indeed, one may look for Harry Potter using a lantern that emits light where one of two non-orthogonal quantum states is encoded. The invisibility cloak should measure the state and return the same one to generate the spoofing. However, the success of that state's determination is probabilistic. Assuming that magic doesn't break quantum mechanics, the emitter may detect when the state doesn't match, break the charm, and detect Harry Potter. Nevertheless, the receiver also has to do a measurement with a probabilistic outcome to determine that. Recently, Blakely et al. [1] introduced this concept of quantum spoofing and demonstrated an advantage with respect to a classical version of the problem. In their approach, that advantage appears when limiting themselves to a small number of photons. Here, we aim to demonstrate that the quantum advantage remains for a large number of photons. We also derive analytically the quantum states needed to attain that advantage. In our approach, single photon sources are not required for a proof-of-principle experiment, thus opening the door for an experimental implementation in a standard quantum optics lab and facilitating further development of quantum radar technology.

J.N. Blakely and S.D. Pethel, Physical Review Research 4, 023178 (2022)