Metamaterial absorber for dual-rail photonic qubit filtering

We demonstrate manipulation of a photonic qubit, encoded in a dual-rail basis, by a plasmonic metamaterial absorber. The metamaterial is designed in a way to transmit a single photon, occupying anti-symmetric superposition of two spatial modes, without losses and to absorb its symmetric superposition completely. Thus, whatever the input state of the dual-rail photon, it can leave the absorber in anti-symmetric state only. Probability to pass through the absorber is defined by the amplitude of the anti-symmetric part in the input photon state. To verify this phenomenon, we assembled actively stabilized in-fibre quantum network containing fully-fiberized metamaterial package. By altering the input state of the photon propagating in the network and passing through metamaterial, we measure the output state of the photon and show that it is coherent and does not depend on the input state of the photon. By varying the input state, we also affect the probability of the photon to pass through the absorber. All the results are in a good agreement with the above discussion. We notice, that absorber can be designed in an opposite way: to transmit symmetric state and to absorb anti-symmetric one. Proposed devices can be employed in quantum computation schemes with dual-rail encoding as well as in other protocols of quantum information, operating with single photons or weak coherent states