# Photonic quantum simulator for unbiased phase covariant cloning

## TL;DR

This paper demonstrates a photonic quantum circuit that simulates phase covariant cloning, showing how an eavesdropper can mitigate noise in quantum key distribution by using mirrored cloning strategies.

## Contribution

It introduces a linear optics implementation of a phase covariant cloner using different degrees of freedom of a single photon, enabling simulation of mirrored and asymmetric cloning machines.

## Key findings

- Mitigates noise in quantum key distribution via random cloning sequences
- Can produce clones biased towards opposite Bloch sphere regions
- Flexible setup for simulating optimal and non-optimal cloning

## Abstract

We present the results of a linear optics photonic implementation of a quantum circuit that simulates a phase covariant cloner, by using two different degrees of freedom of a single photon. We experimentally simulate the action of two mirrored $1\rightarrow 2$ cloners, each of them biasing the cloned states into opposite regions of the Bloch sphere. We show that by applying a random sequence of these two cloners, an eavesdropper can mitigate the amount of noise added to the original input state and therefore prepare clones with no bias but with the same individual fidelity, masking its presence in a quantum key distribution protocol. Input polarization qubit states are cloned into path qubit states of the same photon, which is identified as a potential eavesdropper in a quantum key distribution protocol. The device has the flexibility to produce mirrored versions that optimally clone states on either the northern or southern hemispheres of the Bloch sphere, as well as to simulate optimal and non-optimal cloning machines by tuning the asymmetry on each of the cloning machines.