Effects of Cavity Birefringence on Polarisation Encoded Remote Entanglement Generation

The remote entanglement of ions in distinct traps via single photons distributed over a photonic network is a promising method for scaling ion trap quantum computers. Collecting the single network photons using optical cavities can greatly enhance the entanglement rate.  

 

We present results of a study into the fidelity loss due to cavity birefringence when using polarisation-encoding of the photonic qubit. We then consider three techniques for restoring the fidelity: windowing of the photon detection time; electro-optic rotation of the cavity output photon polarisation; and local rotation of the ion qubit conditioned on the photon arrival times. We demonstrate that these methods can partially restore the lost fidelity at the cost of substantially increased technical complexity. For current standards of mirror fabrication, fidelity loss is likely to be significant even post-correction. 

 

We conclude that cavity birefringence is a crucial consideration for polarisation-encoded entanglement of remote atomic systems, and unless significant improvements are made to micro-mirror fabrication techniques, alternative remote entanglement schemes, such as time-bin encoding, may be superior.