Quantum Communication with non-Gaussian states

In this thesis we investigate two aspects of quantum information processing with continuous variables in networks. Quantum physics is currently undergoing its second revolution, in which the unique properties of quantum superpositions and non-classical entanglement are harnessed and engineered to improve technologies across a wide range of fields.

One particular area of interest is the connection of different quantum devices across a shared network, where the scale can range from a single room to the size of a university or company campus. An obvious choice of channels for networks of this size is optical fibers and so the faithful transfer of quantum states of light across the channels of the network becomes an integral challenge.

Firstly, we generate a continuous variable non-Gaussian state, namely the single photon subtracted squeezed vacuum state (1-PSSqV), and use it as a probe of the transmission efficiency across three different network channels. For the first channel, a 1m single mode fiber (SMF) on the same optical table of the state generation setup, we measure a Wigner negativities of −0:206 ± 0:001Π of the received state.

For the second channel, a 60 m SMF connection between the state generation lab and an adjacent lab, we measure −0:104 ± 0:001Π. For the third channel, a 400 m connection across 3 nodes of the DTU campus fiber-optic network to a separate building, we unfortunately could not measure any Wigner negativity.

Here the main problem was optical loss of the channel. The presence of Wigner negativity confirms the survival of the highly non-classical correlations of the transmitted state. Secondly, we implement a sensing protocol on a small on-table free-space network consisting of four nodes. In the protocol a continuous variable multi-partite entangled state is used to measure the average of individual phase shifts at each node.

Here we show an increased sensitivity to the phase shift, as a ∼ 20% reduction in the root-mean-square estimation error, compared to the sensitivity possible for any measurement protocol not using an entangled probe state.