A quantum cryptographic network enhanced by quantum memories

We report on the creation of an elementary memory assisted quantum communication network and present individual experiments addressing the various challenges to create the quantum connectivity needed to perform long-distance... [ view full abstract ]

We report on the creation of an elementary memory assisted quantum communication network and present individual experiments addressing the various challenges to create the quantum connectivity needed to perform long-distance quantum communication with random polarization qubits and entanglement.

We have shown an ultra-low noise regime of operation in warm Rb87 atomic vapor quantum memories. Fidelities >99% for single-photon level polarization qubits are possible by modeling the quantum dynamics of four-level room-temperature atoms. Employing noise-reduction techniques permits the use of higher optical depths and control field powers, allowing for storage efficiencies above 50% [1]. Storage times at the few-photon-level of ~1ms are also already within reach. We have also investigated memory-integrated photon-shaping techniques necessary to interface several quantum memories efficiently [2].

Our quantum network already combines several different quantum modules: (i) two independent random polarization qubit generators, (ii) two ultra-low noise warm-vapor quantum memories and (iii) a Bell state qubit decoder and reading station, (iv) atom-tuned quantum light and (v) free-space/fiber quantum communication channels. After storage and retrieval, the qubits are analyzed in a four-detector polarization setup. We have observed Hong-Ou-Mandel interference between independent quantum memories and estimated quantum bit error rates sufficient to perform the BB84 and Memory-Assisted Measurement-Device-Independent Quantum Key Distribution protocols [3].

In addition, we use Spontaneous Parametric Down Conversion in a non-linear crystal placed inside a cavity, to generate non-classical light tuned to Rubidium transitions. The quantum light has its wavelength and bandwidth matched to those of an Electromagnetically-Induced Transparency window, thus making it compatible with quantum memory operation. We have performed real-time state reconstruction of the output quantum light using Quantum State Tomography via a homodyne detection.

With further optimization of these components we will reach the parameters required for long-distance entanglement distribution and build a prototype of a quantum repeater node.

[1] Phys. Rev. Applied 8, 034023 (2017)
[2] Phys. Rev. A 92, 033846 (2015)
[3] Phys. Rev. Applied 8, 064013 (2017)