Quantum Repeater

In conventional optical communication, information carrying light pulses can travel in near transparent optical fibres for hundreds of kilometres. In order to extend the communication range of optical telecommunication networks for global usage, optical amplifiers are used to amplify the light signals along the length of the communication line. These amplifier stations, also known as “repeater stations”, can be located along highways, under the ocean, or within an earth orbiting satellite in space.

Figure 1: Schematic block diagram of a quantum repeater.

In a quantum communication network, fragile quantum states are transmitted between a sender and a receiver. Due to decoherence introduced during transmission (either from losses or from environmental contamination), quantum communication is restricted to a range of around 200km beyond which states can no longer be reliably measured. Similar to its classical analogue, a quantum repeater is a device that can extend the range of quantum communication between sender and receiver. In contrast to classical network, however, quantum information cannot be detected or amplified without having its information converted back to classical information. Straightforward adoption of an optical amplifier in a quantum network therefore cannot work. A quantum repeater has to achieve an effective “amplification” or restoration of the quantum information without resorting to a direct measurement of the laser light.

Figures 2a and b: Whispering gallery mode squeezed and entangled light generation.

CQC2T aims to realize an operational quantum repeater that can be used in a quantum key distribution network for extending the communication range to beyond that of a passive network. Realization of an operation quantum repeater requires interfacing and integrating a number of quantum components into one complete system. These components include:

Entanglement Generation

Optical entanglement is a basic resource for quantum communication. Entanglement generation can be based on single-photon pair production or on interference of squeezed light. In CQC2T both approaches are being adopted. As an example, squeezed state of light can be generated in mini-whispering gallery mode resonators formed by nonlinear crystals. The tight and near lossless confinement of the optical mode within the resonator can have large nonlinearity for generating quantum states of light. These WMG resonators can be used to generate multi-partite entanglement.

Figure 3: Fabrication of whispering gallery mode optical resonator using ultra-precision lathe with diamond cutter.

Quantum Memory

In order to synchronize the various stages of the quantum communication line, light pulses need to be stored and retrieved on demand while preserving the embedded quantum information. At CQC2T our schemes are mainly based on the gradient echo memory (GEM) approach. Two complementary approaches adopted by us are (a) Rb gas cell Raman GEM and (b) rare-earth ion crystal GEM. These memories should have high efficiency and have long coherence time.

Figure 4: Experimental setup of a whispering gallery mode optical resonator.

Quantum Measurement & Processing

Entanglement between different part of the communication line needs to be processed. At the Centre we plan to explore a range of measurement and processing protocols. They include entanglement swapping and distillation, conditioned noiseless linear amplification, small to medium scale cluster state quantum computation.

Figure 5: Rb gas cell gradient echo memory.