Figure 2
From: Performing private database queries in a real-world environment using a quantum protocol
Diagram of the experimental setup.
The database (Dave) uses a computer and field-programmable gate-array (FPGA) to control the generation of polarization qubits via an attenuated laser diode (LD1 and ATT) and polarization modular (PM). Quantum frames32 (sequences of strong light for timing and stabilization) are generated by a second laser diode (LD2) and merged using a polarizing beam-splitter (PBS). Light is transmitted from Dave to Ursula through a 12.4 km dark fiber link with 4.5 dB loss between SAIT Polytechnic and the University of Calgary. Ursula splits off 10% of the incoming light (90/10 BS) to a photodiode (PD) used to detect the quantum frames. The 50/50 BS is used to passively select a random measurement basis. The apparatus for each basis consists of a polarization controller (PC), a PBS and two single photon detectors (SPD) to make the projection measurement. Upon detecting a quantum frame, Ursula's FPGA triggers the SPDs and initiates data collection by the computer, or polarization compensation, as appropriate.
