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(Color online) Upper chart: Schematic of the experimental setup in our system. Inside Bob/Jr. Alice: components in Bob/Alice’s package of id Quantique QKD system. Our modifications: : polarization-insensitive phase modulator (detailed structure shown in lower chart); FG: functional generator. Original QKD system: LD: laser diode; APD: avalanche photon diode; : phase modulator; PBS: polarization beam splitter; PD: classical photodetector; FM: Faraday mirror. Lower chart: Detailed structure of the polarization-insensitive phase modulator. ISL: optical isolator; FC: fiber coupler; : electro-optical modulator; FM: Faraday mirror. Solid line: optical fiber; dashed line: electric cable.
Upper chart: correctly implemented phase randomization. A different random phase is applied on each bit. Lower chart: phase modulation for incorrect implementation: signal pulse and reference pulse are modulated by different phases and thus we could expect QBER to be around 50%. Right chart: differently polarized components of the same pulse are modulated at different moments.
(Color online) QBER vs delay time. Alice sent with on average for each point in this diagram. QBER at is surprisingly low , and is comparable to QBER at .
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