Figure 1

Schematics of the LM05 protocol. Bob prepares the qubit with random bases and values and sends it to Alice. Alice encodes data on it with probability $(1-c)$ or measures the qubit with probability $c$. In case of measurement she also prepares a new qubit in the same state resulting from her measurement. Finally Bob measures the qubit in the same basis used for its preparation and decodes the data.Reuse & Permissions

Figure 2

(Color online) LM05 setup using integrated optics over optical fiber. Bob’s station: 1, DFB-LD at $1550\mathrm{nm}$; 2, fixed attenuator; 3, variable attenuator; 4, circulator; 5, polarization maintaining variable coupler; 6, PM fiber $\left(10\mathrm{m}\right)$; 7, phase modulator; 8, polarization combiner; 9, WDM coupler (at $1550–1310\mathrm{nm}$); 10, DFB-LD at $1310\mathrm{nm}$; 11, InGaAs APDs (id-Quantique, id200). Alice’s station: 1, WDM coupler (at $1550–1310\mathrm{nm}$); 2, $80:20$ $1\times 2$ coupler; 3, polarization coupler; 4, Faraday mirror; 5, phase modulator; 6, fast InGaAs $p\text{−}i\text{−}n$ derector. In blue color are drawn single-mode fibers and in red color polarization-maintaining fibers. VA: variable attenuator for simulating a longer optical fiber.Reuse & Permissions

Figure 3

(Color online) Variation of the QBER with different mean photon number for three different simulated fiber lengths (5.86, 11.22, $20.42\mathrm{km}$) connecting the two users. The values of $\mu$ are measured as described in the text. The theoretical curves are plotted according to the noise model given by Eq. (31) with $Q_{\mathit{opt}}=1.35\times {10}^{-2}$, $Q_{\mathit{el}}=0.55\times {10}^{-2}$, and $p_{\mathit{\text{dark}}}=1.3\times {10}^{-4}$.Reuse & Permissions

Figure 4

(Color online) The experimental rates of the LM05 apparatus. The quantities are given as a function of the fiber length connecting the two users. The raw key is the collection of the events in which only one of Bob’s detectors is counted. The partially amplified key is the result of a privacy amplification process which only takes into account the parities $P$ revealed during error correction and the security parameter $S$ (see text for the parameters’ definitions). $F_{\mathrm{LM}05}^{\mathit{DR}}$ and $F_{\mathrm{LM}05}^{\mathit{RR}}$ are the theoretical rates pertaining to the LM05 protocol when intercept-resend and beam-splitter attacks are taken into account, in the case of direct reconciliation and reverse reconciliation, respectively (see text for details).Reuse & Permissions