## 4 Experimental Implementations  
In the previous sections, we have seen that continuous-variable QKD protocols may vary in terms of required resources, in particular for state preparation (squeezed or coherent states) and detection techniques (homodyne or heterodyne). These choices are of great importance for the security achieved by the corresponding implementations (see Table 1), but also affect their performance, which is typically quantified by the maximal distance over which secret keys can be generated and the rate of their production. Another important choice is the medium used for the transmission of the quantum keys, namely optical fiber or free space, which depends on the targeted application of the QKD implementation. It is interesting to remark here on a historical note that initial proposals for CVQKDnecessitated the use of squeezed states and were burdened by a 3-dB loss limit [41, 53], which greatly limited their practical interest in any realistic communication scenario. Later, however, protocols using coherent states appeared (GG02) [22], and the 3-dB limit was lifted [20, 25]. These results enhanced considerably the interest in the use of continuous variables for QKD and were at the basis of a series of works that led to ever better performing systems. As in discrete-variable QKD, PM CVQKD protocols are in general easier to implement in practice. We describe in the following in some detail PM fiber optic implementations of the GG02 protocol, whose principle and security were discussed in Sections 2 and 3, respectively. This protocol is particularly interesting from a practical point of view, since it merely necessitates the generation of coherent states,  
their modulation in phase space and the detection of the quadratures of the received states using homodyne (or heterodyne) techniques. The components required to achieve these functionalities are readily available at a telecommunication wavelength, which is suitable for operation with fiber optic systems. The optical configuration for performing this protocol is shown in Figure 1. In this scheme, the signal and phase reference (or local oscillator) that is necessary for performing the coherent detection are generated from a laser diode source at [PERSON]'s site. The signal is modulated in amplitude and phase following a Gaussian distribution as required by the protocol and then attenuated at a suitable modulation variance level. It is also multiplexed both in time and in polarization with the local oscillator before entering the quantum channel. At [PERSON]'s site, the two signals are demultiplexed using, respectively, a delay line and a polarization beam splitter and superposed in time to interfere on a shot noise-limited balanced pulsed homodyne detector. The quadrature selection required by the GG02 protocol is performed by the phase modulator placed in the local oscillator path. The setup is completed by several active feedforward and control elements, which provide the necessary synchronization and stability conditions for performing the quantum key distribution. Figure 1: Optical layout of a fiber optic CVQKD system implementing the GG02 [22] protocol with homodyne detection. The described system realizes the first part, namely (i) state distribution and measurement, of the full GG02 protocol described in Section 2; the remaining post-processing parts, namely (ii) error reconciliation, (iii) parameter estimation and (iv) privacy amplification, and, in particular, the first two, require sophisticated computational algorithms, as we will discuss further below. The initial realization of the optical setup of Figure 1 was used in the European SECOQC QKD network [4], which was deployed over installed optical fibers and integrated various QKD technologies [4, 54]. It was also used in a field test of a point-to-point classical symmetric encryption link with fast key renewal provided by the quantum layer, which demonstrated the reliability of the CVQKD system operation over a long period of time in a server room environment [55].