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We present a flexible quantum key distribution (QKD) system implementation using quadrature phase-shift keying (QPSK) encoding. Two detection techniques are implemented and compared: a photon counting detection scheme using single photon avalanche diodes (SPAD) and a dual-threshold balanced homodyne detection (BHD) using standard PIN diodes in which the weak signal is time-multiplexed with a strong reference. The interferometer instability and the system phase fluctuations are compensated by an optoelectronic feedback loop that allows an automatic continuous operation. We compare the QKD system performance for both schemes in terms of BER and key generation efficiency. Finally, we analyze the BHD QKD system security under the potential individual intercept-resend attack and the intermediate-base attack.