  Continuous-variable quantum key distribution protocol using coherent states
and heterodyne detection, called No-Switching protocol, is widely used in
practical systems due to the simple experimental setup without basis switching
and easy assessment to phase information. The security of an ideal No-Switching
protocol has been proved against general attacks in finite-size regime and
composable security framework, whose heterodyne detector consists of a beam
splitter with transmittance of $50\%$ and two ideal homodyne detectors.
However, the transmittance of a beam splitter is inaccurate and the two
detectors always have different quantum efficiency and electronic noise, which
introduce asymmetry into the heterodyne detection, and further lead to the
mismatch between the ideal protocol and practical systems, thereby
overestimating the secret key rate and resulting in a practical security
loophole. In this paper, we close this loophole by proposing a modified
No-Switching protocol with biased quadrature detection, where the asymmetry of
the heterodyne detection is modeled to match the practical systems, and the
security of the protocol is analyzed in asymptotic and finite-size regimes.
Further, an optimization strategy is proposed to achieve the optimal secret key
rate by adjusting the transmittance of the beam splitter. Simulation results
show the necessity of considering the asymmetry in heterodyne detection and the
effectiveness of the optimization, which provides a promising way to realize a
practical secure and high-performance No-Switching system.
