Continuous-wave based homodyne detection of pulsed single-photon states

In continuous-variable optical quantum information processing, balanced homodyne detection (BHD) is crucial for measuring information encoded in the quadratures of light. Here, we propose and experimentally demonstrate a novel "pulse-continuous wave (CW) hybrid BHD" scheme, where quantum light generated by pulsed light is measured using a CW local oscillator (LO). We injected pump pulses of tens of picoseconds into a Type-II PPLN waveguide to generate an EPR state via parametric down-conversion. By using a heralding scheme, a single-photon state was heralded in the other mode and measured with BHD using a CW LO. The fidelity of the measured single-photon state was 73.8%, with a wavepacket width of 70 ps, which demonstrates the high-precision measurement of short-wavepacket quantum states thanks to the pulse-CW hybrid BHD scheme.

In conventional schemes, when both pump and LO are CW, wavepacket widths were limited to the nanosecond range due to the BHD bandwidth. On the other hand, using pulsed light for both the pump and LO led to temporal mode mismatches, which reduced the measurement accuracy. Our hybrid scheme overcomes these limitations, enabling high-precision measurement of picosecond wavepackets by employing a broadband BHD with optical parametric amplifier and generating pulsed light via intensity-modulated CW light to maintain coherence between the pulsed and CW light. In this presentation, we report on these experimental results.