  Time-bin qubits, in which quantum information is encoded in a single photon
at different times $\Delta$t, are suitable for long-distance transmission via
optical fibers. However, detection of time-bin qubits has been limited to the
nanosecond range owing to the limited temporal resolution of single-photon
detectors. In this study, we developed an up-conversion single-photon detector
(UCSPD), using commercial nonlinear crystals of different lengths. By changing
the crystal length and pump power, we quantitatively evaluated the efficiency
and temporal resolution of the UCSPD and determined the optimal conditions for
measuring femtosecond time-bin qubits. This detector achieved a temporal
resolution of 415 fs and up-conversion efficiency of 10.1 %. Consequently, we
successfully evaluated single-photon level pseudo femtosecond time-bin qubits
with a pulse interval of only 800 fs.
