In an ideal BSM device, one would be able to project an arbitrary two-qubit entangled state into any of the four so-called Bell states. In practice, this can be done only with limited probability – if the experiment is restricted to the use of linear optics without auxiliary photons the success probability is limited to 50% for ideal detectors. If non-ideal detectors are used, the success probability is reduced by a factor given by the quantum efficiency squared.
BSM can be carried out for qubits encoded in any discrete photonic two-dimensional degree of freedom, for example polarization or time bin. The current paper focuses on time-bin qubits prepared by an attenuated laser beam modulated in time so that approximately a single photon is emitted, in each of two channels, into “early” (labeled as |0>) and “late” (labeled as |1>) time windows, which play an analogous role to horizontal / vertical polarizations for the case of polarization encoding. The BSM operation is accomplished by combining the two channels at a beam splitter and time-resolving the arrival of photons at the two output ports of the beam splitter. Coincident detection at the two output channels with cross “early” / “late” identifiers indicates a Ψ- state, while detection events with both “early” and “late” identifiers in a single channel indicates a Ψ+ + state.
Projection of the two qubits into a Ψ+ state is limited by the dead time inherent in the avalanche photodiodes (APD’s) typically used, since the same detector needs to fire twice, i.e., for each of the “early” and “late” time windows. The essential contribution of this paper is the use of custom superconducting nanowire single photon detectors (SNSPD’s), based on tungsten silicide meanders, for an implementation of a BSM device with a drastically improved performance. There are two important fronts in which the use of SNSPD’s helps. On the one hand, the quantum efficiency of the detectors used is an impressive >76% (as compared to around 15% for indium gallium arsenide APD’s). On the other hand, by using a custom-designed circuit for the SNSPD’s, the deadtime has been reduced to 40ns (compared to around 10 μs for indium gallium arsenide APD’s). A high quantum efficiency with reduced deadtime, combined with operation in non-gated, free-running mode and in the absence of after-pulsing makes this a far superior single-photon detection technology.
The authors have demonstrated a total BSM efficiency of ~28% for the z basis (|0>, |1>) and ~30% for the x basis (|0> ± |1>), which corresponds to a ~30 times enhancement with respect to the best previous result, and clearly constitutes an important step forward towards reaching the 50% ideal efficiency.
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