Using a physical detector we demonstrate high-contrast bi-photon entanglement measurement in few milliseconds with ~10⁸ bi-photons/detection-time, a record-high speed quantum measurement.

In spite of their unique quantum properties, broadband bi-photons are the 'black sheep of the family' in current quantum information, and are rarely used in experiments, mainly because of the bandwidth incompetency between the bi-photons bandwidth (reaching ~100THz in our report) and the photo-detectors in standard detection schemes. When the single photons are directly detected, the photo-detectors response time is far too slow (~100ps with the fastest available detectors) to resolve the ultrafast correlation time (of order 10-100fs), and the maximum detectable flux for standard coincidence circuits is limited to few 10⁶ photons/sec, inhibiting utilization of the ultra-high flux offered by broadband bi-photons. Thus, the broad bandwidth of bi-photons is an impediment for standard detection, not a resource. 

By offering a physical detector, we enable full measurement of the bi-photons quantum wave function, utilizing the full bandwidth of the bi-photons, and enabling an ultrafast high visibility measurement (>50%) in just a few milliseconds. For this purpose we use a well known interference effect, where the possibilities for bi-photon generation in two different nonlinear crystals interfere quantum mechanically. We exploit this pairwise interference to measure the spectral phase and amplitude of ultra-broadband bi-photons and to observe their nonclassical nature with near unity efficiency, thereby fully utilizing the ultra-high flux of photons and speeding the measurement by several orders of magnitude. 

When bi-photons produced in one crystal enter a second crystal, they can either enhance further down conversion, or be up-converted back to the pump, depending on the relative phase between the pump and the bi-photons. This is a quantum mechanical interference between two indistinguishable possibilities to generate bi-photons - either in the first crystal or in the second. 

In order to demonstrate that the observed interference contrast is indeed a nonclassical feature, we compare two experimental scenarios that are classically indistinguishable, yet quantum mechanically lead to very different results: we attenuate the light entering the second crystal in two ways - once by attenuating the pump before the first crystal, thereby reducing also the generated bi-photons flux; and second, by attenuating both the pump and the bi-photons beam between the two crystals. Quantum-mechanically these two possibilities are different procedures, as the first attenuates the generation rate of bi-photons, but does not alter their purity, thereby preserving the interference contrast, whereas the second attenuates every photon independently, reducing the single photon flux linearly, but the ”surviving” bi-photon flux quadratically, thereby reducing the bi-photon state purity and thereby reducing the interference contrast.