Figure 1

The Fock-state filter: a device that blocks the passage of single photons, but allows the coherent passage of photon pairs. As described in the text, a probabilistic Fock-state filter can be created by combining a 50% beam splitter, ancilla photon, quantum interference, and measurement.Reuse & Permissions

Figure 2

Quantum interference in two- and fourfold coincidence counts as a function of the longitudinal position of the input fiber coupler for mode $b$. At zero delay, we see marked preferential absorption of single-photon over two-photon states in mode $c$, as indicated by the larger dip in two- over fourfold counts. The two- and fourfold raw visibilities are $(95.20\pm 0.02)\%$ and $(68\pm 5)\%$, respectively; correcting for background as described in the text, the twofold visibility becomes $(99.6\pm 0.1)\%$ (error bars are smaller than the points in the twofold case and are not shown). The visibilities are in excellent agreement with the theoretically expected two- and four-visibilities of 100% and 66.7% [6, 11, 18]. The input coupler was scanned 1 mm in 630 s: to mitigate drift effects the scan was repeated 63 times, leading to an integration time of 31.5 min per point. The slopes in the data are due to longitudinal-position-dependent coupling to the detectors; the trigger detector was particularly sensitive in this respect, leading to a large slope in the fourfolds; the twofolds show a much smaller slope as the trigger detector plays no role in that data. The visibilities were obtained from curve fits to products of a Gaussian and a linear function.Reuse & Permissions

Figure 3

Density matrices for the Fock-state filter. Ideal output states from the filter when the filtering is (a) turned off, $|DD⟩$, and (b) turned on, $(|HH⟩\mathrm{\text{−}}|VV⟩)/\sqrt{2}$, as described in text. The corresponding experimental tomographic reconstructions, based on raw counts, are shown, respectively, in (c) and (d), the upper (lower) panels are the real (imaginary) components. The fidelity between the ideal and measured states is $93\pm 4\%$ and $69\pm 9\%$, respectively. The state measured in (d) is entangled, with tangle $T=20\pm 9\%$.Reuse & Permissions