Photon-bunching measurement after two $25\text{−}\mathrm{km}$-long optical fibers

To show the feasibility of a long-distance partial Bell-state measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecommunication wavelength are created by parametric down-conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam splitter and transmitted via two fibers of $25\mathrm{km}$. The wave packets are relatively delayed and recombined on a second beam splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the $25\text{−}\mathrm{km}$-long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a net visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long-distance Bell-state measurement with two independent sources, as, e.g., required in an entanglement swapping configuration in the scale of tens of kilometers.

Figure 1

Schematic of the experimental setup. A pair of photons emitted by one source is split by a first beam splitter (BS1). These photons are passed through a Mach-Zehnder interferometer of $2\times 25\mathrm{km}$ and are recombined on a second beam splitter (BS2). Coincidence counts between the two exit modes are regarded, while the length of one arm is changed.

Figure 2

Theoretically expected: second-order interferences in the coincidence count rate versus the phase. The dotted line represents the count rate averaged over several phases. Note that the phase units are arbitrary and the fringe scale has been modified for the reasons of clearness. In reality they are much closer.

Figure 3

Experimental realization of a long-distance photon-bunching measurement over $2\times 25\mathrm{km}$. A PPLN waveguide (PPLN-W), Bragg filter $\left(F\right)$ coupled to a three-port circulator $\left(C\right)$, adjustable path length $\Delta L$, polarization control (PC), beam splitter (BS).

Figure 4

Mandel dip after $2\times 25\mathrm{km}$: experimental results and fitted curve (solid line) with a maximal achievable visibility of 50%. The theoretical curve is represented by the dotted line. Accidental events are plotted below.

Figure 5

Dispersion cancellation scheme: even nonzero dispersion in one arm of a Mach-Zehnder interferometer leads to the same firing scheme of the detectors and result in an HOM dip with a width corresponding to the coherence time of the photons. The dotted line represents the center of the wave packet. BS1 and BS2 are the two beam splitters. It can be seen that in both cases detector $C$ clicks with the same time delay after detector $D$.