Remote preparation of single-plasmon states

Quantum entanglement is a stunning consequence of the superposition principle. This universal property of quantum systems has been intensively explored with photons, atoms, ions, and electrons. Collective excitations such as surface plasmons exhibit quantum behaviors. We report the remote preparation of a single plasmon state through the projective measurement of a photon entangled with the plasmon. We achieved photon-plasmon entanglement by converting one photon of an entangled photon pair into a surface plasmon. The plasmon is tested on a plasmonic platform in a Mach-Zehnder interferometer. A projective measurement on the polarization of the photon allows the remote preparation of the interference state of the plasmon. Entanglement between particles of various nature paves the way to the design of hybrid systems in quantum information networks.

Figure 1

Sketch of the entangled photon source. The laser diode pumps a periodically poled potassium titanyl phosphate (PPKTP) crystal to generate pairs of red photons. They are separated by a PBS before being sent onto a 50:50 fibered beam splitter. The optical path difference between the photons is adjusted with a translation stage that mechanically adds a delay on one arm. We use the coincidences between single photon counting modules (SPCMs) $\alpha$ and $\beta$ to postselect the entangled state described in Eq. (2).

Figure 2

(a) Sketch of the experimental setup. A polarizer (POL) is placed on path $\alpha$ and a polarizing beam splitter (PBS) is placed on path $\beta$. The PBS separates the incoming mode into two distinct photonic modes ${\beta }_1$ and ${\beta }_2$ which are next converted, respectively, into ${\mathrm{SPP}}_1$ and ${\mathrm{SPP}}_2$ modes on a plasmonic platform and recombined on a plasmonic beam splitter. This configuration is equivalent to a Mach-Zehnder (MZ) interferometer, with interference occurring on plasmonic modes. Fringes can be observed by introducing a variable delay on one of the photonic paths of the interferometer. SPCM C detects heralding photon counts, and SPCM A and SPCM B record detection counts heralded from SPCM C. (b) SEM picture of the plasmonic platform. The top and left gratings convert incident photonic modes into directional plasmonic modes. The light incident on each structure is polarized perpendicularly to the grating (TM polarization). The surface plasmon beam splitter (SPBS) consists of two diagonal slits that can be seen in the center of the device's picture. The two black rectangles are out-coupling slits that convert SPPs back to photons. The latter are finally detected by SPCMs A and B.

Figure 3

(a) Heralded photon output rates $R_{A|C}$ as a function of the varying delay ${\delta }_{\mathrm{MZ}}$ between the MZ arms. (a) POL is aligned along $H$ ($\theta =0^{\circ }$), one of the neutral axes of the PBS. No interference pattern appears for this direction of POL. (b) POL is aligned along the diagonal direction $D$ ($\theta ={45}^{\circ }$, with respect to the neutral axis of the PBS). An interference pattern is clearly observed, showing that the single SPP excites indistinguishably the two input arms of the SPBS. The solid line is the sine fit function of the experimental data. For those two sets of data, the source has a Bell parameter around 2.44. (c) POL is still aligned in the diagonal/antidiagonal basis, but the source is now characterized by a Bell parameter $S_{\mathrm{Bell}}=1.38$, meaning that the pairs are no longer entangled.