Observation and Manipulation of Optical Parametric Down-Conversion in the Langevin Regime

Abstract

Quantum fluctuation plays a key role in the parametric down-conversion in the Langevin regime. In this paper, we report the experimental realization of optical parametric down-conversion in the Langevin regime on a chip. By precisely controlling the loss inherently tied to fluctuation, we observe the asymmetric Hong-Ou-Mandel dip - a hallmark of quantum fluctuation in the fluctuation-driven PDC, and the fluctuation-induced compression of single photons by nearly one order of magnitude. These findings pave the way for the manipulation of quantum fluctuation, quantum states, and system-reservoir interaction.

Figures (4)

• Figure 1: (a) Glauber correlation functions and (b) biphoton spectra for $\alpha_{s}=0, 100, 200, 500$ and $1000$ m$^{-1}$. (c) Idler-triggered and (d) signal-triggered Glauber correlation functions for $\alpha_{sum}=100$ m$^{-1}$. (e) The HOM interference as a function of path difference. The black solid line is the calculation with the Langevin terms, and the color lines (or dashed lines) are the results without the Langevin term for $\Delta\alpha =$$-100$, $-50$, $0$, $50$, and $100$ m$^{-1}$. (f) The observed HOM interference (blue square) compared to theoretical predictions. Black and red curves are calculated by the Langevin and non-Langevin theories, respectively, with $\alpha_{s} = 87.00$ m$^{-1}$ and $\alpha_{i} = 29.77$ m$^{-1}$. The inset shows the region near the dip. All calculations consider a lossy region across the crystal length of $L =8$ mm.
• Figure 2: (a) Schematic of the Ti-diffused PPLN waveguide sample. The multilayer structure above the PPLN waveguide consists of 50 nm thick SiO$_{2}$ and 15 nm thick Ti layers, followed by a 200 nm layer of Au. Different lengths of the Ti/Au layers $L_{1}$ are fabricated. (b) The experimental setup of HOM interference (black optical paths) and spectrum measurement (red optical paths). ECDL: external-cavity diode laser, OBJ: objective, F: long-pass filter, M: mirror, PBS: polarization beam splitter, FC: fiber coupler, TS: translation stage, PC: polarization controller, FBS: fiber beam splitter, FBP: fiber band-pass filter, SPD: single photon detector.
• Figure 3: Fluctuation-driven PDC with position-dependent loss for $L=2$ cm, $\alpha_{s1}=1100$ m$^{-1}$, $\alpha_{i1}=60$ m$^{-1}$, and $\alpha_{i2}=\alpha_{s2}=0$ m$^{-1}$. (a) Glauber correlation functions. (b) Biphoton spectra. (c) HOM interferences. The FWHM widths and visibilities are 0.78 mm (1), 0.60 mm (0.981), 0.37 mm (0.967), and 0.14 mm (0.890) for $L_{1} = 0$, $0.3 L$, $0.6 L$, and $0.9 L$, respectively. (d) HOM interferences without the Langevin term.
• Figure 4: Manipulation of optical PDC in the Langevin regime. (a) Measured HOM dips without the SiO$_2$ and Ti/Au layers. The coincidence rates per mW of pump power for $L_1=0$ and $0.6L$ are multiplied by 1.4 and 6.1, respectively. (b) Measured HOM dip with the SiO$_2$ layer. (c) Measured HOM dip with the SiO$_2$ and Ti/Au layers ($L_1=0.6 L$). (d) Measured HOM dip with the SiO$_2$ and Ti/Au layers ($L_1=0.9 L$). The visibilities are 0.746, 0.840, and 0.851 in (b), (c), and (d) at a pump power (before the objective) of 0.3, 4, and 4 mW, respectively. The black curves are the corresponding theoretical predictions in Fig. \ref{['fig:PC_sim']}(c). (e) Measured spectra of the heralded idler photons. (f) Measured spectra of the heralded signal photons.