Narrowband Frequency-Entangled Photon Source for Hong-Ou-Mandel Interferometry

Abstract

Hong-Ou-Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhance the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the sub-millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.

Figures (5)

• Figure 1: (a) HOM interference of frequency-entangled photons and the corresponding (b) Fisher information for various frequency detuning between the signal and idler photons $\Delta f=\Delta\omega/2\pi$. The open circle represents the point where the Fisher information is undefined. The inset shows an enlarge view of the Fisher information calculated with degenerate photon pairs. The interference visibility in the calculations is $V=1$. (c) Fisher information ratio as a function of interference visibility. The angular frequency detuning between the signal and idler photons for frequency-entangled state is $\Delta\omega=10$ THz. In the calculations, the waveform of the state is double-exponential decay function with cavity decay rates $\Gamma_{s}=\Gamma_{i}=$ 1/20 ns$^{-1}$.
• Figure 2: Schematic of the experimental setup. EOM: electro-optic modulator, FG: function generator, M: mirror, PBS: polarizing beam splitter, HWP: half-wave plate, BS: non-polarizing beam splitter, PD: photodetector, PID: proportional–integral–derivative controller, LD: laser diode, LN: x-cut LiNbO$_{3}$ prism, WGMR: whispering gallery mode resonator, D: diamond prism, DM: dichroic mirror, RM: retroreflector mirror, FC: fiber collimator, SNSPD: superconducting nanowire single-photon detector, TM: transverse magnetic modes, TE: transverse electric modes, o: ordinary, and e: extraordinary.
• Figure 3: (a) Reflectance spectra for the pump beams in the CW and CCW directions measured from photodetectors $D_{1}$ and $D_{2}$, respectively. (b) Glauber correlation functions of photon pairs generated in the CW and CCW directions of the WGMR.
• Figure 4: (a) HOM interference of narrowband frequency-entangled photons. The highest interference visibility achieved is 87.3$\%$ based on the best fit (red curve). (b) HOM interference with various optical path delays. The HOM interference fringes are measured under critical coupling conditions, where the coupling rate is approximately equal to the internal loss rate. Under these conditions, the photon-pair bandwidth is approximately 5.6 MHz. The red curves are the best fits to the measured data.
• Figure 5: (a) Glauber correlation functions with various bandwidths of photon pairs $\Delta\Omega$. (b) HOM interference measured under the overcoupling condition exhibits sinusoidal oscillations with a fitted beating period of 3.20 $\mu$m and an interference visibility of 72.6$\%$. The photon-pair bandwidth is approximately 43.53 MHz.