Quantum interference between spectral bandwidth mismatched photons

Abstract

Two-photon interference is a cornerstone of photonic quantum technologies. However, its practical implementation in promising hybrid architectures is severely constrained by the requirement of photon wavepacket indistinguishability, in particular, in terms of the photon linewidth and associated time scale. While narrowband filtering can improve interference visibility, it introduces significant photon loss - a critical limitation for applications. Here, we experimentally demonstrate an efficient approach to enable non-classical two-photon interference between spectral-bandwidth mismatched photons using an electro-optic time lens. We increase the visibility of Hong-Ou-Mandel interference between photons of 10-fold spectral bandwidth mismatch by more than 12 times, achieving non-classical two-photon interference visibility without spectral filtering. This result opens the possibility to efficiently integrate quantum systems operating at different time scales for hybrid quantum communication, teleportation, entanglement swapping, distributed sensing, and hybrid quantum computing.

Figures (9)

• Figure 1: Overview of the experiment. We begin with two photons with mismatched spectral bandwidths. We apply a time-lens-based spectral bandwidth converter to compress the spectrum of the signal photon to the spectral width of the reference photon. Such modification improves the overlap of the photons' spectro-temporal modes and therefore increases the indistinguishability of the photons, improving the two-photon interference visibility.
• Figure 2: The principle of operation of the spectral bandwidth converter. At the input the single-photon wavepacket is short in time and broad in spectrum (with optical frequency denoted by $\omega$ and spectral width denoted by $\Delta\omega$). The application of a quadratic phase in spectrum (denoted by $\phi_s$) using the dispersive medium (exhibiting group delay dispersion, GDD) broadens photon in time and chirps it. The chirp results in arrival of different frequency components (plotted with different colors within the stretched photon's temporal envelope) at different times. The photon is then subjected to quadratic temporal phase ($\phi_t$) modulation (in our experiment it is applied using an electro-optic phase modulator, EOPM). It shifts the spectral components such that they converge at the central frequency, resulting in spectral compression of the single-photon wavepacket (with spectral bandwidth denoted by $\Delta\omega'$).
• Figure 3: Schematic of the experimental setup (a). A pair of photons is generated by the SPDC source. The reference photon is spectrally filtered using a pulse shaper. The second (unfiltered) photon is spectrally compressed using the time lens based bandwidth converter. We drive the electro-optic phase modulator (EOPM) with an RF signal from an arbitrary waveform generator (AWG) amplified by an RF amplifier. We perform two-photon interference at a $50:50$ fiber beam splitter (FBS). In (b,c) we show the action of the time lens on the wideband photon's spectrum. The spectra from (b) are compressed by the converter resulting in the spectra, presented in (c). We show here single (blue) and coincidence counts with the narrowband photon (red) as well as classically measured transmission window of the filter as a reference. The FBS was not used for those measurements.
• Figure 4: Hong-Ou-Mandel interference between bandwidth mismatched photons: normalized coincidence rates between two beam splitter outputs as a function of delay between interfering photons, set at the delay line. In blue the Hong-Ou-Mandel dip after compressing the signal photon, in red the dip without the bandwidth converter. Error bars assume Poisson photon count statistics. We normalized the number of coincidences by the number of singles in both cases to remove residual photon count drifts. The low visibility dip is especially fragile for such drifts. We calculate the two-photon interference visibility from Gaussian functions (plotted with solid lines) fitted to the data. The zero point at the delay axis is calibrated to the dip position (blue data), and the position of the dip (Fig. S5(b)) observed without the spectral filter (red data).
• Figure S1: Maximum visibility as a function of ratio of spectral bandwidth $\sigma_a$ of target photon to the bandwidth ($\sigma_b$) of input photon according to Eq. (\ref{['eq:vis']}). As the compression factor increases, the maximum visibility approaches 100%.