Hybrid biphoton spectrometer for time-resolved quantum spectroscopy across visible and near-infrared regions

TL;DR

The paper addresses the challenge of time-resolved spectroscopy with entangled photon pairs that span visible to near-infrared wavelengths. It proposes a hybrid, non-scanning biphoton spectrometer that combines a visible-path delay-line-anode detector with a near-infrared fibre spectrograph and time-tagging to reconstruct time-resolved joint spectral intensities with a $150~ps$ resolution. Using type-I SPDC in lithium triborate, the authors demonstrate a three-fold non-degenerate biphoton spectrum, reporting experimental Schmidt number $K=2.93$ and purity $P=0.34$, alongside simulations suggesting $K=5.60$ and $P=0.18$ and attributing discrepancies to detector timing jitter. The work establishes a practical platform for time- and frequency-resolved quantum spectroscopy and points toward future upgrades (e.g., streak-tube readout) to access sub-picosecond dynamics and enable two-dimensional entangled-photon spectroscopy for complex molecular systems.

Abstract

Joint spectral measurements are a powerful tool for characterising biphoton spectral correlation, which is crucial for quantum information and communication technologies. In these applications, highly pure biphoton states are essential in any time- and frequency-mode, often obviating the need for time-resolved measurements. Conversely, spectroscopy utilising entangled photon pairs is gaining significant attention for its ability to unveil molecular dynamics, a field that critically demands time-resolved capabilities. Here, we introduce a novel methodology for capturing a biphoton spectrum that comprises visible and near-infrared photons, resulting in a three-fold non-degenerate joint spectrum. Our system employs two non-scanning spectrographs: a fibre spectrograph for near-infrared photons and a delay-line-anode single-photon imager for visible photons. We successfully measure the joint spectral intensity by leveraging a time-tagging acquisition strategy. Furthermore, our approach uniquely enables time-resolved joint spectral measurements with a temporal resolution of approximately 150 ps. This methodology bridges the gap between the requirements for pure biphoton states and the need for dynamic insights in quantum spectroscopy.

Figures (5)

• Figure 1: (a) The principle of the hybrid biphoton spectrometer. The signal photons are measured with the delay-line-anode single-photon detector (DLD), which consists of the photocathode, the microchannel plate (MCP), and the delay-line anode. The idler photons are temporally stretched by a dispersion-compensating fibre, followed by a superconducting nanowire single-photon detector (SNSPD). The JSI is time-resolved according to the laser synchronisation signal (laser sync). (b) The time-tagging strategy illustrates how electrical pulses are processed at the Time-to-Digital Converter (TDC), with a bin size of 25 ps. All data processing is based on the second-order correlation function, or time-difference measurement. The four one-dimensional histograms on the right are used to reconstruct a time-resolved JSI.
• Figure 2: (a) Optical diagram showing the time-resolved detection of three-fold non-degenerate photon pairs using the hybrid biphoton spectrometer. The top inset shows the phase-matching angle between the excitation laser path and the crystal axis of the LBO crystal used for SPDC. The phase-matching angle is ($\varphi = 25^{\circ}$). The bottom left figures show typical examples of time-resolved JSI, where the horizontal and vertical axes represent the signal ($\omega_{s}$) and idler ($\omega_{i}$) frequencies, respectively. L: Lens, LBO: Lithium triborate, SHG: Second-harmonic generation, SPDC: Spontaneous parametric down-conversion, DM: Dichroic mirror, SNSPD: Superconducting nanowire single-photon detector, TDC: Time-to-digital converter. (b) shows the captured image at the DLD, and its projection is (c), corresponding to the spectrum of signal photons. (d) shows a coincidence peak between the microchannel plate and SNSPD. The inset is an enlarged histogram providing the spectrum of idler photons.
• Figure 3: (a) Simulated JSI of the LBO crystal. The horizontal (vertical) axis shows the frequency or wavelength of signal (idler) photons. (b) shows the static JSI triggered by the microchannel plate.
• Figure 4: (a) Instrument response function measured between the microchannel plate and the photodiode. (b) Non-time-resolved JSI includes the whole counts over the IRF. (c) The segmented JSIs are every 150 ps, with a range indicated on the top left which corresponds to the segmented area of IRF.
• Figure 5: Comparison of the spectroscopic systems for JSI measurements