Generation of multi-photon Fock states at telecommunication wavelength using picosecond pulsed light

TL;DR

This work demonstrates the generation of picosecond-pulsed single- and two-photon Fock states in the telecommunications C-band via heralded SPDC in a type-I PPLN waveguide, with Wigner negativity confirmed by pulsed homodyne tomography. A single-pixel photon-number-resolving detector enables high-rate heralding and circumvents detector multiplexing errors, while shaping and timing of the LO ensure correct temporal mode matching. The results show nonclassical quadrature distributions and negative Wigner function values without loss corrections, substantiating the generation of non-Gaussian states in the telecom domain. The setup acts as a prototype for high-speed, ultrafast optical quantum state generation at telecom wavelengths, with potential scalability to GHz repetition rates given faster homodyne detection.

Abstract

Multi-photon Fock states have diverse applications such as optical quantum information processing. For the implementation of quantum information processing, it is desirable that Fock states be generated within the telecommunication wavelength band, particularly in the C-band (1530-1565 nm). This is because mature optical communication technologies can be leveraged for the transmission, manipulation, and detection. Additionally, to achieve high-speed quantum information processing, it is desirable for Fock states to be generated in short optical pulses, as this allows embedding lots of information in the time domain. In this paper, we report the first generation of picosecond pulsed multi-photon Fock states (single-photon and two-photon states) in the C-band with Wigner negativities, which are verified by pulsed homodyne tomography. In our experimental setup, we utilize a single-pixel superconducting nanostrip photon-number-resolving detector (SNSPD), which is expected to facilitate the high-rate generation of various quantum states. This capability stems from the high temporal resolution of SNSPDs (50 ps in our case) allowing us to increase the repetition frequency of pulsed light from the conventional MHz range to the GHz range, although in this experiment the repetition frequency is limited to 10 MHz due to the bandwidth of the homodyne detector. Consequently, our experimental setup is anticipated to serve as a prototype of a high-speed optical quantum state generator for ultrafast quantum information processing at telecommunication wavelength.

Figures (4)

• Figure 1: The schematic diagram of the optical quantum states generation using the heralding method. In the simplest example, Fock states are generated by performing photon-number-resolving detection (PNRD) on one mode of the EPR states. Various non-Gaussian states, such as Schrödinger's cat states or cubic phase states can be also generated by inserting a half-wave plate (HWP) before the polarizing beamsplitter (PBS) or by applying displacement operations before the PNRD.
• Figure 2: The detailed experimental setup for the generation of Fock states. The graph shows the overwritten output of the SNSPD’s signals. The red lines correspond to the two-photon detection event and the black lines correspond to the single-photon detection event. BPF: Bandpass Filter, WS: Waveshaper, AOM: Acousto-Optic Modulator, EDFA: Erbium-Doped Fiber Amplifier, HWP: Half-wave plate, PBS: Polarizing Beamsplitter, DM: Dichroic Mirror, SHG: Second Harmonic Generation, LO: Local Oscillator, WG: Waveguide, VBG: Volume Bragg Grating, SNSPD: Superconducting Nanostrip Photon-number-resolving Detector, HD: Homodyne Detector.
• Figure 3: (a) One of the SNSPD's signals detecting two photons. The black dashed line represents the threshold for detecting one photon, and the red dashed line represents the threshold for detecting two photons. (b) The HD signals when the SNSPD detects two photons. This figure overwrites the signals 20 times. The quadrature is measured every 100 ns, and the third pulse corresponds to the two-photon detection event in this case. Here, the SNSPD signal and the HD signal are triggered by the electrical signal of the photodetector measuring the pulsed light source. (c) The variance of the homodyne measurement values, which is calculated from all HD signals of the two-photon detection events. It can be observed that the variance is significantly larger only for the pulse corresponding to two-photon detection events.
• Figure 4: The results of measurement of vacuum states, single-photon states (pump power = 75 µW), two-photon states (pump power = 300 µW). (a-c) The probability distribution of quadratures obtained from the experiment (histogram) and that obtained from the estimated density matrices (dashed line) for (a) vacuum states, (b) single-photon states, (c) two-photon states. For the calculation of theoretical probability distribution (dashed line) of vacuum states in (a), we use $\dyad{0}$ as the density matrix. (d-e) The estimated photon-number distributions without any loss corrections for (d) single-photon states and (e) two-photon states. (f-g) The estimated Wigner functions without any loss corrections for (f) single-photon states and (g) two-photon states. The estimated Wigner functions have negative values, $W(0,0)= -0.081 \pm 0.007$ for single-photon states, and $W(0,0.065)=-0.082 \pm 0.0034$ for two-photon states.