# Counting of Hong-Ou-Mandel Bunched Optical Photons Using a Fast Pixel Camera

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## Abstract

**:**

## 1. Introduction

## 2. Single-Photon Counting Detectors

## 3. Counting of Bunched Single Photons in a Fast Camera

#### Tpx3Cam Fast Camera

^{2}. The processing electronics in each pixel records the time of arrival (ToA) of hits that cross a preset threshold with 1.6 ns resolution and stores it as time code in a memory inside the pixel. The information about time-over-threshold (ToT), which is related to the deposited energy in each pixel, is also stored. The readout is data driven with pixel dead time of only 475 ns + ToT, which allows multi-hit functionality at the pixel level and fast, 80 Mpix/sec, throughput.

^{TM}assembly [46], which integrates an intensifier, its power supply and relay optics to project the light flashes from the intensifier output screen directly onto the optical sensor in the camera. The image intensifier is a vacuum device with a photocathode followed by a micro-channel plate (MCP) and fast scintillator P47. The quantum efficiency (QE) of the GaAs photocathode in the intensifier (Photonis) is about 30% at 810 nm. The MCP efficiency in this intensifier is close to 100%. A second intensifier with a hi-QE green photocathode was used in series after the first intensifier to ensure efficient detection of the hits. The gains of both intensifiers were optimised to provide the maximum photon detection efficiency while avoiding saturation. Similar configurations of the intensified Tpx3Cam have been used previously for characterization of quantum networks [47,48], quantum target detection [49] and lifetime imaging [50] studies.

## 4. Experimental Setup

## 5. Data Analysis

## 6. Theory: SPDC Bi-Photon Spectrum and Hong-Ou-Mandel Effect

## 7. Results

## 8. Discussion and Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Abbreviations

AOM | acousto-optic modulator |

CW | continuous wave |

MCP | micro-channel plate |

MKID | microwave kinetic inductance device |

PDE | photon detection efficiency |

PMF | polarization maintaining fiber |

QE | quantum efficiency |

SiPM | silicon photomultiplier |

SNSPD | superconducting nanowire single photon detector |

SPDC | spontaneous parametric down conversion |

TES | transition edge sensor |

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**Figure 1.**Schematic illustration of the Hong-Ou-Mandel (HOM) effect. Left: Two near-simultaneous photons with similar wavelength impinge on a beam splitter (BS), and the outputs are registered on two detectors, D1 and D2. When the photons’ arrival is simultaneous, making them indistinguishable, the HOM interference effect causes both photons to exit one side of the splitter or the other, inhibiting the outcome with one photon going to each detector. Right: The signature of the HOM effect is a drop in the rate of coincident detections at D1 and D2, as the photon arrival times become identical. This is typically observed as an “HOM dip” in coincidences as a function of some relative time delay at the BS input.

**Figure 2.**Two photons coming out of a single-mode fiber, focused on to the fast pixel camera and registered individually.

**Figure 3.**The sketch of the experimental setup. Pump beam produced by continuous-wave (CW) narrow-band ($\mathsf{\Delta}{\lambda}_{\mathrm{p}}\approx 0.7\phantom{\rule{4pt}{0ex}}\mathrm{nm}$) laser tuned to the wavelength of (${\lambda}_{\mathrm{p}}=405\phantom{\rule{3.33333pt}{0ex}}\mathrm{nm}$) (spontaneous parametric down-conversion (SPDC) source). The created SPDC photons are coupled to polarization-maintaining fibers (PMF), where input polarization of photons in both arms is controlled by polarization plates P

_{1,2}. In the delay module, one can tune an optical path difference between the two legs using a motorized translation stage with 0.3 μm minimal step and dynamic range of 10 mm. Photon counts are recorded with the intensified Tpx3Cam fast camera.

**Figure 4.**Left: photograph of the HOM interferometer with optical delay and beam splitter. Right: photograph of Tpx3Cam with two fibers pointing to the intensifier photocathode.

**Figure 6.**Left: distribution of number of pixels in the cluster. Right: distribution of time-over-threshold (ToT) for the brightest pixel in the cluster, in ns, corresponding to the pixel intensity.

**Figure 7.**Left: distribution of measured time difference between photons registered in fiber 1 and fiber 2. Right: distribution of measured time difference between two photons in fiber 1. The distributions are fit with a double Gaussian function and a constant, see the text for detail.

**Figure 8.**Six examples of two clusters in a single fiber (fiber 2) separated by less than 100 ns. The hits are shown as boxed pairs of heatmaps in time-over-threshold (ToT) representation (left graph in the boxed pair of graphs) and time of arrival (ToA) representation (right graph).

**Figure 9.**Distribution of distances between the photon pairs in the same fiber for fiber 1 (red) and fiber 2 (blue).

**Figure 10.**Number of coincidences between two different fibers (fiber 1&2) and within the same fibers (fiber 1, fiber 2), shown for experimental data and corresponding fits as function of the delay between two photons. The delay is expressed in mm (bottom horizontal scale) and in fs (top horizontal scale). The HOM dip is obvious around the delay value of 0.18 mm.

**Figure 11.**Sum of two-photon coincidence rates in single fibers and between two fibers as function of the delay.

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**MDPI and ACS Style**

Nomerotski, A.; Keach, M.; Stankus, P.; Svihra, P.; Vintskevich, S. Counting of Hong-Ou-Mandel Bunched Optical Photons Using a Fast Pixel Camera. *Sensors* **2020**, *20*, 3475.
https://doi.org/10.3390/s20123475

**AMA Style**

Nomerotski A, Keach M, Stankus P, Svihra P, Vintskevich S. Counting of Hong-Ou-Mandel Bunched Optical Photons Using a Fast Pixel Camera. *Sensors*. 2020; 20(12):3475.
https://doi.org/10.3390/s20123475

**Chicago/Turabian Style**

Nomerotski, Andrei, Michael Keach, Paul Stankus, Peter Svihra, and Stephen Vintskevich. 2020. "Counting of Hong-Ou-Mandel Bunched Optical Photons Using a Fast Pixel Camera" *Sensors* 20, no. 12: 3475.
https://doi.org/10.3390/s20123475