# Quantum tomography of photon states encoded in polarization and picosecond time-bins

## TL;DR

This paper demonstrates a method for quantum tomography of two-qubit states encoded in a single photon’s polarization and picosecond time-bins, achieving high fidelity in state reconstruction without interferometers.

## Contribution

It introduces a novel approach to analyze two degrees of freedom in a single photon simultaneously using photon bunching and high-speed timing, enabling high-fidelity quantum state tomography.

## Key findings

- Achieved over 96% fidelity in reconstructing two-qubit states.
- Implemented a method to encode and measure polarization and time-bin qubits simultaneously.
- Demonstrated tomography of entangled and non-entangled states in a single photon.

## Abstract

A single photon has many physical degrees of freedom (DOF) that can carry the state of a high-dimensional quantum system. Nevertheless, only a single DOF is usually used in any specific demonstration. Furthermore, when more DOF are being used, they are analyzed and measured one at a time. We introduce a two-qubit information system, realized by two degrees of freedom of a single photon: polarization and time. The photon arrival time is divided into two time-bins representing a qubit, while its polarization state represents a second qubit. The time difference between the two time-bins is created without an interferometer at the picosecond scale, which is much smaller than the detector's response time. The two physically different DOF are analyzed simultaneously by photon bunching between the analyzed photon and an ancilla photon. Full two-qubit states encoded in single photons were reconstructed using quantum state tomography, both when the two DOF were entangled and when they were not, with fidelities higher than 96%.