A robust approach for time-bin encoded photonic quantum information protocols

Abstract

Quantum states encoded in the time-bin degree of freedom of photons represent a fundamental resource for quantum information protocols. Traditional methods for generating and measuring time-bin encoded quantum states face severe challenges due to optical instabilities, complex setups, and timing resolution requirements. Here, we leverage a robust approach based on Hong-Ou-Mandel interference that allows us to circumvent these issues. First, we perform high-fidelity quantum state tomographies of time-bin qubits with a short temporal separation. Then, we certify intrasystem polarization-time entanglement of single photons through a nonclassicality test. Finally, we propose a robust and scalable protocol to generate and measure high-dimensional time-bin quantum states in a single spatial mode. The protocol promises to enable access to high-dimensional states and tasks that are practically inaccessible with standard schemes, thereby advancing fundamental quantum information science and opening applications in quantum communication.

Figures (4)

• Figure 1: Conceptual scheme of HOM-based measurement. The unknown target photon is measured by means of a known and controlled reference photon via HOM interference at a beam splitter (BS). From the coincidence counts, one can deduce the projection value.
• Figure 2: Experimental setup. The states of two single photons are independently encoded using their polarization, and are then mapped into time-polarization states (yellow and blue panels). The polarization is either erased for the tomography experiment, using a PBS (red and green panels), or measured to verify intrasystem entanglement using a suitably rotated HWP and a PBS (green panel). Target and reference photons are recombined to perform projection via HOM interference (purple panel); coincidences are recorded using SNSPDs and counting modules. HWP, half-waveplate; QWP, quarter-waveplate; PBS, polarizing beam splitter; BD, beam displacer; SNSPD, superconducting-nanowire single-photon detector; FBS, fiber-beam splitter.
• Figure 3: Experimental results.a) Experimentally reconstructed mixed states (red) and theoretical expectations (blue) on the Bloch sphere. Insets: Examples for real parts of density matrices. b) Expectation values of the correlators measured for the noncontextuality test of single-photon time-polarization hybrid entangled states. Dashed lines: ideal values for a singlet state and optimal measurements. Error bars are estimated assuming Poissonian statistics.
• Figure 4: Conceptual scheme of the proposal to create and measure high-dimensional states.a) Through quantum walk (QW) dynamics, arbitrary high-dimensional time-bin states can be generated. b) Using the QWs as a building block, high-dimensional entangled states can be prepared, distributed, and measured. HWP, half-waveplate; QWP, quarter-waveplate; PBS, polarizing beam splitter; BS, beam splitter.