Quantum-state purity of heralded single photons

Shengwang Du

We analyze the quantum-state purity of heralded single photons produced from frequency-anticorrelated biphotons. We find that the quantum-state purity in the time-frequency domain depends strongly on the response time uncertainty of the trigger-photon detector that heralds the generation of its paired photon. If the trigger response time is much shorter than the two-photon coherence time, the time-frequency quantum-state purity of heralded single photons approaches unity and the heralded single photon is in a nearly pure state. If the trigger response time is much longer than the two-photon coherence time, the heralded photon is then projected onto a mixed state. Making use of the time-frequency entanglement, heralded single photons with a well-defined temporal wave function or a frequency superposition state can be produced and engineered. This time-frequency entanglement allows for shaping heralded single photons through nonlocal spectral modulation.

Philip F. Schewe

P. Dhilipan, G. Raghavan

Changchen Chen, Jane E. Heyes, Kyung-Han Hong +6 more

, with a Gaussian phase-matching function and a transform-limited Gaussian pump, to achieve near-unity spectral purity in heralded single photons at telecommunication wavelength. Theory shows that these phase-matching and pump conditions are sufficient to ensure that a biphoton state with a circularly symmetric joint spectral intensity profile is transform limited and factorable. We verify the heralded-state spectral purity in a four-fold coincidence measurement by performing Hong-Ou-Mandel interference between two independently generated heralded photons. With a mild spectral filter we obtain an interference visibility of 98.4±1.1% which corresponds to a heralded-state purity of 99.2%. Our heralded photon source is potentially an essential resource for measurement-based quantum information processing and quantum network applications.

Ferney Castro-Simanca, Francisco A. Domínguez-Serna, Mauricio Gómez-Robles +3 more

The generation of pure single-photon states and their characterization are fundamental in quantum information processing applications like quantum computing. In this work, we design integrated optical directional couplers on a silicon nitride platform for heralded single-photon characterization. We assume the generation of such photons in silicon nitride waveguides through the spontaneous four-wave mixing process. We show that manipulating the waveguide geometry and dielectric contrast makes it possible to obtain nearly wavelength-independent directional coupler designs that perform as 50:50 beamsplitters over a broad bandwidth around the central design wavelength. Assuming two independent but identical heralded single-photon sources, we compare the visibility obtained by simulating the Hong-Ou-Mandel interference for three different 50:50 beamsplitters: an ideal one (perfectly wavelength-independent), an engineered one (nearly wavelength-independent), and an unengineered one (significantly wavelength-dependent). The purity obtained with the engineered beamsplitter closely matches the purity obtained with the ideal one, for which the interference visibility is a direct measure of the quantum state purity. This finding implies that the proposed engineered beamsplitter is suitable for characterizing heralded single-photon states, provided its bandwidth is 23 nm.

Shengwang Du