Nonclassical Effects from Spontaneous Parametric Down-Conversion Adventures in Quantum Wonderland.

S. Lakshmibala, V. Balakrishnan

Yosep Kim, Dong-Gil Im, Yong‐Su Kim +5 more

In Alice's Adventures in Wonderland by Lewis Carroll, mischievous grin of a Cheshire cat is sometimes dis-embodied from her body. Quantum theory allows such a counterintuitive phenomenon, known as the quantum Cheshire cat effect [1], to happen for a pre- and post-selected (PPS) system. While observing the quantum Cheshire cat, the physical property and the carrier's location should be weakly probed with a measuring device to leave the quantum state with negligible disturbance. To date, experimental observations of the quantum Cheshire cat effect have been realized with a single neutron as well as with a single photon [2, 3]. However, their observations relied on weakly disturbing elements instead of a true weak measuring device. In this work, we report the experimental observation of the quantum Cheshire cat effect with a weak measuring device. To investigate the quantum Cheshire cat effect, we employ a photon pair generated by spontaneous parametric down conversion (SPDC). The elements of quantum Cheshire cat, namely the body (photon) and the grin (polarization), are indirectly probed with a quantum pointer using another single photon, where both the body and the grin are weakly measured without destroying the photon itself.

Z. Y. Ou

Guillaume Thekkadath, Martin Houde, Duncan G. England +4 more

Strongly driven nonlinear optical processes such as spontaneous parametric down-conversion and spontaneous four-wave mixing can produce multiphoton nonclassical beams of light which have applications in quantum information processing and sensing. In contrast to the low-gain regime, new physical effects arise in a high-gain regime due to the interactions between the nonclassical light and the strong pump driving the nonlinear process. Here, we describe and experimentally observe a gain-induced group delay between the multiphoton pulses generated in a high-gain type-II spontaneous parametric down-conversion source. Since the group delay introduces distinguishability between the generated photons, it will be important to compensate for it when designing quantum interference devices in which strong optical nonlinearities are required.

Alexander N. Penin, G. Kh. Kitaeva, Alexander V. Sergienko

Results of nondestructive measurements of intensity (photons per mode) of light from different sources are discussed. The procedure of measurement does not destroy the state of the optical field. The method is based on using the second order nonlinearity of crystal media lacking a center of symmetry and the nonclassical properties of the process of Spontaneous Parametric Down Conversion (SPDC).

Xingyu Zou