= Polarization preparation and dephasing experiment
A 2018 photonic experiment makes the distinction between populations and <quantum coherence> controllable. A 400-nm <laser> pumped a crystal to generate paired <photons>, filtered near 800 nm. One photon triggered collection; the other entered the apparatus. A <half-wave plate> and <polarizing beam splitter> selected horizontal <photon polarization>; another <half-wave plate> prepared balanced $H,V$ <quantum superpositions>. Separated polarization paths and <diffraction gratings> let a <spatial light modulator> control their frequency-dependent <quantum phases> and spectral weights. Recombined paths passed through <quartz> plates whose adjustable thickness controlled polarization–frequency coupling. Quarter- and <half-wave plates> plus a <polarizing beam splitter> selected tomography bases; jointly registered <photodetector> events supplied the readout. <Quantum state tomography> inferred polarization <density matrices> at each thickness. The <dephasing> model preserves diagonal populations while changing off-diagonal <quantum coherence>. Here the unobserved photon <frequency> acts as the environment for the polarization <qubit>. Finite modulator resolution and imperfect spectral shaping limited agreement with the intended dynamics; this experiment does not establish exact $I/2$ preparation or the ideal probabilities above. <Experimental implementation of fully controlled dephasing dynamics and synthetic spectral densities by Zhao-Di Liu and others>[Experimental implementation of fully controlled dephasing dynamics and synthetic spectral densities, Nature Communications 9, 3453 (2018), Results: Experimental setup, Fig. 1, equations (1)–(5); Using a photon to simulate qubit coupled to Ising chain, systematic-error discussion after Fig. 3]{ref}
Back to article page
