Measurement of Entropy and Quantum Coherence Properties of Two Type-I Entangled Photonic Qubits

Abstract

Using the type-I SPDC process in BBO nonlinear crystal (NLC), we generate a polarization-entangled state near to the maximally-entangled Bell-state with high-visibility (high-brightness) 98.50 ±1.33 % (87.71 ± 4.45 %) for HV (DA) basis. We calculate the CHSH version of the Bell inequality, as a nonlocal realism test, and find a strong violation from the classical physics or any hidden variable theory (HVT), S= 2.71±0.10. Via measuring the coincidence count (CC) rate in the SPDC process, we obtain the quantum efficiency of single-photon detectors (SPDs) around (25.5 ±3.4)% , which is in good agreement to their manufacturer company. As expected, we verify the linear dependency of the CC rate vs. pump power of input CW-laser, which may yield to find the effective second-order susceptibility crystal. Using the theory of the measurement of qubits, includes a tomographic reconstruction of quantum states due to the linear set of 16 polarization-measurement, together with a maximum-likelihood-technique (MLT), which is based on the numerical optimization, we calculate the physical non-negative definite density matrices, which implies on the non-separability and entanglement of prepared state. By having the maximum likelihood density operator, we calculate precisely the entanglement measures such as Concurrence, entanglement of formation, tangle, logarithmic negativity, and different entanglement entropies such as linear entropy, Von-Neumann entropy, and Renyi 2-entropy. Finally, this high-brightness and low-rate entangled photons source can be used for short-range quantum measurements in the Lab.

Nonlocal realism tests and quantum state tomography in Sagnac-based type-II polarization-entanglement SPDC-source

“…This type of NLC has potential to generate the type-II polarization-entangled Bell-state (for more details about the state of SPDC, see Refs. [2,8,9]).…”

“…We are interested in measuring CHSH (Clauser, Horne, Shimony, and Holt) inequality [70] which is slightly different from the original Bell's inequality. In 1964, Bell showed that all hidden-variable theories (HVTs) obey his inequality, while quantum mechanics, for example entangled state, violates [8,67,68].…”

“…Let us consider Bell state |ψ (−) Bell in Eq. ( 1), it can be easily shown that the coincidence count can be obtained as [2,8,67,73,74]…”

“…[3]). Among these, one can remark, for example, EPR realization [5], the entanglement generation [6][7][8][9], quantum state teleportation [10], quantum ellipsometry [11,12], quantum illumination [13][14][15][16][17][18][19][20], quantum spectroscopy [21][22][23][24], squeezing generation [25], quantum imaging [26][27][28][29][30], quantum communication [31][32][33][34][35][36][37][38][39][40], and nonlocal realism tests [5,[41][42][43][44][45].…”