Spectrally uncorrelated biphoton state generated from the spontaneous nonlinear optical process is an important resource for quantum information. Currently such spectrally uncorrelated biphoton state can only be prepared from limited kinds of nonlinear media, thus limiting their wavelengths. In order to explore wider wavelength range, here we theoretically study the generation of spectrally uncorrelated biphoton state from 14 isomorphs of potassium dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the 'KDP family' still maintain similar nonlinear optical properties of KDP, such as KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP and KDA, which satisfy 3 kinds of the group-velocity matching conditions for spectrally uncorrelated biphoton state generation from near-infrared to telecom wavelengths. Based on the uncorrelated biphoton state, we investigate the generation of heralded pure-state single photon by detecting one member of the biphoton state to herald the output of the other. The purity of the heralded single photon is as high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel interference from independent sources can also achieve a visibility of 98%. This study may provide more and better single-photon sources for quantum information processing at near-infrared and telecom wavelengths.
We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states
|
Ψ
+
⟩
and
|
Ψ
−
⟩
with the fidelities of
0.948
±
0.004
and
0.963
±
0.002
. In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the
S
value can achieve
2.78
±
0.01
. This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.
An array of 128×128 digital pixel sensors (DPS) that performs both in pixel light current integration and analog-to-digital conversion is presented. The pixel fabricated on a DP4M CMOS process provides a digital output of ultraviolet light intensity via an integrated multiple-channel bit-serial (MCBS) ADC. Due to low light current (~pA) of ultraviolet focal-plane-array, the architecture of capacitive trans-impedance amplifier (CTIA) is used. The proposed readout integrated circuits have a 12-bit resolution, 70dB dynamic range and 99% of linearity.
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