The parametric process is that the photon is not changing the quantum state of the nonlinear medium when interacting with the non-linear crystal, so the energy momentum is conserved between the photons of this process. Parametric down-conversion refers to high-energy photons splitting into two low energy photon, the process of preparation of entangled photon pairs is the laboratory commonly used methods, high quality of light source is implemented without holes Bell inequality between validation and star of the entanglement of distribution.
Our system USES the non-collinear type I phase matching to produce entangled photon pairs, and the nonlinear medium adopts beta-bbo crystal. For the type I phase matching of the non-collinear, the lower conversion light field is the cone surface with the center of the pump, and the lower conversion photon has the same polarization and is perpendicular to the pump light (e-> o+ o).
The principle of polarization entanglement is shown in the figure below
Adoption of two pieces of the same cutting type I BBO crystal, optic axis is vertically and joint together, for example, the first piece of crystal optical axis direction and the direction of the pump is defined as the vertical plane, the second piece of crystal optical axis direction and the direction of the pump is defined as horizontal plane, and two pieces of crystal optical axis direction and Angle of the pump are all theta. When the pump light is vertical polarization, the first crystal is the e polarization, and the phase matching condition of the type I is only converted under the first crystal, and the lower conversion field is horizontal polarization. When the pump light is horizontal polarization, only the second crystal is converted, and the lower conversion field is vertical polarization. When the polarization direction of the pump is 45 degrees, the probability of the conversion of two crystals is the same, and the two processes are coherent. When collected and pump center symmetrical photon pair in two directions, they in the polarization entanglement
Literature: Ultrabright source of polarization - entangled photons
Device 1 is 404nm half wave plate for adjusting pump power.
Device 2 is 404nm polarized beam splitter, used for deviation.
Device 3 is a 404nm half-wave plate that will rotate the pump light polarization to 45 degrees
Device 4 is 404nm and a quarter wave plate, which is used to adjust the relative phase between the horizontal polarization of the pump and the vertical polarization, thus adjusting the phase of the preparation of entangled state.
Device 5 is a lens for focusing the pump light on the BBO crystal to improve the conversion efficiency.
Devices 6 and 8 are used to reflect light paths.
Device 7 is type I BBO crystal, which is formed by the two vertical BBO crystals.
Device 9 is 808nm half wave plate, which is used to rotate the polarization of the transformed photon.
Device 10-12 is a polarization measuring device, including a quarter wave plate, a half-wave plate and a polarization beam splitter, which can measure the arbitrary polarization direction of Bloch ball.
Device 13 is an interference filter to filter out the background light. The center wavelength is 808nm, FWHM=5nm
Device 14 is a four-dimensional coupler used to collect the parameters of the optical fiber.
Device 15 is single or multimode fiber
Device 16 is the light spot recording board, used for auxiliary adjustment of the light.
Quantum entanglement is one of the most mysterious products of quantum theory, and it is also the core resource of quantum information technology. Properties of entanglement is very strange, that if you are one of the particles were observed, would be instant impact to another state of particles, the effect was instantaneous, a localized, known as "spooky action at a distance". Concept of quantum entanglement, as put forward by Einstein in 1935, but he doesn't agree with this kind of role at a distance, so he thinks that the quantum theory is not perfect, you can find more simple local explanation, namely the hidden variable theory. How to verify the existence of this super-distance in the experiment was not solved until 1964.
The Bell inequality form is:
Where A, A, B, and B represent the four mechanical operators, the eigenvalues are plus or minus 1, and E is the statistical average. If this inequality is violated, the existence of the superdistance can be proved. When preparation of entangled state for , four measuring operator can choose to:
Where X,Z is the Pauli operator.
In the experiment, we measured the polarization direction of the photon, namely, the projection direction of two eigenstates of the Pauli operator. For example, in the inequality
an respectively. A, b two eigen modes, N the coincidence counting for the measuring direction.