Spontaneous parametric down-conversion entanglement using BBO

In summary, the creation of Bell's entanglement state involves using two type I BBO crystals placed orthogonal to each other. Each crystal emits a photon pair with either horizontal-horizontal or vertical-vertical polarization, resulting in a Bell's state at the intersection of the two emitted cones. This is possible because the two crystals act on orthogonal polarization components and the down-conversion processes are coherent, creating quantum uncertainty and leading to the entangled state.
  • #1
Paul159
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4
Hello,

I have a question about the creation of the Bell's entanglement state ##1/\sqrt{2} (|HH> + |VV>)##using type I BBO crystals (https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion).

Two crystals are put orthogonal to each other and each of them emits a photon pair (##|HH>## or ##|VV>##). Then at the intersection of the two emitted cones we have the Bell's state. But I don't understand why.
Indeed, if I do measurement of the photons with two polarizers, one at 90° and the other 0°, I don't understand why I will have 0 correlation. For me I could detect for example the signal photon of the ##|HH>## state and the idler photon of the ##|VV>##.

I hope my "question" is clear.
 
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  • #2
Paul159 said:
Two crystals are put orthogonal to each other and each of them emits a photon pair (##|HH>## or ##|VV>##). Then at the intersection of the two emitted cones we have the Bell's state.

Where are you getting this from? Not from the article you linked to; that just talks about producing one photon pair using one crystal.
 
  • #4
Paul159 said:

Thanks for the reference. It looks to me like the key properties of this setup that allows it to produce the Bell-type state are:

(1) The two crystals act on the two orthogonal polarization components (##H## and ##V##);

(2) The down-conversion processes in each crystal are coherent, so an input photon polarized at 45 degrees (halfway between ##H## and ##V##) will create quantum uncertainty about which crystal is doing the downconversion; this is what creates the Bell-type entangled state.
 
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  • #5
Yes of course, I understand now. Thanks !
 

Related to Spontaneous parametric down-conversion entanglement using BBO

1. What is spontaneous parametric down-conversion entanglement?

Spontaneous parametric down-conversion (SPDC) is a process in which a single photon of a high-energy light beam is split into two lower-energy photons, known as signal and idler photons. Entanglement refers to the quantum phenomenon in which two or more particles become connected in such a way that the state of one particle is dependent on the state of the other(s).

2. How is BBO used in SPDC entanglement?

BBO (beta-barium borate) is a nonlinear optical crystal that is commonly used in SPDC experiments. When a high-energy light beam passes through BBO, it can undergo SPDC, resulting in two entangled photons being produced.

3. What are the potential applications of SPDC entanglement using BBO?

SPDC entanglement using BBO has potential applications in quantum communication, quantum cryptography, and quantum computing. It can also be used in fundamental research to study the foundations of quantum mechanics.

4. How is the entanglement between the two photons measured?

The entanglement between the two photons can be measured using various methods, such as quantum state tomography or Bell inequality tests. These methods involve measuring the properties of the entangled photons and comparing them to theoretical predictions.

5. Are there any challenges in using SPDC entanglement with BBO?

One challenge in using SPDC entanglement with BBO is maintaining the entanglement over long distances. The entanglement between the two photons can be easily disrupted by environmental noise or loss of photons during transmission. Researchers are working on developing techniques to overcome these challenges and make SPDC entanglement a more practical technology.

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