Bell measurement of distant particles

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Discussion Overview

The discussion revolves around the requirements for performing a Bell measurement in the context of quantum teleportation, specifically focusing on the timing and spatial relationship of the qubits involved. Participants explore the implications of photon arrival times and their impact on the measurement process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the qubits need to be close in space and time for a Bell measurement to be performed, referencing a diagram of quantum teleportation.
  • Another participant asserts that A and B can be far apart, but emphasizes the necessity of a classical signal to communicate measurement results.
  • A later post clarifies that the focus is on the two photons that Alice possesses and questions the implications of their arrival times on the Bell measurement.
  • One participant argues that for a Bell State Measurement, the two particles must arrive nearly simultaneously to be indistinguishable, suggesting that practical constraints often prevent this from occurring.
  • It is noted that PDC pairs typically cannot be produced on demand, which complicates the timing of photon arrivals for the measurement.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of the timing and spatial proximity of the qubits for a Bell measurement, indicating that the discussion remains unresolved with competing perspectives on the requirements.

Contextual Notes

Participants highlight limitations related to the timing of photon arrivals and the practical challenges of producing PDC pairs on demand, which may affect the feasibility of performing Bell measurements.

Heidi
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Hi Pf
I am looking at the figure in wiki about quantum teleportation.
https://en.wikipedia.org/wiki/File:Quantum_teleportation_diagram.PNG
A wants to send a qubit to B . She receives another qubit from a spdc.
so she has to make a Bell measurement on them. Has these qbits to be close in space and time to do that?
 
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Heidi said:
Hi Pf
I am looking at the figure in wiki about quantum teleportation.
https://en.wikipedia.org/wiki/File:Quantum_teleportation_diagram.PNG
A wants to send a qubit to B . She receives another qubit from a spdc.
so she has to make a Bell measurement on them. Has these qbits to be close in space and time to do that?

In the diagram you reference: No, A and B can be as far apart as you like. However, a classical signal must go from A to B so that a determination can be made as to the proper operation(s) to execute so that the exiting photon is a proper clone of the original.
 
I am afraid i misspoke my question. it is only about the two photons that Alice has. at t she receives a photon from the spdc but what about the other photon (the ϕ)
What if she receives it at a time t0 before or after t?
can a bell measurement be done with distant qubits at t?
 
Last edited:
Heidi said:
I am afraid i misspoke my question. it is only about the two photons that Alice has. at t she receives a photon from the spdc but what about the other photon (the ϕ)
What if she receives it at a time t0 before or after t?
can a bell measurement be done with distant qubits at t?

No. To do a Bell State Measurement (BSM), the 2 particles must arrive at nearly the same time - so as to be indistinguishable. They are routed into a PBS apparatus together, and then you look for a pair of detections at the output ports.

As a general rule, PDC pairs cannot be produced upon demand. So in many (most) cases, the photons being subjected to the BSM will not arrive together within a small enough time window.
 
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