How Long Does a Photon Retain Its Spin After Dis-Entanglement?

  • Context: Graduate 
  • Thread starter Thread starter San K
  • Start date Start date
  • Tags Tags
    Photon
Click For Summary

Discussion Overview

The discussion revolves around the behavior of a photon’s spin after it has been dis-entangled from another photon, particularly focusing on the time frame and conditions under which the photon retains its spin state. The conversation includes elements of quantum mechanics, measurement, and the implications of entanglement.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how long, on average, a photon retains its spin after dis-entanglement, using a hypothetical scenario involving two entangled photons measured at different times.
  • Another participant suggests that if Bob's photon does not interact with anything after Alice measures hers, it will remain in a spin state (spin down) as determined by Alice's measurement.
  • A participant raises a question about the implications of Bob changing his measurement from spin down to spin up, suggesting a potential change in Alice's measurement outcome.
  • There is a clarification that when Alice measures her photon, Bob's photon is still considered to be in a superposition state until a measurement is made, referencing the linearity of the Schrödinger equation.
  • Participants discuss the concept of "no definite state," with one asserting that the collapse of the wave function is instantaneous and questioning the nature of the system being discussed.
  • Another participant reiterates that Bob's photon cannot remain in an indefinite state after a measurement is made.
  • One participant emphasizes that according to standard quantum mechanics, no measurement occurs until an interaction is made.

Areas of Agreement / Disagreement

Participants express differing views on the implications of measurement and the state of the photons after dis-entanglement. There is no consensus on how long a photon retains its spin or the effects of subsequent measurements.

Contextual Notes

Discussions include assumptions about measurement, the nature of entanglement, and the interpretation of quantum mechanics, which remain unresolved and may depend on specific definitions and interpretations.

San K
Messages
905
Reaction score
1
how long, after dis-entanglement, on average, does an event occur that causes the photon to loose its newly created spin/state?

To illustrate the above, let's say:

Alice measures her entangled photon at 10 am. She gets spin up. We all know if Bob measures at 10:000000000001 am he will get spin down.

However let's assume Bob was having beer and was a bit slow and measured his photon at say, 10:15 am...
 
Physics news on Phys.org


If nothing further happens to Bob's photon (i.e. interacts etc.), then it will remain spin down.

(Technically, though, when Alice measures her photon, the photon at Bob's side is still entangled with Alice's (i.e. in no definite state, according to standard QM). This is a consequence of the linearity of the Schrödinger equation.)
 


and if bob after see his spin down change it to up, alice it will see then her spin down ?...
 


audioloop said:
and if bob after see his spin down change it to up, alice it will see then her spin down ?...

Nope.
 


StevieTNZ said:
(Technically, though, when Alice measures her photon, the photon at Bob's side is still entangled with Alice's (i.e. in no definite state, according to standard QM). This is a consequence of the linearity of the Schrödinger equation.)
What do you mean by "in no definite state"? 'Collapse' is instantaneous as far as we can tell at this point(>10^4 C), and we are talking about one system.
 


Maui said:
What do you mean by "in no definite state"? 'Collapse' is instantaneous as far as we can tell at this point(>10^4 C), and we are talking about one system.

right.
 


Maui said:
What do you mean by "in no definite state"? 'Collapse' is instantaneous as far as we can tell at this point(>10^4 C), and we are talking about one system.

By "in no definite state", I mean in a superposition of spin up and down.
 


StevieTNZ said:
By "in no definite state", I mean in a superposition of spin up and down.



Indefiniteness is broken once a measurement is made on the system. Bob's photon cannot remain in an indefinite state(up and down).
 


As I stated earlier, according to standard QM no measurement occurs.
 

Similar threads

  • · Replies 58 ·
2
Replies
58
Views
5K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 19 ·
Replies
19
Views
4K
  • · Replies 41 ·
2
Replies
41
Views
6K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 15 ·
Replies
15
Views
3K
Replies
18
Views
3K