5 ms·

So, something that always bothered me, and my admittedly ignorant understanding of QM as a non-physicist. I understand that the quantum state of a particle can

So, something that always bothered me, and my admittedly ignorant understanding of QM as a non-physicist. I understand that the quantum state of a particle can be teleported, and that classical information must be passed to observe the particle and collapse the state correctly. My question is, if we can't observe the particle, how do we know that that particle is entangled with the original one?

Technically 6y ago
The particle can observed, but this collapses the entanglement. If the key derived from entangled particles is bad, and communication does not work with the observed key, you know with high probability the particle collapsed pre-entanglement, and and you resync to agree on a new key. There is no way to tell without comparing (classical) communication whether or not your observation collapsed the key. Most of the engineering effort is preserving the entanglement for farther travel from the source of entanglement. Note: I'm using "key" here because it's presumed the shared state would refer to an encryption key, but you could use it for other types of communication (eg agreeing on a novel primary key for some common database).
tsimionescu 6y ago
We take a process that produces entangled pairs, and send the 2 particles to different places. As long as we don't measure their state (whatever that means - TBD :) ), they remain entangled.
- Rooster61 6y ago
  But that begs the question of how we know that we have produced an entangled pair. Do we not need a metric to use to make sure they are entangled?
  - stelfer 6y ago
    The act of measurement on the second particle destroys the entagled state of the pair. That effect can be measured on the first particle's side (which may be far away... on the other side of the galaxy maybe).
    - jiofih 6y ago
      But to see/interpret the effect on the first particle, you need to know what the measurement on the second was, and that information needs to be transferred across the galaxy by conventional means. Honesty quantum mechanics sounds more like a bug in the universe or some quirk we just don’t understand yet.
  - jaggirs 6y ago
    Your question boils down to whether or not the quantum circuit that entangles the qubits works correctly. You can simply test the reliability of the quantum circuit by running it multiple times and checking that the outcomes behave as you expect. For example, if your circuit is supposed to generate the entagled pair [00 or 11 with equal probability] you would expect measurements to either be 00 or 11 and never 01 or 10.
mathgenius 6y ago
Given an unknown state, it's not possible to know if the parts are entangled. There is no experiment to determine this. The "facts" of quantum have this weird disconnect with reality. If you have access to many copies of the same state, then you can do tomography to learn if the state is entangled or not. But this is a different situation to what you are asking in your question. Actually I'm not entirely sure what your question is asking, but I won't delete this comment just yet.

Technically 6y ago

The particle can observed, but this collapses the entanglement. If the key derived from entangled particles is bad, and communication does not work with the observed key, you know with high probability the particle collapsed pre-entanglement, and and you resync to agree on a new key. There is no way to tell without comparing (classical) communication whether or not your observation collapsed the key. Most of the engineering effort is preserving the entanglement for farther travel from the source of entanglement. Note: I'm using "key" here because it's presumed the shared state would refer to an encryption key, but you could use it for other types of communication (eg agreeing on a novel primary key for some common database).

tsimionescu 6y ago

We take a process that produces entangled pairs, and send the 2 particles to different places. As long as we don't measure their state (whatever that means - TBD :) ), they remain entangled.

Rooster61 6y ago
But that begs the question of how we know that we have produced an entangled pair. Do we not need a metric to use to make sure they are entangled?
- stelfer 6y ago
  The act of measurement on the second particle destroys the entagled state of the pair. That effect can be measured on the first particle's side (which may be far away... on the other side of the galaxy maybe).
  - jiofih 6y ago
    But to see/interpret the effect on the first particle, you need to know what the measurement on the second was, and that information needs to be transferred across the galaxy by conventional means. Honesty quantum mechanics sounds more like a bug in the universe or some quirk we just don’t understand yet.
- jaggirs 6y ago
  Your question boils down to whether or not the quantum circuit that entangles the qubits works correctly. You can simply test the reliability of the quantum circuit by running it multiple times and checking that the outcomes behave as you expect. For example, if your circuit is supposed to generate the entagled pair [00 or 11 with equal probability] you would expect measurements to either be 00 or 11 and never 01 or 10.

mathgenius 6y ago

Given an unknown state, it's not possible to know if the parts are entangled. There is no experiment to determine this. The "facts" of quantum have this weird disconnect with reality. If you have access to many copies of the same state, then you can do tomography to learn if the state is entangled or not. But this is a different situation to what you are asking in your question. Actually I'm not entirely sure what your question is asking, but I won't delete this comment just yet.