4 ms·

What's the implication? A faster than light message transmission possibility?

hackandthink 4y ago
This won't happen and the paper does not speculate in any way about it. But they are vague about the implications and I'm generally unsure about the practical relevance of quantum protocols like superdense coding ...
- eigenket 4y ago
  Superdense coding specifically is unlikely to ever be practically relevant. Even for people quite optimistic about the future of quantum computing it is hard to imagine a scenario where a single qubit will be cheaper than two classical bits. Other quantum protocols are much more likely to be useful, perhaps particularly for cryptography.
  - amluto 4y ago
    I sometimes consider a qubit to be worth two classical bits in a very heuristic sense. One can superdense-code two bits into a qubit. Similarly, one can send two classical bits to teleport one qubit. Both of these require spending entanglement, so it’s not a very useful heuristic.
    - hackandthink 4y ago
      "one can send two classical bits to teleport one qubit" How do you do this? (it goes against my intuition of a qubit as state in a hilbert space)
      - hackandthink 4y ago
        "However, by taking advantage of two classical bits and an entangled qubit pair" Yes one needs entanglement. https://qiskit.org/textbook/ch-algorithms/teleportation.html https://qiskit.org/textbook/ch-algorithms/teleportation.html
      - eigenket 4y ago
        The cost is two classical bits and one entangled Bell pair to teleport a qubit. You make the Bell pair, send one half of it to Alice (who wants to transmit the qubit) and one half to Bob (who wants to receive it). Alice then does a Bell basis measurement on the qubit she wants to transmit and the half of Bell pair she owns. The Bell basis measurement has four outcomes. For convenience you can label them 00 01 10 11 Alice tells Bob the measurement outcome she got (this is the part that requires two bits of classical communication), and depending on which outcome she got Bob has to apply either no correction or a Pauli X, Y or Z gate. This is known as quantum teleportation https://en.wikipedia.org/wiki/Quantum_teleportation https://en.wikipedia.org/wiki/Quantum_teleportation and has quite a long history at this point (proposed in the early '90s and first performed in the late '90s).
        hackandthink 4y ago
        Thanks, nice explanation.
eigenket 4y ago
The no-communication theorem [1] fairly comprehensively rules out any attempt to send messages faster than light using quantum theory. It essentially says that in order to send any message using quantum mechanics you also have either physically move your system from place to place, or send some classical information (which can be smaller than your quantum message but have non-zero size). This means that in order to use quantum theory to send a message faster than light you either have to already have a way to move you quantum system faster than light, or a way to send classical messages faster than light. The summary is that you can only send messages ftl using quantum theory if you already know a way to send messages ftl without using quantum theory. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem

This won't happen and the paper does not speculate in any way about it. But they are vague about the implications and I'm generally unsure about the practical relevance of quantum protocols like superdense coding ...

eigenket 4y ago
Superdense coding specifically is unlikely to ever be practically relevant. Even for people quite optimistic about the future of quantum computing it is hard to imagine a scenario where a single qubit will be cheaper than two classical bits. Other quantum protocols are much more likely to be useful, perhaps particularly for cryptography.
- amluto 4y ago
  I sometimes consider a qubit to be worth two classical bits in a very heuristic sense. One can superdense-code two bits into a qubit. Similarly, one can send two classical bits to teleport one qubit. Both of these require spending entanglement, so it’s not a very useful heuristic.
  - hackandthink 4y ago
    "one can send two classical bits to teleport one qubit" How do you do this? (it goes against my intuition of a qubit as state in a hilbert space)
    - hackandthink 4y ago
      "However, by taking advantage of two classical bits and an entangled qubit pair" Yes one needs entanglement. https://qiskit.org/textbook/ch-algorithms/teleportation.html https://qiskit.org/textbook/ch-algorithms/teleportation.html
    - eigenket 4y ago
      The cost is two classical bits and one entangled Bell pair to teleport a qubit. You make the Bell pair, send one half of it to Alice (who wants to transmit the qubit) and one half to Bob (who wants to receive it). Alice then does a Bell basis measurement on the qubit she wants to transmit and the half of Bell pair she owns. The Bell basis measurement has four outcomes. For convenience you can label them 00 01 10 11 Alice tells Bob the measurement outcome she got (this is the part that requires two bits of classical communication), and depending on which outcome she got Bob has to apply either no correction or a Pauli X, Y or Z gate. This is known as quantum teleportation https://en.wikipedia.org/wiki/Quantum_teleportation https://en.wikipedia.org/wiki/Quantum_teleportation and has quite a long history at this point (proposed in the early '90s and first performed in the late '90s).
      - hackandthink 4y ago
        Thanks, nice explanation.

eigenket 4y ago

The no-communication theorem [1] fairly comprehensively rules out any attempt to send messages faster than light using quantum theory. It essentially says that in order to send any message using quantum mechanics you also have either physically move your system from place to place, or send some classical information (which can be smaller than your quantum message but have non-zero size). This means that in order to use quantum theory to send a message faster than light you either have to already have a way to move you quantum system faster than light, or a way to send classical messages faster than light. The summary is that you can only send messages ftl using quantum theory if you already know a way to send messages ftl without using quantum theory. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem