Is there a way to revert two entangled qubits to a separate state without using code...like restarting a quantum computer?

Note: By "restarting a quantum computer", I do not mean restarting a virtual machine/simulator; I mean restarting a physical quantum computer with physical qubits.

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    $\begingroup$ Pick a physical realization, then can be more specific in answer. $\endgroup$ – AHusain Jun 4 '19 at 23:41
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    $\begingroup$ Not sure what you mean: You can just measure the qubit and re-initialize it with some physical process, though this will "collapse" the system that the measured qubit was entangled with. If you want to preserve the state of the entangled qubit, but "reset" the original qubit to $|0\rangle$, I don't think this is possible in all cases (such a map might make 2 orthogonal entangled states become non-orthogonal). $\endgroup$ – Sam Jaques Jun 5 '19 at 2:37
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    $\begingroup$ I mean be more specific like ion or superconducting etc. That way when answering can say what exactly need to do. That way the answer can be something like apply this magnetic field etc. $\endgroup$ – AHusain Jun 5 '19 at 3:08

Well, there's more than one way.

  • You can perform a measurement, though I suppose this doesn't comply with your definition of 'without code' (since you actually have to perform a command)?
  • Since quantum states are fragile and (at the moment) still not error-corrected, you can just wait for the state to decohere/dephase... if you look at the IBM's device summary information here, you can see that those times (T1, T2) are on the order of the tenth's of microseconds... you you wait a couple hundred microseconds, it's pretty likely that they are unentangled now (though of course, they probably assumed some random quantum state and nothing resembling what you initially programmed.)

Whatever restarting a quantum computer may mean, it probably takes more time/steps than just waiting less than a milisecond.

The entanglement was created with a quantum operation. If you wish to undo and return to the previous state this you have to perform again a quantum operation, which involve hardware actions, which are directed by code. Or you just let the quantum states degrade/get destroyed with non-coherent effects, like noise, etc.

Not sure where your question comes from though, and why you'd want to do something without code, since even the restarting operations probably has some commands being executed. Why would you want to operate a computer without code?

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