# How practical is it to isolate two entangled qubits?

Starting with two qubits $$|00>$$, I can obtain a bell state by applying a Hadamard gate and then a CNOT gate [ref].

The two qubits in a bell state are entangled. It is a common part of giving the following illustration when entanglement is introduced as a concept:

If Bob and Alice take one of the entangled qubits each, isolate them to avoid collapse, and then travel far away from each other (even thousands of kilometers), then when Alice measures her qubit, Bob's qubit also will collapse to the same state when measured.

Initially the two qubits must have been spatially close so that the operations could be applied on them that cause them to entangle.

My question is: How practical is it to then isolate the two qubits without collapsing them? We were told that many physical phenomena (photon polarization; electron spin) can model a qubit. Is there any physical phenomena where there already is appratus which can isolate and spatially separate qubits after entanglement?

• Of course, Bell states are regularly made. For example, if the constituent particles are photons, they usually fly out of the apparatus that makes them in different directions. So you don't need to do anything to spatially separate them. – knzhou Jan 16 '20 at 19:04
• What's "of course" about that? How could you have photons as qubits (and hence, a stable model of computation) if they are flying out of the apparatus? – Peeyush Kushwaha Jan 16 '20 at 22:25
• I just meant "qubit" in the sense of any two-state system. If you're asking about practical quantum computers, then it depends very much on the particular computer. – knzhou Jan 16 '20 at 22:37