I What causes qubit superpositions to form?

[oknow]

TL;DR

on the source of qubit superpositions

When I observe a qubit's state, decoherence happens such that I find the qubit in a particular state. After I cease observing a qubit's state, what physical process causes a fresh superposition of states to develop? Is zero-point energy at least a contributor?

 

[tade]

TL;DR Summary: on the source of qubit superpositions

When I observe a qubit's state, decoherence happens such that I find the qubit in a particular state. After I cease observing a qubit's state, what physical process causes a fresh superposition of states to develop? Is zero-point energy at least a contributor?

if you're referring to a quantum computer i think its that the device is being constantly powered by an electric power supply

 

[PeterDonis]

After I cease observing a qubit's state, what physical process causes a fresh superposition of states to develop?

Why do you think a fresh superposition develops?

 

[oknow]

Yes, I'm thinking quantum computing qubits. Since such qubits are generally reusable for additional computations, I believe a fresh superposition must develop at some point after observation ceases. I am curious when that superposition refresh, if you will, is thought to happen, and by what process.

 

[PeterDonis]

Since such qubits are generally reusable for additional computations, I believe a fresh superposition must develop at some point after observation ceases.

Why would you believe that? A qubit doesn't have to start out in a superposition to be useful for a computation. Indeed, usually you don't want it to start out in a superposition, because you want to be able to control exactly which other qubits it's entangled with and in what manner. That means you want to know what state it starts out in.

 

[PeroK]

Yes, I'm thinking quantum computing qubits. Since such qubits are generally reusable for additional computations, I believe a fresh superposition must develop at some point after observation ceases. I am curious when that superposition refresh, if you will, is thought to happen, and by what process.

The answer in general must be quantum computer engineering. Controlling the state of the quibits generally is what QC is all about. In the same way that a classical computer must be able to set its bits and bytes to certain values.

 

[Malodious]

When I observe a qubit's state, decoherence happens such that I find the qubit in a particular state. After I cease observing a qubit's state, what physical process causes a fresh superposition of states to develop? Is zero-point energy at least a contributor?

One answer is that no process is needed: Your newly observed qubit is already in a superposition. Recall that whether or not a state is a superposition, depends on your chosen basis. So when your qubit is in state |0> or |1>, it is also in a superposition, e.g. of |+> and |->.

If you are sticking to the 0/1 basis, the answer is that you use quantum gates. If you pass your qubit through a Hadamard gate, it turns into a superposition of |0> and |1>. Now, however, it is definitely in state |+> or |->, i.e. it is not a superposition in the +/- basis.

Zero-point energy has nothing to do with it.

 

[f95toli]

Yes, I'm thinking quantum computing qubits. Since such qubits are generally reusable for additional computations, I believe a fresh superposition must develop at some point after observation ceases. I am curious when that superposition refresh, if you will, is thought to happen, and by what process.

That is not -generally speaking- what would happen. As has already mentioned we typically want out qubit to be in a known state at the beginning of a computation; and in nearly all cases this will be the qubit ground state. Unless you are using some sort of active reset (which is different topic) many real computations start by waiting "long enough" (something like 5 times the relaxation time) so that we can be reasonably sure that the qubits are in their ground state (aka zero state). The operations that put the qubits into superpositions (or e.g. entangle multiple qubits) are always part of the circuit itself'.

The IBM Qiskit channel on Youtube has some nice tutorials about this.

 

[oknow]

I'll check out the references mentioned, thanks.

I'm wondering about the source of a fresh superposition at a more fundamental level than, say, other circuitry, electricity supply, gates, etc. Yes, in my example the qubit starts at a known state because I just observed it. Assume at that point, while observing the qubit's state, I severe its connection with other circuitry, then I stop observing at it. Does that qubit then remain forever in the last state I observed, or does a fresh superposition along the same basis arise? If the latter, what fundamental physics triggers that fresh uncertainty of state?

 

[PeroK]

I'll check out the references mentioned, thanks.

I'm wondering about the source of a fresh superposition at a more fundamental level than, say, other circuitry, electricity supply, gates, etc. Yes, in my example the qubit starts at a known state because I just observed it. Assume at that point, while observing the qubit's state, I severe its connection with other circuitry, then I stop observing at it. Does that qubit then remain forever in the last state I observed, or does a fresh superposition along the same basis arise? If the latter, what fundamental physics triggers that fresh uncertainty of state?

This post reveals that you haven't understood the basics of states (pure and mixed) and superposition of states. Moreover, system evolution over time is driven by the Hamiltonian.

Before you can understand quantum computing, you'll need to learn the basics of QM.