I have 2 questions about QC that I have not understood or find any answers for. Maybe someone here knows the answer?
When there is examples of entanglement, it is always between two particles. But if I understand QC right, every qubit must be entangled with each other.
1a - Thus this mean that "particles" can be entangled, not only to one "particle", but to many at the same time? Or are they entangled in series or something?
It is often talk about that qubits need to be entangled in a QC, but never how this is done.
1b - So how is the entanglement between different qubits done in practice in a real QC?
As I understand QC you dont get a absolute answer, just a probability it is the right answer. And because of this you have to run the "simulation" many times to verify the answer. As I also understand is that when you read out the state of an qubit the quantum state collapse and so does it do for all the other qubits.
2a - Thus this mean that to read out the answer from all qubits, you always need to run, at minimum, as many simulations as there is qubits in the QC? That after the first run you read out the state of the first qubit, after the second run you read out the next qubit and so on until you have a state from all qubits for that simulation and then you combine them to a final anwser?
2b - How many times must you actually run a simulation in a QC to get an answer?
Not an expert, but I can answer some of your questions at least.
> Thus this mean that "particles" can be entangled, not only to one "particle", but to many at the same time? Or are they entangled in series or something?
There is in principle no limit to how many particles can be entangled together, but in practice it becomes harder and harder to prevent any particles from interacting with the external environment.
You have to be careful with terms here, because of the 'Monogamy of Entanglement', which essentially says that if A is fully entangled with B then B cannot be fully entangled with C.
So how does this not contradict my earlier statement? When three particles are fully mutually entangled then any pair of them in isolation are not entangled at all.
To be more concrete, if you have three qbits which are in a superposition of 000 and 111, then no experiment you can do on the first two qbits can possibly distinguish this from two classically correlated bits unless you allow the third qbit to be involved in the experiment.
>As I understand QC you dont get a absolute answer, just a probability it is the right answer. And because of this you have to run the "simulation" many times to verify the answer. As I also understand is that when you read out the state of an qubit the quantum state collapse and so does it do for all the other qubits.
It's true that all the qbits collapse, but the state that they collapse is still interesting. You can get much more than 1 bit of useful information per simulation.
> How many times must you actually run a simulation in a QC to get an answer?
Depends on both the algorithm and the details of the hardware. Assuming an 'ideal' quantum computer (which will probably never exist), Grover's algorithm will give you the right answer after 2 runs on average.
As also a non-expert, I can add that the superposition and entanglement of qubits are performed using gates as in classical computing. There’s the Hadamard gate which takes two particles outputs a quantum superposition of the particles. Passing the result of this into a controlled-not gate, or CNOT, entangles the particles and induces them into a Bell state. From there, calculations are performed until some sort of measurement, wherein the entangled particles collapse into one possible definitive state.
So, for your first questions, you can definitely entangle as little or as many qubits as you want. In practice, it gets harder the more qubits you want to entangle. But a state like the GHZ state can entangle all qubits in your system.
How do you entangle? You can just let two particles interact, so they "mix" their information. Example: you send laser light (photon qubits) to an atom (another qubits). After a short period, there is a probability that the atom absorbed photons, but that probability is not 100%. You just entangled light with an atom. Only measurement can give you information on whether the photons were absorbed or not.
In practice, each platform has its own entangling mechanism. Usually, it entangles only 2 qubits. Many-qubit entanglement can be achieved by pairwise entangling AB, BC, CD, etc.
The first practical example of qubit entangling operation was the Cirac-Zoller gate, you can check it out.
Regarding your second question, you can actually measure just part of the system, and measure the rest later. It will give you a partial collapse. It's called "tracing out", the quantum analogue of marginalization in probability.
When there is examples of entanglement, it is always between two particles. But if I understand QC right, every qubit must be entangled with each other.
1a - Thus this mean that "particles" can be entangled, not only to one "particle", but to many at the same time? Or are they entangled in series or something?
It is often talk about that qubits need to be entangled in a QC, but never how this is done.
1b - So how is the entanglement between different qubits done in practice in a real QC?
As I understand QC you dont get a absolute answer, just a probability it is the right answer. And because of this you have to run the "simulation" many times to verify the answer. As I also understand is that when you read out the state of an qubit the quantum state collapse and so does it do for all the other qubits.
2a - Thus this mean that to read out the answer from all qubits, you always need to run, at minimum, as many simulations as there is qubits in the QC? That after the first run you read out the state of the first qubit, after the second run you read out the next qubit and so on until you have a state from all qubits for that simulation and then you combine them to a final anwser?
2b - How many times must you actually run a simulation in a QC to get an answer?