4 ms·

It's because unintended measurement is a type of error in a quantum computer. Like, if an electron passing near your qubit would get pushed left if your qubit w

It's because unintended measurement is a type of error in a quantum computer. Like, if an electron passing near your qubit would get pushed left if your qubit was 0 and right if was 1, then you will see errors when electrons pass by. Repeating the 0 or 1 a thousand times just means there's 1000x more places that electrons passing by would cause a problem. That kind of redundancy makes that kind of error mechanism worse instead of better. There are ways of repeating quantum information that protect against accidental measurement errors. For example, if your logical 0 is |000> + |110> + |011> + |101> and your logical 1 is |111> + |001> + |100> + |010> then can recover from one accidental measurement. And there are more complex states that protect against both bitflip errors and accidental measurements simultaneously. They're just more complicated to describe (and implement!) than "use 0000000 instead of 0 and 1111111 instead of 1".

Kerbonut 2y ago
If there's interference, could you do something like when using 7 repetition for each bit, take whatever 5 of 7 is, e.g. 1111100 is 1 and 1100000 is 0.
nomel 2y ago
Is this the correct interpretation? Classical systems: You measure some state, with the measurement containing some error. Averaging the measurement error usually gets closer to the actual value. Quantum systems: Your measurement influences/can influence the state, which can cause an error in the state itself. Multiple measurements means more possible influence.
- Strilanc 2y ago
  Yeah that's roughly it. In classical computers all errors can be simplified as being bit flip errors (0 instead of 1, 1 instead of 0). Like, power loss is a lot of bit flip errors that happened to target the bits that should have been 1. In quantum computers this simplification does not work, there is another type of error called a phase flip. Measurements cause phase flip errors. You can exchange the phase flip and bit flip bases by using a gate called the Hadamard gate. So if you surround measurements with Hadamard gates, you will see bit flip errors. The existence of gates like Hadamard is what makes it possible to see these kinds of things at all, and correspondingly its availability can be thought of as the thing that makes a quantum computer a quantum computer, instead of a classical computer.

Kerbonut 2y ago

If there's interference, could you do something like when using 7 repetition for each bit, take whatever 5 of 7 is, e.g. 1111100 is 1 and 1100000 is 0.

nomel 2y ago

Is this the correct interpretation? Classical systems: You measure some state, with the measurement containing some error. Averaging the measurement error usually gets closer to the actual value. Quantum systems: Your measurement influences/can influence the state, which can cause an error in the state itself. Multiple measurements means more possible influence.

Strilanc 2y ago
Yeah that's roughly it. In classical computers all errors can be simplified as being bit flip errors (0 instead of 1, 1 instead of 0). Like, power loss is a lot of bit flip errors that happened to target the bits that should have been 1. In quantum computers this simplification does not work, there is another type of error called a phase flip. Measurements cause phase flip errors. You can exchange the phase flip and bit flip bases by using a gate called the Hadamard gate. So if you surround measurements with Hadamard gates, you will see bit flip errors. The existence of gates like Hadamard is what makes it possible to see these kinds of things at all, and correspondingly its availability can be thought of as the thing that makes a quantum computer a quantum computer, instead of a classical computer.