Decoherence
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Decoherence occurs when the quantum behavior of qubits decays. The quantum state can be disturbed instantly by vibrations or temperature changes. This can cause qubits to fall out of superposition and cause errors to appear in computing.
Qubits have very short lifespans before they "break" and their state changes unintentionally, ruining your computation.
This causes the qubits to lose their superposition state and settle into a single state, just like the partygoers would settle down and go home if the party was shut down.
This can be a problem for quantum computers because it means that the qubits lose their ability to consider multiple possibilities at once.
Decoherence means the unwanted interaction of your qubits, your quantum computer, and the external environment. Quantum coherence is a desired property for a qubit. Coherence tells us something about how long a qubit retains its information as some sort of lifetime.
Noise is one of the biggest problems in quantum computing.
All quantum computers built today have qubits that interfere with each other and are very noisy. That’s why we hope to get error-corrected quantum computers where qubits act like the abstract qubits that we want them to be.
The trouble is any interaction with the external world that leaks out the information whether a qubit is 1 or 0 to be recorded in the radiation in the room or molecules in the air or the wires it is as if that qubit has been measured.
The state has now collapsed, it has been entangled with its environment and causing it to lose its quantum state. That’s why qubits need to become perfectly isolated from each other. That’s impossible in order to tell them what to do in a quantum computer. This was the reason why in the 90s quantum computer was believed to be impossible.
Qubits are not required to be perfectly isolated but really well isolated. Even if every qubit is leaking its state to the environment at some rate, as long as that rate is low enough we can encode the information that we care about in very clever ways across the collective states of multiple qubits. We are constantly monitoring the qubits for leaks to detect and correct the errors from the remaining qubits.
Error correction imposes an overhead in the number of qubits to build a scalable quantum computer.
If one of the qubits suffers an error, the error correction code can use the other qubits to "fix" it back into the proper state.
Even after a tremendous amount of public and private money is spent pursuing quantum technologies, some papers claim that quantum computing is just too complicated and won’t be effective for at least the next few decades.
In the quantum winter scenario, qubits will remain noisy and quantum computers will never scale.
Eventually, classical computers might remain faster and more practical than owning or using a quantum computer.
