We know that the current quantum computers are noisy/error-prone. They are susceptible to gate errors (single qubit and multi-qubit), decoherence (relaxation and dephasing), readout error, etc. I want to know what is/are the underlying physical reason(s) behind each type of error. Answer for one type of qubit (say, superconducting qubit) is perfectly okay.


Quantum computers are susceptible to these errors/noise because of physical disturbances. An example of this is if some molecule in the surrounding air were to bump or approach the qubit it would transfer some kinetic energy and maybe affect the state. Another example is if a qubit interferes with any adjacent qubit, if they "bump" into one another their wavefunctions can constructively or destructively interfere to give an odd result. This reason is why adding more qubits into the system greatly increases the noise. There's much higher chances of a qubit "spilling over" onto another one because there's more energy contained. These are some of the reasons why quantum systems are super-cooled and kept in a vacuum.

The way that I like to imagine it physically is sort of like the quantum "egg carton" or ions trapped in an optical lattice. The qubits are stuck in those craters but as you add more it becomes harder to keep them from interacting with other ones. Hence placing more qubits into these craters and conducting large amounts of operations increases the probability of error.

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