Encoding the qubit is a topic often talked about in quantum computing.Usually with very little explanation as to what is actually going on under the hood. I think part of the difficulty in describing it is that different laboratories might go about it in different ways. The process involves deriving the information from the qubit into a classically defined answer. This can be difficult, when you consider that qubits generally have no direct classical analogue. Afterall, whereas a regular bit is written as 0 or 1, a qubit inhabits a superposition of both 0 and 1. Whenever we measure these systems of qubits, the Heisenberg uncertainty principle tells us that the wave equation collapses and what we’re left with is something that looks more classical than it does quantum.

This poses an issue, one in which we need to decohere the qubit in order to measure the outcome and get appropriate solutions, but in doing so, we are removing the qubits inherent nature, and interpreting it as a regular bit. The best way to go about this is not to extract information from the system itself, but rather to get the qubits to encode their information -while being in superposition- to a variety of other systems or informational qubits. Encoding in this sense means to leave a marker or some classification as to what the information is in the qubit, without disrupting it. A language is then built up around these encodings to be able to translate the quantum information into something useful.

Oscillators are often used to encode qubit information, as they have the ability to read different electronic and magnetic amplitudes which is used to keep track of information about the qubits state without decomposing the qubit. They are useful because they are able to keep track of miniscule changes in electrical signals, which is needed for the sensitivity of quantum circuits. Furthermore, they are often employed in the field for a variety of other purpose driven electronics.

Another way to encode a qubit is to make use of other qubits' spin motion to encode their information. In this sense, the qubits that become encoded can become informational qubits rather than logical qubits for the purpose of arriving at useful information.

A third way for a qubit to encode its information is to do so by embedding the information in molecules. This can be done in a variety of ways but one example is by using the rotational states of molecules so that the information is encoded within their chirality.

Encoding the qubit is like taking a snapshot, encapsulating it, removing its potential. Encoding a qubit is like taking a picture….

Encoding qubits in oscillators - Berkely Lab (August 2023)

Encoding a qubit in a spin - Jonathan A. Gross (May 25, 2020)

Robust encoding of a qubit in a molecule - Victor V. Albert (November 20, 2019)