What are virtual photons? Why do we use them?
The interaction between qubits is achieved in the following way: The transition frequencies of the two qubits are tuned to differ from the frequency of the cavity photons. In classical physics, such a situation would forbid the energy transfer between the qubit and a the photon because of the energy mismatch. However, in quantum mechanics it is possible to break the energy conservation for a very short time, allowing the energy to be transferred from one qubit to the cavity and then to the other qubit. In this sense, the coupling is mediated via virtual photons. In distinction to real photons, virtual photons do not decay out of the resonator. As a result, we avoid processes where the energy can leak out of the system instead of being transferred from on qubit to the other qubit.
