Effect of a fast flux line on the coherence of 3D transmon qubits

Abstract

With the development of the science and technology, the quantum computers are becoming the possible key to reveal new breakthroughs. Unlike the classical computers, where we have classical bits (0or1), in quantum computation we have qubits as the building block. The primary and most important difference between the classical bit and qubit is the fact that a qubit can be prepared in the superposition state of the ground and 1 st excited state, and this can have finite probability to be detected in any of these states. However, the environment or external noises can lead to the ‘decoherence’ and due to this the system loses its quantum nature. Decoherence can come in two forms: the energy relaxation process or the dephasing when qubit is prepared in superposition state. Now, by energy relaxation it is meant that a qubit which is in a excited state (a stationary state) can eventually decay to a lower energy state (the ground state). By dephasing it is meant that a qubit prepared in a superposition state (non-stationary state)can acquire a random phase as this wavefunction evolves in time. The times after which a quantum system loses its quantum nature, either through a relaxation process or by dephasing process are called decoherence times or more specifically T1 and T2 times respectively. That’s why while designing a qubit, the coherence time is one of most important factors to keep in mind. In this project, our main aim is to observe the effect of a fast-flux line on the decoherence of a special category of superconducting qubit called Transman qubit, which was coupled to a 3D cavity. We perform numerical simulation to estimate the coherences times, and try to understand the decoherence induced by a fast-flux line.