Question

In superconducting transmon qubits, what physical component is responsible for creating the non-linear energy levels required for a two-level quantum system?

Accepted Answer

The physical component responsible for creating the non-linear energy levels in a superconducting transmon qubit is the Josephson junction. A Josephson junction consists of two superconducting electrodes separated by a very thin insulating barrier, typically made of aluminum oxide. In a standard harmonic oscillator, such as an LC circuit containing an inductor and a capacitor, energy levels are equally spaced, which would make it impossible to isolate just two levels for quantum computation. The Josephson junction acts as a non-linear inductor, meaning its inductance changes depending on the current flowing through it. This non-linearity shifts the energy levels so that the transition frequency between the ground state and the first excited state is different from the transition frequency between the first and second excited states. This unique spacing allows researchers to selectively address the lowest two energy levels using microwave pulses while ignoring higher states, effectively creating a controllable two-level quantum system known as a qubit.

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In superconducting transmon qubits, what physical component is responsible for creating the non-linear energy levels required for a two-level quantum system?