Superconducting qubit

Superconducting qubits are artificial atoms built from superconducting circuits, typically using Josephson junctions as the nonlinear element. They operate at microwave frequencies and require millikelvin temperatures to maintain quantum coherence. The two lowest energy levels of the circuit form the qubit basis states |0⟩ and |1⟩. The Josephson junction provides an anharmonic energy spectrum, allowing selective addressing of the qubit transition without exciting higher levels.

The main types are charge qubits, flux qubits, and phase qubits, with modern variants like transmon and Xmon being most prevalent. The transmon, introduced in 2007 by Koch et al., uses a large shunt capacitance to reduce sensitivity to charge noise, achieving longer coherence times. Flux qubits operate in a regime where the persistent current in a loop creates two basis states. Phase qubits use the phase difference across a junction as the qubit variable.

Superconducting qubits are controlled by microwave pulses resonant with the qubit transition frequency. Readout is typically performed by dispersively coupling the qubit to a microwave resonator, where the resonator's response depends on the qubit state. Two-qubit gates are implemented via tunable couplers or fixed coupling, enabling universal quantum computation.

The concept dates back to the 1980s with proposals by Leggett and others. The first superconducting qubit was demonstrated in 1999 by Nakamura et al., a charge qubit. Subsequent improvements led to the transmon, which became the basis for many modern quantum processors. In 2019, Google claimed quantum supremacy using a 53-qubit superconducting processor, Sycamore.

Superconducting qubits are used by IBM, Google, and others in cloud-accessible quantum computers. Major challenges include improving coherence times, reducing error rates, and scaling to thousands of qubits. Error correction schemes require high-fidelity gates, which are being pursued through materials research and circuit design.

Lesser-known aspects include the role of quasiparticle poisoning, which can cause decoherence; the use of 3D cavities for improved coherence; and the development of 'fluxonium' qubits with high anharmonicity. Additionally, superconducting qubits have been used to simulate quantum phenomena, such as the Jaynes-Cummings model, and are being explored for quantum sensing applications.

Josephson junction

A device consisting of two superconductors separated by a thin insulating barrier, which supports tunneling of Cooper pairs and provides nonlinear inductance.

Transmon

A type of superconducting charge qubit designed to be insensitive to charge noise by using a large shunt capacitance.

Cooper pair

A bound state of two electrons that carries supercurrent in a superconductor.