Flux qubit

A flux qubit consists of a superconducting loop interrupted by one or more Josephson junctions. When the loop is threaded by an external magnetic flux near half a flux quantum (Φ₀/2), the energy landscape forms a double-well potential, with the two wells corresponding to clockwise and counterclockwise persistent current states ¹. These states can be put into a quantum superposition, forming the qubit basis. The energy splitting between the two wells is controlled by the applied flux, allowing manipulation of the qubit state.

The qubit is read out by coupling it to a superconducting resonator or a DC SQUID, which senses the magnetic field produced by the circulating current. This readout scheme is fast and has high fidelity, making it suitable for quantum error correction protocols.

The original flux qubit, proposed by Mooij et al. in 1999, used three Josephson junctions, but modern designs include single-junction and two-junction variants with additional shunting capacitors to reduce sensitivity to charge noise ². The persistent-current qubit, developed at TU Delft, is one of the most studied implementations. More recently, the fluxonium qubit, invented by Manucharyan et al., replaces the two-well potential with a multilevel spectrum, while the c-shunt flux qubit (also known as a capacitively shunted flux qubit) combines flux qubit tunability with improved coherence.

These variants have been integrated into multi-qubit processors, where they are coupled via microwave resonators or direct capacitive links, enabling two-qubit gates such as controlled-Z and iSWAP.

Flux qubits are attractive for quantum computation because they can be designed to have large anharmonicity, allowing fast gates, and they are protected against certain types of decoherence. They are also used in quantum annealing, as demonstrated by D-Wave Systems, which uses a related design of superconducting flux qubits in their processors ⁴. In addition, flux qubits are employed in quantum simulation, quantum metrology, and as quantum memory elements.

One of the main challenges is that flux qubits are sensitive to magnetic flux noise, which limits coherence times. However, recent research has shown that coherence times can be extended by using three-dimensional cavities and by operating at an optimal flux bias point, known as the 'sweet spot'.

Flux qubits have been used to demonstrate quantum effects at macroscopic scales, such as macroscopic quantum tunneling and energy-level quantization, in the pioneering experiments of the 1980s and 1990s ⁵. A particularly niche topic is the use of flux qubits to couple to long-lived phonons in superconducting circuits, enabling hybrid quantum systems.

Another little-known fact is that the term 'flux qubit' is sometimes applied to the 'rf-SQUID' qubit, which has a single Josephson junction and is the predecessor of the modern flux qubit. In such devices, the persistent current can be as high as several micromperes, producing a measurable magnetic field.

Josephson junction

A weak link between two superconductors that allows Cooper pairs to tunnel through, providing the nonlinear inductance essential for qubit operation.

Persistent current

A constant, lossless current that circulates in a superconducting loop, induced by an external magnetic field.

Flux quantum (Φ₀)

The fundamental quantum of magnetic flux, 2.07×10⁻¹⁵ Wb, trapped in a superconducting loop.

Charge noise

Random fluctuations in electric charge in the environment that can perturb quantum states, particularly in charge-based qubits.