Integer Fluxonium Qubit: A High-Performance, Superconducting Device for Quantum Computing

The Integer Fluxonium Qubit is a superconducting qubit that operates in a zero magnetic field, with a frequency of about 4 GHz. It has a high energy relaxation quality factor and a Ramsey coherence time exceeding 100 µs. The qubit’s performance is comparable to the best transmons and is expected to improve with optimized fabrication and measurement procedures.

The Integer Fluxonium Qubit operates at a lower frequency than transmons, which allows it to match their performance with less sophisticated procedures. It is a significant addition to the field of quantum computing, offering new possibilities for high-performance superconducting qubits.

What is the Integer Fluxonium Qubit?

The Integer Fluxonium Qubit is a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and the energy relaxation quality factor Q is 0.71*10^7, even though the dielectric loss quality factor of the circuit components is in the low 10^5 range. The Ramsey coherence time exceeds 100 µs and the average fidelity of Clifford gates is benchmarked to F>0.999. These figures are likely to improve by an order of magnitude with optimized fabrication and measurement procedures. The work establishes a ready-to-use, partially protected superconducting qubit with the error rate comparable to the best transmons.

How Does the Integer Fluxonium Qubit Compare to Other Qubits?

In recent years, superconducting fluxonium qubits have reached and may have even exceeded the state-of-the-art coherence time and gate error defined by the industry-standard transmons. The difference between the two devices seems minimal. A transmon is fundamentally a weakly anharmonic electromagnetic oscillator defined by a Josephson junction’s inductance and a shunting capacitance, while a fluxonium contains an additional high-inductance superinductance shunt which dramatically increases the qubit anharmonicity without introducing new decoherence channels. The strongly anharmonic spectrum of fluxoniums is an important new resource for superconducting quantum processors as it can help mitigate the propagation of coherent errors.

What Makes the Integer Fluxonium Qubit Unique?

The Integer Fluxonium Qubit operates at a much lower frequency, usually ranging around 100-1000 MHz, as opposed to the typically 4-6 GHz frequency range for transmons. Given that the dielectric loss is the primary decoherence mechanism in both devices, it is essentially the lower frequency of fluxoniums that allows matching the best transmons with far less sophisticated material science research and fabrication procedures. Other ideas for post-transmon qubits are being actively explored, including developing control techniques for even lower frequency fluxoniums, which are expected to have even longer coherence time albeit not necessarily higher quality factor.

What are the Practical Applications of the Integer Fluxonium Qubit?

The Integer Fluxonium Qubit can operate as relatively high-frequency qubits when biased at the less explored integer flux quantum sweet spot, as opposed to the usual half-integer flux quantum sweet spot. Circuit parameters must be adjusted such that the low-energy dynamics is governed entirely by flux quantization in the loop and is well described by the dual of the Cooper pair box Hamiltonian. In this case, the lowest excited state is a doublet originating from the classical degeneracy of charging an inductance L with a positive vs a negative flux quantum at energy h^2e^2/2L. Tunneling splits the doublet. The lower doublet and the ground states define the integer fluxonium qubit while the parity selection rule forbids the transition to the higher doublet state from the ground state.

What is the Future of the Integer Fluxonium Qubit?

The Integer Fluxonium Qubit is expected to improve by at least an order of magnitude if an active magnetic flux control is performed and with appropriate optimization of fabrication and measurement procedures. This result adds a ready-to-use device, nicknamed integer fluxonium qubit, to the relatively short list of high-performance superconducting qubits. The paper demonstrates the equivalence of the integer and the half-integer flux sweet spots with respect to the energy relaxation rate using an elementary model of flux quantization. This work is a significant contribution to the field of quantum computing and opens up new possibilities for the development of high-performance superconducting qubits.