Bifluxon: Fluxon-Parity-Protected Superconducting Qubit

Abstract

We develop and characterize a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from charge and flux noises, and dephasing from flux noise. The qubit consists of a Cooper-pair box (CPB) shunted by a superinductor, forming a superconducting loop. Provided the offset charge on the CPB island is an odd number of electrons, the qubit potential corresponds to that of a cos (φ/2) Josephson element, preserving the parity of fluxons in the loop via Aharonov-Casher interference. In this regime, the logical-state wavefunctions reside in disjoint regions of Hilbert space, thereby ensuring protection against energy decay. By switching the protection on, we observe a tenfold increase of the decay time, reaching up to 100 μs. Though the qubit is sensitive to charge noise, the sensitivity is much reduced in comparison with the charge qubit, and the charge-noise-induced dephasing time of the current device exceeds 1 μs. Implementation of full dephasing protection can be achieved in the next-generation devices by combining several cos(φ/2) Josephson elements in a small array.

Engineering high-coherence superconducting qubits

“…Recent experiments on this qubit have shown a 10-fold increase in the energy relaxation time up to ∼ 100µs. 169 Although improving materials quality to boost coherence will always be a critical driver for any quantum processor architecture, it is important to quantify what level of perfection is needed to reach a threshold above which coherent lifetimes are sufficiently long to enable quantum error correction. Qubit structures with some degree of noise protection incorporated into the hardware itself promise to achieve fault-tolerant operation with a greater tolerance for materials defects, albeit at the expense of more complex fabrication and gate protocols.…”

“…Spring Nature (Smith et al). Panel d is adapted with permission from Ref 169 ,. American Physical Society (Kalashnikov et al).…”

Materials in superconducting quantum circuits

“…. 量子比特是量子芯片的核心, 实验上将超导电路 作为量子比特已经有20多年的历史, 实现了多种不同 的超导量子比特, 包括Charge [85] 、Flux [86] 、Phase [87] 、 Transmon [82] 、Fluxonium [83] 、0-π [88] 、电容并联的磁通 比特(capacitive shunt flux qubit, CSFQ) [89] 、Quarton [90] 、Bifluxon [91] 等. 大部分比特的结构都可以简化 为一个电容、电感、约瑟夫森结并联的非谐振电路模 型 [92] .…”

Protected hybrid superconducting qubit in an array of gate-tunable Josephson interferometers

“…A promising route to creating protected qubits is to rely on circuits with underlying symmetries and encode the qubit into distinct eigenstates of the corresponding symmetry operator [1][2][3][4][5][6]. Such circuits satisfy the requirements of a protected qubit because (1) the vanishing transition matrix elements between states with different symmetries prevent bitflip errors, and (2) the near-degeneracy of the qubit states suppresses phase-flip errors.…”