Using bi-fluxon tunneling to protect the Fluxonium qubit
arxiv(2024)
摘要
Encoding quantum information in quantum states with disjoint wave-function
support and noise insensitive energies is the key behind the idea of qubit
protection. While fully protected qubits are expected to offer exponential
protection against both energy relaxation and pure dephasing, simpler circuits
may grant partial protection with currently achievable parameters. Here, we
study a fluxonium circuit in which the wave-functions are engineered to
minimize their overlap while benefiting from a first-order-insensitive flux
sweet spot. Taking advantage of a large superinductance (L∼ 1 μH),
our circuit incorporates a resonant tunneling mechanism at zero external flux
that couples states with the same fluxon parity, thus enabling bifluxon
tunneling. The states |0⟩ and |1⟩ are encoded in wave-functions
with parities 0 and 1, respectively, ensuring a minimal form of protection
against relaxation. Two-tone spectroscopy reveals the energy level structure of
the circuit and the presence of 4 π quantum-phase slips between different
potential wells corresponding to m=± 1 fluxons, which can be precisely
described by a simple fluxonium Hamiltonian or by an effective bifluxon
Hamiltonian. Despite suboptimal fabrication, the measured relaxation (T_1 =
177± 3 μ s) and dephasing (T_2^E = 75± 5 μs) times not only
demonstrate the relevance of our approach but also opens an alternative
direction towards quantum computing using partially-protected fluxonium qubits.
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