Bosonic quantum error correction (QEC) offers a hardware-efficient route to fault-tolerant quantum computing. To date, however, superconducting circuit implementations of bosonic codes have utilized centimeter-scale 3D microwave cavities controlled by fixed-frequency transmon qubits, with logical lifetimes limited by transmon bit-flip errors. Here, we realize bosonic QEC in a fully planar architecture by pairing a heavy fluxonium, whose $451 \pm 70~μ\mathrm{s}$ bit-flip lifetime exceeds that of any control qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of $1.59 \pm 0.05$ . These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting circuit architecture.Bosonic quantum error correction (QEC) offers a hardware-efficient route to fault-tolerant quantum computing. To date, however, superconducting circuit implementations of bosonic codes have utilized centimeter-scale 3D microwave cavities controlled by fixed-frequency transmon qubits, with logical lifetimes limited by transmon bit-flip errors. Here, we realize bosonic QEC in a fully planar architecture by pairing a heavy fluxonium, whose $451 \pm 70~μ\mathrm{s}$ bit-flip lifetime exceeds that of any control qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of $1.59 \pm 0.05$. These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting circuit architecture.

quant-pharXiv:2609.26880Sep 24, 20260 citations

Engineering Light-Matter Interactions with Metastable States in a Heavy Fluxonium

Nathan Earnest

In the past few years there has been an increasing amount of media coverage on quantum computing, largely due to the substantial investment in the superconducting circuit quantum electrodynamics (cQED) technologies by several governments and technology and finance companies throughout the world. Since their inception, superconducting qubits have realized several orders of magnitude improvement in information lifetimes, but individual qubit lifetimes must be further improved to realize a fault-tolerant quantum computer. A critical component of these circuits, which allows quantum states to be addressed, is the non-linear inductive element known as the Josephson junction. In this thesis, we will present experimental studies showing how to design and fabricate the Josephson element to leverage a recently developed `superinductor' as a way to engineer new qubit regimes, such as the heavy fluxonium. We create the heavy fluxonium with a chain of large Josephson junctions and a large shunting capacitor. This shunting capacitor increases the effective mass of a fictitious particle inside the wells of the heavy fluxonium potential energy landscape, giving rise to two characteristically different transitions: intra-well transmon-like plasmon transitions and inter-well fluxon transitions. The fluxon transitions are heavily suppressed by a reduced dipole matrix element realized from the increased effective mass, and result in metastable fluxon transitions, with $T_1$ 's as large as 8ms. This qubit design offers the possibility to study qubit interaction with the thermal bath, the implementation of atomic state preparation techniques such as Raman gates, optical pumping and fluorescent style readout, as well as the possibility to make substantial improvements in qubit lifetimes and create protected qubits in the cQED platform.In the past few years there has been an increasing amount of media coverage on quantum computing, largely due to the substantial investment in the superconducting circuit quantum electrodynamics (cQED) technologies by several governments and technology and finance companies throughout the world. Since their inception, superconducting qubits have realized several orders of magnitude improvement in information lifetimes, but individual qubit lifetimes must be further improved to realize a fault-tolerant quantum computer. A critical component of these circuits, which allows quantum states to be addressed, is the non-linear inductive element known as the Josephson junction. In this thesis, we will present experimental studies showing how to design and fabricate the Josephson element to leverage a recently developed `superinductor' as a way to engineer new qubit regimes, such as the heavy fluxonium. We create the heavy fluxonium with a chain of large Josephson junctions and a large shunting capacitor. This shunting capacitor increases the effective mass of a fictitious particle inside the wells of the heavy fluxonium potential energy landscape, giving rise to two characteristically different transitions: intra-well transmon-like plasmon transitions and inter-well fluxon transitions. The fluxon transitions are heavily suppressed by a reduced dipole matrix element realized from the increased effective mass, and result in metastable fluxon transitions, with $T_1$'s as large as 8ms. This qubit design offers the possibility to study qubit interaction with the thermal bath, the implementation of atomic state preparation techniques such as Raman gates, optical pumping and fluorescent style readout, as well as the possibility to make substantial improvements in qubit lifetimes and create protected qubits in the cQED platform.

Mar 1, 20190 citations

Realization of a Λ System with Metastable States of a Capacitively Shunted Fluxonium

Nathan Earnest, Srivatsan Chakram, Yao Lu +9 more

We realize a Λ system in a superconducting circuit, with metastable states exhibiting lifetimes up to 8 ms. We exponentially suppress the tunneling matrix elements involved in spontaneous energy relaxation by creating a "heavy" fluxonium, realized by adding a capacitive shunt to the original circuit design. The device allows for both cavity-assisted and direct fluorescent readouts, as well as state preparation schemes akin to optical pumping. Since direct transitions between the metastable states are strongly suppressed, we utilize Raman transitions for coherent manipulation of the states.We realize a Λ system in a superconducting circuit, with metastable states exhibiting lifetimes up to 8 ms. We exponentially suppress the tunneling matrix elements involved in spontaneous energy relaxation by creating a "heavy" fluxonium, realized by adding a capacitive shunt to the original circuit design. The device allows for both cavity-assisted and direct fluorescent readouts, as well as state preparation schemes akin to optical pumping. Since direct transitions between the metastable states are strongly suppressed, we utilize Raman transitions for coherent manipulation of the states.

arXiv:1707.00656Physical Review LettersApr 13, 2018131 citations

451 \pm 70~μ\mathrm{s}

bit-flip lifetime exceeds that of any control qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of

1.59 \pm 0.05

. These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting circuit architecture.

T_1

's as large as 8ms. This qubit design offers the possibility to study qubit interaction with the thermal bath, the implementation of atomic state preparation techniques such as Raman gates, optical pumping and fluorescent style readout, as well as the possibility to make substantial improvements in qubit lifetimes and create protected qubits in the cQED platform.