arXiv subjects

Jeremy B. Kline

Publications and source records attributed to Jeremy B. Kline.

3 recordsLinked to original sources

arXiv2025

The Arm Qubit: A Superconducting Qubit Co-Designed for Coherence and Coupling

We present a superconducting qubit which consists of two strongly coupled modes: one for data storage and one for coupling, allowing faster, higher-fidelity entangling gates and readout. The use of a dedicated coupling mode allows nonlinear couplings of several hundred MHz between the data mode and other elements, with minimal linear coupling to the data mode. Including decoherence, simulations show that this architecture enables microwave-only CZ gates with an infidelity of $8.6\times10^{-5}$ in 17 ns and always-on ZZ interaction less than 0.4 kHz. Numerical simulations also show readout with state assignment error of $1\times10^{-4}$ in 27 ns (assuming quantum efficiency $η=0.5$), Purcell-limited lifetime of 167 ms without a Purcell filter, and a mechanism to suppress shot-noise dephasing ($1/Γ_ϕ=15.8$ ms). Single-qubit gate infidelities are below $1\times10^{-5}$ including decoherence. These beyond experimental state-of-the-art gate and readout fidelities rely only on capacitive coupling between arm qubits, making the arm qubit a promising scalable building block for fault-tolerant quantum computers.

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arXiv2024

Near-ultrastrong nonlinear light-matter coupling in superconducting circuits

The interaction between an atom and an electromagnetic mode of a resonator is of both fundamental interest and is ubiquitous in quantum technologies. Most prior work studies a linear light-matter coupling of the form $g \widehatσ_x (\widehat{a} + \widehat{a}^\dagger)$, where $g$ measured relative to photonic ($ω_a$) and atomic ($ω_b$) mode frequencies can reach the ultrastrong regime ($g/ω_{a}\!>\!10^{-1}$). In contrast, a nonlinear light-matter coupling of the form $\fracχ{2} \widehatσ_z \widehat{a}^\dagger \widehat{a}$ has the advantage of commuting with the atomic $\widehatσ_z$ and photonic $\widehat{a}^\dagger\widehat{a}$ Hamiltonian, allowing for fundamental operations such as quantum-non-demolition measurement. However, due to the perturbative nature of nonlinear coupling, the state-of-the-art $χ/\text{max}(ω_a, ω_b)$ is limited to $\!<\!10^{-2}$. Here, we use a superconducting circuit architecture featuring a quarton coupler to experimentally demonstrate, for the first time, a near-ultrastrong $χ/\text{max}(ω_a, ω_b)= (4.852\pm0.006)\times10^{-2}$ nonlinear coupling of a superconducting artificial atom and a nearly-linear resonator. We also show signatures of light-light nonlinear coupling ($χ\widehat{a}^\dagger\widehat{a}\widehat{b}^\dagger\widehat{b}$), and $χ/2π= 580.3 \pm 0.4 $ MHz matter-matter nonlinear coupling ($\fracχ{4}\widehatσ_{z,a}\widehatσ_{z,b}$) which represents the largest reported $ZZ$ interaction between two coherent qubits. Such advances in the nonlinear coupling strength of light, matter modes enable new physical regimes and could lead to applications such as orders of magnitude faster qubit readout and gates.

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arXiv2024

Ultrafast Superconducting Qubit Readout with the Quarton Coupler

Fast, high-fidelity, and quantum nondemolition (QND) qubit readout is an essential element of quantum information processing. For superconducting qubits, state-of-the-art readout is based on a dispersive cross-Kerr coupling between a qubit and its readout resonator. The resulting readout can be high-fidelity and QND, but readout times are currently limited to the order of 50 ns due to the dispersive cross-Kerr of magnitude 10 MHz. Here, we present a new readout scheme that uses the quarton coupler to facilitate a large (greater than 250 MHz) cross-Kerr between a transmon qubit and its readout resonator. Full master equation simulations show a 5 ns readout time with greater than 99% readout and QND fidelity. Unlike state-of-the-art dispersive readout, the proposed "quartonic readout" scheme relies on a transmon with linearized transitions as the readout resonator. Such operational points are found from a detailed theoretical treatment and parameter study of the coupled system. The quartonic readout circuit is also experimentally feasible and preserves the coherence properties of the qubit. Our work reveals a new path for order-of-magnitude improvements of superconducting qubit readout by engineering nonlinear light-matter couplings in parameter regimes unreachable by existing designs.

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