Proposals for experimentally realizing (mostly) quantum-autonomous gates

TL;DR

The paper tackles reducing reliance on time-dependent classical control in quantum devices by proposing experimentally feasible quantum-autonomous gates across Rydberg-atom, trapped-ion, and superconducting-qubit platforms. It presents concrete gate schemes—Rydberg-blockade CZ and ultrafast entanglement for atoms, Z and MS gates via tailored potentials for ions, and Z and XY gates in circuit QED—all powered by autonomous clocks or passive lasers. The work analyzes the necessary components, including autonomous clocks, pulse resources, and parameter regimes, and discusses fidelity and timing considerations. By providing building blocks for fully or partially autonomous circuits, the results point toward lower energy costs, reduced wiring complexity, and improved scaling in quantum processors.

Abstract

Autonomous quantum machines (AQMs) execute tasks without requiring time-dependent external control. Motivations for AQMs include the restrictions imposed by classical control on quantum machines' coherence times and geometries. Most AQM work is theoretical and abstract; yet an experiment recently demonstrated AQMs' usefulness in qubit reset, crucial to quantum computing. To further reduce quantum computing's classical control, we propose realizations of (fully and partially) quantum-autonomous gates on three platforms: Rydberg atoms, trapped ions, and superconducting qubits. First, we show that a Rydberg-blockade interaction or an ultrafast transition can quantum-autonomously effect entangling gates on Rydberg atoms. One can perform $Z$ or entangling gates on trapped ions mostly quantum-autonomously, by sculpting a linear Paul trap or leveraging a ring trap. Passive lasers control these gates, as well as the Rydberg-atom gates, quantum-autonomously. Finally, circuit quantum electrodynamics can enable quantum-autonomous $Z$ and $XY$ gates on superconducting qubits. The gates can serve as building blocks for (fully or partially) quantum-autonomous circuits, which may reduce classical-control burdens.

Figures (9)

• Figure 1: Energy-level diagram for a $^{87}$Rb atom (simplified from Gaetan09). A two-photon process couples the qubit state $\lvert 1 \rangle$ to the Rydberg state $\lvert {\rm r} \rangle$. The lasers are detuned from $\lvert 0 \rangle$–$\lvert 1 \rangle$ and $\lvert 1 \rangle$–$\lvert \rm r \rangle$ transitions, so the process does not suffer from $\lvert \text{i} \rangle$'s lossiness.
• Figure 2: Tailored potential slide. A potential (dashed curve) confines the ions before the protocol, then is switched off. The ions initially slide down a potential gradient, then traverse the gate zones. Laser beams (yellow) implement gates.
• Figure 3: Quantum-autonomous $Z$ gate implemented with a ring trap. The black disks represent ions. Their trajectory forms the dashed line. A laser shines on two ring segments, driving internal transitions in the ions.
• Figure 4: Controlled-$(-Z)$ gate implemented in circuit QED. An incoming photon interacts with a transmon–cavity system, which reflects the photon. The photon acquires a phase dependent on the transmon's initial state; equivalently, the transmon acquires a phase dependent on the photon's initial state. Figure simplified from Besse18.
• Figure 5: $\Lambda$ system in an autonomous quantum clock. A clockwork mechanism (Fig. \ref{['fig:engine']}) pumps the system into $\lvert \text{e} \rangle$. The system spontaneously emits frequency-$\omega_\text{se}$ photons, which serve as clock ticks.