Coherent manipulation of interacting electron qubitson solid neon

Abstract

Electrons trapped on solid neon surfaces serve as low-noise charge qubits with long coherence times and high operational fidelities. Such charge qubits offer full electrical control and compact device footprints, convenient for scaling up with quantum circuits. Realizing two-qubit gates on this platform is a critical step towards practical quantum information processing. In this work, we report the first experimental demonstration of coherent manipulation of multiple interacting electron-on-solid-neon (eNe) charge qubits. By exploiting the electrons naturally confined in close proximity by the surface structures of solid neon, we have achieved a direct qubit-qubit coupling strength of up to 62.5 MHz, as well as implemented cross-resonance (CR) and bSWAP two-qubit gates using global microwave drives. The natural electron confinement by solid neon mitigates the high-density-wiring challenge, simplifies the multi-qubit control, and establishes a unique path to scale up the eNe qubit platform.

Figures (9)

• Figure 1: Schematic of multi-qubit coupling on solid neon.a, Solid neon creates multi-qubit host spots and supports inter-qubit coupling at a short distance. b, Schematic of the interactions between a two-qubit system on solid neon and a superconducting resonator, where a bright qubit $\mathrm{Q_b}$ and a dark qubit $\mathrm{Q_d}$ are coupled with strength $J$, and $\mathrm{Q_b}$ is coupled to the resonator with strength $g_\mathrm{b}$. Due to the orthogonal dipole alignment to the field, as marked by the black arrow, $\mathrm{Q_d}$ is invisible to the resonator. c, Energy-level schematic of the two-qubit system in b, illustrating the possible transitions. Both the cross-resonance (CR) and the bSWAP type two-qubit operations are shown, where the $\mathrm{Q_d}$ are driven by microwave gates mediated through the $\mathrm{Q_b}$.
• Figure 2: Spectroscopic characterization of coherent two-qubit coupling on solid neon.a, Continuous-wave (CW) two-tone qubit spectroscopy with avoided crossings on the resonator's phase response. The dashed lines indicate the bare transition frequencies of the dark qubit $\mathrm{Q_d}$ and the bright qubit $\mathrm{Q_b}$. b, The line cut at the avoided crossing shows a splitting corresponding to a charge-charge interaction strength of $J/2\pi=3.35$ MHz, which exceeds the hybrid linewidth $(\gamma_\textrm{b}+\gamma_\textrm{d})/4\pi=0.56$ MHz. c, Qubits spectroscopy measured with high-power pulsed readout, in which $\mathrm{Q_b}$ served as the mediator between $\mathrm{Q_d}$ and the resonator. The inset shows the pulse sequence: a long square pulse (10 µ s) with variable frequency $f_\mathrm{dr}$ and power $P_\mathrm{dr}$ drive the system to a specific state, followed by a short square probe pulse (0.7 µs) at the resonator frequency $f_\mathrm{r}$ with approximately $-120$ dBm power reaching the resonator. The readout pulse induces the frequency crossing of the two qubits and the swap of their populations, revealing the state information of the dark qubit $\mathrm{Q_d}$. Drive pulse power $P_{\text{dr}}$ reaching the resonator's input coupler increases from $-73$ dBm to $-63$ dBm, activating transitions corresponding to the CR ($|0_\mathrm{b}0_\mathrm{d}\rangle \rightarrow |0_\mathrm{b}1_\mathrm{d}\rangle$) and bSWAP ($|0_\mathrm{b}0_\mathrm{d}\rangle \rightarrow |1_\mathrm{b}1_\mathrm{d}\rangle$) two-qubit operations. The measurements were taken when the system was biased at $\Delta V_{\rm tg}=\,$0 V. The phase curves are off-set for visualization.
• Figure 3: Time-domain all-microwave two-qubit operations on solid neon.a and c, Drive pulse length-frequency (a) and drive pulse amplitude-frequency (c) Rabi measurements, following the pulse sequences depicted in the inset. In (a), the drive pulse power is fixed at $-70$ dBm, with varying length from 0 to 2.0 µ s. In (c), the drive pulse length is fixed at 0.8 µs, with output amplitude varying from 0.0 V to 0.25 V, corresponding to zero power to $-57$ dBm reaching the resonator input coupler. The parameters of the readout pulses are the same as described in the Two-qubit Spectroscopy section. The system is biased with a single DC gate at $\Delta V_\mathrm{tg}=0$. Measurement results show the two oscillations corresponding to cross-resonance (CR, purple arrows) and bSWAP (magenta arrows) two-qubit gates. b and d, Numerically simulated excited state population of $\mathrm{Q_b}$ after the readout pulses, corresponding to the Rabi measurements in (b) and (d). The CR and bSWAP oscillation features are reproduced with experimentally measured parameters from the spectroscopy and decoherence characterization, see Methods for details.
• Figure 4: Spectroscopic characterization of a three-qubit coupled system on solid neon.a, Two-tone qubit spectroscopy reveals a three-qubit coupled system, with the inter-qubit coupling strengths $J_\mathrm{12}$ and $J_\mathrm{23}$ shown in the inset. Via $\mathrm{Q}_1$, the system is probed at the resonator frequency while being driven with a second tone at varied frequencies. The dashed line marks the avoided crossing as in (b). b, The line cut shows the large charge-charge interaction strength between $\mathrm{Q}_1$ and $\mathrm{Q}_2$ of $J_\mathrm{12}/2\pi=62.5$ MHz and hybrid linewidth $(\gamma_1+\gamma_2)/4\pi=3.5$ MHz. c, Calculated eigenstate energy diagram of the three-qubit coupled system, with bare qubit states shown as dashed lines. See Methods for detailed qubit parameters used in the calculation.
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