Deterministic Quantum Communication Between Fixed-Frequency Superconducting Qubits via Broadband Resonators

TL;DR

This work tackles the challenge of scalable quantum communication between spatially separated superconducting processors by using fixed-frequency qubits connected through broadband transfer resonators and a frequency-tunable photon-generation method. The authors implement two coupled-resonator transfer channels that provide a spectral overlap exceeding $>100$ MHz, enabling deterministic quantum state transfer and remote entanglement over a $30$-MHz photon-frequency range centered near $\omega_{\rm ph}/2\pi \approx 9.37$ GHz. They achieve process fidelities around $\mathcal{F}_p \approx 0.78$ for state transfer and Bell-state fidelities around $\mathcal{F}_{\mathrm{Bell}} \approx 0.73$ across the tested frequencies, with photon absorption efficiency ~95% and propagation loss ~29%. The approach removes the need for flux-tunable circuit elements, offering a scalable path toward quantum networks and potential for frequency multiplexing, though it currently relies on adiabatic wave-packet shaping that constrains photon bandwidth.

Abstract

Quantum communication between remote chips is essential for realizing large-scale superconducting quantum computers. For such communication, itinerant microwave photons propagating through transmission lines offer a promising approach. However, demonstrations to date have relied on frequency-tunable circuit elements to compensate for fabrication-related parameter variations between sender and receiver devices, introducing control complexity and limiting scalability. In this work, we demonstrate deterministic quantum state transfer and remote entanglement generation between fixed-frequency superconducting qubits on separate chips. To compensate for the sender-receiver mismatch, we employ a frequency-tunable photon-generation technique which enables us to adjust the photon frequency without modifying circuit parameters. To enhance the frequency tunability, we implement broadband transfer resonators composed of two coupled coplanar-waveguide resonators, achieving a bandwidth of more than 100 MHz. This broadband design enables successful quantum communication across a 30-MHz range of photon frequencies between the remote qubits. Quantum process tomography reveals state transfer fidelities of around 78% and Bell-state fidelities of around 73% across the full frequency range. Our approach avoids the complexity of the control lines and noise channels, providing a flexible pathway toward scalable quantum networks.

Figures (7)

• Figure 1: Quantum communication between fixed-frequency superconducting qubits with broadband resonators. (a) Schematic of the system. (b) Energy-level diagram of the system. A microwave photon is emitted from the sender device (left) via a resonator-assisted Raman transition (red arrow) and absorbed by the receiver device (right). The broadband designs of the communication resonators (yellow Lorentzians) compensate for frequency mismatch, and the frequency-tunable photon generation method miyamura_generation_2025 enables quantum communication between fixed-frequency qubits despite such variations.
• Figure 2: (a) False-colored photograph of the device. A fixed-frequency transmon qubit (green) is capacitively coupled to a readout resonator (orange) and a transfer resonator for single-photon emission and absorption. The qubit is driven via a dedicated control line (red). Both the sender and receiver devices have the same circuit structure. (b) Equivalent circuit diagram of each device. (c) Schematic of the qubit--transfer-resonator system. (d) Equivalent schematic of (c). (e)(f) Resonator design strategies employed in this work. $\Gamma_f$ is the photon emission rate that corresponds to the resonator spectrum. Dashed curves represent individual resonance modes and solid curves represent the combined resonator spectrum. The black dotted line indicates the threshold of the photon-emission rate used for designing. (g) Reflection spectroscopy of the transfer resonator. The qubit is prepared in the $\ket{g}$ ($\ket{e}$) state, and the reflection coefficient $S_{11,g(e)}$ is measured. Blue (red) dots represent the phase of the ratio $S_{11,g}/S_{11,e}$ for the sender (receiver) device, with corresponding fits shown as solid lines. The shaded region indicates the frequency range of the emitted photons used in the quantum communication experiments.
• Figure 3: Propagation loss and absorption efficiency. (a) Pulse sequence to estimate photon loss during propagation. A microwave photon is emitted from the sender and receiver devices separately. The presence of a circulator in the connecting cable (see Fig. \ref{['figure_wiring']} in Appendix \ref{['app:setup']}) allows an independent measurement of photon loss during propagation and absorption efficiency at the receiver kurpiers_deterministic_2018. (b) Fourier amplitudes of the emitted photons at each target frequency $\omega_{\mathrm{ph}}$. Solid curves represent photons emitted from the sender device, while dashed curves represent photons emitted from the receiver device. Vertical dotted lines indicate the target photon frequencies $\omega_{\mathrm{ph}}$. (c) Estimated photon loss for each photon frequency $\omega_{\mathrm{ph}}$. (d) Pulse sequence for measuring the photon-absorption efficiency. A microwave photon emitted from the sender is absorbed at the receiver using a time-reversed drive pulse. (e) Measured photon flux at $\omega_{\mathrm{ph}}/2\pi = 9.39$ GHz. The solid curve shows the photon flux without absorption, and the dashed curve shows the residual photon flux after the absorption at the receiver. (f) Measured absorption efficiency for each photon frequency $\omega_{\mathrm{ph}}$.
• Figure 4: Quantum state transfer and remote entanglement generation. (a) Pulse sequence for quantum state transfer between the sender and receiver qubits. (b) Process matrix for quantum state transfer at $\omega_{\mathrm{ph}}/2\pi = 9.39$ GHz. Gray and blue wireframes indicate the ideal process and the simulated result, respectively. (c) Process fidelities for quantum state transfer at each photon frequency $\omega_{\mathrm{ph}}$. The dashed frames represent the simulated results. (d) Pulse sequence for remote Bell-state generation. (e) Reconstructed density matrix of the two-qubit Bell state at $\omega_{\mathrm{ph}}/2\pi = 9.39$ GHz. Gray and blue wireframes indicate the ideal state and the simulated result, respectively. (f) Bell-state fidelities at each photon frequency $\omega_{\mathrm{ph}}$. The dashed frames indicate the simulated results.
• Figure 5: Experimental setup. AWG, arbitrary waveform generator; ADC, analog-to-digital converter; SSB, single-sideband mixer; IR, image-reject mixer; LPF, low-pass filter; BPF, band-pass filter; HEMT, high-electron-mobility transistor; Ecco., eccosorb filter; JPA, Josephson parametric amplifier. The dotted line indicates the alternative signal path used for transfer resonator spectroscopy, which is manually connected by reconfiguring the sender readout line.