High-performance quantum interconnect between bosonic modules beyond transmission loss constraints

Abstract

Distributed quantum computing architectures require high-performance quantum interconnects between quantum information processing units, while previous implementations have been fundamentally limited by transmission line losses. Here, we demonstrate a low-loss interconnect between two superconducting modules using an aluminum coaxial cable, achieving a bus mode quality factor of 1.7e6. By employing SNAIL as couplers, we realize inter-modular state transfer in 0.8 μs via a three-wave mixing process. The state transfer fidelity reaches 98.2% for quantum states encoded in the first two energy levels, achieving a Bell state fidelity of 92.5%. Furthermore, we show the capability to transfer high-dimensional states by successfully transmitting binomially encoded logical states. Systematic characterization reveals that performance constraints have shifted from transmission line losses (contributing merely 0.2% infidelity) to module-channel interface effects and local Kerr nonlinearities. Our work advances the realization of quantum interconnects approaching fundamental capacity limits, paving the way for scalable distributed quantum computing and efficient quantum communications.

Figures (5)

• Figure 1: (a) Schematic of two superconducting modules connected via a high-quality aluminum coaxial cable as the transmission channel. Each module consists of a 3D cavity, a SNAIL coupler, and a transmon qubit. (b) The three-wave mixing process enabled by the SNAIL. The frequency mismatches between the cavity and the channel mode are compensated by selected driving frequencies. (c) 3D schematic of the module-channel interface, highlighting the peeled length $L$ of the cable for engineering the transmission loss. The energy of the bus mode is dissipated at the clamp-cable interface (not shown) at the bottom of the package. The waveguide above the transmission line is to spatially extend the bus mode and enhance the bus-SNAIL coupling. (d-e) Trade-off between quality factor $Q$ and coupling strength $g$ versus $L$. The crosses indicate the operating point in our experiment at $L=2$ mm.
• Figure 2: (a) Experimental sequence for characterizing the coherent conversion interaction. (b) Extracted conversion coupling strength under different pump amplitudes ($|\xi|$). (c) Cavity population evolution as a function of pump detuning and interaction time, with dashed lines marking resonant pumping conditions. (d) Line cuts at resonant pumping, with solid lines representing the fitted results for extracting the conversion coupling strength. (e) Pulse sequence for characterizing the bus mode's energy relaxation time. (f) Decay measurements showing single-exponential fits (lines) to the experimental data (dots), yielding $T_1=74\,\mu$s for both modules (module 1: orange; module 2: purple).
• Figure 3: (a) and (b) Pulse sequence and corresponding population dynamics for single-photon transfer between two modules. (c) and (d) Pulse sequence and measured process fidelity for variable numbers of transfer, respectively. Solid line: exponential fit, yielding a process fidelity of 98.2% per state transfer. (e) Time evolution of the populations in the two cavities under detuned pumping $\Delta=\sqrt{8/3}g_\mathrm{BS}$. Arrow at 580 ns indicates the 50:50 conversion. (f) Joint Pauli measurement results of the two modules (red bar) compared to the ideal value (black frames), demonstrating entanglement fidelity of 92.5%.
• Figure 4: Binomial logical state transfer. (a) Pulse sequence for the binomially encoded state transfer. Left: Wigner tomography is performed immediately after state preparation in module M2 to measure the infidelity due to SPAM errors. Right: After state preparation, the quantum state is transferred to module M1 for Wigner function measurement. By comparing the fidelity before and after the state transfer, we can eliminate the SPAM errors. (b) Prepared and received binomial logical states $\left| 0_L \right\rangle$, $\left|1_L \right\rangle$, and $\left|+_L \right\rangle$ that are characterized by Wigner functions. The deterministic phase shift from the state transfer and measurement process is compensated virtually.
• Figure 5: Simulated self-Kerr effect on multi-photon state transfer. (a) Schematic of self-Kerr effect in a cavity. As photon numbers increase, the nonlinear effect becomes more pronounced, inducing photon-number-dependent frequency shifts. These shifts lead to temporally chirped wavepackets after conversion from the module. (b) Average received state fidelity for the binomial code as a function of self-Kerr. The blue curve represents the originally received state fidelity, while the red curve shows the fidelity after applying a corrective phase gate. (c) Received state fidelity for the superposition state $\left|0\right\rangle+\left|N\right\rangle$ as a function of photon number $N$, using the same self-Kerr strength as in the experimental device sm.