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Quantum Noise Frequency Conversion (QNFC)

Jincheng Zhang

Published Aug 19, 2026• 0 citations

Abstract

Quantum noise is an intrinsic element of any quantum system, arising from vacuum fluctuations, thermal excitations, and the measurement back‑action of quantum devices. Conventional treatments regard noise as a deleterious influence that must be suppressed to preserve coherence. Recent advances in superconducting quantum circuits, however, have revealed that quantum noise possesses a richer dynamical structure than previously appreciated. In this work we propose and analyze a novel phenomenon, termed Quantum Noise Frequency Conversion (QNFC), in which the spectral density of quantum noise can be coherently transferred from one frequency band to another without the conventional amplification or attenuation mechanisms. We develop a theoretical framework based on a multi‑mode bosonic Hamiltonian that couples a primary noise mode at frequency \omega_1 to a secondary mode at \omega_2 via a parametric interaction mediated by a superconducting transmon. By solving the Heisenberg‑Langevin equations of motion under realistic parameter regimes, we demonstrate that the noise spectral density at \omega_1 can be mapped onto \omega_2 with high fidelity, while preserving the total noise power. This process is fundamentally distinct from standard squeezing or parametric amplification, as it does not alter the quadrature variance of either mode. We discuss potential applications of QNFC in noise spectroscopy, quantum error mitigation, and the design of noise‑immune quantum gates. The feasibility of experimental realization is examined in the context of current superconducting quantum hardware, and we outline a concrete protocol that employs flux‑bias tuning of the transmon to enable the required parametric coupling. Finally, we highlight the broader implications of QNFC for uncovering new dynamical regimes of quantum noise, thereby opening a pathway toward leveraging noise as a resource rather than a liability in quantum information processing.

DOI: 10.5281/zenodo.22005171

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Quantum computing is a relatively new computing paradigm that leverages entanglement to implement algorithms beyond the reach of classical hardware. Superconducting quantum circuits have emerged as a platform to build quantum processors, with demonstrations of quantum supremacy over classical supercomputers and quantum error correction beyond fault tolerance. However, current noisy devices require significant improvements to unlock the full potential of quantum algorithms.This thesis investigates intrinsic noise channels in superconducting circuits and the design of noise-resilient qubits. By characterizing high-quality superconducting resonators, we identify lossy dielectric materials at the interfaces as the dominant loss mechanism, which we mitigate via targeted surface treatments. Subsequently, we leverage Josephson junction nonlinearity to engineer noise-biased qubits utilizing the Kerr parametric oscillator (KPO). By analyzing the efficiency of strong, far-off-resonance driving to facilitate three- and four-wave mixing processes, we demonstrate high-fidelity operations on Fock, Kerr-cat, and dual-rail qubits. For the Kerr-cat qubit, we introduce a novel 2D architecture that enables strong light-matter coupling with suppressed Purcell decay, yielding millisecond-scale bit-flip lifetimes. Within this same architecture, we demonstrate a longitudinal qubit readout with a

99.6\%

quantum nondemolition score, alongside a high-fidelity universal single-qubit gate set, all while maintaining bit-flip error suppression. Finally, we exploit the efficient wave-mixing of strongly coupled, far-detuned KPOs to realize a dynamically protected dual-rail qubit exhibiting both erasure bias and high-fidelity single-qubit gates.

Critical current fluctuations with a 1∕f spectral density (f is frequency) are potentially a limiting source of intrinsic decoherence in superconducting quantum bits (qubits) based on Josephson tunnel junctions. Prior measurements of this noise were made at nonzero voltages whereas qubits are operated in the zero voltage state. We report measurements of 1∕f noise in a dc superconducting quantum interference device first, coupled to a resonant tank circuit and operated in a dispersive mode at zero voltage, and, second, operated conventionally with a current bias in the voltage regime. Both measurements yield essentially the same magnitude of critical current 1∕f noise.

Quantum frequency conversion (QFC) which converts the frequencies of photons while preserving the quantum state is an essential technology for realizing the quantum internet and quantum interconnect. For the QFC based on the frequency downconversion from visible to the telecom wavelengths around 1500 nm, it is widely known that noise photons produced by the strong pump light used for QFC contaminate the frequency-converted photon, which degrades the quality of the quantum property of the photon after QFC. In conventional QFC experiments, noise photons are removed using external narrowband frequency filter systems. In contrast, in this study, we implement a compact QFC device integrating the cavity structure only for the converted mode. While the cavity structure can enhance not only the desired QFC efficiency but also the noise photon generation rate, we show that the cavity-enhanced QFC followed by a relatively wide bandpass filter achieves the signal-to-noise ratio comparable to the QFCs with external narrowband filters. We experimentally demonstrate the cavity-enhanced QFC using a single photon at 780 nm to 1540 nm, in which the non-classical photon statistics is clearly observed after QFC.