THE QUANTUM INTELLIGENCER

Detecting Axion Winds with Quantum Spin Sensors: A Modulated Signal Approach Using Earth's Motion In Plain English: Scientists are searching for invisible particles called axions that might make up dark matter, the mysterious stuff that holds galaxies together. This study proposes using tiny quantum devices—like super-sensitive compass needles made from single electrons—to detect a faint, wind-like signal from these particles. Because Earth spins and moves around the Sun, any real signal from axions would rise and fall in a predictable daily and yearly pattern, helping scientists tell it apart from random noise. The method could detect incredibly weak interactions that current tools miss, bringing us closer to solving one of physics’ biggest mysteries. Summary: This paper proposes a phase-coherent, narrowband magnetometry technique using gate-defined silicon quantum-dot spin qubits to detect couplings between axions or axion-like particles (ALPs) and electron spins. By employing repeated Ramsey echo sequences and dispersive readout, the system achieves sub-Hertz spectral resolution, enabling precise tracking of qubit precession in response to potential axion-induced fields. The method leverages two natural modulations: the daily sidereal signal due to Earth’s rotation relative to the fixed stars, and an annual amplitude modulation from Earth’s orbital motion around the Sun. These combine to produce a distinctive sideband triplet in the frequency spectrum—central peak at the sidereal frequency with sidebands spaced ±Ω⊕ (≈ 2π/1 year)—which serves as a unique fingerprint of the axion wind. To maximize sensitivity and reject instrumental or stationary noise, the analysis incorporates both modulation patterns into the likelihood function. Filtering protocols are applied to mitigate dominant noise sources such as $1/f$ noise and readout errors. For axion masses in the 1–10 μeV range, the setup can probe axion-electron coupling strengths $g_{ae}$ from $10^{-14}$ to $10^{-10}$, approaching sensitivities suggested by astrophysical observations (e.g., stellar energy loss rates). While the proposal focuses on silicon spin-qubit systems, the methodology is generalizable to other spin-based quantum sensors. This represents a promising, laboratory-based avenue for discovering axion-electron interactions, complementing astrophysical and cosmological probes. Key Points: - Quantum dot spin qubits in silicon are used as ultra-sensitive magnetometers to detect potential axion-electron interactions. - The method uses repeated Ramsey echo sequences and dispersive readout to achieve sub-Hz spectral resolution. - Earth’s rotation creates a daily (sidereal) modulation, while its orbit adds an annual envelope, generating a sideband triplet signal. - The sidebands appear at fixed frequencies ±Ω⊕ around the main sidereal peak, forming a distinctive fingerprint of axion wind. - Incorporating both modulation patterns improves signal discrimination against stationary or instrumental noise. - Sensitivity covers axion masses of 1–10 μeV and coupling strengths $g_{ae} = 10^{-14}$ to $10^{-10}$. - Noise filtering accounts for $1/f$ noise and readout errors to enhance detection capability. - The approach is applicable beyond silicon systems to other spin-based quantum sensors. - This technique offers a laboratory-based complement to astrophysical searches for axions. Notable Quotes: - “The accessible axion mass window is determined using a series of filtering protocols that take into account sensing noise, including readout errors and $1/f$ noise.” - “Earth’s orbital motion produces an annual amplitude envelope that generates sidebands at fixed frequency spacing $\pm \Omega_\oplus$ around the sidereal component.” - “Including both daily and annual modulation patterns in the likelihood analysis enhances the rejection of stationary or instrumental noise.” - “Our results indicate that spin-qubit magnetometry can achieve sensitivities approaching those suggested by astrophysical considerations…” Data Points: - Axion mass range: 1–10 μeV - Detectable axion-electron coupling strength: $g_{ae} = 10^{-14}$ to $10^{-10}$ - Spectral resolution: sub-Hz - Sideband spacing: ±Ω⊕, where Ω⊕ corresponds to Earth’s orbital angular frequency (≈ 2π radians per year) - Sensor platform: gate-defined silicon quantum-dot spin qubits - Signal modulation: sidereal (daily) + annual amplitude envelope - Noise sources considered: $1/f$ noise, readout errors Controversial Claims: - The claim that quantum dot spin qubits can reach sensitivities comparable to astrophysical bounds for axion-electron coupling ($g_{ae} \sim 10^{-14}$) may be optimistic, as it assumes ideal noise suppression and long-term coherence stability not yet demonstrated at scale. - The assumption that the sideband triplet structure will be cleanly resolvable in real experimental conditions—despite $1/f$ noise and environmental decoherence—represents a strong and potentially debatable extrapolation of current quantum sensing capabilities. - The paper implies that laboratory detection of axion wind via electron coupling is feasible in the near term, which contrasts with the prevailing view that such couplings are too weak to detect without major technological breakthroughs. Technical Terms: - Axion wind - Axion-like particles (ALPs) - Quantum dot spin qubit - Gate-defined silicon quantum dots - Ramsey echo sequence - Dispersive readout - Phase-coherent magnetometry - Sub-Hz spectral resolution - Sidereal modulation - Annual modulation - Sideband triplet - $1/f$ noise - Axion-electron coupling ($g_{ae}$) - Likelihood analysis - Filtering protocols - Coherence time - Dark matter detection - Narrowband sensing —Ada H. Pemberley Dispatch from The Prepared E0