Search for Parity-Violating Long-Range Spin-Dependent Interactions Using a Quantum Comagnetometer ✦
Context
Parity violation, first discovered in weak interactions, became a cornerstone of the Standard Model of particle physics. However, the existence of dark matter and the hierarchy problem suggest the need to extend this model. Light bosons, such as axions or Z' bosons, are prime candidates for mediating new forces and could give rise to long-range parity-violating spin-dependent interactions. The search for such exotic interactions not only probes fundamental symmetries predicted by Dirac but may also shed light on the nature of dark matter, as these hypothetical particles are considered promising dark matter candidates.
Methods
The experimental setup includes rotating lead blocks (mass source) and an ultrasensitive SERF (spin-exchange relaxation-free) comagnetometer operating in the hybrid spin resonance (HSR) mode. Unlike conventional quantum measurement techniques, HSR offers a broad bandwidth (up to 25 Hz) and high stability, critical for long-duration precision measurements. The 3 Hz rotation of the masses modulates the exotic field at 6 Hz, which, via a pseudomagnetic field, acts on the nuclear spins of 21Ne, described by Pauli matrices. The signal is read out optically—by monitoring the rotation of the polarization plane of a spectroscopic probe beam. The dominant vibrational noise was suppressed by more than a factor of 700 using a multi-stage isolation system, including a separate vacuum chamber and a vibration-isolated foundation—reminiscent of technologies employed in gravitational-wave observatories. Response calibration was performed with classical magnetic fields, linking the measured signal to the coupling constants of exotic interactions. Modeling the system response with phase delays accounted for systematic effects.
Results
Over 108 hours of data taking, a pseudomagnetic response of (1.8±4.2_stat±2.0_syst) aT (attotesla) was measured. For the neutron-nucleon interaction at a force range λ = 5 m, the coupling constant gA^n gV^N = (5.3±12.4_stat±5.9_syst)×10^{-39} was obtained, corresponding to an upper limit of |gA^n gV^N| ≤ 2.9×10^{-38} at 95% confidence. This is three orders of magnitude (1000 times) better than previous laboratory constraints. For the electron-nucleon interaction, gA^e gV^N = (2.8±6.7_stat±6.2_syst)×10^{-36} with a limit of ≤1.9×10^{-35}. The wide HSR bandwidth allowed the inclusion of up to three signal harmonics, boosting the signal-to-noise ratio. The achieved sensitivity rivals that needed for detecting gravitational waves, but for hunting exotic forces predicted by axion models.
Implications
The obtained limits close a significant region of parameter space for models with light Z' bosons and axions that could previously account for dark matter. The result shows that tabletop quantum experiments can compete with astrophysical observations in testing fundamental symmetries. The success of the HSR mode paves the way for more stable quantum sensors, which could lead to detection not only of new forces but also gravitational waves via atom interferometry. This work continues the tradition, pioneered by Feynman, of searching for invariance violations in quantum systems.
Future development
Future plans include improving spin coherence with new materials and multilayer magnetic shielding to reduce systematic errors. Applying machine learning for active noise cancellation and nonclassical squeezed states, as in quantum metrology, will enable surpassing the standard quantum limit. This will open the possibility to probe interactions at even shorter distances and with greater precision, approaching scales where quantum gravity effects may manifest.
Impact
The developed vibration isolation and bandwidth extension methods will find applications not only in fundamental physics but also in gravitational-wave detectors, precision magnetometry, and quantum computing.
Next steps
The next step will be adapting the setup to search for parity-conserving spin-dependent forces and investigate axion-mediated dipole-dipole interactions, which will require even finer control of systematics.
Key open problems
The experiment is directly connected to the problems of the nature of dark matter and the mass hierarchy in the Standard Model, since the sought-after axions and Z' bosons could be key to their solutions.
🎯 The lead blocks in the experiment have a density of 11.3 g/cm³ — almost one and a half times denser than steel, making them ideal for generating a strong flux of virtual particles. And the 700-fold vibration reduction is like trying to hear a whisper during a rock concert by hiding in a soundproof room.
Key numbers
- Vibration suppression: more than 700 times
- Improvement in limits (λ=5 m): 1000 times
- Limit |g_A^n g_V^N| (95% CL): ≤ 2.9×10^{-38}
- Field sensitivity: attotesla level