image: Schematic of space-based search for ultralight exotic bosons and the prototype space quantum sensor, including vapor cell, magnetic shield, fiber-optic gyroscope, and radiation shielding box.
Credit: ©Science China Press
Core Principle of SQUIRE: Space-Based Quantum precision measurement on Exotic Interactions
Exotic-boson-mediated interactions comprise 16 forms, of which 15 are spin-dependent and 10 are velocity-dependent. These interactions mat induce energy shifts in atomic energy levels, which can be detected as pseudomagnetic fields by quantum spin sensors. The SQUIRE project plans to deploy quantum spin sensors on space platforms such as the China Space Station to search for such pseudomagnetic fields induced by exotic interactions between sensor spins and Earth’s geoelectrons. By integrating quantum precision measurement with space technology, SQUIRE overcomes the terrestrial bottleneck of simultaneously enhancing two critical parameters—relative velocity and polarized spin number.
The key advantages of space-based detection lie in: (i) The China Space Station operates in low Earth orbit at a stable velocity of 7.67 km/s relative to Earth—nearly the first cosmic velocity and ~400 times faster than moving sources in terrestrial experiments. (ii) Earth itself serves as a massive natural polarized spin source, with unpaired geoelectrons in the mantle and crust—polarized by the geomagnetic field—providing approximately 10⁴² polarized electron spins, exceeding laboratory SmCo₅ spin sources by ~10¹⁷. (iii) Orbital motion modulates exotic interaction signals into periodic oscillations. For the China Space Station (orbital period ~1.5 hours), the signal is modulated to ~0.189 mHz, a frequency band with inherently lower noise than DC regimes.
Thanks to these unique space advantages, even under the most stringent current coupling constant constraints, the amplitude of exotic fields in the SQUIRE scheme can reach up to 20 pT—far exceeding terrestrial detection limits (0.015 pT). The expected sensitivity for velocity-dependent exotic interactions with force ranges >10⁶ m is enhanced by 6–7 orders of magnitude.
Prototype Space Quantum Sensor: Engineering a Detector for Space Conditions
Developing the space quantum sensor prototype is central to realizing the SQUIRE mission, requiring high sensitivity and long-term stability in the complex space environment. Space-based spin sensors face three primary interference sources: geomagnetic fluctuations, platform mechanical vibration, and cosmic radiation.
To address these, the SQUIRE team developed a prototype integrating three breakthrough technologies: (i) Dual Noble-Gas Spin Sensor: Using ¹²⁹Xe and ¹³¹Xe isotopes with opposite gyromagnetic ratios, the sensor suppresses common-mode magnetic noise while preserving sensitivity to SSVI signals. This achieves 10⁴-fold magnetic noise suppression, and combined with multi-layer magnetic shielding, reduces geomagnetic fluctuations to sub-femtotesla. (ii) Vibration Compensation Technology: Equipped with a fiber-optic gyroscope, the system actively compensates for platform vibration, reducing noise to a negligible 0.65 fT. (iii) Radiation-Hardened Architecture: A 0.5 cm aluminum enclosure and triple modular redundancy in control circuits mitigate cosmic ray impacts. This ensures functionality even if two of three redundant circuits fail, reducing disruptions to <1 per day.
Integrating these technologies, the SQUIRE prototype achieves a single-shot sensitivity of 4.3 fT @ 1165 s—ideal for detecting SSVI signals with a 1.5-hour period—laying a solid technical foundation for on-orbit high-precision dark matter detection.
Broader Scientific Impact: A Space-Ground Integrated Sensing Network
Beyond exotic interaction searches, quantum spin sensors on the China Space Station will enable a wide range of fundamental physics research in space. SQUIRE envisions a “space-ground integrated” quantum sensing network, linking orbital and terrestrial sensors to dramatically enhance sensitivity across multiple dark matter models and beyond-Standard-Model phenomena, including other exotic interactions, Axion halos, and CPT violation probes.
Specifically, high-speed orbital motion enhances coupling between axion halos and nucleon spins, achieving a 10-fold sensitivity improvement over terrestrial direct dark matter searches. As China’s deep space exploration advances, the SQUIRE framework will inspire the use of distant planets (e.g., Jupiter and Saturn, rich in polarized particles) as natural polarized sources, expanding the frontiers of physics exploration on cosmic scales.
Journal
National Science Review