This approach lays the groundwork for a global and interplanetary sensing system that could reveal hidden particles and forces.

Understanding SQUIRE and Its Space-Based Quantum Strategy

Exotic-boson-mediated interactions fall into 16 categories. Of these, 15 depend on particle spin and 10 depend on relative velocity. These interactions can produce small shifts in atomic energy levels, and quantum spin sensors detect those shifts as pseudomagnetic fields. The SQUIRE mission intends to place such sensors on space platforms, including the China Space Station, to look for pseudomagnetic fields generated by exotic interactions between the sensors and Earth's geoelectrons. By combining space access with quantum precision tools, SQUIRE avoids a major limitation of ground experiments, which struggle to increase both relative velocity and the total number of polarized spins at the same time.

Why Low Earth Orbit Greatly Improves Sensitivity

Several features of the orbital environment provide strong advantages.

1. The China Space Station travels in low Earth orbit at 7.67 km/s relative to Earth, nearly the first cosmic velocity and about 400 times faster than typical moving sources used in laboratory tests.
2. Earth acts as an enormous natural source of polarized spins. Unpaired geoelectrons within the mantle and crust, aligned by the geomagnetic field, supply roughly 10⁴² polarized electrons, exceeding the capabilities of SmCo₅ laboratory spin sources by approximately 10¹⁷.
3. Orbital motion turns exotic interaction signatures into periodic signals. For the China Space Station (orbital period ~1.5 hours), this produces modulation near 0.189 mHz, a region with lower intrinsic noise than DC measurement bands.

Projected Performance Gains in Orbit

With these space-enabled benefits, the SQUIRE concept allows exotic field amplitudes to reach up to 20 pT even under strict current limits on coupling constants. This is dramatically higher than the best terrestrial detection threshold of 0.015 pT. For velocity-dependent interactions with force ranges >10⁶ m, the projected sensitivity improves by 6 to 7 orders of magnitude.

Building a Space-Ready Quantum Spin Sensor

Developing the prototype quantum sensor is essential for putting SQUIRE into operation. The instrument must remain extremely sensitive and stable over long periods while operating in a challenging orbital environment. In space, spin sensors encounter three dominant sources of interference: variations in the geomagnetic field, mechanical vibrations of the spacecraft, and cosmic radiation.

Reducing Noise and Increasing Stability

To overcome these challenges, the SQUIRE team created a prototype using three major innovations.

1. Dual Noble-Gas Spin Sensor: The device uses ¹²⁹Xe and ¹³¹Xe isotopes with opposite gyromagnetic ratios, which allows it to cancel shared magnetic noise while remaining responsive to SSVI signals. This approach provides 10⁴-fold noise suppression. With multilayer magnetic shielding, geomagnetic disturbances fall to the sub-femtotesla level.
2. Vibration Compensation Technology: A fiber-optic gyroscope tracks spacecraft vibrations and enables active correction, bringing vibration noise to roughly 0.65 fT.
3. Radiation-Hardened Architecture: A 0.5 cm aluminum enclosure and triple modular redundancy in its control electronics protect the system from cosmic rays. The design can continue functioning even if two of the three modules fail, reducing radiation-related interruptions to fewer than one per day.

On-Orbit Sensitivity and Scientific Readiness

By combining these technologies, the prototype achieves a single-shot sensitivity of 4.3 fT @ 1165 s, which is well matched to detecting SSVI signals that follow the 1.5-hour orbital period. This capability establishes a strong technological basis for precision dark matter searches conducted directly in orbit.

Expanding Toward a Space-Ground Quantum Sensing Network

Quantum spin sensors aboard the China Space Station can do far more than search for exotic interactions. SQUIRE proposes a "space-ground integrated" quantum sensing network that links orbital detectors with those on Earth, enabling far greater sensitivity across many dark matter models and other beyond-Standard-Model possibilities. These include additional exotic interactions, Axion halos, and CPT violation studies.

Future Opportunities Across the Solar System

The high-speed motion of orbiting sensors increases the coupling between axion halos and nucleon spins, producing a tenfold sensitivity improvement compared with Earth-based dark matter searches. As China expands deeper into the solar system, the SQUIRE approach may eventually employ distant planets such as Jupiter and Saturn (e.g., planets rich in polarized particles) as large natural spin sources. This long-term vision opens the door to exploring physics across much broader cosmic scales.