This paper describes SPT-3G+, a new survey receiver planned for the 10‑meter South Pole Telescope (SPT) and scheduled for installation in early 2029. The device is built to study the cosmic microwave background (CMB), the faint leftover light from the Big Bang. Compared with the current SPT-3G receiver, SPT-3G+ is designed to map the sky nearly an order of magnitude faster and to produce much deeper polarization maps.

The new receiver will include 6,020 polarization‑sensitive dichroic pixels. “Dichroic” means each pixel measures two frequency bands at once, centered at 90 gigahertz (GHz) and 150 GHz. In total the receiver has 24,080 individual detectors. These sensors are transition‑edge sensors, a kind of superconducting thermometer that is extremely sensitive to small changes in incoming radiation. The detectors will be cooled to about 100 millikelvin (0.1 kelvin) by a dilution refrigerator to reduce noise. Signals from the detectors will be read out with microwave SQUID multiplexing; SQUID stands for superconducting quantum interference device, a very sensitive electronic amplifier used in low‑temperature physics.

The receiver’s optics give a 4‑degree diameter field of view. That field is divided into 14 separate optics tubes, each containing cold lenses made from alumina, silicon, and nylon to focus the CMB light onto the detectors. The planned survey will run for six years and will overlap the area observed by the BICEP survey. By combining data from both instruments at 90 GHz and 150 GHz, the teams aim to reach a combined CMB map depth of 0.5 microkelvin‑arcminute, a measure of how faint a temperature fluctuation the map can detect.

One main science goal is to make unprecedentedly deep maps of CMB lensing. CMB lensing is the subtle bending of the CMB’s path by intervening matter; it converts one type of polarized pattern, called E modes, into another called B modes. With a very detailed lensing map, researchers can subtract or “delens” that lensing signal to search for B modes that would come from primordial gravitational waves generated during cosmic inflation. The teams plan to combine SPT-3G+ maps with data from the BICEP Array to constrain the tensor‑to‑scalar ratio r, a number that quantifies the strength of those primordial gravitational waves. Their stated goal is an uncertainty σ(r) = 0.001, which would be a sensitive test of inflation models.