  Degenerate quantum gases are instrumental in advancing many-body quantum
physics and underpin emerging precision sensing technologies. All
state-of-the-art experiments use evaporative cooling to achieve the ultracold
temperatures needed for quantum degeneracy, yet evaporative cooling is
extremely lossy: more than 99.9% of the gas is discarded. Such final particle
number limitations constrain imaging resolution, gas lifetime, and applications
leveraging macroscopic quantum coherence. Here we show that atomic Bose gases
can be cooled to quantum degeneracy using real-time feedback, an entirely new
method that does not suffer the same limitations as evaporative cooling.
Through novel quantum-field simulations and scaling arguments, we demonstrate
that an initial low-condensate-fraction thermal Bose gas can be cooled to a
high-purity Bose-Einstein condensate (BEC) by feedback control, with
substantially lower atomic loss than evaporative cooling. Advantages of
feedback cooling are found to be robust to imperfect detection, finite
resolution of the control and measurement, time delay in the control loop, and
spontaneous emission. Using feedback cooling to create degenerate sources with
high coherence and low entropy enables new capabilities in precision
measurement, atomtronics, and few- and many-body quantum physics.
