  Precise phase control of microwaves and millimeter waves is critical for
today's wireless communication and signal processing electronics. For decades,
achieving even modest phase-shifting resolution has demanded a complex mix of
transistor switches and passive electromagnetic structures. These true-phase
delay, true-time-delay or quasi-true-time-delay circuits operate near
resonances that heavily attenuate and distort signals, limiting transmission
bandwidth. Despite numerous attempts, passive phase shifters have remained
lossy and incompatible with high-channel-capacity beamforming, irrespective of
the semiconductor fabrication process. In this article, we introduce a
mechanism whereby waveguide reflectors based on coupled resonances can be
reprogrammed to evade loss. Central to their operation is that loss from
internal resonances is confined to low frequencies, while at high frequencies,
their reflectivity is maximized and broadband phase variations, induced by
those resonances, still persist. Since performance in the low-loss
post-resonance spectrum is largely agnostic to the number of switches,
ultra-fine digital tuning is possible. This breaks the historical tradeoff
between loss and precision to achieve resolution surpassing state-of-the-art
integrated circuits by over three orders (bits) of magnitude. The device does
not distort the transmitted signal and consumes no power. Moreover, the chip
occupies a sub-wavelength footprint in a Complementary Metal Oxide
Semiconductor platform. This makes it an optimal candidate for seamless
integration in on-chip multi-gigabit data links, radio astronomy transceivers
and control hardware for millimeter-wave qubits.
