LAUSR

Highly sensitive phase- and frequency-resolved detection of microwave electric fields is of central importance in a wide range of fields, including cosmology 1 , 2 , meteorology 3 , communication 4 and microwave quantum technology 5 . Atom-based electrometers 6 , 7 promise traceable standards for microwave electrometry, but their best sensitivity is currently limited to a few μV cm −1  Hz −1/2 (refs. 8 , 9 ) and they only yield information about the field amplitude and polarization 10 . Here, we demonstrate a conceptually new microwave electric field sensor—the Rydberg-atom superheterodyne receiver (superhet). The sensitivity of this technique scales favourably, achieving even 55 nV cm −1  Hz −1/2 with a modest set-up. The minimum detectable field of 780 pV cm −1 is three orders of magnitude smaller than what can be reached by existing atomic electrometers. The Rydberg-atom superhet allows SI-traceable measurements, reaching uncertainty levels of 10 −8  V cm −1 when measuring a sub-μV cm −1 field, which has been inaccessible so far with atomic sensors. Our method also enables phase and frequency detection. In sensing Doppler frequencies, sub-μHz precision is reached for fields of a few hundred nV cm −1 . This work is a first step towards realizing electromagnetic-wave quantum sensors with quantum projection noise-limited sensitivity. Such a device will impact diverse areas like radio astronomy, radar technology and metrology. The Rydberg-atom superhet, based on microwave-dressed Rydberg atoms and a tailored electromagnetically induced transparency spectrum, allows SI-traceable measurements of microwave electric fields with unprecedented sensitivity.