We report the discovery of a new form of quantum squeezing leading to re-

duced noise in certain quantum mechanical measurements. This phenomenon

has avoided earlier investigations, probably because of its invisibility to conven-

tional detection systems. We provide the description of the necessary detector

and prove that the exploitation of this new, "complex" squeezing can eliminate

the quantum back-action of a force measurement.

Continously measuring a quantum system disturbs it, an effect known as

measurement back-action. This disturbance is the imprint of quantum

fluctuations from the probe - typically a beam of light - unto the system. With the

information about the measured system, the outgoing probe also carries back-

action noise, i.e. a record of its own fluctuations from previous times. The same

fluctuations appearing at different times give rise to complex squeezing in the

probe.

The detector we propose, called a "synodyne detector", allows to subtract

noise from previous times from the present measurement record. This cancel-

lation allows the elimination of the back-action in the detector and results in

force sensitivity only limited by the noise present in the measured system.

We expect complex correlations to occur in other instances of quantum mea-

surements, and our results may improve the performance of quantum sensors

used in future devices, laboratories or observatories. The arXiv paper is available here.