  Quantum mechanics allows the existence of "virtual states" that have no
classical analogue. Such virtual states defy direct observation through strong
measurement, which would destroy the volatile virtual state. Here we show how a
virtual state of an interacting many-body system can be detected employing a
weak measurement protocol with postselection. We employ this protocol for the
measurement of the time it takes an electron to tunnel through a virtual state
of a quantum dot (cotunneling). Contrary to classical intuition, this
cotunneling time is independent of the strength of the dot-lead coupling and
may deviate from that predicted by time-energy uncertainty relation. Our
approach, amenable to experimental verification, may elucidate an important
facet of quantum mechanics which hitherto was not accessible by direct
measurements.
