  We use a double quantum dot as a frequency-tunable on-chip microwave detector
to investigate the radiation from electron shot-noise in a near-by quantum
point contact. The device is realized by monitoring the inelastic tunneling of
electrons between the quantum dots due to photon absorption. The frequency of
the absorbed radiation is set by the energy separation between the dots, which
is easily tuned with gate voltages. Using time-resolved charge detection
techniques, we can directly relate the detection of a tunneling electron to the
absorption of a single photon.
