Quantum transducer

The quantum transducer or "Ghost Machine" allowed humans a form of mental time travel.

History
In the 21st century, Ed Morgan acquired the machine and then Bernie Harris, a small-time fence in grey-market alien artefacts who filched it out of a biscuit tin found in a lockup owned by Ed Morgan presumably.

Torchwood Three acquired the machine by tracing alien energy signals from the device while still in Bernie's possession. After a chase that led her to a train station, Gwen Cooper ended up with Bernie's hoodie, with the machine in a pocket. She mentally time-slipped back in time and saw a ghost from World War II, not a phantom in the conventional sense, but a quantum echo of the long-ago emotional state of a living person, Tom Flanagan. The other half of the machine, also at Bernie's, snapped in neatly with the first and picked up visions of the future.

The transducer eventually was put away with other dangerous alien artefacts in Torchwood Three. (TV: Ghost Machine)

It was seen again when Owen Harper tried to find the blue prints for the Rift Manipulator. (TV: Captain Jack Harkness)

Technology
The transducer used nanotechnology. It converted the quantum traces of emotional events, both past and future, into a form that humans could mentally experience.

When activated, apparently by touching the buttons on the front, the user was shown an emotional event from the past or future. As well as showing the actual event, the transducer also transmitted the thoughts of the people in the event. (TV: Ghost Machine)

Torchwood website
The series 1 version of the Torchwood website had an article written by Captain Jack Harkness titled "Ghost machine explained". It said that instead of looking at the transducer from the perspective of flesh and blood beings like humans, the device had to be seen as how a transdimensional being — who can travel back and forth through time as if it was water — would observe the universe. Jack explained that "every" traveller needs a map — some equivalent of GPS. Jack speculated that the quantum transducer functioned as a "ghost map" that locked onto quantum "hot spots", which humanity perceived as ghosts to help the transdimensional being find out where he was in time and space. He says that this is why it kept switching itself on around a "big emotional echo" — it's essentially "a mobile phone scanning for a signal".