It gets better. What if B is also tethered to a quantum object that is in a superposition of two locations? So the quantum state of A now blurs in two different ways, depending on the possible locations of B. Because the determination of the quantum state of B determines the state of A, A and B are now entangled.
In the example above, two quintessential properties of quantum systems (superposition and entanglement) turn out to depend on the reference frame. “The main message is that many of the properties that we think are very important, and in some ways absolute, are relational” or relative, he said. Anne Catherine de la Hametteco-author of the recent article.
Even the order of events succumbs to the rigors of quantum reference frames. For example, from a frame of reference, we could observe the click of a detector at a certain time. But from a different frame of reference, the click could end in a superposition of events before and after some other event. Whether you observe the click occurring at a particular time or as a superposition of different orders of events depends on the choice of frame of reference.
A springboard into gravity
Researchers hope to use these changing quantum perspectives to make sense of the mystifying nature of gravity. Einstein’s general relativity, which is a classical theory of gravity, says that gravity is the deformation of the fabric of space-time by a massive object. But how will spacetime be warped if the object itself is in a superposition of two locations? “That’s very difficult to answer with regular quantum physics and gravity,” he said. Victoria Kabelresearcher in Brukner’s group and co-author of the new article.
However, if you change to a reference frame whose origin is in a superposition, the massive object may end up at a defined location. It is now possible to calculate its gravitational field. “By finding a convenient quantum reference frame, we can take a problem we can’t solve (and turn it) into a problem for which we can simply use known standard physics,” Kabel said.
These changes in perspective should be useful to analyze future experiments which aim to put extremely small masses on superpositions. For example, physicists Chiara Marletto and Vlatko Vedral from the University of Oxford have proposed put two masses each in a superposition of two locations and then study how this affects their gravitational fields. Growing attempts to formally describe quantum frames of reference could help make sense of these investigations into the interplay between gravity and quantum theory, an essential stepping stone toward a theory of quantum gravity.