This is better. What if B is also anchored to a quantum object that is in a superposition of two locations? Then the quantum state of A is now painted in two different ways, depending on the possible locations of B. Since the determination of the quantum state of B determines the state of A, A and B are now linked.

Renato Renner, a physicist at the Swiss Federal Institute of Technology Zurich, thinks that a careful consideration of quantum reference frames can solve the paradoxes of our understanding of the quantum world.

Credit: Giulia Marthaler

In the example above, two quintessential properties of quantum systems – superposition and entanglement – depend on the frame of reference. “The main message is that many properties that we think are very important, and in some way absolute, are related” or relative, he said. Anne-Catherine de la Hametteis a co-author of the new paper.

Even the sequence of events falls under the strain of quantum reference frames. For example, from a reference frame, we can observe the click of a detector occurring in one hour. But from a different reference frame, the click can end up being a superposition that happens before and after other events. Whether you observe the click as occurring at a particular time or as a superposition of different sequences of events depends on the choice of reference frame.

Gravity’s Stepping Stone

Researchers hope to use these shifting quantum perspectives to understand the puzzling nature of gravity. Einstein’s general relativity, which is a classical theory of gravity, states that gravity is the disruption of the space-time fabric of a massive object. But how does space-time warp when the object itself is in a superposition of two locations? “That’s very difficult to answer with conventional quantum physics and gravity,” he said Victoria Cableis a researcher in Brukner’s group and co-author of the new paper.

Move a reference frame whose origin is in a superposition, however, and the large object may end up in a fixed location. It is now possible to calculate its gravitational field. “By finding a convenient quantum reference frame, we can take a problem that we cannot solve (and make it) a problem that we can only use standard known physics,” Kabel said.

Such perspective changes should be useful for analysis future experiments which aims to place extremely small masses in superpositions. For example, physicists Chiara Marletto and Vlatko Vedral at the University of Oxford has proposed placing two masses each in a superposition of two locations and then studying how this affects their gravitational fields. Emerging attempts to formally describe the quantum reference frame will help to understand these investigations of the interaction between gravity and quantum theory—an important step in a theory of quantum gravity.