Viewers of Quantum Events Are Also Subject to Uncertainty

It gets better. What if B is also anchored to a quantum object that’s in a superposition of two locations? Then A’s quantum state is now smeared out in two different ways, depending on the possible locations of B. Because determining B’s quantum state determines A’s state, A and B are now entangled.

In the above example, two quintessential properties of quantum systems—superposition and entanglement—turn out to depend on the frame of reference. “The main message is that a lot of the properties that we think are very important, and in a way absolute, are relational” or relative, said Anne-Catherine de la Hamette, a coauthor of the recent paper.

Even the order of events succumbs to the rigors of quantum reference frames. For example, from one reference frame, we might observe the click of a detector happening at a certain time. But from a different reference frame, the click might end up in a superposition of happening before and after some other event. Whether you observe the click as happening at some particular time or as being in a superposition of different orders of events depends on the choice of reference frame.

Stepping Stone to Gravity

Researchers hope to use these shifting quantum perspectives to make sense of the puzzling nature of gravity. Einstein’s general relativity, which is a classical theory of gravity, says that gravity is the warping of the fabric of space-time by a massive object. But how will space-time warp if the object itself is in a superposition of two locations? “That’s very hard to answer with usual quantum physics and gravity,” said Viktoria Kabel, a researcher in Brukner’s group and a coauthor of the new paper.

Switch to a reference frame whose origin is in a superposition, though, and the massive object can end up in a definite location. It now becomes 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 just use standard known physics for,” Kabel said.

Such perspective changes ought to be useful for analyzing future experiments that aim to put extremely small masses in superpositions. For example, the physicists Chiara Marletto and Vlatko Vedral of the University of Oxford have proposed putting two masses each in a superposition of two locations and then studying how this affects their gravitational fields. The burgeoning attempts to formally describe quantum reference frames could help make sense of these investigations of the interaction between gravity and quantum theory—an essential stepping stone to a theory of quantum gravity.