The group, led by Caltech's Assistant Professor of Physics Yu-xi Liu and MIT's Professor Alexej Jerschow, describes the new concept in a paper that appears in the August 2019 issue of Nature Physics.
"With the technology demonstrated in our paper, we can directly measure the early quantum dynamics of a large class of quantum systems," Liu says, adding, "This is similar in spirit to the idea of time-lapse photography; except in our case, the time-travel effect does not apply to people or objects but to quantum information."
The scientists realized their time-traveling quantum sensors based on a phenomenon called quantum entanglement. Quantum entanglement occurs when two or more particles are linked together in such a way that the state of one particle cannot be described independently of the state of the other. This concept is counterintuitive to our everyday experience of classical physics but has been repeatedly confirmed by experiments.
In the new research, Liu and colleagues took advantage of quantum entanglement to create a kind of quantum eraser that allowed them to retrieve information about the early-time dynamics of a particular quantum system.
To illustrate the concept, the scientists used a quantum system made of two trapped, spinning particles (atoms), referred to as quantum bits or qubits for short. The spins of these two qubits were entangled, and the time evolution of their spins was affected by the presence of a magnetic field.
At a precisely controlled time, the researchers turned on the magnetic field and allowed it to affect the spins of the two qubits. This had the effect of scrambling the quantum information stored in the spins. Subsequently, the researchers implemented the quantum eraser, which restored the quantum information from the early time before the magnetic field was applied.
The ability to travel back in time—albeit only with respect to quantum information—opens up exciting new possibilities for understanding a variety of physical processes. For example, the technique could be used to probe the early-stage behavior of chemical reactions, study intricate energy dissipation mechanisms, and even uncover fundamental properties of black holes.
Liu is particularly enthusiastic about the applications of the new time-traveling quantum sensors in condensed matter physics. "It is quite amazing to think about applying the power of quantum time travel to a solid with trillions of atoms, where many different quantum phenomena are constantly happening," he says. "A new way to revisit the evolution of these phenomena could reveal hidden physics that we normally do not see."
