Cottrell Scholar Awards - 2017
Seeking Quantum Limits with Cold Atoms and Light: from Sensing and Control to Scrambling
At first perceived as “spooky action at a distance,” in the words of Albert Einstein, quantum entanglement is increasingly viewed as an astoundingly powerful resource for applications ranging from precision measurement to computation.
Quantum entanglement is the process by which pairs (or larger groups) of atoms or subatomic particles are generated such that the quantum state of each atom or particle cannot be described independently of the others, even when they are separated by a large distance.
Monika Schleier-Smith, physics, Stanford University, has received a Cottrell Scholar Award from Research Corporation for Science Advancement to perform a set of experiments investigating fundamental limits in the dynamics of entanglement.
Her first area of investigation involves quantum metrology (scientific measurement), and aims to answer the question: How can we design a sensor that reaches a given measurement precision as fast as possible with finite resources?
Schleier-Smith notes, “To perform a fast and precise measurement of a magnetic field, we need a small change in the field to swiftly perturb the state of some sensor. While a common choice of sensor is a spin-polarized gas of atoms, the same collection of atoms can attain higher sensitivity if prepared in a suitably entangled quantum state.”
To engineer a maximally sensitive quantum state, Schleier-Smith and her colleagues will attempt to construct a “Schrodinger’s cat” device. In Schrodinger’s famous thought experiment (in the making of which no real animals were harmed), the fate of a cat inside a closed box hangs in the balance due to a single radioactive atom: if the atom decays, it triggers a device that kills the cat; if not, then the cat stays alive. But quantum mechanics posits that the atom’s state (decayed or not decayed) is indeterminate until observed, which counterintuitively implies the animal exists as both alive and dead until the box is opened and the cat is observed.
“In our lab, the role of the cat will be played by an ensemble of laser-cooled rubidium atoms confined in an optical resonator, the latter mimicking the box that isolates the cat from outside observers,” Schleier-Smith said. “The role of the ‘radioactive’ atom will be played by an ancillary rubidium atom that is trapped in the same resonator and optically excited to a state that can decay by emitting a photon into the resonator. The effect of this photon on the ensemble atoms’ spins will be to rotate their orientation in a direction that depends on the handedness of the photon’s polarization: right circular polarization will save the ‘cat’s’ life, whereas left circular polarization will kill it.”
“Eventually,” she said, “the photon will leak out of the resonator, and we must take care to detect it in a manner that does not reveal whether it killed the ‘cat.’ We will thereby project the ensemble into a macroscopic superposition of ‘alive’ and ‘dead.’” At this point, the state of the cat will be completely indeterminate, but measuring the state of the ancilla atom would reveal it instantaneously through spooky action at a distance.
The highly entangled ‘cat’ state is a resource for reaching the theoretical limit of precision measurement imposed by the laws of quantum mechanics. Achieving such a high degree of control over a quantum mechanical system may also have implications for other processes that rely on entanglement, such as quantum computation.
The Cottrell Scholar Award also contains an education component. Schleier-Smith intends to use some of the funding to inspire students to pursue careers in physics by developing a freshman-level course on exciting topics in quantum information science. “Introducing creative young minds to these topics will provide a foundation for tackling difficult problems in quantum engineering in the decades to come,” she said.