Researchers have taken a peek into the box containing Schrodinger’s cat with the publication of a study Thursday that mapped the most likely trajectory of a superconducting circuit.
In the study, co-authored by UC Berkeley researchers and published in the journal Nature, the team confirmed its theory about the most likely trajectory, which allows scientists to understand how quantum systems change until they reach a permanent condition. By probing circuits as they moved from an initial state to a final state, researchers came to a discovery that represents a significant advance for the world of quantum physics.
Quantum physics’ principle of superposition is commonly recognized in the Schrodinger’s cat thought experiment, conceived by Erwin Schrodinger in 1935. It proposes that if a cat is put in a box with three objects — a Geiger counter, a radioactive substance and poison that will kill the cat if the Geiger counter detects radioactive decay — the cat exists as both alive and dead until the box is opened, when it instantaneously becomes dead or alive.
The study, however, demonstrates that the process is not an immediate one. Rather, the metaphorical cat follows a continuous trajectory from its initial state until the lid is opened and its condition is revealed — although in this case, the cat is a very cold superconducting circuit made of aluminum. By probing the system, researchers can determine where on the spectrum the cat lies between alive or dead without even opening the box.
“The idea that has puzzled and troubled people for a long time with this postulate is, how is it that, instantaneously, you look at something and suddenly ‘poof,’ it’s either alive or dead?” said Irfan Siddiqi, a campus associate professor of physics who co-authored the paper. “It’s not instantaneous. There is information flowing out of the system.”
In the study, researchers used a circuit that can only occupy two states, called a ground state and an excited state. By the principle of superposition, the circuit — a type of qubit, or unit of quantum information — exists in part at every combination of those two states. The qubit is placed in a cavity and brought down to a temperature of 20 millikelvin, close to absolute zero, to reduce its resistance. There, it is probed by microwaves that very weakly affect the circuit — the equivalent of barely opening the lid of the box.
The circuit’s container, made from aluminum, responds to a certain frequency that is dependent on the particular combination of ground and excited states that the circuit occupies. Researchers then amplify that signal to measure that combination.
Siddiqi said the process of probing the qubit itself is what spurs the trajectory.
“It’s the act of measurement that is actually driving the system. If the cat were dead and alive, it would stay that way,” he said. “The only reason it leaves that state is because you’re perturbing it by measuring that state.”
Kater Murch, a co-author of the study and assistant professor at Washington University in St. Louis, compared the most likely trajectory the qubit takes to shortcuts students might make across a grassy lawn.
“Even though not every student takes that path, enough students take that path so that a path forms,” said Andrew Jordan, an associate professor of physics at the University of Rochester and a co-author of the study.
The study’s findings offer new abilities to control quantum systems, which in turn hold the promise of a new technological revolution, said Alexander Lvovsky, a physics professor at the University of Calgary, who has done research on superposition.
Siddiqi said the findings could be applied to the field of quantum computers, which have the potential to be more powerful than today’s computers, by developing a system to improve continuous error processing.
“From a fundamental point of view, these are really groundbreaking experiments,” Jordan said. “This really is a new window onto the microscopic level.”