A cornerstone of quantum physics is uncertainty. Heisenberg’s uncertainty principle states the more precisely you pinpoint the position of a particle, the less precisely you can know its momentum at the same time, and vice versa.
However, a new study reveals that scientists have now discovered a way to sidestep this quantum tradeoff. This could lead to next-generation quantum sensors that can simultaneously measure both the position and momentum of particles with unprecedented precision.
“We take it for granted that the Heisenberg uncertainty principle is a fundamental law that cannot be broken,” says Tingrei Tan, a research fellow at the University of Sydney Nano Institute and School of Physics. “I want to be clear that we have not broken the Heisenberg uncertainty principle. But in certain cases, we can get around it.”
Other kinds of uncertainty are known in quantum physics as well. For example, if an atom is excited to a higher orbital, one cannot precisely know both the energy and the duration of its excited state at the same time.
Previously, scientists have taken advantages of these kinds of tradeoffs to “squeeze” or reduce the uncertainty in the measurements of a given variable while increasing the uncertainty in the measurement of another variable the researchers can ignore. For example, researchers have shown they can make qubits—the key components in quantum computers—highly resistant to a common source of error known as bit flip, when a qubit’s state flips from 1 to 0 or the reverse, while making them more vulnerable to a different common source of error known as phase flip, when a qubit switches between one of two opposite phases. They make this tradeoff because having just one common source of error to correct instead of two can drastically simplify quantum-computer design.
Now Tan and his colleagues have found they can make a similar tradeoff to sidestep Heisenberg’s uncertainty principle. “We are increasing the precision with which we can measure momentum and position at the same time within a small sensing range, while increasing the uncertainty with which we can measure those properties simultaneously outside of that sensing range,” Tan says.
Since the kind of quantum sensing applications the researchers want to carry out with this new technique are roughly on the atomic scale anyhow, gaining precision on a tiny scale while losing it on larger ones is a worthwhile tradeoff. It’s a bit like using a magnifying glass — you care about what you want to focus on under the lens, not around it.
In the new study, the researchers experimented with a single ytterbium ion held in place and controlled with electric and magnetic fields. They generated sets of specific patterns of vibrations in the ion known as grid states or Gottesman-Kitaev-Preskill (GKP) states.
Scientists have long investigated GKP states for quantum error correction strategies in quantum computing. In the case of trapped ions, because of the way in which the patterns of vibrations are entangled together, any small disturbance would generate changes in these patterns, which quantum error correction techniques can detect and account for in quantum computations.
In the new study, by preparing grid states in the vibrations of a ytterbium ion, the researchers found they could simultaneously measure both its position and momentum with a precision beyond the standard quantum limit — the best achievable using only classical sensors.
Dr Tingrei Tan in the Sydney Nanoscience Hub at the University of Sydney, where he manages the Quantum Control Laboratory.Fiona Wolf/The University of Sydney
“We are borrowing techniques from quantum error correction to do quantum sensing,” Tan says.
Tan says a key application for this research is spectroscopy — the analysis of atoms or molecules based on the specific wavelengths of light they absorb or reflect, which finds wide use in medical research, navigation in environments where GPS does not work, and searches for dark matter. In general, however, “this opens up a whole new way to do precision measurements,” Tan says. “It was taken for granted that there were pairs of variables that you could not measure precisely at the same time, and our work provides a framework for how we can get around those limitations.”
The scientists detailed their findings 24 September in the journal Science Advances.
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