A physics cornerstone known as the Heisenberg Uncertainty Principle, developed in the early 20th century, has shown signs of wear in some recent experiments involving optics. The principle says it’s impossible to measure two related properties of a system with high precision at the same time, because the very act of measuring one property with high precision will disturb the system so much that precision on the other property will decrease dramatically. It may not be as bad as we thought.
A general diagram of an apparatus to measure the polarization of a pair of entangled photons
Last October, some researchers from Toronto reported that they had been able to measure, with high precision, the different polarization states of a photon. The Heisenberg Uncertainty Principle involves what are known as “complementary physical properties,” such as the location and momentum of an electron. But it has been generalized to refer to any two properties that are complementary, such as the different polarization states of a photon. These are subject to the generalized Heisenberg uncertainty relationship.
The researchers’ main goal was to quantify how much the act of measuring the polarization disturbed the photons, which they did by observing the light particles both before and after the measurement. However, if the “before shot” disturbed the system, the “after shot” would be tainted. They were able to get around this quantum mechanical Catch-22 by sneaking “non-disruptive peeks” at the photons before their polarization was measured.
“If you interact very weakly with your quantum particle, you won’t disturb it very much,” said Lee Rozema, a PhD candidate in quantum optics research at the University of Toronto and lead author of the study. Weak interactions, however, can be like grainy photographs: they yield very little information about the particle. “If you take just a single measurement, there will be a lot of noise in that measurement,” said Rozema. “But if you repeat the measurement many, many times, you can build up statistics and can look at the average.”
By comparing thousands of “before” and “after” views of the photons, the researchers revealed that their precise measurements disturbed the system much less than predicted by the original Heisenberg formula. The team’s results provide the first direct experimental evidence that a new measurement-disturbance relationship, mathematically computed by physicist Masanao Ozawa, at Nagoya University in Japan, in 2003, is more accurate.
“Precision quantum measurement is becoming a very important topic, especially in fields like quantum cryptography where we rely on the fact that measurement disturbs the system in order to transmit information securely,” said Rozema. “In essence, our experiment shows that we are able to make more precise measurements and give less disturbance than we had previously thought.”
And there’s more evidence today
Publishing their work online in the March 3 edition of Nature Photonics, researchers at the University of Rochester and the University of Ottawa have applied the above technique to directly measure for the first time the polarization states of light. The work overcomes some important challenges of Heisenberg’s Uncertainty Principle and can also be applied to qubits, the building blocks of quantum information theory.
Direct measurements of the wavefunction, a way of determining the state of a quantum system, had long seemed impossible because of a key tenet of the uncertainty principle—the idea that certain properties of a quantum system could be known only poorly if certain other related properties were known with precision. The ability to make these measurements directly challenges the idea that full understanding of a quantum system could never come from direct observation.
The Rochester/Ottawa researchers, led by Robert Boyd, who has appointments at both universities, measured the polarization states of light—the directions in which the electric and magnetic fields of the light oscillate. Their key result, like that of the team that pioneered direct measurement, is that it is possible to measure key related variables, known as “conjugate” variables, of a quantum particle or state directly. The polarization states of light can be used to encode information, which is why they can be the basis of qubits in quantum information applications.
“The ability to perform direct measurement of the quantum wavefunction has important future implications for quantum information science,” Dr Boyd said. “Ongoing work in our group involves applying this technique to other systems, for example, measuring the form of a ‘mixed’ (as opposed to a pure) quantum state.”
