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Peeking into Schrodinger’s box: direct measurement method continues to show its potential for quantum information

According to a news release from the University of Rochester, researchers have been able to apply a newly developed method called direct measurement to measure a 27-dimensional quantum state in a single experiment with no post-processing. Utilizing a technique known as quantum tomography, this would have normally been a multistage process. Methods such as direct measurement could be crucial in forming high security quantum communications system, as well as to exploring our understanding of quantum mechanics.

“Our work shows that direct measurement offers an exciting alternative to quantum tomography,” posited Robert Boyd, Professor of Optics and Physics at the University of Rochester and Canada Excellence Research Chair in Quantum Nonlinear Optics at the University of Ottawa. “As the field of quantum information continues to advance, we expect direct measurement to play an increasingly important role in this.”

Boyd added the lack of post-processing is a significant factor in accelerating direct measurements.

The direct measurement method offers a way to directly measure the state of a quantum system. It was first created in 2011 by scientists at the National Research Council Canada, who utilized it to measure the position and momentum of photons. In 2013, Boyd and his colleagues revealed that direct measurement could be used to measure the polarization state of light. The new paper represents that first time this technique has been utilized to measure the state of a discrete, high dimensional system.

Direct measurement consists of two kinds of measurements conducted one after the other, first a “weak” measurement followed by a “strong” measurement. In quantum mechanics the act of measuring a quantum state disturbs it irreversibly, a phenomenon known as the collapse of the wavefunction. However, the first measurement in this technique is so mild that it only somewhat disturbs the system and does not lead to the collapse of the wavefunction.

“It is sort of like peeking into the box to see if Schrodinger’s cat is alive, without fully opening the box,” explained lead author Dr. Mehul Malik, who was a Ph.D. in Boyd’s group when the work was conducted. “The weak measurement is essentially a bad measurement, which leaves you mostly uncertain about whether the cat is alive or dead. It does, however, give partial information on the health of the cat, which when repeated many times can lead to near certain information as to whether the cat is alive or dead.”

The study’s findings are described in greater detail in the journal Nature Communications.