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Quantum measurement carries information even when the measurement outcome is unread

Posted August 28, 2013
Quantum measurement carries information even when the measurement outcome is unread

In the communication protocol, Bob performs a measurement on a particle from Alice but does not read the outcome. After he sends the particle back to Alice, she can deduce Bob’s choice of measurement, which carries information. Image credit: Kalev, et al. ©2013 American Physical Society

Some tasks that are impossible in classical systems can be realized in quantum systems. This fact is exemplified by a new protocol that highlights an important difference between classical and quantum measurements. In classical mechanics, performing a measurement without reading the measurement outcome does not carry any information and is therefore equivalent to not performing the measurement at all. But in the new protocol, a quantum measurement that is performed but not read can carry information because the information can be encoded in the choice of the type of measurement that was performed.

The physicists, Amir Kalev at the University of New Mexico in Albuquerque, along with Ady Mann and Michael Revzen at the Technion—Israel Institute of Technology in Haifa, Israel, have published their paper on the unique features of quantum measurements in a recent issue of Physical Review Letters.

When a measurement is performed but not read, it is called “nonselective.” The difference between classical nonselective measurements and quantum nonselective measurements is that the latter cause an inevitable disturbance to the measured system. By tracking this disturbance, the physicists here have shown that it can be used to carry and communicate information.

The proposed protocol involves two parties, Alice and Bob. First, Alice prepares two entangled qudits (D-dimensional quantum systems) and sends one to Bob. Bob performs a measurement on his qudit using an instrument with a certain alignment of his choice, does not read the outcome, and then sends the qudit back to Alice. Finally, Alice measures the resulting two-qudit state, which allows her to deduce Bob’s choice of measurement. At no point do Alice or Bob read the outcome of Bob’s measurement, but the two parties can still use the measurement to communicate information.

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