Quantum measurement carries information even when the measurement outcome is unread

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…

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.
The protocol relies on the uniquely quantum features of the system, since performing nonselective measurements on classical systems cannot carry information.
“In this protocol, we use a non-selective measurement in different bases (different alignments of the measurement apparatus, if you like) to transmit information,” Kalev told Phys.org. “To the best of our knowledge, this protocol is the first protocol which uses the disturbance on the system caused by the measurement in different bases, regardless of the actual outcome (!), for a communication task. It seems that, in classical mechanics, one cannot use a similar protocol for communication since there is no notion of ‘different’ or ‘complementary’ bases, and moreover a non-selective measurement on a classical system is equivalent to not making a measurement at all.”
As the physicists explain, the protocol suggests a novel way to view quantum measurement as consisting of two stages. The first stage involves performing the measurement, while the second stage involves reading the outcome. In the first stage, the state of the system is mixed (i.e., undetermined), but a particular set of variables associated with the choice of the measurement do have a determined value. In the second stage, when the measurement is read, the state of the system is determined. Classical measurement can be thought of as consisting only of the second stage; the first stage is a uniquely quantum component.
The proposal that nonselective quantum measurements carry information is not only of fundamental interest, but could potentially have applications for information tasks. As the scientists explain, Bob’s sending of his qudit to Alice can be viewed as a form of dense coding. Although the protocol in its present form cannot be used for secure communication, the physicists are now investigating whether variations of the protocol might be useful for cryptography applications.

Explore further:

Playing quantum tricks with measurements

More information: Amir Kalev, et al. “Choice of Measurement as the Signal.” PRL 110, 260502 (2013). DOI: 10.1103/PhysRevLett.110.260502

Journal reference:

Physical Review Letters

view popular

4.5 /5 (11 votes)

Related Stories

Playing quantum tricks with measurements

Feb 15, 2013

A team of physicists at the University of Innsbruck, Austria, performed an experiment that seems to contradict the foundations of quantum theory—at first glance. The team led by Rainer Blatt reversed a …

Entanglement recycling makes teleportation more practical

Jan 17, 2013

(Phys.org)—Working in the exotic-sounding field of quantum teleportation, physicists are trying to make it easier to transmit quantum information in the form of qubits from one location to another without having the qubits …

Physicists publish solution to the quantum measurement problem

Jul 17, 2013

(Phys.org) —Quantum mechanics is a highly successful theory, but its interpretation has still not been settled. In their recent opus magnum, Theo Nieuwenhuizen (Institute of Physics, UvA) and colleagues claim to have found …

New principle sets maximum limit on quantum information communication

Jun 04, 2013

(Phys.org) —When two parties use a quantum system to share information, the amount of quantum information that can be communicated is fundamentally limited by quantum properties. Now in a new paper, Damián …

Teleportation just got easier—but not for you, unfortunately

Aug 21, 2013

Thanks to two studies published in Nature last Thursday, the chance of successful teleportation has considerably increased. Which is a good thing, right? …

Quantum physics mimics spooky action into the past

Apr 23, 2012

Physicists of the group of Prof. Anton Zeilinger at the Institute for Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Vienna Center for Quantum Science and Technology (VCQ) …

Recommended for you

Quantum inverted pendulum: Control scheme dynamically maintains unstable quantum system

Aug 27, 2013

A simple pendulum has two equilibrium points: hanging in the “down” position and perfectly inverted in the “up” position. While the “down” position is a stable equilibrium, the inverted position is definitely …

Quantum computing: Manipulating a single nuclear spin qubit of a laser cooled atom

Aug 27, 2013

It is advantageous to implement a quantum bit (qubit) with a single nuclear spin because the nuclear spin is robust against any stray magnetic fields. This robustness can be attributed to the small magnitude …

Physicist disentangles ‘Schrodinger’s cat’ debate

Aug 26, 2013

Physicist Art Hobson has offered a solution, within the framework of standard quantum physics, to the long-running debate about the nature of quantum measurement.

Physicist proves impossibility of quantum time crystals

Aug 22, 2013

(Phys.org) —Is it possible that a moving object could have zero energy? The common sense answer is no, since motion itself is kinetic energy, but this answer has been challenged recently by the concept …

Teleportation just got easier—but not for you, unfortunately

Aug 21, 2013

Thanks to two studies published in Nature last Thursday, the chance of successful teleportation has considerably increased. Which is a good thing, right? …

User comments : 9

Adjust slider to filter visible comments by rank

Display comments: newest first

More news stories

Fastest rotating man-made object created

(Phys.org) —A team of researchers at the University of St Andrews has created the world’s fastest spinning man-made object.

Magnetic charge crystals imaged in artificial spin ice

A team of scientists has reported direct visualization of magnetic charge crystallization in an artificial spin ice material, a first in the study of a relatively new class of frustrated artificial magnetic …

Magnetic materials: Forging ahead with a back-to-basics approach

Scientists have recently started to explore the possibility of using an intrinsic property of the electron known as spin for processing and storing information. Magnetic fields can influence the dynamics …

Laser fusion experiment yields record energy

(Phys.org) —In the early morning hours of Aug.13, Lawrence Livermore’s National Ignition Facility (NIF) focused all 192 of its ultra-powerful laser beams on a tiny deuterium-tritium filled capsule. In …

Physicist disentangles ‘Schrodinger’s cat’ debate

Physicist Art Hobson has offered a solution, within the framework of standard quantum physics, to the long-running debate about the nature of quantum measurement.

China to add 1,500 gigawatts of power capacity by 2030, study says

China will add some 1,500 gigawatts of power production capacity by 2030, or the equivalent of Britain’s existing capacity every year, a study showed on Wednesday.

Slowdown in global warming only temporary, new study finds

A slowing in global warming that climate sceptics say undermines the greenhouse theory is simply a “hiatus” from higher temperatures, scientists said on Wednesday.

Two deaths, six new MERS virus cases in Saudi: WHO

Eight new cases of the deadly coronavirus MERS, a SARS-like infection, have been registered in Saudi Arabia, the World Health Organisation said on Wednesday.

Study: Marijuana top illegal drug used worldwide

Marijuana is the most popular illegal drug used worldwide, but addictions to popular painkillers like Vicodin, Oxycontin and codeine kill the most people, according to the first-ever global survey of illicit drug abuse.

US physician who told of handling UFO debris dies

Dr. Jesse Marcel Jr., who said he handled debris from the 1947 crash of an unidentified flying object near Roswell, New Mexico, has died at the age of 76.