Quantum measurement carries information even when the measurement outcome is unread

August 27, 2013 by Lisa Zyga, Phys.org feature

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
(Phys.org) —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 , 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 .

The proposed involves two parties, Alice and Bob. First, Alice prepares two entangled qudits (D-dimensional ) 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 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 , 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 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

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1 / 5 (7) Aug 27, 2013
This implies that, provided Alice and Bob prearrange their understanding of what the measurement methods encode as meaningful information- for instance, measurement X = "Go!"; measurement Y= "Stay!"- one could establish an instantaneous signaling system. Such things are supposed to be unavailable to those of us existing in decohered states. Where's the gotcha?
1.7 / 5 (11) Aug 27, 2013
Bob performs a measurement on a particle from Alice but does not read the outcome.
I wish they had been more specific on how this is done.

Obviously it can't be something as stupid as measuring the particle while keeping your eyes shut.
1 / 5 (1) Aug 27, 2013
Isn't a quantum system "hanging" in supposition, until it's "resolved"/watched, read/ on a macroscopic level?
If so, what and how they "measure"?
It's looking like something here is going in opposition with the famous cat :~
not rated yet Aug 27, 2013
I'm more confused because, at least from the abstract, this is functionally the same as the existing entanglement communication schemes. Ie, Bob chooses a measurement basis, and its that measurement basis that serves as message. But the standard communication protocol is for Alice to message Bob by choosing a measurement basis and then transmitting to Bob the results. Well here's the paper on Arxiv anyway, I'll try to peruse for answers and come back.

1.4 / 5 (11) Aug 27, 2013
Wasn't there an experiment recently which showed entanglement backwards in time as long as certain conditions were met? Could the measurement Bob does be automated (in isolation) and the choice of measurements be controlled by a separate retro-entanglement so that Bob could wait until the original entangled particle was sent back then use the retro-entanglement to choose the measurement and thus send a real piece of information instantaneously? I know the answer is almost certainly "no" but no harm in asking.
not rated yet Aug 28, 2013
Is it something like a measurement resulting in a superposition of up and down in the x/y/or z-direction, or whatever axis you choose to measure on? So the superposition is of both spin results (up or down), but oriented in a particular direction, based on the axis the 'measurement' applies to?? (I haven't been able to view the Arxiv paper yet.)
1.3 / 5 (13) Aug 28, 2013
Bob performs a measurement on a particle from Alice but does not read the outcome.
I wish they had been more specific on how this is done.

Obviously it can't be something as stupid as measuring the particle while keeping your eyes shut.

It would have been helpful. The above mentioned "alignment", so maybe they used a Stern-Gerlach apparatus. So if Bob chooses and measures an X alignment for spin, it would destroy information about spin in Y and Z.
1 / 5 (2) Aug 28, 2013
So they run the photon through the magnetic field part of a Stern-Gerlach apparatus, but without any detector screen, so the deflection is never actually observed?
1 / 5 (8) Aug 29, 2013
1 / 5 (11) Aug 30, 2013
Voodoo physics
1.4 / 5 (11) Aug 31, 2013
There are no physical differences between the world of the small and the world of the big. Discussing characteristic differences between quantum and classical mechanics is nothing but philosophical mumbo jumbo. The 2 realms belong to a single reality. The mathematical incompatibility of the 2 is a result of a lack of human ingenuity. A single theory will one day be formulated which will encompass both the very big and the very small without invoking hidden dimensions and/or hidden/dark variables.
1.3 / 5 (12) Aug 31, 2013
The natural world is purely deterministic. Nature at the classical scale is deterministic. Nature at the quantum scale, which is said to be indeterministic (probabilistic), is also purely deterministic. Our equations (models), which we use to predict outcomes and to define nature, give us results in terms of probabilities of occurrence. When one attempts to explain formulas using words, we encounter a problem. We at this point start giving attributes to nature itself. But, nature is not defined by our interpretations of it. Discussing physical theories is tricky business. You must take extreme care to explain the mathematical model and not nature itself. This can get very confusing at times.

The only thing unnatural is life itself. We are designed to counteract nature in order to self preserve. Only the actions of living things are indeterministic. And I would venture so far as to even question this: Is choice also an illusion?
1 / 5 (2) Aug 31, 2013
Hidden variables, here we come.
5 / 5 (4) Sep 01, 2013
In an entangled two qudit system Bob can measure something about one qudit's state and, after sending it back to Alice, she can measure both qudits and determine what Bob did (which measurement he made). The idea is that the system as a whole is still undetermined, but the possible measurement outcomes have been limited by Bob's measurement of one of the qudits (which is an observation whether he looked or not). Since it is a complex system, there are many possible measurements Bob can make, essentially encoding information for Alice to detect. Nothing spooky here. Alice needs both qudits. Only Alice can decode the message since she has the entangled partner.
2.4 / 5 (5) Sep 01, 2013
In the caption for the diagram at the beginning of the article, it says that Bob performs a non-selective measurement on the photon and then sends it back to Alice. So, does that mean he runs it through the alignment part of the apparatus but doesn't actually detect/observe the outcome of the alignment? Instead, he just sends the unobserved photon back to Alice?

Also, it says that most of time, Alice will be able to figure out what Bob did. How much of time is most?

Can Alice send non-selectively measured unobserved photons to Bob to tell him that she can't figure out the meaning of the non-selectively measured unobserved photons that he is sending back to her?

To paraphrase Socrates, "an unobservered photon is not worth transmitting"
3.7 / 5 (3) Sep 03, 2013
I don't see where it says "most of the time".

This isn't about a photon, BTW. It is a qudit. (not just a qubit) It is an entangled multidimensional quantum system.

If it is measured, it is as observed as it needs to be. Bob doesn't need to actually look or convey any information about the actual measurement. Merely interacting disturbs the system and leaves an adequate trace. Note that observation doesn't mean someone looked at a measurement. It just means the system interacted with a measurement system.
2.3 / 5 (3) Sep 05, 2013
In the diagram at the beginning, in step 3, it says "almost always."
3 / 5 (2) Sep 07, 2013
Yep --- missed that. Strange
1 / 5 (1) Sep 09, 2013
from the Paper: "Bob may or may not record the measurement
outcome. This is of no relevance to the protocol."

There is an inconclusive returnstate in which nothing really changed so Alice doesn't know if bob ever performed a measurement (as far as I understand the paper)
Captain Stumpy
1.4 / 5 (9) Sep 10, 2013
In an entangled two qudit system Bob can measure something about one qudit's state and, after sending it back to Alice,...........Alice needs both qudits. Only Alice can decode the message since she has the entangled partner.

ok, this made more sense to me than the article...

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