Two defining features of quantum mechanics never appear together

Two of the most important ideas that distinguish the quantum world from the classical one are nonlocality and contextuality.

Previously, physicists have theoretically shown that both of these phenomena cannot simultaneously exist in a quantum system, as they are both just different manifestations of a more fundamental concept, the assumption of realism. Now in a new paper, physicists have for the first time experimentally confirmed that these two defining features of quantum mechanics never appear together.

The physicists, Xiang Zhan, et al., have published a paper on the nonlocality-contextuality tradeoff in a recent issue of Physical Review Letters.

In the everyday world that we observe, an object can only be affected by nearby objects (locality), and when we make a measurement, the outcome does not depend on other independent measurements being made at the same time (noncontextuality).

In contrast, the quantum world is nonlocal, as demonstrated by quantum entanglement where two objects can influence each other even when separated by large distances. And in the quantum world, measurements are contextual, so quantum systems do not have predetermined values but instead their values depend on how measurements are made.

To show that a quantum system is nonlocal or contextual, physicists have defined inequalities that assume a system is the opposite (local or noncontextual). Then they perform experiments that attempt to violate these inequalities to show that the system is not local or noncontextual. So far, these two types of inequalities have never been tested simultaneously.

In the new study, the researchers have attempted to violate both inequalities at the same time, but have found that only one inequality can be violated at once. Their experiment uses entangled photons to generate photonic qutrit-qubit systems (a qubit is a superposition of two states, whereas a qutrit is a superposition of three states). By performing various measurements on these photons, the researchers could violate the inequalities separately, but not at the same time.

"The greatest significance of our work is that we provide experimental evidence of the assumption that quantum entanglement and contextuality are intertwined quantum resources," Peng Xue, a physicist at the University of Science and Technology of China and one of the lead authors of the paper, told Phys.org.

As the physicists explain, the reason for the nonlocality-contextuality tradeoff arises from the fact that both properties have the same root: the assumption of realism, which is the assumption that the physical world exists independent of our observations, and that the act of observation does not change it.

Since nonlocality and contextuality can be thought of as two different manifestations of the basic assumption of realism, then one of them can be transformed into the other, but both cannot exist at the same time because they are essentially the same thing.

"We think the contextuality-nonlocality monogamy suggests the existence of a quantum resource of which entanglement is just a particular form," Xue said.

"The resource required to violate the noncontextuality inequality and that required to violate the locality inequality are fungible through entanglement. That is, to violate the locality inequality costs entanglement as a resource, while to violate the noncontextuality inequality costs contextuality as a resource. In a quantum system, only one of the two inequalities can be violated because nothing is left to violate the other one."

The researchers hope that the new experiment will open the doors to further exploring the mutual resource in the future, as well as lead to potential applications.

"We plan to study contextuality as a resource for experimental quantum information processing, such as for quantum computation," Xue said.

http://phys.org/news/2016-03-features-quantum-mechanics.html

http://webcache.googleusercontent.com/search…

http://arxiv.org/abs/1512.03334

https://en.wikipedia.org/wiki/Quantum_contextuality

The Big Short

Based on the non-fiction book of the same name by Michael Lewis, The Big Short is a 2015 biographical comedy-drama co-written and directed by Adam McKay and starring Christian Bale, Steve Carell, Ryan Gosling and Brad Pitt.

An exploration of the the 2008 financial crisis through the eyes of various eccentric insiders who saw what was coming before anyone else did. Michael Burry (Bale) is a hedge fund manager who is the first to realise that the housing market is extremely unstable as its based on subprime loans. Jared Vennett (Gosling) is a trader who gets wind of Burry's insights and realises he's onto something. Joined by hedge fund manager Mark Baum (Carell) and his team in betting against the housing market. Meanwhile newcomers Charlie Geller (John Magaro) and Jamie Shipley (Finn Wittrock) also start getting into credit default swap after they find a paper by Vennett and convince Geller's former neighbour and retired banker Ben Rickert (Pitt) to help them out to get the necessary credentials to be able to do so.

An amazing change of pace for director Adam Mckay, who also co-wrote the screenplay and for his Will Ferrell comedies, The Big Short is a highly successful mixture of comedy and docu-drama. Brandishing a distinct style featuring some great editing and 4th wall breaking narration by Gosling as well as fantastic performances from all involved (with especially Bale, Carrell and Gosling standing out), The Big Short is an insightful as well as highly entertaining expose of the 2008 collapse of the housing market and economic crisis in its wake. Additionally the film also benefits from its screenplay which not only manages to clearly explain all the financial lingo but also gives the actors some great written characters to work with. The Big Short was nominated for two Academy Awards, winning one for Best Adapted Screenplay, one Golden Globe for Best Screenplay and two BAFTA Awards, once again winning one for Best Adapted Screenplay. It also won 12 more awards in this category from various critics associations. A remarkable different type of comedy from director McKay, The Big Short is a must-see film for anyone interested into the origins of the 2008 financial crisis or fans of fresh docu-dramas.

Sleep suppresses brain rebalancing

Why humans and other animals sleep is one of the remaining deep mysteries of physiology. One prominent theory in neuroscience is that sleep is when the brain replays memories "offline" to better encode them ("memory consolidation").

A prominent and competing theory is that sleep is important for re-balancing activity in brain networks that have been perturbed during learning while awake. Such "rebalancing" of brain activity involves homeostatic plasticity mechanisms that were first discovered at Brandeis University, and have been thoroughly studied by a number of Brandeis labs including the lab of Brandeis professor of biology Gina Turrigiano.

Now, a study from the lab just published in the journal Cell shows that these homeostatic mechanisms are indeed gated by sleep and wake, but in the opposite direction from that theorized previously: homeostatic brain rebalancing occurs exclusively when animals are awake, and is suppressed by sleep.

These findings raise the intriguing possibility that different forms of brain plasticity - for example those involved in memory consolidation and those involved in homeostatic rebalancing - must be temporally segregated from each other to prevent interference.

The requirement that neurons carefully maintain an average firing rate, much like the thermostat in a house senses and maintains temperature, has long been suggested by computational work. Without homeostatic ("thermostat-like") control of firing rates,