Nanophononics Workshop

Last week, we hosted a CECAM Workshop on Nanophononics. The conference had the top speakers in the field from all around the world, as you can see in the program.

 

The conference was also mentioned in the local newspaper. Here is a scan from the Weser Kurier of the article (in German).

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New Paper: Thermodynamics of quantum coherence

The bizarre microscopic quantum world is exemplified by Schrödinger’s cat, where a quantum mechanical “cat” state is said to be both death and alive simultaneously. This non-classical state is called a quantum coherence. Coherence is at odds with macroscopic realism. Our experience is dominated by thermodynamics, which destroys quantum coherences at our length and time scales.

We decided to study the reverse situation: In the microscopic world, can quantum coherence affect thermodynamics? We have posted a new manuscript titled "Thermodynamics of quantum coherence".

Thermodynamics of quantum coherence [arXiv:1308.1245]
César A. Rodríguez-Rosario, Thomas Frauenheim, Alán Aspuru-Guzik

Quantum decoherence is seen as an undesired source of irreversibility that destroys quantum resources. Quantum coherences seem to be a property that vanishes at thermodynamic equilibrium. Away from equilibrium, quantum coherences challenge the classical notions of a thermodynamic bath in a Carnot engines, affect the efficiency of quantum transport, lead to violations of Fourier's law, and can be used to dynamically control the temperature of a state. However, the role of quantum coherence in thermodynamics is not fully understood. Here we show that the relative entropy of a state with quantum coherence with respect to its decohered state captures its deviation from thermodynamic equilibrium. As a result, changes in quantum coherence can lead to a heat flow with no associated temperature, and affect the entropy production rate. From this, we derive a quantum version of the Onsager reciprocal relations that shows that there is a reciprocal relation between thermodynamic forces from coherence and quantum transport. Quantum decoherence can be useful and offers new possibilities of thermodynamic control for quantum transport.

In this paper, we showed that quantum coherences are useful in thermodynamics in an exactly reciprocal manner to the way thermodynamics destroys coherences. This theory suggest that this interplay can lead to improved molecular devices, and to a deeper understanding of energy transport in photosynthesis.The main results of this paper include a generalization of the laws of thermodynamics and of the Onsager reciprocal relations for the quantum regime.  These allowed us to interpret quantum coherences as a new thermodynamic resource. This new theory provides a framework to unify previous results on quantum Carnot engines , thermal control by quantum measurements, quantum coherences in photosynthetic complexes and transport in molecular devices.

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The Boring World of Niels Bohr

The espionage scandals in the news prompts us to revisit how physicists have been under surveillance by the US government, to sometimes hilarious results. During World War II, Niels Bohr (and his son) visited Washington D.C., where they were under secret surveillance. The following declassified report confirms the standard suspicion of quantum physics.

I type here the relevant part of the report on Niels Bohr:

Both the father and son appear to be extremely absent-minded individuals, engrossed in themselves, and go about paying little attention to any external influences. As they did a great deal of walking, this Agent had occasion to spend considerable time behind them and observe that it was rare when either of them paid much attention to stop lights or signs, but proceeded on their way much the same as if they were walking in the woods. On one occasion, subjects proceeded across a busy intersection against the red light in a diagonal fashion, taking the longest route possible and one of greatest danger. The resourceful work of Agent Maiers in blocking out one half of the stream of automobile traffic with his car prevented their possibly incurring serious injury in this instance.

In conclusion, yes, quantum physicists are very dangerous, but to themselves.

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Paper: Vanishing quantum discord is not necessary for completely positive maps

Our paper:

Vanishing quantum discord is not necessary for completely positive maps

The description of the dynamics of a system that may be correlated with its environment is only meaningful within the context of a specific framework. Different frameworks rely upon different assumptions about the initial system-environment state. We reexamine the connections between complete positivity and quantum discord within two different sets of assumptions about the relevant family of initial states. We present an example of a system-environment state with nonvanishing quantum discord that leads to a completely positive map. This invalidates an earlier claim about the necessity of vanishing quantum discord for completely positive maps. In our final remarks, we discuss the physical validity of each approach.

has been published in Physical Review A! This paper challenges some of the main claims of one of the most cited papers in the quantum discord community. We hope this will lead to fruitful discussion on the subject.

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CECAM Workshop: Nanophononics

I am one of the organizers for the CECAM Workshop on Nanophononics.

It will take place here in Bremen this August, and promises to have all the most important researchers working on the control of phonon transport in the nanoscale. This is going to be an amazing conference. You can register here.

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Paper: Positivity in the presence of initial system-environment correlation

Our paper Positivity in the presence of initial system-environment correlation has been published in Physical Review A!

The constraints imposed by the initial system-environment correlation can lead to nonpositive dynamical maps. We find the conditions for positivity and complete positivity of such dynamical maps by using the concept of an assignment map. Any initial system-environment correlations make the assignment map nonpositive, while the positivity of the dynamical map depends on the interplay between the assignment map and the system-environment coupling. We show how this interplay can reveal or hide the nonpositivity of the assignment map. We discuss the role of this interplay in Markovian models.

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Blackboards of Physics

Theoretical physicists rely on their blackboards like priests on their pulpits, or like news anchors on their desks. We have very strong opinions about them. Markers vs Chalk? If you like markers, what brand of markers, and how thick? And then, do you write on the whiteboard, the glass window, or just simply, special wall paint? If you like chalk, what kind of chalk? Different brands crumble in different ways. Do you like colored chalk? Do you prefer to write on green or black board?

I am old fashioned about this. I like white chalk on black boards. But, I found that I'm allergic to chalk dust, and I've been forced to use markers on white boards now.

There is a photographic art exhibit about the Blackboards of Quantum Physicists. Some pictures are cool. In some others, I feel that the physicists tried to ruin how authentic the writing were, either by making them too nice, or by trying to sabotage them. Either way, I think this kind of works as a way to convince the public to not be scared by our crazy equations.

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Visiting Hong Kong University

I have just come back from Hong Kong, where I was collaborating with the group of Prof. GuanHua Chen. We are interested in studying mathematical properties of the Hierarchical Equations of Motion from a Master Equation point of view. This was a very productive visit.

I also had a wonderful time exploring the city, and finding great food everywhere.

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Graphene CECAM Workshop

The Bremen Center for Computational Materials Science was host to the International CECAM-Workshop
Graphene: From band structure to many-body physics. See the workshop program here. This was a fantastic conference, with many of the leaders in the field.Conference Attendandts in front of the Bremen Townhall

Here is one of the Workshop pictures in front of the Bremen Townhall

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The Sins of Isaac Newton

I just came back from a delightful vacation where I reread The Baroque Cycle. The Baroque Cycle is a series of novels published in three volumes Quicksilver, The Confusion, and The System of the World in the genre of historical fiction about the rise of natural philosophy, important developments in economics and politics during the enlightenment. It is insanely ambitious, imitating literary styles from the era. It is insanely detailed. One of its major characters is Isaac Newton, both in his roles of the greatest natural philosopher ever, and as the master of the Mint, reforming the financial system in England.

Through out the books, references are made to a list of sins that Isaac Newton kept. In this list, he detailed every sin he ever committed. Curiously, the real documents including this list has been found by historians, and published on the internet. Check out the real list of Isaac Newton's sins here.

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