Tag: research

Quantum Minesweeper

If you are vaguely interested in quantum mechanics, you must check out the game Quantum Minesweeper. You might want to start with the video tutorial before you play online.

The game differs from classical Minesweeper in the following ways:

  • The board is really a quantum superposition of two boards. It is your goal to figure out the superpositions. It is simplified, as only one kind of phase is allowed.
  • There are three different kind of measurements that you can do, each one a limited number of times. The measurements are:
  1. classical measurement – collapse that can trigger a mine probabilistically. Very risky!
  2. entropy measurement – it indicates if there is a superposition or not, but doesn’t tell you if there is a mine or not!
  3. interaction-free measurements – it is very magical, doesn’t collapse the wave function, actually gives you the phase information. Very powerful!

This game is fantastic!

Technical digression:

I have a question that might be a good undergraduate research project for someone interested in quantum information. What is the optimal strategy for the game? That is, if you thought of this game as a kind of state tomography problem, is there a general protocol to extract the state with high fidelity, given the constrains of the number of measurements? To make it more interesting, imagine a version of quantum minesweeper where the boards could have between them any kind of phase, how much harder would solving it be?

—

Give it one last try
til the next
one more
last try.
-A Wilhelm Scream

Video: Quantum Effects in Photosynthesis – FermiLab

The video and slides for my talk for undergraduates at FermiLab can be found here:

Quantum Effects in Photosynthesis [RealPlayer video link].

This is a good introduction to my research. If you are curious about what I do, by all means watch it.

[I know RealPlayer is so 1998 and sucks, I’m trying to get the file in another format from the FermiLab people.]

[Previous post here.]

Program: Quantum Effects in Biological Systems

This thursday, the second conference on Quantum Effects in Biological System will be held here at Harvard University. We are very excited to have experts from all over the world here. The aim of this series of conferences is to establish the new field of Quantum Biology. The conference program can be found here.

Why does the world look classical?

A few days ago we posted a new paper.

General Bound on the Rate of Decoherence [arXiv:10045405]

Cesar A. Rodriguez-Rosario, Gen Kimura, Hideki Imai, Alan Aspuru-Guzik

We establish the necessary and sufficient conditions for a quantum system to be stable under any general system-environment interaction. Quantum systems are stable when the time-derivative of their purity is zero. This stability provides a dynamical explanation of the classicality of measurement apparatus. We also propose a protocol to detect global quantum correlations using only local dynamical information. We show how quantum correlations to the environment provide bounds to the purity rate, which in turn can be used to estimate dissipation rates for general non-Markovian open quantum systems.

[SciRate]

The paper could have been alternatively titled: “Necessary and Sufficient Conditions for System Stability Under Any Coupling to the Environment”. In this post, I want to discuss briefly our first result of the paper:

$$left[ frac{d}{dt}mathbf{P}^mathcal{S}_tright]_{t=tau} = 0; Leftrightarrow ; left[rho^mathcal{S}_tauotimes I^mathcal{E},rho^mathcal{SE}_tauright] =0$$.

We were interested in finding universal decoherence stability criteria that depended on the structure of the system-environment state, but was independent of the particular Hamiltonian dynamics. We focused on the measure of decoherence called “Purity”, in particular the rate of change of purity. We found that there exist system-environment states that preserve the purity of the system independent of the details of the interaction Hamiltonian. These states are given by the commutator in the equation above vanishing, and we call them “Stable System States” or SSS for lack of a better name.

SSS states are sparse topologically and not-dense: they are quite rare. But, at the same time, they include states whose system part looks very classical. On first sight, since they are rare, this would raise the question of why does the world looks classical to us. However, the equation above also implies that these states are stable under decoherence, and thus can be long-lived.

In other words, we can prove how classical states emerge naturally in the world without any assumptions of the dynamics! This provides a non-equilibrium thermodynamical explanation to why our universe looks classical.

Reuters Interviews Daniel Lidar

Reuter’s ScienceWatch recently posted an interview with Prof. Daniel Lidar from USC where they discuss Daniel’s most important papers, focusing on decoherence free subspaces. Daniel was very kind to mention my own work on Open Quantum Systems with initial correlations during the interview.

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“The theory of its operation is rudimentary and attempts to improve its performance are still made in an almost haphazard way”
-Sadi Carnot on engines

Picture of Obama Feigning Interest in Excitonics

Flipping through the NY Mag photo gallery titled A History of Obama Feigning Interest in Mundane Things, I found a picture of Excitonics Center‘s Vladimir Bulovic.

Click for A picture of Obama Feigning Interest in the Excitonics Center

The old story of Obama’s visit can be found here.

—
Dr. Peter Venkman : Back off man. I’m a scientist.
-Ghost Busters

Quantum Stochastic Walks

It took some time with the printing proofs, but finally, the paper has been published.

Quantum stochastic walks: A generalization of classical random walks and quantum walks

We introduce the quantum stochastic walk (QSW), which determines the evolution of a generalized quantum-mechanical walk on a graph that obeys a quantum stochastic equation of motion. Using an axiomatic approach, we specify the rules for all possible quantum, classical, and quantum-stochastic transitions from a vertex as defined by its connectivity. We show how the family of possible QSWs encompasses both the classical random walk (CRW) and the quantum walk (QW) as special cases but also includes more general probability distributions. As an example, we study the QSW on a line and the glued tree of depth three to observe the behavior of the QW-to-CRW transition.

Phys. Rev. A 81, 022323 (2010)

Previously: video abstract

—

Man, you come right out of a comic book. -Enter the Dragon

How not to lie about Quantum Mechanics?

Writing for the general public about science news is hard. ArsTech has an article where they accuse many news organizations of deliberately lying in their science coverage, and discuss how they can get away with it do to double standards.

As a scientist with interest in informing the public of my research, are there any guidelines to follow when talking to the press? I want them to see them as allies, but most of the science news are so bad I can’t help but hating them.

I’ve thought much about how to describe my research to family and friends, and haven’t found any good and concise way to do it. More specifically, can any one suggest any good, simple, cocktail-party style one-liners to explain what is quantum mechanics and quantum computing, but that doesn’t make me feel like I’m lying? If I read again the phrase “what Einstein called spooky action at a distance” I might vomit.

Any ideas?

—
When Men fly from danger, it is natural for them to run farther than they need.
-The Mischiefs that ought justly to be apprehended from a Whig-goverment

Quantum Stochastic Walks

We just posted a paper in the arXiv.

Quantum stochastic walks: A generalization of classical random walks and quantum walks

We introduce the quantum stochastic walk (QSW), which determines the evolution of generalized quantum mechanical walk on a graph that obeys a quantum stochastic equation of motion. Using an axiomatic approach, we specify the rules for all possible quantum, classical and quantum-stochastic transitions of a vertex as defined from its connectivity. We show how the family of possible QSW encompasses both the classical random walk (CRW) and the quantum walks (QW) as special cases, but also includes more general probability distributions. As an example, we study the QSW on the line, its QW to CRW transition and transitions to genearlized QSWs that go beyond the CRW and QW. QSWs provide a new framework to the study of quantum walks with environmental effects as well as quantum algorithms.

I promise a simple explanation of Classical Random Walks soon!