**DP-PMI Day**** 2015 **- Abstracts

10h05

Maxwell’s demon was born in 1867 and his sole function was to illustrate some potential problems with the Second Law of thermodynamics. He still thrives in modern physics and plays an important role in clarifying connections between thermodynamics and information theory. In my talk I will present a variety of different demons which, when restricted by the Second Law, will lead to interesting consequences in electromagnetism, optics, gravity, quantum mechanics as well as quantum information. Finally, I will speculate if the concept of information could in some sense be considered deeper than the entities typically though of as fundamental in physics and will mention various efforts to derive quantum physics from simpler information theoretic axioms.

11h20

A continuous-time analogue of Grover's algorithm can be formulated as a quantum walk on a complete graph of N vertices. We analyse the robustness of this algorithm against the loss of edges in the graph, as well as against static disorder. Firstly, when a few edges are removed from a complete graph, the dynamics of the algorithm is restricted to a three dimensional subspace and its optimality is retained. Subsequently, in the more general scenario where each edge of the graph remains with probability p, the underlying structure can be treated as an Erdos-Renyi random graph. We show that in the asymptotic limit, when N goes to infinity, search is optimal on almost all graphs as long as p = c N^(-z), where c is a constant and 0<=z<1. Secondly, we analyze the robustness of this algorithm to random static disorder in the diagonal and off-diagonal elements of the search Hamiltonian. We show that the algorithm retains its optimality as long as the standard deviation of the disorder is of O(N^(-1/2)).

11h40

We study the entanglement of families of Unruh modes in the Bell states $|\Phi^\pm\rangle =1/\sqrt{2}(|00\rangle\pm|11\rangle)$ and $|\Psi^\pm\rangle=1/\sqrt{2}(|01\rangle\pm|10\rangle)$. Thereby we find fundamental differences in the robustness of entanglement against acceleration for these states. So are states $\Psi^\pm$ entangled for all finite accelerations, while, due to the Unruh effect, states $\Phi^\pm$ lose their free entanglement for finite accelerations. This is true for Bell states of two bosonic modes as well as for Bell states of a bosonic and a fermionic mode that are studied for the first time in this work. But also for Bell states of fermionic modes there are differences in the degradation of entanglement. We reveal the origin of these distinct characteristics of entanglement degradation and discuss the role that is played by particle statistics. Our studies suggest that the behavior of entanglement in accelerated frames heavily depends on the occupation patterns of the constituent states, whose superposition constitutes the entangled state, where especially states $\Phi^\pm$ and $\Psi^\pm$ inherit distinct characteristics regarding entanglement degradation. Finally, we point out possible implications of hovering over a black hole for these states.

14h00

Quantum data hiding, originally invented as a limitation on local operations and classical communications (LOCC) in distinguishing globally orthogonal states, is actually a phenomenon arising generically in statistics whenever comparing a 'strong' set of measurements (i.e., decision rules) with a 'weak' one. The classical statistical analogue of this would be secret sharing, in which two perfectly distinguishable multi-partite hypotheses appear to be indistinguishable when accessing only a marginal. The quantum versions are richer in that for example LOCC allows for state tomography, so the states cannot be come perfectly indistinguishable but only nearly so, and hence the question is one of efficiency. The issues covered in the talk are going to be the following:

1. Every restriction on the allowed measurements, but with arbitrary processing of the measurement data at the end, gives rise to a norm on density matrices, the "distinguishability norm"; we will review the general theory of these [Matthews/Wehner/AW, CMP 291:813-843, 2009].

2. LOCC is perhaps the most natural restriction in multi-partite systems, and we will revisit LOCC data hiding and its efficiency.

3. Gaussian operations and classical computation (GOCC): Not very surprisingly, GOCC cannot distinguish optimally even two coherent states of a single mode [Takeoka & Sasaki, PRA 78:022320, 2008].

But we can find states, each a mixture of multi-mode coherent states, which are almost perfectly distinguishable by suitable measurements, by when restricted to GOCC, i.e. linear optics and postprocessing, the states appear almost identical. The construction is random and relies on coding arguments. Open questions include whether one can give a constructive version of the argument, and whether for instance even thermal states can be used, or how efficient the hiding is.

14h45

Recently, the Automated Reasoning community has turned its attention to the area of Satisfiability Modulo Theories (SMT). In short, SMT deals with problems that are a generalization of boolean SAT problems, in the sense that we are dealing with first-order formulas and not just propositional formulas. On the other hand, the Security community has been very interested in the prevention and detection of side-channel attacks. Here, we will see how we can use SMT techniques in the mitigation of power side-channel leaks.

15h05

We present the developments in a Magneto Optical Trap (MOT) recently assembled by the Group of Quantum Plasmas - IPFN.

A distinct collective behavior is identified through a comparison between the experimental density profiles and the theory previously developed by the group. These collective processes are mediated by the multiple scattering of light, in conditions of high number of atoms and low laser detuning, which may lead to exotic phenomena such as photon bubbles, phonon lasing, classical rotons, twisted phonons, etc.

The current experimental efforts to excite high-lying Rydberg states are also discussed. We intend to investigate the Rydberg bloackade process, the building block of quantum information with neutral atoms.

Go back to the DP-PMI Day 2015 page.

**The Physics of Impossible Machines***Vlatko Vedral (**University of Oxford)*Maxwell’s demon was born in 1867 and his sole function was to illustrate some potential problems with the Second Law of thermodynamics. He still thrives in modern physics and plays an important role in clarifying connections between thermodynamics and information theory. In my talk I will present a variety of different demons which, when restricted by the Second Law, will lead to interesting consequences in electromagnetism, optics, gravity, quantum mechanics as well as quantum information. Finally, I will speculate if the concept of information could in some sense be considered deeper than the entities typically though of as fundamental in physics and will mention various efforts to derive quantum physics from simpler information theoretic axioms.

11h20

**Robustness of Spatial Quantum Search***Shantanav Chakraborty (PDEEC, IST)*A continuous-time analogue of Grover's algorithm can be formulated as a quantum walk on a complete graph of N vertices. We analyse the robustness of this algorithm against the loss of edges in the graph, as well as against static disorder. Firstly, when a few edges are removed from a complete graph, the dynamics of the algorithm is restricted to a three dimensional subspace and its optimality is retained. Subsequently, in the more general scenario where each edge of the graph remains with probability p, the underlying structure can be treated as an Erdos-Renyi random graph. We show that in the asymptotic limit, when N goes to infinity, search is optimal on almost all graphs as long as p = c N^(-z), where c is a constant and 0<=z<1. Secondly, we analyze the robustness of this algorithm to random static disorder in the diagonal and off-diagonal elements of the search Hamiltonian. We show that the algorithm retains its optimality as long as the standard deviation of the disorder is of O(N^(-1/2)).

11h40

**Degradation of Entanglement between Two Accelerated Parties***Benedikt Richter (PDF, IST)*We study the entanglement of families of Unruh modes in the Bell states $|\Phi^\pm\rangle =1/\sqrt{2}(|00\rangle\pm|11\rangle)$ and $|\Psi^\pm\rangle=1/\sqrt{2}(|01\rangle\pm|10\rangle)$. Thereby we find fundamental differences in the robustness of entanglement against acceleration for these states. So are states $\Psi^\pm$ entangled for all finite accelerations, while, due to the Unruh effect, states $\Phi^\pm$ lose their free entanglement for finite accelerations. This is true for Bell states of two bosonic modes as well as for Bell states of a bosonic and a fermionic mode that are studied for the first time in this work. But also for Bell states of fermionic modes there are differences in the degradation of entanglement. We reveal the origin of these distinct characteristics of entanglement degradation and discuss the role that is played by particle statistics. Our studies suggest that the behavior of entanglement in accelerated frames heavily depends on the occupation patterns of the constituent states, whose superposition constitutes the entangled state, where especially states $\Phi^\pm$ and $\Psi^\pm$ inherit distinct characteristics regarding entanglement degradation. Finally, we point out possible implications of hovering over a black hole for these states.

14h00

**Reflections on Quantum Data Hiding***Andreas Winter (**Universitat Autònoma de Barcelona)*Quantum data hiding, originally invented as a limitation on local operations and classical communications (LOCC) in distinguishing globally orthogonal states, is actually a phenomenon arising generically in statistics whenever comparing a 'strong' set of measurements (i.e., decision rules) with a 'weak' one. The classical statistical analogue of this would be secret sharing, in which two perfectly distinguishable multi-partite hypotheses appear to be indistinguishable when accessing only a marginal. The quantum versions are richer in that for example LOCC allows for state tomography, so the states cannot be come perfectly indistinguishable but only nearly so, and hence the question is one of efficiency. The issues covered in the talk are going to be the following:

1. Every restriction on the allowed measurements, but with arbitrary processing of the measurement data at the end, gives rise to a norm on density matrices, the "distinguishability norm"; we will review the general theory of these [Matthews/Wehner/AW, CMP 291:813-843, 2009].

2. LOCC is perhaps the most natural restriction in multi-partite systems, and we will revisit LOCC data hiding and its efficiency.

3. Gaussian operations and classical computation (GOCC): Not very surprisingly, GOCC cannot distinguish optimally even two coherent states of a single mode [Takeoka & Sasaki, PRA 78:022320, 2008].

But we can find states, each a mixture of multi-mode coherent states, which are almost perfectly distinguishable by suitable measurements, by when restricted to GOCC, i.e. linear optics and postprocessing, the states appear almost identical. The construction is random and relies on coding arguments. Open questions include whether one can give a constructive version of the argument, and whether for instance even thermal states can be used, or how efficient the hiding is.

14h45

**Mitigation of Power Side-Channel Leaks with Satisfiability Modulo Theories***Filipe Casal (PDInfoSec, IST)*Recently, the Automated Reasoning community has turned its attention to the area of Satisfiability Modulo Theories (SMT). In short, SMT deals with problems that are a generalization of boolean SAT problems, in the sense that we are dealing with first-order formulas and not just propositional formulas. On the other hand, the Security community has been very interested in the prevention and detection of side-channel attacks. Here, we will see how we can use SMT techniques in the mitigation of power side-channel leaks.

15h05

**A Cold Atoms Experiment: Collective Behavior and Rydberg Physics***João Rodrigues (PDF, IST)*We present the developments in a Magneto Optical Trap (MOT) recently assembled by the Group of Quantum Plasmas - IPFN.

A distinct collective behavior is identified through a comparison between the experimental density profiles and the theory previously developed by the group. These collective processes are mediated by the multiple scattering of light, in conditions of high number of atoms and low laser detuning, which may lead to exotic phenomena such as photon bubbles, phonon lasing, classical rotons, twisted phonons, etc.

The current experimental efforts to excite high-lying Rydberg states are also discussed. We intend to investigate the Rydberg bloackade process, the building block of quantum information with neutral atoms.

Go back to the DP-PMI Day 2015 page.