ITP-quantumseminare

itp-quantumseminare@lists.phys.ethz.ch

1 participants
1357 discussions

no meeting tomorrow / new time next week
by Joe Renes 07 Sep '20

07 Sep '20

Hi all, We have a little break in the calendar this week --- no meeting tomorrow. Next week we will start at the new time for the fall semester, 2pm Tuesdays, with a talk by Alberto Rolandi. He'll report on his master thesis with Henrik. I'll send around the details of the talk next week. Best, -joe

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tomorrow's group meeting 1500
by Joe Renes 31 Aug '20

31 Aug '20

Hi all, Tomorrow Yanglin Hu will tell us about his semester project, entitled "Confined Relativistic Quantum Clock". See below for the abstract. The zoom link is https://ethz.zoom.us/j/362994444. Best, -joe Title: Confined Relativistic Quantum Clock Abstract: A potentially detectable quantum modification to time in delocalised clocks are proposed previously in order to examine the correctness of quantum gravity theory. Here we aim to further investigate the possibility to design an experiment. We derive the Hamiltonian of a relativistic quantum clock in the electromagnetic field in the Rindler spacetime. Based on that, we calculate the time dilation of a quantum clock in a Penning trap. The numerical result shows that the time dilation is detectable for a confined quantum clock. We also explore the effect of decoherence via a heat bath of harmonic oscillators on a free quantum clock. The decoherence does not change the time average of the clock. Results till now show that designing an experiment to detect the quantum modification to time are a possibility.

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tomorrow's group meeting at 15:00
by Joe Renes 24 Aug '20

24 Aug '20

Hi all, Tomorrow Sven Jandura will tell us about his semester project at IBM, entitled "Time Propagation of a Wave Packet using a Variational Quantum Simulator". See below for the abstract. The zoom link is https://ethz.zoom.us/j/362994444. Best, -joe Abstract: Quantum computers can speed up the calculation of the time evolution of quantum states, a task that is classically hard because just storing the wavefunction of a state of multiple particles requires resources exponential in the number of particles. Here we present a variational hybrid quantum-classical algorithm based on McLachlans variational principle that approximates the time evolution of particles moving in arbitrary potentials. We then numerically study the performance of this algorithm on a free Gaussian wave packet in one dimension and the scattering of a Gaussian wave packet on an Eckart barrier. We find that for a free particle the number of parameters in the variational form used by us can be significantly smaller than the real dimension of the Hilber space, while for the Eckart barrier it has to be close to the real dimension of the Hilbert space to obtain an high fidelity approximation.

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tomorrow's group meeting 1500
by Joe Renes 17 Aug '20

17 Aug '20

Hi all, Tomorrow Boxi Li will tell us about his master thesis at TU Delft, entitled "Efficient Optimization of Cut-offs in Quantum Repeater Chains". See below for the abstract. Zoom link: https://ethz.zoom.us/j/362994444. Best, -joe Abstract: Quantum communication enables the implementation of tasks that are unachievable with classical resources. However, losses on the communication channel preclude the direct long-distance transmission of quantum information in many relevant scenarios. In principle quantum repeaters allow one to overcome losses. However, realistic hardware parameters make long-distance quantum communication a challenge in practice. For instance, in many protocols an entangled pair is generated that needs to wait in quantum memory until the generation of an additional pair. During this waiting time the first pair decoheres, impacting the quality of the final entanglement produced. At the cost of a lower rate, this effect can be mitigated by imposing a cut-off condition. For instance, a maximum storage time for entanglement after which it is discarded. In this work, we optimize the cut-offs for quantum repeater chains. First, we develop an algorithm for computing the probability distribution of the waiting time and fidelity of entanglement produced by repeater chain protocols which include a cut-off. Then, we use the algorithm to optimize cut-offs in order to maximize secret-key rate between the end nodes of the repeater chain. We find that the use of the optimal cut-off extends the parameter regime for which secret key can be generated and moreover significantly increases the secret-key rate for a large range of parameters. arXiv preprint: http://arxiv.org/abs/2005.04946

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tomorrow's talks at 15:00
by Joe Renes 10 Aug '20

10 Aug '20

Hi all, Tomorrow we again have two master thesis talks, this time by Romain Moyard and Christian Bertoni. Romain worked with Raban on circuit optimization, and Christian worked with me on information theory and renormalization. Titles and abstracts for their talks are below. Here's the zoom link: https://ethz.zoom.us/j/362994444 Best, -joe ---------------- Romain Moyard ~ Title: Implementation, improvement and testing of an efficient template matching algorithm Abstract: Given a large and a small quantum circuit, an algorithm to find all maximal matches of the small circuit, called template, in the large circuit under consideration of pairwise commutation relations between quantum gates was suggested by Iten et al. in arXiv:1909.05270. In this talk, we present the result of the implementation of this algorithm into the software library Qiskit-terra. We check its correctness by comparing it with another, less efficient, algorithm that finds all the maximal matches. We find two kinds of cases that were not correctly handled in the efficient matching algorithm and correct them. Our implementation shows that the average time complexity of the algorithm scales much better than the theoretical worst-case time complexity. In addition, we present different heuristics to speed up the matching algorithm with the trade-off of probably not finding all maximal matches. We apply the matching algorithm for circuit optimization on random circuits and also on benchmark circuits that are already heavily optimized. In both cases, we can significantly improve the implementation cost of the circuits. Finally we present an adapted version of the algorithm that finds all maximal gate sequences acting on a given qubit set in a circuit. This is promising for peephole optimization. Preprint: arXiv:1909.05270 ------ Christian Bertoni ~ Title: Information theory and renormalization in statistical mechanics Abstract: Renormalization in statistical mechanics is a process in which a system is mapped to a different scale. It arises naturally when considering phase transitions, since statistical systems exhibit scale invariance at criticality. It is natural to think that this process eliminates information, and that it should have an information theoretic interpretation. In this talk I will present two different ways in which tools from information theory can be applied to the problem of renormalization, focusing on the question of whether one can select an appropriate renormalization map by using simple information theoretic criteria.

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tomorrow's group meeting at 15:00
by Joe Renes 03 Aug '20

03 Aug '20

Hi all, Tomorrow we are back from the summer break with two master thesis talks (of 25 minutes each). The speakers are Ludovico Machet, who worked with Jinzhao, and Tim Möbus, who worked with David. See below for titles and abstracts. We start as usual at 1500 on zoom, at https://ethz.zoom.us/j/362994444. Best, -joe ++ Ludovico Machet: "On the gravitational action and horizon entropy in Causal Set Theory”. Abstract: The formulation of a quantum theory of gravity has been one of the major challenges for theoretical physicists in the last century. In this framework, the Causal Set Theory (CST) approach saw recent and rapid progress with the discovery of a discrete equivalent for the Einstein-Hilbert action. In this talk, after a rapid introduction of the CST formalism, I will describe the derivation of the causal set action, illustrate its continuum limit and how it can extract geometric information from the causal set structure. I will then focus on the presence of boundary terms in the continuum limit of the action and extend the discussion to a general curved space time. I will show that the action of a causal diamond in a Riemann normal neighbor effectively localizes to the null-null joint. I will then discuss how the information encoded in a causal set can be useful to define a kinematical gravitational entropy. I will first consider the Horizon Molecules proposal, then I will introduce the Spacetime Mutual Information concept. I will argue that the SMI follows an area law when evaluated on causal diamonds cut by causal horizons. This result is promising in the search for a quantity giving a kinematical, then dynamical, discrete entropy for causal horizons in CST. Preprint: https://arxiv.org/abs/2007.13192 ++ Tim Möbus: "On Chain Rules for Quantum Rényi Divergences". Abstract: The chain rule of the Rényi divergence, in its classical form, decomposes the divergence on a multipartite system into a sum of conditional divergences with respect to certain subsystems. The aim of this master thesis is to recap the existing chain rule results for the minimal and maximal quantum Rényi divergences, compare them, and discuss the role of the regularisation. Especially, the case $\alpha=\frac{1}{2}$ is investigated and counterexamples for the minimal divergence show the non-additivity of the channel divergence and the necessity of the regularisation of the chain rule. For that purpose, we introduce semi-definite programs and prove a SDP representation for the channel divergence.

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tomorrow's group meeting @ 1500
by Joe Renes 13 Jul '20

13 Jul '20

Hi all, Tomorrow Giulia Mazzola will tell us about her master thesis at IBM, on "Simulating Yang-Mills Lattice Gauge Theories on Digital Quantum Computers". See below for the abstract. Here's the link: https://ethz.zoom.us/j/362994444. Best, -joe Abstract: Gauge theories represent the most successful description of elementary particles and their fundamental interactions. In computing the real-time dynamics in lattice gauge theories however, standard classical numerical methods suffer from an exponential scaling of the required resources with growing system size and thus, display severe limitations for the simulation of real-time phenomena. Quantum computers might provide a potential framework to tackle this problem as lattice gauge theories can be efficiently represented using resources which are polynomial in the system size. Here, we review different encoding schemes for non-abelian Yang-Mills lattice gauge theories with dynamical fermionic matter such as quantum chromodynamics in arbitrary dimensions on digital circuit-based quantum computers. On the other hand, we apply promising variational quantum algorithms for near-term quantum computation to study the groundstate properties of small abelian lattice gauge theory systems like quantum electrodynamics in (1+1)- and (2+1)-spacetime dimensions. Specifically, we investigate the performance of physically motivated variational forms that preserve the gauge symmetry of the theory, thereby allowing for an efficient sampling in the physical Hilbert space of states. Therewith, we demonstrate the phenomenon of flux-string breaking in form of a phase diagram and further present the results of simulations on real superconducting quantum hardware as a proof-of-principle. By generalizing this scheme to non-abelian Yang-Mills theories, such gauge invariant variational forms might be included in variational real-time evolution algorithms, providing a potential path towards simulating real-time dynamics of lattice gauge theories on near-term quantum devices.

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tomorrow's group meeting at 3pm
by Joe Renes 06 Jul '20

06 Jul '20

Hi all, Tomorrow Anne-Catherine de la Hamette will tell us about her master thesis with Thomas Galley at Perimeter Institute, entitled "Quantum reference frames: a relational approach to quantum theory". See the abstract below for more details. Zoom link: https://ethz.zoom.us/j/362994444. Best, -joe Abstract Reference frames are essential in the description of physical phenomena. Often used implicitly, they appear as idealized classical systems. Following a more fundamental approach and considering them as physical systems subject to the laws of quantum mechanics, they become quantum reference frames (QRFs). A rigorous treatment of QRFs is indispensable both in the construction of a relational theory of quantum mechanics and of quantum gravity. Not only do the properties of QRFs give rise to numerous applications in quantum information; they are also expected to improve our understanding of observer-dependent settings such as the Wigner's friend experiment. In this talk, we present a relational formalism based on the group and representation theoretic nature of reference frames. By identifying coordinate systems with elements of a symmetry group G, we define a general operator for reversibly transforming between QRFs associated to the group G. We explore the properties of this operator in different cases and show how it gives rise to transformations between coordinate systems which are `in a superposition' relative to other coordinate systems. Finally, we apply the relational formalism and the change of reference system developed in this work to the Wigner's friend thought experiment.

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tomorrow's group meeting 1500
by Joe Renes 29 Jun '20

29 Jun '20

Hi all, Tomorrow Ladina Hausmann will tell us about her semester project with Nuriya and Lídia, "On multi-agent logical paradoxes in epistemically-restricted hidden variable theories". See below for the abstract. The zoom link is https://ethz.zoom.us/j/362994444. Best, -joe Abstract: The Frauchiger-Renner paradox shows that inconsistencies appear when multiple agents reason about each other in quantum mechanics [1]. But is this a distinct feature of quantum theory, or can other physical theories exhibit similar paradoxes? We know that it is possible to find a paradoxical setting in box world, a theory less local than quantum mechanics, for a specific model of physical agents [2]. In this project we investigate multi-agent logical paradoxes in Spekkens' toy theory. Spekkens' toy theory exhibits many phenomena of quantum mechanics, but is a non-contextual local hidden variable theory [3]. The quantum-like effects stem from an epistemic restriction akin to the uncertainty principle, and a measurement disturbance rule. We were able to prove that there is no Frauchiger-Renner-like paradox in Spekkens' toy theory. In addition, we refined the formalism of Spekkens' toy theory and were able to model agents' physical memories and deterministic reasoning. In this talk, I will give an introduction to Spekkens' toy theory. Additionally, I will present how memory update and reasoning work in the toy theory. In the end, I'll discuss the Frauchiger-Renner paradox in Spekkens' toy theory. [1] Daniela Frauchiger and Renato Renner. Quantum theory cannot consistently describe the use of itself. Nature Communications, 9(1), sep 2018. doi:10.1038/s41467-018-05739-8. [2] V Vilasini, Nuriya Nurgalieva, and Lídia del Rio. Multi-agent paradoxes beyond quantum theory. New J. Phys., 21(11):113028, nov 2019. doi:10.1088/1367-2630/ab4fc4. [3] Robert W. Spekkens. Evidence for the epistemic view of quantum states: A toy theory. Physical Review A, 75(3), mar 2007. doi:10.1103/physreva.75.032110.

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tomorrow's group meeting at 1500
by Joe Renes 22 Jun '20

22 Jun '20

Hi all, Tomorrow we'll hear from Gillen Beck on "Conditional Mutual Information in Quantum Circuits", his semester project with David Sutter. See below for the abstract. Meeting link: https://ethz.zoom.us/j/362994444. Best, -joe Abstract: For a tripartite quantum state ρABC, the conditional mutual information is an entropic value which quantifies the correlations between subsystems A and C, conditioned on B. When this value is zero, the state can be considered aquantum Markov chain, and there exists a map which can precisely recover ρABC from ρAB alone. A related map exists for states with small, but nonzero, conditional mutual information, which approximately recovers ρABC, with the recovery fidelity approaching unity as the conditional mutual information approaches zero. Motivated by the potential of universal state recovery in the context of qubit loss and noisy circuits, this project examined the evolution of conditional mutual information through a few standard circuits: the quantum Fourier transform, phase estimation, and Grover's search. I first present the general findings, which suggest that recoverable settings can arise naturally in quantum circuits. Lastly, I discuss both the potential application of the recovery map as a new approach to circuit optimization, as well as the next steps required to determine its viability.

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ITP-quantumseminare