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7234 discussions

[Aspuru-Guzik Group List] [qip] talk in 30 minutes

by Aram Harrow

Dear quanta, a reminder that Anurag is speaking today at 10:30. > Monday, December 16, 2019 > 10:30am > 6C-442 > > Title: Bounds on entanglement entropy of ground states and scar states > > speaker: Anurag Anshu (University of Waterloo) > abstract: > Relevant many-body quantum states, such as gapped ground states and gibbs > state, can be approximated as polynomials of the underlying hamiltonian. > This simple idea has been remarkably useful; such as in the proofs of 1D > area laws and PEPS representation of gibbs state. In this talk, we provide > two new applications. First we show a subvolume law for frustration-free > locally gapped spin systems in two dimensions, following the well known > Approximate ground state projector (AGSP) approach. Our bound shows an > entanglement entropy bound of approx L^{5/3}, for vertical and rectangular > bipartitions of perimeter L. Second, we show that recently studied scar > quantum states have subvolume bound on entanglement entropy on lattices of > finite dimension, complementing a recent result of Alhambra and Wilming. > Based on joint works with Itai Arad and David Gosset and with Alvaro > Alhambra. > _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] [qip] group meeting and seminar tomorrow

by Aram Harrow

Dear quanta, Tomorrow we will meet at 11am in 6-310 and go around the room and give updates. This will be our last group meeting for the semester. In the afternoon we will have a talk which gives a sense of how up for grabs the basic categories of quantum computers still are. Friday, Dec 13, 2019, 1:30pm, 6C-442 Laser-free trapped-ion quantum logic using an oscillating magnetic field gradient at radio frequency Raghavendra Srinivas (NIST & U.C. Boulder) Trapped-ion quantum logic is usually performed using laser-induced coupling of the ions’ internal spin states to their motion. Laser-free spin-motion coupling methods, which eliminate photon scattering errors and offer benefits for scalability, have been proposed and demonstrated using static magnetic field gradients or magnetic field gradients oscillating at GHz frequencies [1-4]. We present a new method of spin-motion coupling for trapped ions without lasers, instead using microwaves and a magnetic field gradient oscillating at radio frequency [5]. We demonstrate and characterize this method with trapped Mg+ ions, cooling a single mode of motion to close to its ground state, and generating entangled states of two ions. This implementation offers important technical advantages over other laser-free techniques, while also enabling laser-free entangling gates with reduced sensitivity to qubit frequency errors [6]. These experiments are performed in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave fields and the oscillating magnetic field gradient. Currently, we achieve a Bell-state fidelity of 0.996(2) with ground-state-cooled ions and 0.991(3) for ions cooled to the Doppler limit (nbar = 2). [1] Mintert and Wunderlich PRL 87, 257904 (2001) [2] Weidt et al. PRL 117, 220501 (2016) [3] Ospelkaus et al. Nature 476, 181 (2011) [4] Harty et al. PRL 117, 140501 (2016) [5] Srinivas et al. PRL 122, 163201 (2019) [6] Sutherland et al. NJP 21, 033033 (2019) _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] [qip] two talks next week: Mon 10:30 and Fri 1:30

by Aram Harrow

Dear quanta, We will have two talks next week. We will also have a group meeting Friday at 11 where we go around the room and give updates. This will be our last group meeting for the year. The talks are 1. Mon, 10:30, Jonathan Oppenheim A post-quantum theory of classical gravity? 2. Fri, 1:30, Raghavendra Srinivas Laser-free trapped-ion quantum logic using an oscillating magnetic field gradient at radio frequency Here is more detail. 1. Monday, December 9, 2019 10:30am 6C-442 title: A post-quantum theory of classical gravity? speaker: Jonathan Oppenheim (University College London) abstract: We present a consistent theory of classical systems coupled to quantum ones. The dynamics is linear in the density matrix, completely positive and trace-preserving. We apply this to construct a theory of classical gravity coupled to quantum field theory. The theory doesn't suffer the pathologies of semi-classical gravity and reduces to Einstein's equations in the appropriate limit. The assumption that gravity is classical necessarily modifies the dynamical laws of quantum mechanics -- the theory must be fundamentally information destroying involving finite sized and stochastic jumps in space-time and in the quantum field. Nonetheless the quantum state of the system can remain pure conditioned on the classical degrees of freedom. The measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical space-time necessarily causes collapse of the wave-function. The theory can be regarded as fundamental, or as an effective theory of quantum field theory in curved space where backreaction is consistently accounted for. 2. Friday, Dec 13, 2019 1:30pm 6C-442 Laser-free trapped-ion quantum logic using an oscillating magnetic field gradient at radio frequency Raghavendra Srinivas (NIST & U.C. Boulder) Trapped-ion quantum logic is usually performed using laser-induced coupling of the ions’ internal spin states to their motion. Laser-free spin-motion coupling methods, which eliminate photon scattering errors and offer benefits for scalability, have been proposed and demonstrated using static magnetic field gradients or magnetic field gradients oscillating at GHz frequencies [1-4]. We present a new method of spin-motion coupling for trapped ions without lasers, instead using microwaves and a magnetic field gradient oscillating at radio frequency [5]. We demonstrate and characterize this method with trapped Mg+ ions, cooling a single mode of motion to close to its ground state, and generating entangled states of two ions. This implementation offers important technical advantages over other laser-free techniques, while also enabling laser-free entangling gates with reduced sensitivity to qubit frequency errors [6]. These experiments are performed in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave fields and the oscillating magnetic field gradient. Currently, we achieve a Bell-state fidelity of 0.996(2) with ground-state-cooled ions and 0.991(3) for ions cooled to the Doppler limit (nbar = 2). [1] Mintert and Wunderlich PRL 87, 257904 (2001) [2] Weidt et al. PRL 117, 220501 (2016) [3] Ospelkaus et al. Nature 476, 181 (2011) [4] Harty et al. PRL 117, 140501 (2016) [5] Srinivas et al. PRL 122, 163201 (2019) [6] Sutherland et al. NJP 21, 033033 (2019) _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] [qip] tomorrow and upcoming talks

by Aram Harrow

Dear quanta, We will cancel group meeting tomorrow because there is a crypto seminar that may be of interest to many of us. At 1:30pm in 6C-442, Adam will tell us about his recent work (with me and Nicole) on macroscopic Bell inequalities. Next Monday we will have a talk by Jonathan Oppenheim at 10:30am in 6C-442 on his proposal for classical gravity. It should be accessible to QI people. The following Monday (Dec 16) will be Anurag Anshu at 10:30am speaking about his recent paper on a subvolume law in 2D systems. Details below. Tomorrow's morning talk. https://toc.csail.mit.edu/node/1360 Dhiraj Holden: No-Signaling Proofs with sqrt(log⁡ n) Provers is in PSPACE Friday, December 6, 2019 - 10:30am to 12:00pm Location: Hewlett, G882 Abstract: No-signaling proofs, motivated by quantum computation, have found applications in cryptography and hardness of approximation. An important open problem is characterizing the power of no-signaling proofs. It is known that 2-prover no-signaling proofs are characterized by PSPACE, and that no-signaling proofs with poly(n)-provers are characterized by EXP. However, the power of k-prover no-signaling proofs, for 2 < k < poly(n) remained an open problem. We show that k-prover no-signaling proofs (with negligible soundness) for k = √(log⁡ n) are contained in PSPACE. We prove this via two different routes that are of independent interest. In both routes we consider a relaxation of no-signaling called sub-no-signaling. Our main technical contribution (which is used in both our proofs) is a reduction showing how to convert any sub-no-signaling strategy with value at least 1 - 2^(-Omega(k^2)) into a no-signaling one with value at least 2^-O(k^2). In the first route, we show that the classical prover reduction method for converting k-prover games into 2-prover games carries over to the no-signaling setting with the following loss in soundness: if a k-player game has value less than 2^(−ck^2) (for some constant c>0), then the corresponding 2-prover game has value at most 1−2(-dk^2) (for some constant d>0). In the second route we show that the value of a sub-no-signaling game can be approximated in space that is polynomial in the communication complexity and exponential in the number of provers. Afternoon talk 1:30pm, Fri, Dec 6, 6C-442 Adam Bene Watts Nonlinear Bell inequality for macroscopic measurements https://arxiv.org/abs/1911.09122 -aram _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] [qip] talk on Mon, Dec 2

by Aram Harrow

Dear quanta, There is no meeting this week because of Thanksgiving. Next week, we will have a special seminar on Monday by Marissa Giustina on Google's hardware. --------- Monday December 2, 2019 11:00 6C-442 Title: Building Google’s Quantum Computer Presenter: Marissa Giustina, Senior Research Scientist and Quantum Electronics Engineer, Google LLC. Abstract: The Google AI Quantum team develops chip-based circuitry that one can interact with (control and read out) and which behaves reliably according to a simple quantum model. Such quantum hardware holds promise as a platform for tackling problems intractable for classical computing hardware. While the demonstration of a universal, fault-tolerant, quantum computer remains a goal for the future, it has informed the design of a prototype with which we have recently controlled a quantum system of unprecedented scale. This talk introduces Google’s quantum computing effort from both hardware and quantum-information perspectives, including an overview of recent technological developments and recent results. _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] [qip] talks this week

by Aram Harrow

Wed at 1pm we will have a remote talk from Ken Brown about our STAQ collaboration. This will be a broad overview of work on quantum algorithms and architectures. The Thursday physics colloquium (4pm, 10-250) should interest some of us. It is "Quantum Simulation of Abelian and non-Abelian Gauge Theories" by Uwe-Jens Wiese. https://web.mit.edu/physics/events/colloquia.html Fri at 11am we will have Yi-Xiang Liu from the Cappellaro group speaking at the group meeting about q simulation. Below are her title & abstract. Title: High-fidelity Trotter expansions for digital quantum simulation Abstract: Quantum simulation promises to address many challenges in fields ranging from quantum chemistry to material science and high-energy physics, and could be implemented in noisy intermediate scale quantum devices. A challenge in building good digital quantum simulators is the fidelity of the engineered dynamics given a finite set of elementary operations. The goal of this work is to find a proper ordering of elementary operations (Trotter expansion), so that they approximate as well as possible the desired evolution. However, when the quantum system is large, even calculating one elementary operation is computationally expensive. In this talk I will introduce a geometric framework for optimizing the order of operations without considering the details of the operations themselves, thus achieving computational efficiency. Based on the geometric framework, I will present two alternative orderings. One has optimal fidelity at a short time scale, and the other one is robust at a long time scale. Thanks to the improved fidelity at different time scales, the two different orderings can form the basis for experimental-constrained digital quantum simulation. https://arxiv.org/abs/1903.01654 _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

4 years, 5 months

[Aspuru-Guzik Group List] Special Seminar - TODAY @ 3PM - Jeremy Young

by Ploucha, Clare

HQI Special Seminar Thursday, November 21 3:00 PM, Jefferson 356 Jeremy Young (UMD), Postdoc Candidate Driven-dissipative coupled Ising models: a new non-equilibrium universality class Driven-dissipative systems can potentially exhibit non-equilibrium phenomena that are absent in their equilibrium counterparts. However, phase transitions present in these systems generically exhibit an effectively classical, equilibrium behavior in spite of their non-equilibrium origin. To illustrate this, I will begin by showing how the driven-dissipative Bose-Hubbard model gives rise to emergent thermal behavior near the critical point. I will then investigate an experimentally-motivated model where two Ising-like order parameters interact and form a multicritical point and discuss how at such a multicritical point, new non-equilibrium criticality can emerge. These non-equilibrium multicritical points exhibit a variety of exotic phenomena with no counterpart in equilibrium, including spiraling phase boundaries, the emergence of discrete scale invariance rather than the more familiar continuous scale invariance, and the violation of the fluctuation-dissipation theorem at all length scales, resulting in a sytem which becomes “hotter” and “hotter” at longer and longer wavelengths. Finally, I will discuss some future directions based on these results, such as non-equilibrium quantum criticality.

4 years, 5 months

[Aspuru-Guzik Group List] HQI Special Seminar - TODAY @ 12PM - Prof. Vahid Sandoghdar

by Ploucha, Clare

HQI Special Seminar Wednesday, November 20 12 PM, Jefferson 250 Lunch will be served Prof. Vahid Sandoghdar, Max Planck Institute for the Science of Light Efficiency in the Interaction of Light and Matter: From Nano-quantum Optics to Nanobiophotonics Light-matter interaction at the nanometer scale lies at the heart of elementary optical processes such as absorption, emission or scattering. Over the past two decades, we have realized a series of experiments to investigate the interaction of single photons, single molecules and single nanoparticles. In this presentation, I will report on recent studies, where we reach unity efficiency in the coupling of single photons to single molecules and describe our efforts to exploit this for the realization of polaritonic states involving a controlled number of molecules and photons. Furthermore, I will show how the underlying mechanisms that play a central role in quantum optics, help image and track single biological nanoparticles such as viruses and small proteins with high spatial and temporal resolutions. -- Clare Ploucha Director of Programs Harvard Quantum Initiative 18 Hammond Street, Palfrey 105 Cambridge, MA 02138

4 years, 5 months

[Aspuru-Guzik Group List] Special Seminar - Thursday, November 21 @ 3PM - Jeremy Young

by Ploucha, Clare

4 years, 6 months

[Aspuru-Guzik Group List] HQI Special Seminar - Wednesday, November 20 @ 12PM - Prof. Vahid Sandoghdar

by Ploucha, Clare

4 years, 6 months

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