## Recursos de colección

#### Caltech Authors (165.044 recursos)

Repository of works by Caltech published authors.

Group = Institute for Quantum Information and Matter

1. #### Floquet Engineering in Quantum Chains

Kennes, D. M.; de la Torre, A.; Ron, A.; Hsieh, D.; Millis, A. J.
We consider a one-dimensional interacting spinless fermion model, which displays the well-known Luttinger liquid (LL) to charge density wave (CDW) transition as a function of the ratio between the strength of the interaction, $U$, and the hopping, $J$. We subject this system to a spatially uniform drive which is ramped up over a finite time interval and becomes time-periodic in the long time limit. We show that by using a density matrix renormalization group (DMRG) approach formulated for infinite system sizes, we can access the large-time limit even when the drive induces finite heating. When both the initial and long-time states are in the gapless (LL) phase, the...

2. #### Subradiant states of quantum bits coupled to a one-dimensional waveguide

Albrecht, Andreas; Henriet, Loïc; Asenjo-Garcia, Ana; Dieterle, Paul B.; Painter, Oskar; Chang, Darrick E.
The properties of coupled emitters can differ dramatically from those of their individual constituents. Canonical examples include sub- and super-radiance, wherein the decay rate of a collective excitation is reduced or enhanced due to correlated interactions with the environment. Here, we systematically study the properties of collective excitations for regularly spaced arrays of quantum emitters coupled to a one-dimensional (1D) waveguide. We find that, for low excitation numbers, the modal properties are well-characterized by spin waves with a definite wavevector. Moreover, the decay rate of the most subradiant modes obeys a universal scaling with a cubic suppression in the number...

3. #### Superconducting metamaterials for waveguide quantum electrodynamics

Mirhosseini, Mohammad; Kim, Eunjong; Ferreira, Vinicius S.; Kalaee, Mahmoud; Sipahigil, Alp; Keller, Andrew J.; Painter, Oskar
The embedding of tunable quantum emitters in a photonic bandgap structure enables the control of dissipative and dispersive interactions between emitters and their photonic bath. Operation in the transmission band, outside the gap, allows for studying waveguide quantum electrodynamics in the slow-light regime. Alternatively, tuning the emitter into the bandgap results in finite range emitter-emitter interactions via bound photonic states. Here we couple a transmon qubit to a superconducting metamaterial with a deep sub-wavelength lattice constant ($\lambda/60$). The metamaterial is formed by periodically loading a transmission line with compact, low loss, low disorder lumped element microwave resonators. We probe the coherent and dissipative dynamics of the system by measuring the...

4. #### Multiple-scale structures: from Faraday waves to soft-matter quasicrystals

Savitz, Samuel; Babadi, Mehrtash; Lifshitz, Ron
For many years, quasicrystals were only observed as solid-state metallic alloys, yet current research is actively exploring their formation in a variety of soft materials, including systems of macromolecules, nanoparticles, and colloids. Much effort is being invested in understanding the thermodynamic properties of these soft-matter quasicrystals in order to predict and possibly control the structures that form, and to hopefully shed light on the broader, yet unresolved, general questions of quasicrystal formation and stability. Moreover, the ability to control the self-assembly of soft quasicrystals may contribute to the development of novel photonic or other applications based on self-assembled metamaterials. Here we...

5. #### Beyond heat baths II: framework for generalized thermodynamic resource theories

Yunger Halpern, Nicole
Thermodynamics, which describes vast systems, has been reconciled with small scales, relevant to single-molecule experiments, in resource theories. Resource theories have been used to model exchanges of energy and information. Recently, particle exchanges were modeled; and an umbrella family of thermodynamic resource theories was proposed to model diverse baths, interactions, and free energies. This paper motivates and details the family's structure and prospective applications. How to model electrochemical, gravitational, magnetic, and other thermodynamic systems is explained. Szilárd's engine and Landauer's Principle are generalized, as resourcefulness is shown to be convertible not only between information and gravitational energy, but also among...

6. #### Evidence of an Improper Displacive Phase Transition in Cd_2Re_2O_7 via Time-Resolved Coherent Phonon Spectroscopy

Harter, J. W.; Kennes, D. M.; Chu, H.; de la Torre, A.; Zhao, Z. Y.; Yan, J.-Q.; Mandrus, D. G.; Millis, A. J.; Hsieh, D.
We have used a combination of ultrafast coherent phonon spectroscopy, ultrafast thermometry, and time-dependent Landau theory to study the inversion symmetry breaking phase transition at T_c=200  K in the strongly spin-orbit coupled correlated metal Cd_2Re_2O_7. We establish that the structural distortion at T_c is a secondary effect through the absence of any softening of its associated phonon mode, which supports a purely electronically driven mechanism. However, the phonon lifetime exhibits an anomalously strong temperature dependence that decreases linearly to zero near T_c. We show that this behavior naturally explains the spurious appearance of phonon softening in previous Raman spectroscopy experiments and...

7. #### Practical device-independent quantum cryptography via entropy accumulation

Arnon-Friedman, Rotem; Dupuis, Frédéric; Fawzi, Omar; Renner, Renato; Vidick, Thomas
Device-independent cryptography goes beyond conventional quantum cryptography by providing security that holds independently of the quality of the underlying physical devices. Device-independent protocols are based on the quantum phenomena of non-locality and the violation of Bell inequalities. This high level of security could so far only be established under conditions which are not achievable experimentally. Here we present a property of entropy, termed “entropy accumulation”, which asserts that the total amount of entropy of a large system is the sum of its parts. We use this property to prove the security of cryptographic protocols, including device-independent quantum key distribution, while...

8. #### Fermion condensation and super pivotal categories

Aasen, David; Lake, Ethan; Walker, Kevin
We study fermionic topological phases using the technique of fermion condensation. We give a prescription for performing fermion condensation in bosonic topological phases which contain a fermion. Our approach to fermion condensation can roughly be understood as coupling the parent bosonic topological phase to a phase of physical fermions, and condensing pairs of physical and emergent fermions. There are two distinct types of objects in fermionic theories, which we call "m-type" and "q-type" particles. The endomorphism algebras of q-type particles are complex Clifford algebras, and they have no analogues in bosonic theories. We construct a fermionic generalization of the tube...

9. #### Absence of thermalization in finite isolated interacting Floquet systems

Seetharam, Karthik; Titum, Paraj; Kolodrubetz, Michael; Refael, Gil
Conventional wisdom suggests that the long-time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal-entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy nonthermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit nonthermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even with no static...

10. #### Absence of thermalization in finite isolated interacting Floquet systems

Seetharam, Karthik; Titum, Paraj; Kolodrubetz, Michael; Refael, Gil
Conventional wisdom suggests that the long-time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal-entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy nonthermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit nonthermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even with no static...

11. #### Snowflake phononic topological insulator at the nanoscale

Brendel, Christian; Peano, Vittorio; Painter, Oskar; Marquardt, Florian
We show how the snowflake phononic crystal structure, which recently has been realized experimentally, can be turned into a topological ins ulator for mechanical waves. This idea, based purely on simple geometrical modifications, could be readily implemented on the nanoscale.

12. #### Equivalence between contextuality and negativity of the Wigner function for qudits

Delfosse, Nicolas; Okay, Cihan; Bermejo-Vega, Juan; Browne, Dan E.; Raussendorf, Robert
Understanding what distinguishes quantum mechanics from classical mechanics is crucial for quantum information processing applications. In this work, we consider two notions of non-classicality for quantum systems, negativity of the Wigner function and contextuality for Pauli measurements. We prove that these two notions are equivalent for multi-qudit systems with odd local dimension. For a single qudit, the equivalence breaks down. We show that there exist single qudit states that admit a non-contextual hidden variable model description and whose Wigner functions are negative.

13. #### Recoupling Coefficients and Quantum Entropies

Christandl, Matthias; Sahinoğlu, M. Burak; Walter, Michael
We prove that the asymptotic behavior of the recoupling coefficients of the symmetric group S_k is characterized by a quantum marginal problem: they decay polynomially in k if there exists a quantum state of three particles with given eigenvalues for their reduced density operators and exponentially otherwise. As an application, we deduce solely from symmetry considerations of the coefficients the strong subadditivity property of the von Neumann entropy, first proved by Lieb and Ruskai (J Math Phys 14:1938–1941, 1973). Our work may be seen as a non-commutative generalization of the representation-theoretic aspect of the recently found connection between the quantum...

14. #### Recoupling Coefficients and Quantum Entropies

Christandl, Matthias; Sahinoğlu, M. Burak; Walter, Michael
We prove that the asymptotic behavior of the recoupling coefficients of the symmetric group S_k is characterized by a quantum marginal problem: they decay polynomially in k if there exists a quantum state of three particles with given eigenvalues for their reduced density operators and exponentially otherwise. As an application, we deduce solely from symmetry considerations of the coefficients the strong subadditivity property of the von Neumann entropy, first proved by Lieb and Ruskai (J Math Phys 14:1938–1941, 1973). Our work may be seen as a non-commutative generalization of the representation-theoretic aspect of the recently found connection between the quantum...

15. #### Dynamics of many-body localization in the presence of particle loss

van Nieuwenburg, Evert; Yago Malo, Jorge; Daley, Andrew J.; Fischer, Mark Hannes
At long times, residual couplings to the environment become relevant even in the most isolated experiments, a crucial difficulty for the study of fundamental aspects of many-body dynamics. A particular example is many-body localization in a cold-atom setting, where incoherent photon scattering introduces both dephasing and particle loss. Whereas dephasing has been studied in detail and is known to destroy localization already on the level of non-interacting particles, the effect of particle loss is less well understood. A difficulty arises due to the 'non-local' nature of the loss process, complicating standard numerical tools using matrix product decomposition. Utilizing symmetries of...

16. #### Dynamics of many-body localization in the presence of particle loss

van Nieuwenburg, Evert; Yago Malo, Jorge; Daley, Andrew J.; Fischer, Mark Hannes
At long times, residual couplings to the environment become relevant even in the most isolated experiments, a crucial difficulty for the study of fundamental aspects of many-body dynamics. A particular example is many-body localization in a cold-atom setting, where incoherent photon scattering introduces both dephasing and particle loss. Whereas dephasing has been studied in detail and is known to destroy localization already on the level of non-interacting particles, the effect of particle loss is less well understood. A difficulty arises due to the 'non-local' nature of the loss process, complicating standard numerical tools using matrix product decomposition. Utilizing symmetries of...

17. #### Explicit construction of quasiconserved local operator of translationally invariant nonintegrable quantum spin chain in prethermalization

Lin, Cheng-Ju; Motrunich, Olexei I.
We numerically construct translationally invariant quasiconserved operators with maximum range M, which best commute with a nonintegrable quantum spin chain Hamiltonian, up to M = 12. In the large coupling limit, we find that the residual norm of the commutator of the quasiconserved operator decays exponentially with its maximum range M at small M, and turns into a slower decay at larger M. This quasiconserved operator can be understood as a dressed total “spin-z” operator, by comparing with the perturbative Schrieffer-Wolff construction developed to high order reaching essentially the same maximum range. We also examine the operator inverse participation ratio...

18. #### Quantum information in quantum cognition

Yunger Halpern, Nicole; Crosson, Elizabeth
Matthew Fisher recently postulated a mechanism by which quantum phenomena could influence cognition: Phosphorus nuclear spins may resist decoherence for long times. The spins would serve as biological qubits. The qubits may resist decoherence longer when in Posner molecules. We imagine that Fisher postulates correctly. How adroitly could biological systems process quantum information (QI)? We establish a framework for answering. Additionally, we apply biological qubits in quantum error correction, quantum communication, and quantum computation. First, we posit how the QI encoded by the spins transforms as Posner molecules form. The transformation points to a natural computational basis for qubits in Posner molecules....

19. #### Characterization of surface-plasmon polaritons at lossy interfaces

Sang-Nourpour, Nafiseh; Lavoie, Benjamin R.; Kheradmand, R.; Rezaei, M.; Sanders, Barry C.
We characterize surface-plasmon polaritons at lossy planar interfaces between one dispersive and one nondispersive linear isotropic homogeneous media, i.e. materials or metamaterials. Specifically, we solve Maxwell's equations to obtain strict bounds for the permittivity and permeability of these media, such that satisfying these bounds implies surface-plasmon polaritons successfully propagate at the interface, and violation of the bounds impedes propagation, i.e. the field delocalizes from the surface into the bulk. Our characterization of surface-plasmon polaritons is valuable for checking the viability of a proposed application, and, as an example, we employ our method to falsify a previous prediction that surface-plasmon propagation...

20. #### Fast optimization algorithms and the cosmological constant

Bao, Ning; Bousso, Raphael; Jordan, Stephen; Lackey, Brad
Denef and Douglas have observed that in certain landscape models the problem of finding small values of the cosmological constant is a large instance of a problem that is hard for the complexity class NP (Nondeterministic Polynomial-time). The number of elementary operations (quantum gates) needed to solve this problem by brute force search exceeds the estimated computational capacity of the observable Universe. Here we describe a way out of this puzzling circumstance: despite being NP-hard, the problem of finding a small cosmological constant can be attacked by more sophisticated algorithms whose performance vastly exceeds brute force search. In fact, in...

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