Recursos de colección

Caltech Authors (155.447 recursos)

Repository of works by Caltech published authors.

Group = IQIM

Mostrando recursos 1 - 20 de 101

  1. Anisotropic exchange and spin-wave damping in pure and electron-doped Sr_2IrO_4

    Pincini, D.; Vale, J. G.; Donnerer, C.; de la Torre, A.; Hunter, E. C.; Perry, R.; Moretti Sala, M.; Baumberger, F.; McMorrow, D. F.
    The collective magnetic excitations in the spin-orbit Mott insulator (Sr_(1−x)La_x)_2IrO_4 (x=0, 0.01, 0.04, 0.1) were investigated by means of resonant inelastic x-ray scattering. We report significant magnon energy gaps at both the crystallographic and antiferromagnetic zone centers at all doping levels, along with a remarkably pronounced momentum-dependent lifetime broadening. The spin-wave gap is accounted for by a significant anisotropy in the interactions between J_(eff)=1/2 isospins, thus marking the departure of Sr_2IrO_4 from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.

  2. Nanophotonic rare-earth quantum memory with optically controlled retrieval

    Zhong, Tian; Kindem, Jonathan M.; Bartholomew, John G.; Rochman, Jake; Craiciu, Ioana; Miyazono, Evan; Bettinelli, Marco; Cavalli, Enrico; Verma, Varun; Nam, Sae Woo; Marsili, Francesco; Shaw, Matthew D.; Beyer, Andrew D.; Faraon, Andrei
    Optical quantum memories are essential elements in quantum networks for long distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of its readout time. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at...

  3. An equality between entanglement and uncertainty

    Berta, Mario; Coles, Patrick J.; Wehner, Stephanie
    Heisenberg's uncertainty principle implies that if one party (Alice) prepares a system and randomly measures one of two incompatible observables, then another party (Bob) cannot perfectly predict the measurement outcomes. This implication assumes that Bob does not possess an additional system that is entangled to the measured one; indeed the seminal paper of Einstein, Podolsky and Rosen (EPR) showed that maximal entanglement allows Bob to perfectly win this guessing game. Although not in contradiction, the observations made by EPR and Heisenberg illustrate two extreme cases of the interplay between entanglement and uncertainty. On the one hand, no entanglement means that...

  4. Rényi divergences as weighted non-commutative vector valued L_p-spaces

    Berta, Mario; Scholz, Volkher B.; Tomamichel, Marco
    We show that Araki and Masuda's weighted non-commutative vector valued L_p-spaces [Araki & Masuda, Publ. Res. Inst. Math. Sci., 18:339 (1982)] correspond to an algebraic generalization of the sandwiched Rényi divergences with parameter α=p/2. Using complex interpolation theory, we prove various fundamental properties of these divergences in the setup of von Neumann algebras, including a data processing inequality and monotonicity in α. We thereby also give new proofs for the corresponding finite-dimensional properties. We discuss the limiting cases α→{12,1,∞} leading to minus the logarithm of Uhlmann's fidelity, Umegaki's relative entropy, and the max-relative entropy, respectively. As a contribution that might...

  5. Gaussian hypothesis testing and quantum illumination

    Wilde, Mark M.; Tomamichel, Marco; Lloyd, Seth; Berta, Mario
    Quantum hypothesis testing is one of the most basic tasks in quantum information theory and has fundamental links with quantum communication and estimation theory. In this paper, we establish a formula that characterizes the decay rate of the minimal Type-II error probability in a quantum hypothesis test of two Gaussian states given a fixed constraint on the Type-I error probability. This formula is a direct function of the mean vectors and covariance matrices of the quantum Gaussian states in question. We give an application to quantum illumination, which is the task of determining whether there is a low-reflectivity object embedded...

  6. Disentanglement Cost of Quantum States

    Berta, Mario; Majenz, Christian
    We show that the minimal rate of noise needed to catalytically erase the entanglement in a bipartite quantum state is given by the regularized relative entropy of entanglement. This offers a solution to the central open question raised in [Groisman et al., PRA 72, 032317 (2005)] and complements their main result that the minimal rate of noise needed to erase all correlations is given by the quantum mutual information.

  7. Fixed Points of Wegner-Wilson Flows and Many-Body Localization

    Pekker, David; Clark, Bryan K.; Oganesyan, Vadim; Refael, Gil
    Many-body localization (MBL) is a phase of matter that is characterized by the absence of thermalization. Dynamical generation of a large number of local quantum numbers has been identified as one key characteristic of this phase, quite possibly the microscopic mechanism of breakdown of thermalization and the phase transition itself. We formulate a robust algorithm, based on Wegner-Wilson flow (WWF) renormalization, for computing these conserved quantities and their interactions. We present evidence for the existence of distinct fixed point distributions of the latter: a Gaussian white-noise-like distribution in the ergodic phase, a 1/f law inside the MBL phase, and scale-free...

  8. Quantum Markov chains and logarithmic trace inequalities

    Sutter, David; Berta, Mario; Tomamichel, Marco
    A Markov chain is a tripartite quantum state ρABC where there exists a recovery map RB→BC such that ρABC = RB→BC(ρAB). More generally, an approximate Markov chain ρABC is a state whose distance to the closest recovered state RB→BC(ρAB) is small. Recently it has been shown that this distance can be bounded from above by the conditional mutual information I(A : C|B)ρ of the state. We improve on this connection by deriving the first bound that is tight in the commutative case and features an explicit recovery map that only depends on the reduced state pBC. The key tool in...

  9. A meta-converse for private communication over quantum channels

    Wilde, Mark M.; Tomamichel, Marco; Berta, Mario
    We establish a converse bounds on the private transmission capabilities of a quantum channel. The main conceptual development builds firmly on the notion of a private state, which is a powerful, uniquely quantum method for simplifying the tripartite picture of privacy involving local operations and public classical communication to a bipartite picture of quantum privacy involving local operations and classical communication. This approach has previously led to some of the strongest upper bounds on secret key rates, including the squashed entanglement and the relative entropy of entanglement. Here we use this approach along with a “privacy test” to establish a...

  10. Symmetry-protected topological order at nonzero temperature

    Roberts, Sam; Yoshida, Beni; Kubica, Aleksander; Bartlett, Stephen D.
    We address the question of whether symmetry-protected topological (SPT) order can persist at nonzero temperature, with a focus on understanding the thermal stability of several models studied in the theory of quantum computation. We present three results in this direction. First, we prove that nontrivial SPT order protected by a global onsite symmetry cannot persist at nonzero temperature, demonstrating that several quantum computational structures protected by such onsite symmetries are not thermally stable. Second, we prove that the three-dimensional (3D) cluster-state model used in the formulation of topological measurement-based quantum computation possesses a nontrivial SPT-ordered thermal phase when protected by...

  11. Towards the Fundamental Quantum Limit of Linear Measurements of Classical Signals

    Miao, Haixing; Adhikari, Rana X.; Ma, Yiqiu; Pang, Belinda; Chen, Yanbei
    The quantum Cramér-Rao bound (QCRB) sets a fundamental limit for the measurement of classical signals with detectors operating in the quantum regime. Using linear-response theory and the Heisenberg uncertainty relation, we derive a general condition for achieving such a fundamental limit. When applied to classical displacement measurements with a test mass, this condition leads to an explicit connection between the QCRB and the standard quantum limit that arises from a tradeoff between the measurement imprecision and quantum backaction; the QCRB can be viewed as an outcome of a quantum nondemolition measurement with the backaction evaded. Additionally, we show that the...

  12. Finite correlation length implies efficient preparation of quantum thermal states

    Brandão, Fernando G. S. L.; Kastoryano, Michael J.
    Preparing quantum thermal states on a quantum computer is in general a difficult task. We provide a procedure to prepare a thermal state on a quantum computer with a logarithmic depth circuit of local quantum channels assuming that the thermal state correlations satisfy the following two properties: (i) the correlations between two regions are exponentially decaying in the distance between the regions, and (ii) the thermal state is an approximate Markov state for shielded regions. We require both properties to hold for the thermal state of the Hamiltonian on any induced subgraph of the original lattice. Assumption (ii) is satisfied...

  13. Deconstruction and conditional erasure of quantum correlations

    Berta, Mario; Brandão, Fernando G. S. L.; Majenz, Christian; Wilde, Mark M.
    We define the deconstruction cost of a tripartite quantum state on systems ABE as the minimum rate of noise needed to apply to the AE systems, such that there is negligible disturbance to the marginal state on the BE systems and the system A of the resulting state is locally recoverable from the E system alone. We refer to such actions as deconstruction operations and protocols implementing them as state deconstruction protocols. State deconstruction generalizes Landauer erasure of a single-party quantum state as well the erasure of correlations of a two-party quantum state. We find that the deconstruction cost of...

  14. Entanglement area laws for long-range interacting systems

    Gong, Zhe-Xuan; Foss-Feig, Michael; Brandão, Fernando G. S. L.; Gorshkov, Alexey V.
    We prove that the entanglement entropy of any state evolved under an arbitrary 1/r^α long-range-interacting D-dimensional lattice spin Hamiltonian cannot change faster than a rate proportional to the boundary area for any α>D+1. We also prove that for any α>2D+2, the ground state of such a Hamiltonian satisfies the entanglement area law if it can be transformed along a gapped adiabatic path into a ground state known to satisfy the area law. These results significantly generalize their existing counterparts for short-range interacting systems, and are useful for identifying dynamical phase transitions and quantum phase transitions in the presence of long-range...

  15. Finite-size scaling of out-of-time-ordered correlators at late times

    Huang, Yichen; Brandão, Fernando G. S. L.; Zhang, Yong-Liang
    Chaotic dynamics in quantum many-body systems scrambles local information so that at late times it can no longer be accessed locally. This is reflected quantitatively in the out-of-time-ordered correlator of local operators which is expected to decay to zero with time. However, for systems of finite size, out-of-time-ordered correlators do not decay exactly to zero and we show in this paper that the residue value can provide useful insights into the chaotic dynamics. In particular, we show that when energy is conserved, the late-time saturation value of out-of-time-ordered correlators for generic traceless local operators scales inverse polynomially with the system...

  16. BQP-completeness of Scattering in Scalar Quantum Field Theory

    Jordan, Stephen P.; Krovi, Hari; Lee, Keith S. M.; Preskill, John
    Recent work has shown that quantum computers can compute scattering probabilities in massive quantum field theories, with a run time that is polynomial in the number of particles, their energy, and the desired precision. Here we study a closely related quantum field-theoretical problem: estimating the vacuum-to-vacuum transition amplitude, in the presence of spacetime-dependent classical sources, for a massive scalar field theory in (1+1) dimensions. We show that this problem is BQP-hard; in other words, its solution enables one to solve any problem that is solvable in polynomial time by a quantum computer. Hence, the vacuum-to-vacuum amplitude cannot be accurately estimated by any efficient classical algorithm, even if the field theory...

  17. Achieving the Heisenberg limit in quantum metrology using quantum error correction

    Zhou, Sisi; Zhang, Mengzhen; Preskill, John; Jiang, Liang
    We study the fundamental limits on precision for parameter estimation using a quantum probe subject to Markovian noise. The best possible scaling of precision $\delta$ with the total probing time t is the Heisenberg limit (HL) $\delta \propto 1/t$, which can be achieved by a noiseless probe, but noise can reduce the precision to the standard quantum limit (SQL) $\delta \propto 1 /\sqrt{t}$. We find a condition on the probe's Markovian noise such that SQL scaling cannot be surpassed if the condition is violated, but if the condition is satisfied then HL scaling can be achieved by using quantum error correction to protect the probe from damage, assuming...

  18. Entanglement entropy from SU(2) Chern-Simons theory and symmetric webs

    Chun, Sungbong; Bao, Ning
    A path integral on a link complement of a three-sphere fixes a vector (the "link state") in Chern-Simons theory. The link state can be written in a certain basis with the colored link invariants as its coefficients. We use symmetric webs to systematically compute the colored link invariants, by which we can write down the multi-partite entangled state of any given link. It is still unknown if a product state necessarily implies that the corresponding components are unlinked, and we leave it as a conjecture.

  19. A quantum linearity test for robustly verifying entanglement

    Natarajan, Anand; Vidick, Thomas
    We introduce a simple two-player test which certifies that the players apply tensor products of Pauli σ_X and σ_Z observables on the tensor product of n EPR pairs. The test has constant robustness: any strategy achieving success probability within an additive of the optimal must be poly(ε)-close, in the appropriate distance measure, to the honest n-qubit strategy. The test involves 2n-bit questions and 2-bit answers. The key technical ingredient is a quantum version of the classical linearity test of Blum, Luby, and Rubinfeld. As applications of our result we give (i) the first robust self-test for n EPR pairs; (ii) a...

  20. Hardness amplification for entangled games via anchoring

    Bavarian, Mohammad; Vidick, Thomas; Yuen, Henry
    We study the parallel repetition of one-round games involving players that can use quantum entanglement. A major open question in this area is whether parallel repetition reduces the entangled value of a game at an exponential rate - in other words, does an analogue of Raz's parallel repetition theorem hold for games with players sharing quantum entanglement? Previous results only apply to special classes of games. We introduce a class of games we call anchored. We then introduce a simple transformation on games called anchoring, inspired in part by the Feige-Kilian transformation, that turns any (multiplayer) game into an anchored game....

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