2014 Posters
Increasing ion trap capabilities: demonstrations of in vacuum control electronics, integrated diffractive optics, and ball grid arrays.*
Jason Amini, Georgia Tech Research Institute
Jason Amini, Curtis Volin, Chris Shappert, Harley Hayden, C.S. Pai, Nicholas Guise, Spencer Fallek, Kenton Brown, True Merrill, and Alexa Harter, Georgia Tech Research Institute; IEMIT collaboration: Lisa Lust, Doug Carlson, Jerry Budach, Kelly Muldoon, and Alan Cornett, Honeywell International; IDM collaboration: Dave Kielpinski, Griffith University; SMIT-BGA collaboration: Daniel Youngner and Matthew Marcus, Honeywell International. We report on three IARPA seedling projects that address issues in scaling of microfabricated ion traps to large numbers of qubits. The first project (IEMIT), in collaboration with Honeywell International, is a successful demonstration of a compact, in-vacuum 80 channel DAC system controlling a microfabricated surface-electrode ion trap. This system reduces the number of through vacuum connections by a factor of ten. Results include ion loading with 40Ca+, 70 m ion transport in the dark at 1 m/s, and a measured ion heating rate that is comparable to external DAC systems. The second project (IDM), in collaboration with Griffith University, takes multiple diffractive optical elements and etches them into the surface of a surface electrode ion trap. We demonstrate optical elements for both collimation and refocusing of light from 171Yb+. For the third project (IDM), Honeywell International is fabricating ion traps with back-side ball-grid-array connections to eliminate wirebonds and to reduce the physical die size. The first run of these traps is nearing completion and we will report on the current state of this project. * This material is based upon work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) under U.S. Army Research Office (ARO) contracts W911NF1210605 and W911NF1210600 and under Space and Naval Warfare Systems (SPAWAR) contract N6600112C2007.
Toward Quantum Communication with Qudits: Measuring Orbital Angular Momentum Entangled Photon Pairs from SPDC
Fangzhao An, Harvey Mudd College
Presenters: Fangzhao A. An and David Spierings van der Wolk Co-authors: David Berryrieser, Julien Devin, and Theresa W. Lynn We describe experimental progress in manipulating orbital angular momentum (OAM) of entangled photon pairs from spontaneous parametric down-conversion (SPDC). OAM provides an infinite-dimensional basis for encoding information in entangled states. Our present work is restricted to the OAM {-1,0,+1} subspace, enabling quantum communication with qutrit and (polarization qubit) x (OAM qutrit) states. OAM measurements are performed with forked-hologram blazed gratings produced photographically in our lab. These diffract into the first order with 37% efficiency while imparting a unit shift in OAM to the diffracted beam. Measurements in an arbitrary superposition of the OAM 0 and +/- 1 states are performed by positioning holograms in the down-converted beams and coupling the first-order diffraction into single-mode fibers. Our OAM-entanglement measurements have in the past been limited by poor mode matching due to the complex spatial mode and spectral profile from SPDC; we present current efforts to measure OAM with higher signal to noise using a multi-mode fiber prefilter in each down-converted beam.
The role of the global phase in optimal quantum control to implement partial isometries
Charles Baldwin, University of New Mexico
Controlling quantum systems is an important step towards the implementation of quantum information protocols. We consider "geometric control," whereby time-dependent waveforms modulate a set of Hamiltonians that are generators of the Lie algebra su(d) for a d-dimensional Hilbert space. In such a scenario, there is a "quantum speed limit," i.e., the minimum time that it is needed to produce a specified control task for a given set of time dependent Hamiltonians. This speed limit is typically studied for two tasks: state-to-state mappings and the implementation of a full unitary map on the Hilbert space. We study the range of intermediate cases -- partial isometries that map an under-complete set of orthogonal states to another under-complete set of orthogonal states. For full unitary control, it was recently shown that the global phase of the target unitary, restricted to root of unity phases, affects the quantum speed limit. We observe that, in the partial isometry case as well as state-to-state mappings, the idea of global phase is not restricted to root of unity phases but can take any value. This means that each control task has a range of speed limits that must be understood in order to implement the control.
Arbitrary 2D-Lattices of Ions
Todd Barrick, Sandia National Laboratories
Arbitrary 2D-Lattices of Ions Todd A. Barrick, Matthew Blain, Peter Maunz, Eric Shaner, Daniel Stick, and Craig R. Clark Sandia National Laboratories Robert Jördens, Dietrich Leibfried, and David Wineland National Institute of Standards and Technology 325 Broadway, Boulder CO 80305 A major aspect of quantum information processing and quantum simulation is the exponential growth in complexity of quantum states as the number of quantum degrees of freedom is scaled up. A cooperative effort at Sandia National Laboratory and the NIST Ion Storage Group has set out to develop ion traps that enable freely configurable quantum interactions over a two-dimensional lattice of trapped ions and have the potential to scale to ion numbers where conventional simulation of the system becomes infeasible. Our near-term goal is to design, fabricate, and test a new set of trap geometries which hold ions in a lattice of individual traps and perform entangling operations mediated by Coulomb interactions between neighboring ions. A configurable interaction between two ions in a double well has been previously demonstrated at NIST. To expand on this work, we plan on fabricating three trap geometries, a 3 and 4 well triangular lattice trap and a 7 well trap with a hexagonal ring around one central well. These designs will be tested at both room temperature and <10K. This poster will present the theoretical motivation for the project along with new trap designs and Sandia National Laboratories’ custom cryogenic chamber design. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000
Frequency translation with single ions
Francisco Benito, Sandia National Laboratories - University of New Mexico
Frequency translation with single ions Francisco Benito, Hayden McGuinness, Susan Clark, Dan Stick Sandia National Laboratories Here we present an experimental scheme to interact two ion species by creating a photonic link between them. The photons from each ion are frequency converted to an intermediate wavelength by difference frequency generation. These photons can then be interfered on a beam splitter to verify their indistinguishability. In our experiment we use single calcium and ytterbium ions trapped on separate microfabricated ion traps. This technique could have applications in hybrid quantum computing and quantum communication. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U. S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
Efficient simulation of three-level open quantum systems
Marduk Bolaños, National Autonomous University of Mexico
The Hilbert space of a system of N three-level atoms interacting with classical radiation has dimension 3^N. When spontaneous emission is taken into account, the state of the system is specified by a density matrix obtained as the solution to a master equation. That is, 9^N equations have to be solved. If the evolution of the system, unitary and non-unitary, is symmetric under the exchange of atoms, we show that the state of the system can be described with a basis of symmetric states of polynomial size. This allows for an efficient calculation of the numerical solution to the master equation and also enlarges the class of problems that can be solved analytically. Authors: Marduk Bolaños, Pablo Barberis Institute for Research on Applied Mathematics and Systems, UNAM, Mexico
Searching for quantum optimal controls in the presence of control constraints
Constantin Brif, Sandia National Laboratories
Wide success enjoyed by quantum optimal control for a variety of theoretical and experimental objectives has been attributed to the trap-free topology of the corresponding control landscapes. In this work, extensive sets of gradient searches are used to explore how the landscape topology is affected by the inevitable presence of constraints on control fields. We identify several essential control resources, including the number of control variables, control duration, and field strength, and quantify the limits on them. Exceeding these limits produces artificial local traps on the control landscape and can thereby prevent gradient searches from reaching a globally optimal solution. These results suggest that severe field constraints are the primary source of failed searches in both optimal control theory and experiments. We demonstrate how careful choice of relevant control parameters can help to eliminate artificial traps and facilitate successful optimization. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
Improved Bounds for Eigenpath Traversal
Hao-tien Chiang, University of New Mexico
We present an improved bound on the length of the path defined by the ground states of a continuous family of Hamiltonians in terms of the spectral gap δ. We use this bound to obtain a better cost of recently proposed methods for quantum adiabatic state transformations and eigenpath traversal. In particular, we prove that a method based on evolution randomization, which is a simple extension of adiabatic quantum computation, has an average cost of order 1/δ^2, and a method based on fixed-point search has a maximum cost of order 1/δ^{3/2}. Additionally, if the Hamiltonians satisfy a frustration-free property, such costs can be further improved to order 1/δ^{3/2} and 1/δ, respectively. Our methods offer an important advantage over adiabatic quantum computation when the gap is small, where the cost is of order 1/δ^3.
Studies of electric field noise near metal surfaces using a trapped ion sensor.
Nikos Daniilidis, University of California, Berkeley
Single ions are an extremely sensitive probe for oscillating electric fields in the frequency range between 100 kHz and few MHz. This allows their use to measure electric field noise near conducting surfaces in ultra high vacuum, and several experiments have found the noise to be orders of magnitude higher than expected. Recent work revealed that the noise is related to carbon contamination of the surface, and can be reduced by more than two orders of magnitude by cleaning the surface in vacuum. We report on ongoing progress in using a vacuum system which combines surface cleaning and analysis capabilities with ion trapping. We performed noise measurements, combined with surface cleaning and in-situ analysis of an aluminum-copper alloy surface. Cleaning reduced the noise by between one and two orders of magnitude, but the surface did not need to be carbon or oxide free to show low noise. An analysis of residual gases in our system revealed possible dependence of the noise on the size and type of carbon contaminants on the surface.
Weak values considered harmful
Chris Ferrie, University of New Mexico
We show using statistically rigorous arguments that the technique of weak value amplification (WVA) does not perform better than standard statistical techniques for the tasks of single parameter estimation and signal detection. Specifically we prove that post-selection, a necessary ingredient for WVA, decreases estimation accuracy and, moreover, arranging for anomalously large weak values is a suboptimal strategy. In doing so, we explicitly provide the optimal estimator, which in turn allows us to identify the optimal experimental arrangement to be the one in which all outcomes have equal weak values (all as small as possible) and the initial state of the meter is the maximal eigenvalue of the square of the system observable. Finally, we give precise quantitative conditions for when weak measurement (measurements without post-selection or anomalously large weak values) can mitigate the effect of uncharacterized technical noise in estimation.
Building quantum hybrids from wires and single ions
Dylan Gorman, University of California at Berkeley
We report experimental and theoretical progress towards constructing hybrid quantum systems from single trapped ions and solid state devices. An instructive proof-of-principle experiment is to use a wire to create entanglement between distant (d ≈ 500μm) ions. Two distant ions trapped near (≈ 100μm) a conducting wire will experience an interaction potential mediated by their Coulomb interaction with the wire. With reasonable parameters, such an experiment would generate entanglement between the motional states of two ions in about 50 ms, suggesting that coupling experiments can be constructed from the wire with heating rates already achieved in surface-electrode ion traps. This experiment is important for developing an experimental toolbox to study the interactions of ions with quantum circuits. At present, we have mounted a wire on a moveable stage above a surface-electrode ion trap. We have moved the wire to within 100μm of a single ion, and measured heating rates as the ion-wire distance is varied. The heating rates appear to remain acceptably low as the wire approaches, suggesting that a coupling experiment is immediately feasible. Current work focuses on moving to a new trap design where the wire is integrated directly into the trap for performing the first coupling experiments.
Simulation of Stochastic Quantum Systems Using Polynomial Chaos Expansions
Matthew Grace, Sandia National Labs
We present an approach to the simulation of quantum systems driven by classical stochastic processes that is based on the polynomial chaos expansion, a well-known technique in the field of uncertainty quantification. The polynomial chaos technique represents the density matrix as an expansion in orthogonal polynomials over the principle components of the stochastic process and yields a sparsely coupled hierarchy of linear differential equations. We provide practical heuristics for truncating this expansion based on results from time-dependent perturbation theory and demonstrate, via an experimentally relevant one-qubit numerical example, that our technique can be significantly more computationally efficient than Monte Carlo simulation.
Quantum Hamiltonian Learning
Christopher Granade, Institute for Quantum Computing
A long-standing problem in the development of practical quantum simulators is how to certify that a given quantum device implements a desired Hamiltonian. For devices on the 100-qubit scale, as are currently being proposed, classical simulation cannot certify the dynamics of a quantum device. Here, we address this problem by providing an algorithm that exploits trusted quantum simulation resources in order to characterize and certify the Hamiltonian dynamics of an untrusted quantum system. Moreover, our algorithm provides a powerful resource for the characterization of quantum information processing devices, thus allowing for processors to be used as resources in the development of further processors. We show that our algorithm, in some analytically-tractable cases, admits near-optimal performance. Moreover, we demonstrate analytic and numeric evidence that our algorithm is robust to sampling errors, decoherence and excluded terms. By using quantum simulation resources together with classical statistical inference techniques, our algorithm provides a powerful tool for certifying quantum simulators and for developing new quantum information processing devices.
Stochastic Master Equations in the Circuit Model
Jonathan Gross, University of New Mexico
We present a derivation of several stochastic master equations that model the trajectory of a system interacting with a continuously monitored Gaussian field. Our approach differs from previous work in using a continuum of finite-dimensional ancillary systems to model the field, allowing the derivation to make use of techniques common in quantum information and measurement theory instead of the traditional quantum-optics-based approach. In particular, we can, before taking the continuous limit, draw circuit diagrams that portray the interaction of the system with the finite-dimensional ancillas.
Controlled-Phase Gate using Rydberg-Dressed States in Cesium
Aaron Hankin, Sandia National Laboratories and University of New Mexico
We are implementing a controlled-phase gate based on trapped neutral atoms whose coupling is mediated by the dipole-dipole interaction of Rydberg states. Ground state cesium atoms are dressed by an off-resonant laser field in a manner conditional on the Rydberg blockade mechanism [1,2,3], providing the required entangling interaction. We will present the calculated controlled-phase gate fidelity for realistic experimental parameters as well as preliminary measurements of the Rydberg-dressed state interaction. Sandia National Laboratories is a multi-program laboratory managed and operated b Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] S. Rolston, et al. Phys. Rev. A, 82, 033412 (2010) [2] T. Keating, et al. Phys. Rev. A, 87, 052314 (2013) [3] A. Hankin, et al. to be published
Donor-quantum-dot qubit in silicon
Patrick Harvey-Collard, Sandia National Laboratories
We propose and show experimental progress towards a donor-quantum-dot silicon qubit. Electron spins in silicon are of increasing interest because of recent successes in demonstrating single quantum bits (qubits) including demonstrations of electron spin resonance (ESR) of a single electron spin bound to a phosphorus donor and formation of a two spin logical qubit (i.e. the singlet-triplet qubit in the m=0 subspace) using a double quantum dot in the SiGe/Si system. Silicon is of particular interest because very long decoherence times can be achieved in isotopically enriched 28Si, however, this spin depleted material also lacks any built-in magnetic field gradient or spin-orbit coupling necessary for rotating the qubit states as is utilized in GaAs and other III-V materials. Therefore, alternative methods of control are being pursued, like local inductors (e.g. for ESR), micro-magnets or more complex multi-electron logical qubit encodings (e.g. triple dots). We examine, instead, coupling a single quantum dot to a donor for which the donor nucleus provides a built-in magnetic field gradient, therefore eliminating the need for magnets, inductors or spin-orbit coupling in a very compact and natural way. In addition, it provides a qubit platform to test many ideas relevant to both Si quantum dot and donor qubit systems.
Dressed-state master equation for an optomechanical system in the ultra-strong coupling regime
Dan Hu, School of Natural Sciences, University of California, Merced
We study the open system dynamics of an optomechanical system in the ultra-strong coupling regime. In our system, the mechanical oscillator couples to a cavity mode via radiation pressure force, and the coupling strength is comparable to the mechanical frequency. The environmental degrees of freedom of both the mechanical mode and the cavity mode are modeled as bosonic baths coupling linearly with the system modes. In contrast to the standard approach to describing the effects of the environment, we derive the Lindblad master equation in the normal-mode basis of the optomechanical system (dressed states). We find that the mechanical damping in our approach depends sensitively on the state of the cavity mode. We illustrate this result using numerical results of the correlations of the cavity field, the optomechanical entanglement, and the Wigner functions.
Quantum Fisher information for states in exponential form
Zhang Jiang, University of New Mexico
We derive explicit expressions for the quantum Fisher information and the symmetric logarithmic derivative (SLD) of a quantum state in the exponential form; the SLD is expressed in terms of the generator. Applications include quantum metrology problems with Gaussian states and general thermal states. Specifically, we give the SLD for a Gaussian state in two forms, in terms of its generator and its moments; its Fisher information is also calculated with the latter form. Special cases are discussed, including pure and very noisy Gaussian states.
Mutually unbiased measurements in finite dimensions
Amir Kalev, University of New Mexico
We generalize the concept of mutually unbiased bases (MUB) to measurements which are not necessarily described by rank one projectors. As such, these measurements can be a useful tool to study the long standing problem of the existence of MUB. We derive their general form, and show that in a finite, d-dimensional Hilbert space, one can construct a complete set of d+1 mutually unbiased measurements. Beside of their intrinsic link to MUB, we show, that these measurements' statistics provide complete information about the state of the system. Moreover, they capture the physical essence of unbiasedness, and in particular, they satisfy non-trivial entropic uncertainty relation similar to d+1 MUB.
Off-resonant CPHASE Gate in Neutral Atoms
Tyler Keating, University of New Mexico
The dipole blockade effect between Rydberg atoms is a promising tool for quantum information processing in neutral atoms. There have been numerous proposals to exploit this effect in order to perform a controlled-phase quantum logic gate between two neutral atom qubits, but most use near- or on-resonance pulses to excite the Rydberg state. By instead using significantly off-resonant lasers to adiabatically dress the atomic ground states, one can make a gate that is more robust against atomic motion at finite temperature. We analyze the benefits of such a scheme as compared to near-resonance approaches and show how we can attain fidelities greater than 0.99, limited primarily by the finite lifetime of the Rydberg state. We also describe how the off-resonant dressing mechanism can be generalized to produce multi-qubit gates, such as the Toffoli gate.
Towards a rigorous link between anyonic excitations and 2D topological codes
Olivier Landon-Cardinal, Caltech
Topological codes are the best candidates for a self-correcting quantum memory. However, they are thermally unstable in 2D. The intuitive argument is that low-energy excitations are anyons whose free diffusion changes the ground state and require only finite energy. However, no formal link has been proven between generic 2D topological codes and anyons, even if it holds for all known model Hamiltonians. The thermal instability of 2D codes was thus proven using a stochastic construction. In this work, we improve it to build unitary operators which move the excitations. Our construction sheds light unto the emergence of anyons from the Hamiltonian. Joint work with David Poulin.
Progress in Quantum Information Processing with Trapped Ions at NIST
Dietrich Leibfried, National Institute of Standards and Technology
This poster will provide an overview of the progress in quantum information processing and quantum simulation with trapped ions at NIST. In particular, improvements of ion transport and cooling within a scalable architecture for quantum information processing and experiments entangling the internal states of ions held in separate trapping wells, a basic building block for quantum simulation, will be discussed.
Scalable Source of Multipartite Continuous Variable Entangled Beams of Light
Alberto Marino, University of Oklahoma
The development of efficient and scalable sources of multipartite entanglement is required for the further development of quantum information. We propose a scalable configuration based on cascaded four-wave mixing (FWM) processes for the generation of multipartite continuous variable (CV) entanglement. The FWM process is based on a double-lambda configuration in rubidium vapor and has been previously used to generate highly entangled twin beams. In the proposed configuration, one of the twin beams is used to seed another FWM process. This leads to the amplification of the beam used as the seed and the generation of an additional entangled beam of light, thus increasing the number of entangled parties by one. One of the advantages of the proposed source is that is phase insensitive, which makes it easily scalable to a large number of parties by cascading multiple FWM processes. We have experimentally verified that a configuration of two cascaded FWM processes leads to the generation of three beams that contain quantum correlations in the form of intensity-difference squeezing and show that the level of squeezing produced by the first FWM process is increased by the second one. We also derive a sufficient and necessary criterion for the presence of multipartite entanglement for the proposed configuration that shows that one should expect the beams generated by the cascaded FWM processes to be entangled.
A generalization of Schur-Weyl duality with applications in quantum estimation
Iman Marvian, University of Southern California
Schur-Weyl duality is a powerful tool in representation theory which has many applications to quantum information theory. We provide a generalization of this duality and demonstrate some of its applications. In particular, we use it to develop a general framework for the study of a family of quantum estimation problems wherein one is given n copies of an unknown quantum state according to some prior and the goal is to estimate certain parameters of the given state. In particular, we are interested to know whether collective measurements are useful and if so to find an upper bound on the amount of entanglement which is required to achieve the optimal estimation. In the case of pure states, we show that commutativity of the set of observables that define the estimation problem implies the sufficiency of unentangled measurements.
CNOT Decompositions for Clifford Operators
Adam Meier, Georgia Tech Research Institute
The Clifford group of unitary operators shows up in quantum error correction, randomized benchmarking, and many fault-tolerant techniques for quantum computing. Experimentally, any operator in this group can be implemented by multiple applications of the Hadamard, “phase” or CNOT operators on subsets of qubits. Of these generating operators, the CNOT is almost certain to have the lowest experimental fidelity, so it is worthwhile to optimize such operator decompositions to reduce the number of CNOT applications. After a brief introduction to the Clifford group, I will describe attempts to understand the structure of the Clifford group with respect to the minimal number of CNOTs needed to generate its elements. These attempts include both exhaustive characterizations of optimal decompositions for each group element and efficient algorithms for nearly optimal decompositions. This work was performed in collaboration with Emanuel Knill.
Progress towards quantum control and squeezing of collective spins
Enrique Montano, University of Arizona
We report preliminary results from an experiment that will implement quantum control of the collective spin of an atomic ensemble. In our setup, a weak probe laser interacts with a cold, trapped sample of cesium atoms with high optical depth, leading to Faraday rotation of the probe light proportional to the atomic magnetization. If the atom-light coupling is strong enough, polarimetry of the probe light will provide a QND measurement of the atomic magnetization with resolution better than the spin projection noise, at which point measurement back-action will become significant enough to be used for quantum control of the spin. Thus far, we have prepared an ensemble of atoms with high optical depth in a crossed optical dipole trap and have generated squeezing of the collective spin of the ensemble. To achieve metrologically relevant spin squeezing, we have implemented a two color probe scheme to suppresses the detrimental effects of tensor light shifts. We are now working to increase atom-light coupling in our experiment, by optimizing the 3D geometry and by using individual-atom control to prepare initial states that exhibit greater spin projection noise.
Quantum limits on Probabilistic Amplifiers
Shashank Pandey, Center for Quantum Information and Control, Department of Physics, University of New Mexico
An ideal phase-preserving linear amplifier is a deterministic device that adds to an input signal the minimal amount of noise consistent with the constraints imposed by quantum mechanics. A noiseless linear amplifier takes an input coherent state to an amplified coherent state, but only works part of the time. Such a device is actually better than noiseless, since the output has less noise than the amplified noise of the input coherent state; for this reason we refer to such devices as immaculate. We bound the working probabilities of probabilistic and approximate immaculate amplifiers and construct theoretical models that achieve some of these bounds. Our chief conclusions are the following: (i) the working probability of any phase-insensitive immaculate amplifier is very small in the phase-plane region where the device works with high fidelity; (ii) phase-sensitive immaculate amplifiers that work only on coherent states sparsely distributed on a phase-plane circle centered at the origin can have a reasonably high working probability.
Autoresonance control protocols in an open quantum system
Arjendu Pattanayak, Carleton College
A classical nonlinear oscillator can be driven to increasingly higher energy by chirping the driving frequency with a chirp rate chosen by various protocols, including one that analyzes the Teager-Kaiser energy operator. We report on the effect of applying this protocol to an open quantum system, particularly as the system size is changed so that the effective Planck's constant increases in size and the behavior becomes more quantum-mechanical. We comment on the connection with the Quantum Ladder Climbing protocol applicable in the extreme quantum limit. (Henry Luo, Ali Ehlen, Zhilu Zhang, and Arjendu Pattanayak)
Control of Quantum Chaos
Bibek Pokharel, Carleton College
We have recently computed Lyapunov exponents describing the chaotic behavior of the quantum trajectories of an open quantum nonlinear oscillator using the Quantum State Diffusion formalism. We have seen several interesting features as a function of changing system parameters. We report on progress towards controlling the observed quantum chaotic behavior using the classical Ott‐Grebogi‐Yorke protocol. [With Arjendu K. Pattanayak, Carleton College]
Progress towards experimentally realizing movable atom traps behind an array of pinholes for quantum computing
Ian Powell, California Polytechnic State University, San Luis Obispo
The neutral atom quantum computing community has successfully demonstrated all criteria for the implementation of a quantum computer except for scalability. We propose to use atoms trapped in the diffraction pattern behind a two-dimensional array of pinholes as a scalable, addressable array of quantum bits (qubits). Changing the angle of incidence of the laser beams illuminating the pinhole array will facilitate two-qubit gates by bringing pairs of atoms together and apart controllably. The current areas of focus of our work are to directly measure the pinhole diffraction pattern for laser beams at large incident angles and to experimentally achieve the transfer of rubidium atoms from a magneto-optical trap (MOT) to the pinhole traps.We have designed and built a circuit for quickly switching off the MOT magnetic field in order to transfer the cold atoms to the pinhole traps. We are building optical setups for projecting the diffraction pattern into the MOT and for characterizing the MOT cloud and pinhole traps using a high-speed camera and photodiode. We are in the process of developing a LabVIEW program for controlling the atom transfer sequence and recording images and photodiode signals of the trapped atoms.We will present the latest progress and results of our work. This work was performed in collaboration with Sanjay Khatri, Jason Schray, Taylor Shannon,Glen D. Gillen, and Katharina Gillen-Christandl (PI). We acknowledge helpful discussions with Thomas D. Gutierrez, Ivan H. Deutsch, and Marianna Safronova. This work was supported by the National Science Foundation Grant No. PHY-0855524.
Local Detection of Quantum Correlations with a Single Trapped Ion
Thaned Pruttivarasin, University of California, Berkeley
Entanglement is one of the most important feature of quantum mechanics. In small systems, full state tomography can reveal such quantum correlations between subsystems and has been implemented in modern experiments routinely. For larger system, full state tomography is time consuming and tedious. We show a realization of quantum correlations detection scheme between two subsystems, represented by two degree-of-freedoms of a single trapped ion, namely, the electronics state and motional states, by accessing only one of the subsystem. Using this protocol, we can infer a lower bound of quantum correlations between them without having to do full state tomography.
Towards space-time crystals with trapped ions
Anthony Ransford, University of California, Berkeley
Recent work has shown that spontaneous symmetry breaking can lead to a crystal not only in space but also in time [F. Wilczek, (2012)]. For instance, there exist static situations with time dependent (quasi) ground states. A proposal for constructing such a phase of matter has been presented for ions in a cylindrically symmetric RF trap with a constant magnetic field [T. Li et al, (2012)]. We present some experimental challenges towards implementing such time crystals with trapped ions. For 100 ions trapped in a ring structure with diameter of 100 micrometers, we expect a level spacing of 1 kHz between the quasi ground states and the next excited state. Thus, exceptionally low heating is required to maintain the ion ring in the quasi ground state and to study the time crystal. We discuss plans to adiabatically cool the ion ring to its motional ground state with a variable pinning potential. Zig-zag phase transitions exhibiting Kibble-Zurek topological defects might also be studied in a similar set up.
What constitutes a resource state for measurement-based quantum computation?
Eleanor Rieffel, NASA Ames Research Center
We consider the issue of what should count as a resource for measurement-based quantum computation (MBQC), and propose some minimal criteria. The term “resource” is used most frequently when discussing which states support universal MBQC and which do not [1-6]. Universality is a sufficient condition, but seems too strong as a necessary condition given known classes of MBQCs that likely give an advantage over classical computing but which are not universal [7-9]. One could try to characterize which resource states support (or do not support) computationally interesting MBQC. The problem is that, almost certainly, not all the interesting MBQC tasks are known. Instead, we concentrate on a weaker property, namely what makes families of MBQCs worthy to be called “measurement-based.” We introduce the notion of inherently measurement-based computations, and give a series of necessary conditions for families of MBQCs to be considered inherently measurement-based. We propose that for a state to be considered a resource for MBQC it must support a family of MBQCs that is inherently measurement-based. Using these criteria, we explain why discord-free states cannot be resources for MBQC, in spite of claims to the contrary [9]. We do not answer the question as to whether entanglement is required. Joint work with Howard M. Wiseman. [1] M. Van den Nest, A. Miyake, W. Du ̈r, and H. J. Briegel, Physical Review Letters 97, 150504 (2006). [2] D. Gross, J. Eisert, N. Schuch, and D. Perez-Garcia, Physical Review A 76, 052315 (2007). [3] H. Briegel, D. Browne, W. Du ̈r, R. Raussendorf, and M. Van den Nest, Nature Physics 5, 19 (2009). [4] D. Gross, S. T. Flammia, and J. Eisert, Physical Review Letters 102, 190501 (2009). [5] M. J. Bremner, C. Mora, and A. Winter, Physical Review Letters 102, 190502 (2009). [6] R. A. Low, Large deviation bounds for k-designs, arXiv:0903.5236 (2009). [7] J. Anders and D. E. Browne, Physical Review Letters 102, 050502 (2009). [8] M. J. Bremner, R. Jozsa, and D. J. Shepherd, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 467, 459 (2011). [9] M. J. Hoban, J. J. Wallman, H. Anwar, N. Usher, R. Raussendorf, and D. E. Browne, Exact sampling and entanglement-free resources for measurement-based quantum computation, arXiv:1304.2667v1 (2013).
Strong converse rates for classical communication over thermal bosonic channels
Bhaskar Roy Bardhan, Louisiana State University
We prove that several known upper bounds on the classical capacity of thermal bosonic channels are actually strong converse rates. Our results strengthen the interpretation of these upper bounds, in the sense that we now know that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds these upper bounds. In order for these theorems to hold, we need to impose a maximum photon number constraint on the states input to the channel (the strong converse property need not hold if there is only a mean photon number constraint). Our first theorem demonstrates that a capacity upper bound due to Koenig and Smith is a strong converse rate, and we prove this result by utilizing their structural decomposition of a thermal channel into a pure-loss channel followed by an amplifier channel. Our second theorem demonstrates that an upper bound due to Giovannetti et al. corresponds to a strong converse rate, and we prove this result by relating success probability to rate, the effective dimension of the output space, and the purity of the channel as measured by the Renyi collision entropy.
Optimal phase estimation in the presence of dephasing noise using photon number and parity measurements
Kaushik Seshadreesan, Louisiana State University
We study interferometric phase estimation in the presence of dephasing noise using photon number, and photon number parity, measurements. We show that both the above measurements can achieve phase sensitivities at the quantum Cramer-Rao bound of the optimal probe state preparation. Furthermore, we show that when operated using a Bayesian update protocol, photon number measurement can be made optimally sensitive to phase fluctuations independently of the actual value of the unknown phase.
Towards a Quantum Memory with Telecom-wavelength Conversion
Daniel Stack, United States Army Research Laboratory
Fiber-based transmission of quantum information over long distances may be achieved using quantum memory elements and quantum repeater protocols. However, atom-based quantum memories typically involve interactions with light fields outside the telecom window needed to minimize absorption in transmission by optical fibers. We report on progress towards a quantum memory based on the generation of 780 nm spontaneously emitted single photons by a write-laser beam interacting with a cold 87Rb ensemble. The single photons are then frequency-converted into (out of) the telecomm band via difference (sum) frequency generation in a PPLN crystal. Finally, the atomic state is read out via the interaction of a read-pulse with the quantum memory. With such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.
Progress towards attaining an equidistant ion chain in an annular segmented surface ion trap
Boyan Tabakov, Sandia National Laboratories and University of New Mexico
Over almost a decade, a primary drive for developing microfabricated segmented surface electrode ion traps has been the application of trapped ions as a quantum information processing platform. At Sandia National Laboratories we design, fabricate, and test such traps, the utility of which extends beyond the realm of quantum computation. One highly symmetric design that has the potential to provide periodic boundary conditions allowing studies of quantum phase transitions, or could be a testbed for observing Hawking radiation from acoustic black holes, is the ring trap. We demonstrate forming a ring of hundreds of Ca+ ions in that trap, and report on the challenges and our progress towards attaining equal spacing in the ion chain. In the absence of undesired stray fields, we envision the ions separated by their mutual Coulomb repulsion and moving on the ring.
Trapped Atoms and Polarimetry in a Nanofiber-based Quantum Interface
Kyle Taylor, University of Arizona
D. Melchior, K. Taylor, P. G. Mickelson, and P. S. Jessen We describe an experiment that will use the evanescent-wave field of a tapered optical fiber (nanofiber) to trap cold atoms and control their collective spin. When probe laser light interacts with a trapped atomic sample with high optical depth, the probe light undergoes Faraday rotation proportional to the atomic magnetization. If the atom-light coupling is strong enough, polarimetry of the probe light will provide a measurement of the magnetization with resolution better than the spin projection noise, at which point measurement back-action can be used for quantum control of the spin. When atoms are trapped in the evanescent mode of a nanofiber, probe light traveling through the nanofiber is particularly well mode-matched to the atom sample, and high optical depth on the order of 10^2 is expected. Here, we report experimental progress towards loading cold atoms samples into these nanofiber traps.
Quantum simulation of chemical systems based on the sparsity of the CI-matrix
Borzu Toloui, Haverford College/ visting Harvard University
Quantum chemistry is an area where quantum simulation algorithms can make considerable contributions in science and technology. The majority of algorithms for simulating electronic structures to date have used a second-quantized representation of the respective Hamiltonian. The qubit requirements for such algorithms that scale linearly with the maximum number of orbitals that are included in the problem. However, storing the full Fock space of the orbitals is unnecessary because the number of electrons is a fixed and known parameter of the problem. Representing the wave function in a basis of slater determinants for fixed electron number suffices. We show how to apply techniques developed for the simulation of sparse Hamiltonians to the CI-matrix that is expressed in such basis. We show that it is possible to use the minimal number of qubits to represent the wave function. We also show that these methods can offer improved scaling in the number of gates required by cleverly exploiting the structure of the CI-matrix.
Distinguishability of Qudit Hyperentangled States with Linear Evolution and Local Measurement
Andrew Turner, Harvey Mudd College
Measurement of an entangled state in the Bell state basis is an integral part of many protocols in quantum communication. Of particular interest is measurement that uses only linear evolution and local measurement (LELM). Previous work has shown that for two identical particles entangled in n qubits, 2^(n+1)-1 classes of the 4^n hyperentangled Bell states can be distinguished using an LELM device. I will present recent progress on the Bell state distinguishability problem for general hyperentangled states using LELM. This includes limits on distinguishability for the qutrit and (qubit) x (qutrit) entangled states of two particles.
General relativistic quantum information
James van Meter, National Institute of Standards and Technology
Relativistic quantum information theory is an emerging field concerned with new phenomena and methods that may emerge from a fully relativistic treatment of quantum information theory. Of particular interest are the effects of curved spacetime, which may for example have measurable effects on quantum communication with satellites. Here we consider the sensitivity of quantum devices to the gravitational field, and the potential for relativistic quantum metrology.
Microwave shot noise and quantum motional sideband asymmetry in an electro-mechanical device
Aaron Weinstein, California Institute of Technology
A quantum harmonic oscillator has an asymmetric position noise spectral density between the positive and negative frequencies, which is the direct outcome of the Heisenberg uncertainty principle. Here, we report a measurement of the up and down-converted sidebands of a radio-frequency mechanical resonator parametrically coupled to a super-conducting microwave transducer. By accounting for the classical microwave noise in the device, we measure a sideband imbalance of 1.2+-0.2 quanta at mechanical occupations near the ground state. Finally, we show that the interpretation of this imbalance must incorporate the type of detection scheme used in the measurement. For amplitude detection of the sidebands presented here, the asymmetry arises solely from the quantum fluctuations of the microwave field, not of the mechanics, and shows good agreement with the imbalance observed in this measurement.
Nonlinear Analog Quantum Computation
Thomas Wong, University of California, San Diego
Extensive experimental work has shown that the effect of any fundamental nonlinear generalization of quantum mechanics must be tiny. Nevertheless, there are quantum mechanical systems with multiple interacting particles whose effective evolution is governed by a nonlinear Schrödinger equation with a term proportional to f(|ψ|2)ψ. This includes the Gross-Pitaevskii equation with a cubic nonlinearity that describes Bose-Einstein condensates, the cubic-quintic nonlinear Schrödinger equation that describes light propagation in nonlinear Kerr media with defocusing corrections, and the logarithmic nonlinear Schrödinger equation that describes Bose liquids under certain conditions. We quantify the computational speedup that this general nonlinearity has in solving the unstructured search problem. In doing so, we identify a host of physically realistic nonlinear quantum systems that can be used to perform continuous-time computation faster than (linear) quantum computation, up to a bound on the size of the problem such that the nonlinear equation is a good approximation of the linear dynamics of the system, unless the nonlinearity is fundamental.
