
Miao, G.  X., & Moodera, J. S. (2015). Spin manipulation with magnetic semiconductor barriers. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 17(2), 751–761.
Abstract: Magnetic semiconductors are a class of materials with special spinfiltering capabilities with magnetically tunable energy gaps. Many of these materials also possess another intrinsic property: indirect exchange interaction between the localized magnetic moments and the adjacent free electrons, which manifests as an extremely large effective magnetic field applying only on the spin degrees of freedom of the free electrons. Novel device concepts can be created by taking advantage of these properties. We discuss in the article the basic principles of these phenomena, and potential ways of applying them in constructing spintronic devices.


Ying, M., Li, Y., Yu, N., & Feng, Y. (2014). ModelChecking LinearTime Properties of Quantum Systems. ACM TRANSACTIONS ON COMPUTATIONAL LOGIC, 15(3).
Abstract: We define a formal framework for reasoning about lineartime properties of quantum systems in which quantum automata are employed in the modeling of systems and certain (closed) subspaces of state Hilbert spaces are used as the atomic propositions about the behavior of systems. We provide an algorithm for verifying invariants of quantum automata. Then, an automatabased modelchecking technique is generalized for the verification of safety properties recognizable by reversible automata and omegaproperties recognizable by reversible Buchi automata.


Piani, M., Narasimhachar, V., & Calsamiglia, J. (2014). Quantumness of correlations, quantumness of ensembles and quantum data hiding. NEW JOURNAL OF PHYSICS, 16.
Abstract: We study the quantumness of correlations for ensembles of biand multipartite systems and relate it to the task of quantum data hiding. Quantumness is here intended in the sense of minimum average disturbance under local measurements. We consider a very general framework, but focus on local complete von Neumann measurements as the cause of the disturbance, and, later on, on the tracedistance as a quantifier of the disturbance. We discuss connections with entanglement and previously defined notions of quantumness of correlations. We prove that a large class of quantifiers of the quantumness of correlations are entanglement monotones for pure bipartite states. In particular, we define an entanglement of disturbance for pure states, for which we give an analytical expression. Such a measure coincides with negativity and concurrence for the case of two qubits. We compute general bounds on disturbance for both single states and ensembles, and consider several examples, including the uniform Haar ensemble of pure states, and pairs of qubit states. Finally, we show that the notion of ensemble quantumness of correlations is most relevant in quantum data hiding. Indeed, while it is known that entanglement is not necessary for a good quantum datahiding scheme, we prove that ensemble quantumness of correlations is necessary.


Huber, T., Predojevic, A., Khoshnegar, M., Dalacu, D., Poole, P. J., Majedi, H., et al. (2014). Polarization Entangled Photons from Quantum Dots Embedded in Nanowires. NANO LETTERS, 14(12), 7107–7114.
Abstract: In this Letter, we present entanglement generated from a novel structure: a single InAsP quantum dot embedded in an InP nanowire. These structures can grow in a sitecontrolled way and exhibit high collection efficiency; we detect 0.5 million biexciton counts per second coupled into a single mode fiber with a standard commercial avalanche photo diode. If we correct for the known setup losses and detector efficiency, we get an extraction efficiency of 15(3) %. For the measured polarization entanglement, we observe a fidelity of 0.76(2) to a reference maximally entangled state as well as a concurrence of 0.57(6).


Moussa, O., Hincks, I., & Cory, D. G. (2014). Preparing and preserving the double quantum coherence in NV centers in Diamond at low fields. JOURNAL OF MAGNETIC RESONANCE, 249, 24–31.
Abstract: We present and demonstrate a simple idea to excite and preserve the doublequantumcoherence (DQC) in the ground state of the electron spin of the Nitrogenvacancy (NV) color center in diamond. We measure the coherence time of the DQC and compare it to the single quantum coherence time, both, in a Ramsey fringe experiment and under a Hahn echo sequence. We also demonstrate a robust pulse sequence based on the DANTE pulse sequence for selectively isolating the signal from the electron transitions conditional on the state of the alwayspresent Nitrogen spin. (C) 2014 Elsevier Inc. All rights reserved.


Koenig, R., & Smolin, J. A. (2014). How to efficiently select an arbitrary Clifford group element. JOURNAL OF MATHEMATICAL PHYSICS, 55(12).
Abstract: We give an algorithm which produces a unique element of the Clifford group on n qubits (Cn) from an integer 0 <= i < Cn (the number of elements in the group). The algorithm involves O(n(3)) operations and provides, in addition to a canonical mapping from the integers to group elements g, a factorization of g into a sequence of at most 4n symplectic transvections. The algorithm can be used to efficiently select random elements of Cn which are often useful in quantum information theory and quantum computation. We also give an algorithm for the inverse map, indexing a group element in time O(n(3)). (C) 2014 AIP Publishing LLC.


Marvian, I., & Spekkens, R. W. (2014). Modes of asymmetry: The application of harmonic analysis to symmetric quantum dynamics and quantum reference frames. PHYSICAL REVIEW A, 90(6).
Abstract: Finding the consequences of symmetry for opensystem quantum dynamics is a problem with broad applications, including describing thermal relaxation, deriving quantum limits on the performance of amplifiers, and exploring quantum metrology in the presence of noise. The symmetry of the dynamics may reflect a symmetry of the fundamental laws of nature or a symmetry of a lowenergy effective theory, or it may describe a practical restriction such as the lack of a reference frame. In this paper, we apply some tools of harmonic analysis together with ideas from quantum information theory to this problem. The central idea is to study the decomposition of quantum operationsin particular, states, measurements, and channelsinto different modes, which we call modes of asymmetry. Under symmetric processing, a given mode of the input is mapped to the corresponding mode of the output, implying that one can only generate a given output if the input contains all of the necessary modes. By defining monotones that quantify the asymmetry in a particular mode, we also derive quantitative constraints on the resources of asymmetry that are required to simulate a given asymmetric operation. We present applications of our results for deriving bounds on the probability of success in nondeterministic state transitions, such as quantum amplification, and a simplified formalism for studying the degradation of quantum reference frames.


Grusdt, F., Shashi, A., Abanin, D., & Demler, E. (2014). Bloch oscillations of bosonic lattice polarons. PHYSICAL REVIEW A, 90(6).
Abstract: We consider a singleimpurity atom confined to an optical lattice and immersed in a homogeneous BoseEinstein condensate (BEC). Interaction of the impurity with the phonon modes of the BEC leads to the formation of a stable quasiparticle, the polaron. We use a variational meanfield approach to study dispersion renormalization and derive equations describing nonequilibrium dynamics of polarons by projecting equations of motion into meanfieldtype wave functions. As a concrete example, we apply our method to study dynamics of impurity atoms in response to a suddenly applied force and explore the interplay of coherent Bloch oscillations and incoherent drift. We obtain a nonlinear dependence of the drift velocity on the applied force, including a subOhmic dependence for small forces for dimensionality d > 1 of the BEC. For the case of heavy impurity atoms, we derive a closed analytical expression for the drift velocity. Our results show considerable differences with the commonly used phenomenological EsakiTsu model.


Namiki, R., Gittsovich, O., Guha, S., & Luetkenhaus, N. (2014). Gaussianonly regenerative stations cannot act as quantum repeaters. PHYSICAL REVIEW A, 90(6).
Abstract: Higher transmission loss diminishes the performance of optical communicationbe it the rate at which classical or quantum data can be sent reliably, or the secure key generation rate of quantum key distribution (QKD). Loss compounds with distanceexponentially in an optical fiber, and inverse square with distance for a freespace channel. In order to boost classical communication rates over long distances, it is customary to introduce regenerative relays at intermediate points along the channel. It is therefore natural to speculate whether untended regenerative stations, such as phaseinsensitive or phasesensitive optical amplifiers, could serve as repeaters for longdistance QKD. The primary result of this paper rules out all bosonic Gaussian channels to be useful as QKD repeaters, which include phaseinsensitive and phasesensitive amplifiers as special cases, for any QKD protocol. We also delineate the conditions under which a Gaussian relay renders a lossy channel entanglement breaking, which in turn makes the channel useless for QKD.


Jain, N., Anisimova, E., Khan, I., Makarov, V., Marquardt, C., & Leuchs, G. (2014). Trojanhorse attacks threaten the security of practical quantum cryptography. NEW JOURNAL OF PHYSICS, 16.
Abstract: A quantum key distribution (QKD) system may be probed by an eavesdropper Eve by sending in bright light from the quantum channel and analyzing the backreflections. We propose and experimentally demonstrate a setup for mounting such a Trojanhorse attack. We show it in operation against the quantum cryptosystem Clavis2 from ID Quantique, as a proofofprinciple. With just a few backreflected photons, Eve discerns Bob's (secret) basis choice, and thus the raw key bit in the ScaraniAcinRibordyGisin 2004 protocol, with higher than 90% probability. This would clearly breach the security of the cryptosystem. Unfortunately, Eve's bright pulses have a side effect of causing a high level of afterpulsing in Bob's singlephoton detectors, resulting in a large quantum bit error rate that effectively protects this system from our attack. However, in a Clavis2like system equipped with detectors with lessnoisy but realistic characteristics, an attack strategy with positive leakage of the key would exist. We confirm this by a numerical simulation. Both the eavesdropping setup and strategy can be generalized to attack most of the current QKD systems, especially if they lack proper safeguards. We also propose countermeasures to prevent such attacks.


MartinMartinez, E., & Sutherland, C. (2014). Quantum gates via relativistic remote control. PHYSICS LETTERS B, 739, 74–82.
Abstract: We harness relativistic effects to gain quantum control on a stationary qubit in an optical cavity by controlling the noninertial motion of a different probe atom. Furthermore, we show that by considering relativistic trajectories of the probe, we enhance the efficiency of the quantum control. We explore the possible use of these relativistic techniques to build 1qubit quantum gates. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.


Huber, M. G., Arif, M., Chen, W. C., Gentile, T. R., Hussey, D. S., Black, T. C., et al. (2014). Neutron interferometric measurement of the scattering length difference between the triplet and singlet states of nHe3. PHYSICAL REVIEW C, 90(6).
Abstract: We report a determination of the nHe3 scattering length difference = b = = b = 1 b = 0 = [5.411 = 0.031 (statistical) = 0.039 (systematic)] fm between the triplet and singlet states using a neutron interferometer. This revises our previous result = b = = [5.610 = 0.027 (statistical) = 0.032 (systematic)] fm obtained using the same technique in 2008 [ Huber et al., Phys. Rev. Lett. 102, 200401 (2009);,103, 179903(E) (2009)]. This revision is attributable to a reanalysis of the 2008 experiment that now includes a systematic correction caused by magneticfield gradients near the 3He cell which had been previously underestimated. Furthermore, we more than doubled our original data set from 2008 by acquiring 6 months of additional data in 2013. Both the new data set and a reanalysis of the older data are in good agreement. Scattering lengths of lowZ isotopes are valued for use in fewbody nuclear effective field theories, provide important tests of modern nuclear potential models, and, in the case of 3He, aid in the interpretation of neutron scattering from quantum liquids. The difference = b = was determined by measuring the relative phase shift between two incident neutron polarizations caused by the spindependent interaction with a polarized 3He target. The target 3He gas was sealed inside a small, flatwindowed glass cell thatwas placed in one beam path of the interferometer. The relaxation of 3He polarization was monitored continuously with neutron transmission measurements. The neutron polarization and spinflipper efficiency were determined separately using 3He analyzers and two different polarimetry analysis methods. A summary of the measured scattering lengths for n3He with a comparison to nucleon interaction models is given.


Kieferova, M., & Wiebe, N. (2014). On the power of coherently controlled quantum adiabatic evolutions. NEW JOURNAL OF PHYSICS, 16.
Abstract: We provide a new approach to adiabatic state preparation that uses coherent control and measurement to average different adiabatic evolutions in ways that cause their diabatic errors to cancel, allowing highly accurate state preparations using less time than conventional approaches. We show that this new model for adiabatic state preparation is polynomially equivalent to conventional adiabatic quantum computation by providing upper bounds on the cost of simulating such evolutions on a circuitbased quantum computer. Finally, we show that this approach is robust to small errors in the quantum control register and that the system remains protected against noise on the adiabatic register by the spectral gap.


Swingle, B., & Kim, I. H. (2014). Reconstructing Quantum States from Local Data. PHYSICAL REVIEW LETTERS, 113(26).
Abstract: We consider the problem of reconstructing global quantum states from local data. Because the reconstruction problem has many solutions in general, we consider the reconstructed state of maximum global entropy consistent with the local data. We show that unique ground states of local Hamiltonians are exactly reconstructed as the maximal entropy state. More generally, we show that if the state in question is a ground state of a local Hamiltonian with a degenerate space of locally indistinguishable ground states, then the maximal entropy state is close to the ground state projector. We also show that local reconstruction is possible for thermal states of local Hamiltonians. Finally, we discuss a procedure to certify that the reconstructed state is close to the true global state. We call the entropy of our reconstructed maximum entropy state the “reconstruction entropy,” and we discuss its relation to emergent geometry in the context of holographic duality.


Berta, M., Coles, P. J., & Wehner, S. (2014). Entanglementassisted guessing of complementary measurement outcomes. PHYSICAL REVIEW A, 90(6).
Abstract: 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 Bob's predictions must display some uncertainty. Yet on the other hand, maximal entanglement means that there is no more uncertainty at all. Here we follow an operational approach and give an exact relation an equalitybetween the amount of uncertainty as measured by the guessing probability and the amount of entanglement as measured by the recoverable entanglement fidelity. From this equality, we deduce a simple criterion for witnessing bipartite entanglement and an entanglement monogamy equality.


