
Zhang, F., Yang, H., & Lehner, L. (2014). Towards an understanding of the forcefree magnetosphere of rapidly spinning black holes. Phys. Rev. D, 90(12), 12 pp.
Abstract: The ability of a plasma surrounding spinning black holes to extract rotational energy and power energetic emissions has been recognized as a key astrophysical phenomenon. Important insights into the nature of this process are obtained through the analysis of the interplay between a forcefree magnetosphere and the black hole. This task involves solving a complicated system of equations, often requiring complex numerical simulations. Recent analytical attempts at tackling this problem have exploited the fact that the nearhorizon region of extreme Kerr (NHEK) is endowed with an enhanced symmetry group. We continue in this direction and show that for some conformally selfsimilar solutions, the NHEK forcefree equations reduce to a single nonlinear ordinary differential equation which is difficult to solve with straightforward integration. We here introduce a new approach specifically tailored to this problem and describe how one can obtain physically meaningful solutions.


Ahmadzadegan, A., Mann, R. B., & MartinMartinez, E. (2014). Measuring motion through relativistic quantum effects. Phys. Rev. A, 90(6), 7 pp.
Abstract: We show that the relativistic signatures on the transition probability of atoms moving through optical cavities are very sensitive to their spatial trajectory. This allows for the use of internal atomic degrees of freedom to measure small timedependent perturbations in the proper acceleration of an atomic probe, or in the relative alignment of a beam of atoms and a cavity.


Li, X. H., & Ghose, S. (2015). Efficient hyperconcentration of nonlocal multipartite entanglement via the crossKerr nonlinearity. Opt. Express, 23(3), 3550–3562.
Abstract: We propose two schemes for concentration of hyperentanglement of nonlocal multipartite states which are simultaneously entangled in the polarization and spatial modes. One scheme uses an auxiliary singlephoton state prepared according to the parameters of the lessentangled states. The other scheme uses two lessentangled states with unknown parameters to distill the maximal hyperentanglement. The procrustean concentration is realized by two parity check measurements in both the two degrees of freedom. Nondestructive quantum nondemolition detectors based on crossKerr nonlinearity are used to implement the parity check, which makes the unsuccessful instances reusable in the next concentration round. The success probabilities in both schemes can be made to approach unity by iteration. Moreover, in both schemes only one of the N parties has to perform the parity check measurements. Our schemes are efficient and useful for quantum information processing involving hyperentanglement. (C) 2015 Optical Society of America


Layden, D., MartinMartinez, E., & Kempf, A. (2015). Perfect Zenolike effect through imperfect measurements at a finite frequency. Phys. Rev. A, 91(2), 6 pp.
Abstract: The quantum Zeno effect is usually thought to require infinitely frequent and perfect projective measurements to freeze the dynamics of quantum states. We show that perfect freezing of quantum states can also be achieved by more realistic nonprojective measurements performed at a finite frequency.


Emms, D., Severini, S., Wilson, R. C., & Hancock, E. R. (2015). Coined quantum walks lift the cospectrality of graphs and trees (vol 42, pg 1988, 2009). PATTERN RECOGNITION, 48(4), 1574–1575.


Robert H. Jonsson, E. M.  M., and Achim Kempf. (2015). Information Transmission Without Energy Exchange. Phys. Rev. Lett., 114(11).
Abstract: We show that it is possible to use a massless field in the vacuum to communicate in such a way that the signal travels arbitrarily slower than the speed of light and such that no energy is transmitted from the sender to the receiver. Instead, the receiver has to supply a signaldependent amount of work to switch his detector on and off. This type of communication is related to Casimirlike interactions, and it is made possible by dimension—and curvature—dependent subtleties of Huygens’ principle.


Chen, L., Chen, J., Dokovic, D. Z., & Zeng, B. (2015). Universal Subspaces for Local Unitary Groups of Fermionic Systems. COMMUNICATIONS IN MATHEMATICAL PHYSICS, 333(2), 541–563.
Abstract: Let V = boolean AND VN be the Nfermion Hilbert space with Mdimensional single particle space V and 2N <= M. We refer to the unitary group G of V as the local unitary (LU) group. We fix an orthonormal (o.n.) basis v(1)>, ... , v(M)> of V. Then the Slater determinants e(i1), ... , i(N) := v(i1). boolean AND v(i2) boolean AND ... boolean AND v(iN) > with i(1) < ... < i(N) form an o.n. basis of V. Let S subset of V be the subspace spanned by all e(i1), ... , i(N) such that the set {i(1), ... , i(N)} contains no pair {2k – 1, 2k}, k an integer. We say that the psi > is an element of S are single occupancy states (with respect to the basis v(1)>, ... , v(M)>). We prove that for N = 3 the subspace S is universal, i.e., each Gorbit in V meets S, and that this is false for N > 3. If M is even, the well known BCS states are not LUequivalent to any single occupancy state. Our main result is that for N = 3 and M even there is a universal subspace W subset of S spanned by M(M – 1)(M – 5)/6 states e(i1), ..., i(N). Moreover, the number M(M – 1)(M – 5)/6 is minimal.


Stacey, W., Annabestani, R., Ma, X., & Luetkenhaus, N. (2015). Security of quantum key distribution using a simplified trusted relay. PHYSICAL REVIEW A, 91(1).
Abstract: We propose a QKD protocol for trusted node relays. Our protocol shifts the communication and computational weight of classical postprocessing to the end users by reassigning the roles of error correction and privacy amplification, while leaving the exchange of quantum signals untouched. We perform a security analysis for this protocol based on the BennettBrassard 1984 protocol on the level of infinite key formulas, taking into account weak coherent implementations involving decoy analysis.


Hwang, W.  Y., Bae, J., & Killoran, N. (2015). Nosignaling quantum key distribution: solution by linear programming. QUANTUM INFORMATION PROCESSING, 14(2), 687–696.
Abstract: We outline a straightforward approach for obtaining a secret key rate using only nosignaling constraints and linear programming. Assuming an individual attack, we consider all possible joint probabilities. Initially, we study only the case where Eve has binary outcomes, and we impose constraints due to the nosignaling principle and given measurement outcomes. Within the remaining space of joint probabilities, by using linear programming, we get bound on the probability of Eve correctly guessing Bob's bit. We then make use of an inequality that relates this guessing probability to the mutual information between Bob and a more general Eve, who is not binaryrestricted. Putting our computed bound together with the CsiszarKorner formula, we obtain a positive key generation rate. The optimal value of this rate agrees with known results, but was calculated in a more straightforward way, offering the potential of generalization to different scenarios.


Aida Ahmadzadegan, E. M.  M., Achim Kempf. (2015). Amplifying the Unruh effect. Bulletin of the American Physical Society, 60(1).
Abstract: In this talk we will explore the effect of nonuniform acceleration on the transition probability
of particle detectors. Our main goal is to find a trajectory that optimizes the sensitivity of the
detector's response and amplifies the celebrated Unruh effect such that it becomes
perceptible with current technology, thus bringing the Unruh effect closer to observability.


Holloway, G. W., Shiri, D., Haapamaki, C. M., Willick, K., Watson, G., LaPierre, R. R., et al. (2015). Magnetoconductance signatures of subband structure in semiconductor nanowires. PHYSICAL REVIEW B, 91(4).
Abstract: The radial confining potential in a semiconductor nanowire plays a key role in determining its quantum transport properties. Previous reports have shown that an axial magnetic field induces fluxperiodic conductance oscillations when the electronic states are confined to a shell. This effect is due to the coupling of orbital angular momentum to the magnetic flux. Here, we perform calculations of the energy level structure, and consequently the conductance, for more general cases ranging from a flat potential to strong surface band bending. The transverse states are not confined to a shell, but are distributed across the nanowire. It is found that, in general, the subband energy spectrum is aperiodic as a function of both gate voltage and magnetic field. In principle, this allows for precise identification of the occupied subbands from the magnetoconductance patterns of quasiballistic devices. The aperiodicity becomes more apparent as the potential flattens. A quantitative method is introduced for matching features in the conductance data to the subband structure resulting from a particular radial potential, where a functional form for the potential is used that depends on two free parameters. Finally, a shortchannel InAs nanowire fieldeffect transistor device is measured at low temperature in search of conductance features that reveal the subband structure. Features are identified and shown to be consistent with three specific subbands. The experiment is analyzed in the context of the weak localization regime; however, we find that the subband effects predicted for ballistic transport should remain visible when backscattering dominates over interband scattering, as is expected for this device.


Granade, C., Ferrie, C., & Cory, D. G. (2015). Accelerated randomized benchmarking. NEW JOURNAL OF PHYSICS, 17.
Abstract: Quantum information processing offers promising advances for a wide range of fields and applications, provided that we can efficiently assess the performance of the control applied in candidate systems. That is, we must be able to determine whether we have implemented a desired gate, and refine accordingly. Randomized benchmarking reduces the difficulty of this task by exploiting symmetries in quantum operations. Here, we bound the resources required for benchmarking and show that, with prior information, we can achieve several orders of magnitude better accuracy than in traditional approaches to benchmarking. Moreover, by building on stateoftheart classical algorithms, we reach these accuracies with nearoptimal resources. Our approach requires an order of magnitude less data to achieve the same accuracies and to provide online estimates of the errors in the reported fidelities. We also show that our approach is useful for physical devices by comparing to simulations.


Zhang, J., Burgarth, D., Laflamme, R., & Suter, D. (2015). Experimental implementation of quantum gates through actuator qubits. PHYSICAL REVIEW A, 91(1).
Abstract: Universal quantum computation requires the implementation of arbitrary control operations on the quantum register. In most cases, this is achieved by external control fields acting selectively on each qubit to drive singlequbit operations. In combination with a drift Hamiltonian containing interactions between the qubits, this allows the implementation of arbitrary gate operations. Here, we demonstrate an alternative scheme that does not require local control for all qubits: we implement one and twoqubit gate operations on a set of target qubits indirectly, through a combination of gates on directly controlled actuator qubits with a drift Hamiltonian that couples actuator and target qubits. Experiments are performed on nuclear spins, using radiofrequency pulses as gate operations and magneticdipole couplings for the drift Hamiltonian.


Vermeyden, L., Ma, X., Lavoie, J., Bonsma, M., Sinha, U., Laflamme, R., et al. (2015). Experimental test of environmentassisted invariance. PHYSICAL REVIEW A, 91(1).
Abstract: Envariance, or environmentassisted invariance, is a recently identified symmetry for maximally entangled states in quantum theory with important ramifications for quantum measurement, specifically for understanding Born's rule. We benchmark the degree to which nature respects this symmetry by using entangled photon pairs. Our results show quantum states can be (99.66 +/ 0.04)% envariant as measured using the quantum fidelity, and (99.963 +/ 0.005)% asmeasured using a modifiedBhattacharya coefficient, as comparedwith a perfectly envariant system which would be 100% in either measure. The deviations can be understood by the lessthanmaximal entanglement in our photon pairs.


Coles, P. J., Kaniewski, J., & Wehner, S. (2014). Equivalence of waveparticle duality to entropic uncertainty. NATURE COMMUNICATIONS, 5.
Abstract: Interferometers capture a basic mystery of quantum mechanics: a single particle can exhibit wave behaviour, yet that wave behaviour disappears when one tries to determine the particle's path inside the interferometer. This idea has been formulated quantitatively as an inequality, for example, by Englert and Jaeger, Shimony and Vaidman, which upper bounds the sum of the interference visibility and the path distinguishability. Such waveparticle duality relations (WPDRs) are often thought to be conceptually inequivalent to Heisenberg's uncertainty principle, although this has been debated. Here we show that WPDRs correspond precisely to a modern formulation of the uncertainty principle in terms of entropies, namely, the minand maxentropies. This observation unifies two fundamental concepts in quantum mechanics. Furthermore, it leads to a robust framework for deriving novel WPDRs by applying entropic uncertainty relations to interferometric models. As an illustration, we derive a novel relation that captures the coherence in a quantum beam splitter.


