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Gharibian, S., Piani, M., Adesso, G., Calsamiglia, J., & Horodecki, P. (2011). Characterizing quantumness via entanglement creation. International Journal of Quantum Information, 9, 1701–1713.
Abstract: In [M. Piani et al., arXiv:1103.4032 (2011)] an activation protocol was introduced which maps the general non-classical (multipartite) correlations between given systems into bipartite entanglement between the systems and local ancillae by means of a potentially highly entangling interaction. Here, we study how this activation protocol can be used to entangle the starting systems themselves via entanglement swapping through a measurement on the ancillae. Furthermore, we bound the relative entropy of quantumness (a naturally arising measure of non-classicality in the scheme of Piani et al. above) for a special class of separable states, the so-called classical-quantum states. In particular, we fully characterize the classical-quantum two-qubit states that are maximally non-classical.
Keywords: quant-ph
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Streltsov, A., Adesso, G., Piani, M., & Bruss, D. (2011). Are general quantum correlations monogamous? arXiv preprint, .
Abstract: Quantum entanglement and quantum non-locality are known to exhibit monogamy, that is, they obey strong constraints on how they can be distributed among multipartite systems. Quantum correlations that comprise and go beyond entanglement are quantified by, e.g., quantum discord. It was observed recently that for some states quantum discord is not monogamous. We prove in general that any measure of correlations that is monogamous for all states and satisfies reasonable basic properties must vanish for all separable states: only entanglement measures can be strictly monogamous. Monogamy of other than entanglement measures can still be satisfied for special, restricted cases: we prove that the geometric measure of discord satisfies the monogamy inequality on all pure states of three qubits.
Keywords: quant-ph; cond-mat.stat-mech; math-ph; math.MP; physics.comp-ph
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Chen, Y. F., Hover, D., Sendelbach, S., Maurer, L., Merkel, S. T., Pritchett, E. J., et al. (2011). Microwave Photon Counter Based on Josephson Junctions. Phys. Rev. Lett., 107(21), 5 pp.
Abstract: We describe a microwave photon counter based on the current-biased Josephson junction. The junction is tuned to absorb single microwave photons from the incident field, after which it tunnels into a classically observable voltage state. Using two such detectors, we have performed a microwave version of the Hanbury Brown-Twiss experiment at 4 GHz and demonstrated a clear signature of photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons.
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Prevedel, R., Schreiter, K. M., Lavoie, J., & Resch, K. J. (2011). Classical analog for dispersion cancellation of entangled photons with local detection. Phys. Rev. A, 84(5), 4 pp.
Abstract: Energy-time entangled photon pairs remain tightly correlated in time when the photons are passed through equal magnitude, but opposite in sign, dispersion. A recent experimental demonstration has observed this effect on ultrafast time scales using second-harmonic generation of the photon pairs. However, the experimental signature of this effect does not require energy-time entanglement. Here, we demonstrate a direct analogue to this effect in narrow-band second-harmonic generation of a pair of classical laser pulses under similar conditions. Perfect cancellation is observed for fs pulses with dispersion as large as 850 fs(2), comparable to the quantum result, but with an 10(13)-fold improvement in signal brightness.
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Lydersen, L., Akhlaghi, M. K., Majedi, A. H., Skaar, J., & Makarov, V. (2011). Controlling a superconducting nanowire single-photon detector using tailored bright illumination. New J. Phys., 13, 14 pp.
Abstract: We experimentally demonstrate that a superconducting nanowire single-photon detector is deterministically controllable by bright illumination. We found that bright light can temporarily make a large fraction of the nanowire length normally conductive, can extend deadtime after a normal photon detection, and can cause a hotspot formation during the deadtime with a highly nonlinear sensitivity. As a result, although based on different physics, the superconducting detector turns out to be controllable by virtually the same techniques as avalanche photodiode detectors. As demonstrated earlier, when such detectors are used in a quantum key distribution system, this allows an eavesdropper to launch a detector control attack to capture the full secret key without this being revealed by too many errors in the key.
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Ross, K. A., Yaraskavitch, L. R., Laver, M., Gardner, J. S., Quilliam, J. A., Meng, S., et al. (2011). Dimensional evolution of spin correlations in the magnetic pyrochlore Yb(2)Ti(2)O(7). Phys. Rev. B, 84(17), 6 pp.
Abstract: The pyrochlore material Yb(2)Ti(2)O(7) displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T = 265 mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short-range three-dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasielastic (QE) scattering below similar to 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265 mK, which we discuss in the context of recent mean field theoretical analysis.
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Ghamsari, B. G., & Majedi, A. H. (2011). Surface Plasmon-Enhanced Coupling of Optical Guided Waves to High-Temperature Superconducting Optoelectronic Structures. IEEE Trans. Appl. Supercond., 21(6), 3646–3651.
Abstract: A novel technique is proposed for coupling near-infrared and visible optical power to high-temperature superconducting (HTS) optoelectronic structures, fabricated on a high-index substrate, by means of the excitation of surface plasmon polariton (SPP) waves at the interface of the HTS layer and a metal cladding. The modal characteristics of these guided waves differ from those of the SPP modes of a metal slab bounded by symmetric or asymmetric dielectric layers because of the presence of a high-index semi-infinite substrate at close proximity. The modal dispersion of the guided mode exhibits a cutoff with increasing HTS thicknesses. Inasmuch as the HTS layer possesses a large extinction factor, it absorbs most of the optical power, whereas in a conventional dielectric-metal structure, the power is virtually absorbed by the metal. Furthermore, the variation of the coupling efficiency as a function of the HTS thickness is examined, and it will be demonstrated that the surface plasmon-assisted coupling technique outperforms unguided illumination schemes. The proposed technique and structure are particularly useful for guided-wave superconducting optoelectronic devices, including superconducting photodetectors and photomixers.
Keywords: Photodetectors; superconducting optical waveguides; superconducting optoelectronics; surface plasmon polaritons (SPPs)
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Cubitt, T. S., Chen, J. X., & Harrow, A. W. (2011). Superactivation of the Asymptotic Zero-Error Classical Capacity of a Quantum Channel. IEEE Trans. Inf. Theory, 57(12), 8114–8126.
Abstract: The zero-error classical capacity of a quantum channel is the asymptotic rate at which it can be used to send classical bits perfectly so that they can be decoded with zero probability of error. We show that there exist pairs of quantum channels, neither of which individually have any zero-error capacity whatsoever (even if arbitrarily many uses of the channels are available), but such that access to even a single copy of both channels allows classical information to be sent perfectly reliably. In other words, we prove that the zero-error classical capacity can be superactivated. This result is the first example of superactivation of a classical capacity of a quantum channel.
Keywords: Additivity violation; channel coding; communication channels; information rates; quantum theory; superactivation; zero-error capacity
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Ansari, M. H., & Wilhelm, F. K. (2011). Noise and microresonance of critical current in Josephson junction induced by Kondo trap states. Phys. Rev. B, 84(23), 17 pp.
Abstract: We analyze the impact of trap states in the oxide layer of a superconducting tunnel junction on the fluctuation of the Josephson critical current, thus on coherence in superconducting qubits. Two mechanisms are usually considered: the current blockage due to repulsion at the occupied trap states and the noise from electrons hopping across a trap. We extend previous studies of noninteracting traps to the case where the traps have onsite electron repulsion inside one ballistic channel. The repulsion not only allows the appropriate temperature dependence of 1/f noise, but also the control of the coupling between the computational qubit and the spurious two-level systems inside the oxide dielectric. We use second-order perturbation theory, which allows to obtain analytical formulas for the interacting bound states and spectral weights, limited to small and intermediate repulsions. Remarkably, it still reproduces the main features of the model as identified from the numerical renormalization group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights. We show that interactions can reverse the supercurrent across the trap. We finally work out the spectrum of junction resonators for qubits in the presence of onsite repulsive electrons and analyze its dependence on microscopic parameters that may be controlled by fabrication.
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Coish, W. A., & Qassemi, F. (2011). Leakage-current line shapes from inelastic cotunneling in the Pauli spin blockade regime. Phys. Rev. B, 84(24), 10 pp.
Abstract: We find the leakage current through a double quantum dot in the Pauli spin blockade regime accounting for inelastic (spin-flip) cotunneling processes. Taking the energy dependence of this spin-flip mechanism into account allows for an accurate description of the current as a function of applied magnetic fields, gate voltages, and an interdot tunnel coupling. In the presence of an additional local dephasing process or nonuniform magnetic field, we obtain a simple closed-form analytical expression for the leakage current giving the full dependence on an applied magnetic field and energy detuning. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e. g., to hyperfine coupling to nuclear spins or spin-orbit coupling) are weak, including silicon and carbon-nanotube or graphene quantum dots.
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Ferrie, C. (2011). Quasi-probability representations of quantum theory with applications to quantum information science. Rep. Prog. Phys., 74(11), 24 pp.
Abstract: This paper comprises a review of both the quasi-probability representations of infinite-dimensional quantum theory (including the Wigner function) and the more recently defined quasi-probability representations of finite-dimensional quantum theory. We focus on both the characteristics and applications of these representations with an emphasis toward quantum information theory. We discuss the recently proposed unification of the set of possible quasi-probability representations via frame theory and then discuss the practical relevance of negativity in such representations as a criteria for quantumness.
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Das, A., Garnerone, S., & Haas, S. (2011). Entanglement and its evolution following a quench in the presence of an energy current. Phys. Rev. A, 84(5), 8 pp.
Abstract: We study the Ising spin chain with a Dzyaloshinskii-Moriya interaction, focusing on the static and dynamic properties of the entanglement entropy following a quantum quench. We show that the effects of the additional anisotropic interaction on the phase diagram and on the dynamics of the system are captured by the properties of the entanglement entropy. In particular, the model provides a way to study the quench dynamics in a system with an energy current. We consider quenches starting from an initial excited state of the Ising spin chain, and we analyze the effects of different initial conditions.
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Shor, P. W., Smith, G., Smolin, J. A., & Zeng, B. (2011). High Performance Single-Error-Correcting Quantum Codes for Amplitude Damping. IEEE Trans. Inf. Theory, 57(10), 7180–7188.
Abstract: We construct families of high performance quantum amplitude damping codes. All of our codes are nonadditive and most modestly outperform the best possible additive codes in terms of encoded dimension. One family is built from nonlinear error-correcting codes for classical asymmetric channels, with which we systematically construct quantum amplitude damping codes with parameters better than any prior construction known for any block length n >= 8 except n = 2(r) – 1. We generalize this construction to employ classical codes over GF(3) with which we numerically obtain better performing codes up to length 14. Because the resulting codes are of the codeword stabilized (CWS) type, conceptually simple (though potentially computationally expensive) encoding and decoding circuits are available.
Keywords: Amplitude damping; asymmetric codes; nonadditive codes; quantum codes
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Prevedel, R., Hamel, D. R., Colbeck, R., Fisher, K., & Resch, K. J. (2011). Experimental investigation of the uncertainty principle in the presence of quantum memory and its application to witnessing entanglement. Nat. Phys., 7(10), 757–761.
Abstract: Heisenberg's uncertainty principle(1) provides a fundamental limitation on the ability of an observer holding classical information to predict the outcome when one of two measurements is performed on a quantum system. However, an observer with access to a particle (stored in a quantum memory) which is entangled with the system generally has a reduced uncertainty: indeed, if the particle and system are maximally entangled, the observer can perfectly predict the outcome of whichever measurement is chosen. This effect has recently been quantified(2) in a new entropic uncertainty relation. Here we experimentally investigate this relation, showing its effectiveness as an efficient entanglement witness. We use entangled photon pairs, an optical delay line serving as a simple quantum memory and fast, active feed-forward. Our results quantitatively agree with the new uncertainty relation. Our technique acts as a witness for almost all entangled states in our experiment as we obtain lower uncertainties than would be possible without the entangled particle(3-5).
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Berry, D. W., & Lvovsky, A. I. (2011). Preservation of loss in linear-optical processing. Phys. Rev. A, 84(4), 7 pp.
Abstract: We propose a measure of quantum efficiency of a multimode state of light that quantifies the amount of optical loss this state has experienced, and prove that this efficiency cannot increase in any linear-optical processing with destructive conditional measurements. Any loss that has affected a state can neither be removed nor redistributed so as to further increase the efficiency in higher-efficiency modes at the expense of lower-efficiency modes. This result eliminates the possibility of catalytically improving photon sources.
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