Journal: New Journal of Physics

Abbreviation

New J. Phys.

Publisher

IOP Publishing

Journal Volumes

ISSN

1367-2630

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Publications1 - 10 of 249
  • A density matrix model of transport and radiation in quantum cascade lasers
    Item type: Journal Article
    Terazzi, Romain; Faist, Jérôme (2010)
    New Journal of Physics
    A transport model for quantum cascade lasers based on density matrix formalism that incorporates the laser optical field is confronted with experiment. For a typical mid-infrared laser, very good agreement is found for both the current–voltage and current–optical power characteristics. Forcing thermal distribution with a unique temperature in all subbands was found to lead to an overestimate of electron heating in the injector. The model can then be used further to optimize and design new structures.
  • Quantum compression of tensor network states
    Item type: Journal Article
    Bai, Ge; Yang, Yuxiang; Chiribella, Giulio (2020)
    New Journal of Physics
    We design quantum compression algorithms for parametric families of tensor network states. We first establish an upper bound on the amount of memory needed to store an arbitrary state from a given state family. The bound is determined by the minimum cut of a suitable flow network, and is related to the flow of information from the manifold of parameters that specify the states to the physical systems in which the states are embodied. For given network topology and given edge dimensions, our upper bound is tight when all edge dimensions are powers of the same integer. When this condition is not met, the bound is optimal up to a multiplicative factor smaller than 1.585. We then provide a compression algorithm for general state families, and show that the algorithm runs in polynomial time for matrix product states.
  • Toys can’t play: physical agents in Spekkens’ theory
    Item type: Journal Article
    Hausmann, Ladina; Nurgalieva, Nuriya; del Rio, Lídia (2023)
    New Journal of Physics
    Information is physical (Landauer 1961 IBM J. Res. Dev. 5 183-91), and for a physical theory to be universal, it should model observers as physical systems, with concrete memories where they store the information acquired through experiments and reasoning. Here we address these issues in Spekkens’ toy theory (Spekkens 2005 Phys. Rev. A 71 052108), a non-contextual epistemically restricted model that partially mimics the behaviour of quantum mechanics. We propose a way to model physical implementations of agents, memories, measurements, conditional actions and information processing. We find that the actions of toy agents are severely limited: although there are non-orthogonal states in the theory, there is no way for physical agents to consciously prepare them. Their memories are also constrained: agents cannot forget in which of two arbitrary states a system is. Finally, we formalize the process of making inferences about other agents’ experiments and model multi-agent experiments like Wigner’s friend. Unlike quantum theory (Nurgalieva and del Rio Lidia 2019 Electron. Proc. Theor. Comput. Sci. 287 267-97; Fraser et al 2020 Fitch’s knowability axioms are incompatible with quantum theory arXiv:2009.00321; Frauchiger and Renner 2018 Nat. Commun. 9 3711; Nurgalieva and Renner 2021 Contemp. Phys. 61 1-24; Brukner 2018 Entropy 20 350) or box world (Vilasini et al 2019 New J. Phys. 21 113028), in toy theory there are no inconsistencies when physical agents reason about each other’s knowledge.
  • Increasing the ν = 5/2 gap energy: an analysis of MBE growth parameters
    Item type: Journal Article
    Reichl, Christian; Chen, Jun; Baer, Stephan; et al. (2014)
    New Journal of Physics
    The fractional quantized Hall state at the filling factor ν = 5/2 is of special interest due to its possible application for quantum computing. Here we report on the optimization of growth parameters that allowed us to produce two-dimensional electron gases (2DEGs) with a 5/2 gap energy up to 135 mK. We concentrated on optimizing the molecular beam epitaxy (MBE) growth to provide high 5/2 gap energies in 'as-grown' samples, without the need to enhance the 2DEGs properties by illumination or gating techniques. Our findings allow us to analyse the impact of doping in narrow quantum wells with respect to conventional DX-doping in AlxGa1−xAs. The impact of the setback distance between doping layer and 2DEG was investigated as well. Additionally, we found a considerable increase in gap energy by reducing the amount of background impurities. To this end growth techniques like temperature reductions for substrate and effusion cells and the reduction of the Al mole fraction in the 2DEG region were applied.
  • Optical fingerprints of the electronic band reconstruction in van der Waals magnetic materials
    Item type: Journal Article
    Corasaniti, Matteo; Yang, Rong; Liu, Yu; et al. (2022)
    New Journal of Physics
    We report a broadband study of the charge dynamics in the van der Waals magnetic materials 2H-MₓTaS₂ (M = Mn and Co), which span the onset of both long-range antiferromagnetic (AFM) and ferromagnetic (FM) order, depending on the intercalation M and its concentration x. We discover a spectral weight (SW) shift from high to low energy scales for FM compositions, while reversely SW is removed from low towards high spectral energies for AFM compounds. This maps the related reconstruction of the electronic band structure along the crossover from the FM to AFM order, which restores an occupation balance in the density of states between spin majority and minority bands of the intercalated 3d elements.
  • Quantum optics with single quantum dot devices
    Item type: Journal Article
    Zwiller, Valéry; Aichele, Thomas; Benson, Oliver (2004)
    New Journal of Physics
    A single radiative transition in a single-quantum emitter results in the emission of a single photon. Single quantum dots are single-quantum emitters with all the requirements to generate single photons at visible and near-infrared wavelengths. It is also possible to generate more than single photons with single quantum dots. In this paper we show that single quantum dots can be used to generate non-classical states of light, from single photons to photon triplets. Advanced solid state structures can be fabricated with single quantum dots as their active region. We also show results obtained on devices based on single quantum dots.
  • Multi-agent paradoxes beyond quantum theory
    Item type: Journal Article
    Vilasini, Venkatesh; Nurgalieva, Nuriya; del Rio, Lídia (2019)
    New Journal of Physics
    Which theories lead to a contradiction between simple reasoning principles and modelling observers' memories as physical systems? Frauchiger and Renner have shown that this is the case for quantum theory (Frauchiger and Renner 2018 Nat. Commun. 9 3711). Here we generalize the conditions of the Frauchiger–Renner result so that they can be applied to arbitrary physical theories, and in particular to those expressed as generalized probabilistic theories (GPTs) (Hardy 2001 arXiv:quant-ph/0101012; Barrett 2007 Phys. Rev. A 75 032304). We then apply them to a particular GPT, box world, and find a deterministic contradiction in the case where agents may share a PR box (Popescu and Rohrlich 1994 Found. Phys. 24 379–85), which is stronger than the quantum paradox, in that it does not rely on post-selection. Obtaining an inconsistency for the framework of GPTs broadens the landscape of theories which are affected by the application of classical rules of reasoning to physical agents. In addition, we model how observers' memories may evolve in box world, in a way consistent with Barrett's criteria for allowed operations (Barrett 2007 Phys. Rev. A 75 032304; Gross et al 2010 Phys. Rev. Lett. 104 080402).
  • Absorption imaging of a quasi-two-dimensional gas
    Item type: Journal Article
    Chomaz, Lauriane; Corman, Laura; Yefsah, Tarik; et al. (2012)
    New Journal of Physics
    Absorption imaging with quasi-resonant laser light is a commonly used technique for probing ultra-cold atomic gases in various geometries. In this paper, we investigate some non-trivial aspects of this method when applying the method to in situ diagnosis of a quasi-two-dimensional (2D) gas. Using Monte Carlo simulations we study the modification of the absorption cross-section of a photon when it undergoes multiple scattering in the gas. We determine the variations of the optical density with various parameters, such as the detuning of the light from the atomic resonance and the thickness of the gas. We compare our results to the known 3D result (the Beer–Lambert law) and outline the specific features of the 2D case.
  • Energy consumption for ion-transport in a segmented Paul trap
    Item type: Journal Article
    Tobalina, Ander; Alonso, Joseba; Muga, Juan G. (2018)
    New Journal of Physics
    There is recent interest in determining energy costs of shortcuts to adiabaticity (STA), but different definitions of 'cost' have been used. We demonstrate the importance of taking into account the control system (CS) for a fair assessment of energy flows and consumptions. We model the energy consumption and power to transport an ion by a STA protocol in a multisegmented Paul trap. The ion is driven by an externally controlled, moving harmonic oscillator. Even if no net ion-energy is gained at destination, setting the time-dependent control parameters is a macroscopic operation that costs energy and results in energy dissipation for the short time scales implied by the intrinsically fast STA processes. The potential minimum is displaced by modulating the voltages on control (dc) electrodes. A secondary effect of the modulation, usually ignored as it does not affect the ion dynamics, is the time-dependent energy shift of the potential minimum. The non trivial part of the energy consumption is due to the electromotive forces to set the electrode voltages through the low-pass filters required to preserve the electronic noise from decohering the ion's motion. The results for the macroscopic CS (the Paul trap) are compared to the microscopic power and energy of the ion alone. Similarities are found—and may be used quantitatively to minimize costs—only when the CS-dependent energy shift of the harmonic oscillator is included in the ion-energy.
  • Normal modes of trapped ions in the presence of anharmonic trap potentials
    Item type: Journal Article
    Home, Jonathan; Hanneke, David; Jost, John D.; et al. (2011)
    New Journal of Physics
    We theoretically and experimentally examine the effects of anharmonic terms in the trapping potential for linear chains of trapped ions. We concentrate on two different effects that become significant at different levels of anharmonicity. The first is a modification of the oscillation frequencies and amplitudes of the ions' normal modes of vibration for multi-ion crystals, resulting from each ion experiencing a different curvature in the potential. In the second effect, which occurs with increased anharmonicity or higher excitation amplitude, amplitude-dependent shifts of the normal-mode frequencies become important. We evaluate normal-mode frequency and amplitude shifts, and comment on the implications for quantum information processing and quantum state engineering. Since the ratio of the anharmonic to harmonic terms typically increases as the ion–electrode distance decreases, anharmonic effects will become more significant as ion trap sizes are reduced. To avoid unwanted problems, anharmonicities should therefore be taken into account at the design stage of trap development.
Publications1 - 10 of 249