Measuring the size of a Schroedinger cat state

We propose a measure for the "size" of a Schroedinger cat state, i.e. a quantum superposition of two many-body states with (supposedly) macroscopically distinct properties, by counting how many single-particle operations are needed to map one state onto the other. This definition gives sensible results for simple, analytically tractable cases and is consistent with a previous definition restricted to Greenberger-Horne-Zeilinger-like states. We apply our measure to the experimentally relevant, nontrivial example of a superconducting three-junction flux qubit put into a superposition of left- and right-circulating supercurrent states and find this Schroedinger cat to be surprisingly small.Comment: 5 pages, 3 figure

Optomechanical creation of magnetic fields for photons on a lattice

We propose using the optomechanical interaction to create artificial magnetic fields for photons on a lattice. The ingredients required are an optomechanical crystal, i.e. a piece of dielectric with the right pattern of holes, and two laser beams with the right pattern of phases. One of the two proposed schemes is based on optomechanical modulation of the links between optical modes, while the other is an lattice extension of optomechanical wavelength-conversion setups. We illustrate the resulting optical spectrum, photon transport in the presence of an artificial Lorentz force, edge states, and the photonic Aharonov-Bohm effect. Moreover, wWe also briefly describe the gauge fields acting on the synthetic dimension related to the phonon/photon degree of freedom. These can be generated using a single laser beam impinging on an optomechanical array

Quantum techniques using continuous variables of light

We present schemes for the generation and evaluation of continuous variable entanglement of bright optical beams and give a brief overview of the variety of optical techniques and quantum communication applications on this basis. A new entanglement-based quantum interferometry scheme with bright beams is suggested. The performance of the presented schemes is independent of the relative interference phase which is advantageous for quantum communication applications.Comment: 11 pages, 5 figures; minor correction, accepted versio

Dimensional Crossover of the Dephasing Time in Disordered Mesoscopic Rings: From Diffusive through Ergodic to 0D Behavior

We analyze dephasing by electron interactions in a small disordered quasi-one dimensional (1D) ring weakly coupled to leads, where we recently predicted a crossover for the dephasing time \tPh(T) from diffusive or ergodic 1D (\tPh^{-1} \propto T^{2/3}, T^{1}) to $0D$ behavior (\tPh^{-1} \propto T^{2}) as $T$ drops below the Thouless energy \ETh. We provide a detailed derivation of our results, based on an influence functional for quantum Nyquist noise, and calculate all leading and subleading terms of the dephasing time in the three regimes. Explicitly taking into account the Pauli blocking of the Fermi sea in the metal allows us to describe the $0D$ regime on equal footing as the others. The crossover to $0D$, predicted by Sivan, Imry and Aronov for 3D systems, has so far eluded experimental observation. We will show that for T \ll \ETh, $0D$ dephasing governs not only the $T$-dependence for the smooth part of the magnetoconductivity but also for the amplitude of the Altshuler-Aronov-Spivak oscillations, which result only from electron paths winding around the ring. This observation can be exploited to filter out and eliminate contributions to dephasing from trajectories which do not wind around the ring, which may tend to mask the $T^{2}$ behavior. Thus, the ring geometry holds promise of finally observing the crossover to $0D$ experimentally.Comment: in "Perspectives of Mesoscopic Physics - Dedicated to Yoseph Imry's 70th Birthday", edited by Amnon Aharony and Ora Entin-Wohlman (World Scientific, 2010), chap. 20, p. 371-396, ISBN-13 978-981-4299-43-

Reduction of Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers

Guided Acoustic Wave Brillouin Scattering (GAWBS) generates phase and polarization noise of light propagating in glass fibers. This excess noise affects the performance of various experiments operating at the quantum noise limit. We experimentally demonstrate the reduction of GAWBS noise in a photonic crystal fiber in a broad frequency range using cavity sound dynamics. We compare the noise spectrum to the one of a standard fiber and observe a 10-fold noise reduction in the frequency range up to 200 MHz. Based on our measurement results as well as on numerical simulations we establish a model for the reduction of GAWBS noise in photonic crystal fibers.Comment: 4 pages, 7 figures; added numerical simulations, added reference

Lessons from Germany: Tenant power in the rental market

The second of our series of reports comparing the English and German housing markets explores the lessons that policymakers in England can learn from Germany – where renting, the dominant tenure, appears to offer both stability and security to its 40-million-plus tenants. The private rented sector (PRS) in England is growing rapidly, in part in response to the increasing unaffordability of home ownership and the declining supply of social housing. There is mounting concern that across a range of indicators it is a poor substitute for both of these main alternatives. Tenants enjoy limited rights, their tenancies are short, and rents – while in the short-term more affordable than buying – are rising faster than incomes, preventing tenants from saving for mortgage deposits or even meeting the everyday costs of living. The PRS does not need to be a poor relation to home ownership or social renting, however, and we can turn our attention to other countries in which the challenges presented by the PRS are managed with more success. This paper, the second of our series comparing the English and German housing markets, explores the lessons policymakers can learn from Germany – a country in which renting is the dominant tenure and which appears able to offer both stability and security to its 40-million-plus tenants. England can learn from Germany in areas of tenancy security, controls on cost, and tenant representation. We recommend greater balance between the rights of a tenant and the rights of a landlord in England through longer tenancies, help with the costs associated with renting (such as deposits and letting fees), and stronger, more formalised representation

German model homes? A comparison of the UK and German housing markets

Can Britain learn from Germany to tackling the housing crisis? A new paper from IPPR finds Germany has higher rates of housebuilding, a much less volatile housing market, and a larger private rented sector than the UK. IPPR looks at why this has happened. Germany has higher rates of housebuilding, a much less volatile housing market, and a larger private rented sector than the UK. This paper, the first of three, explores the reasons for these differences, looking at both supply- and demand-side dimensions. On the supply side: Germany has a more diverse mix of housebuilders, both small and large, who build a wide variety of homes; a broader mix of investors, including build-to-rent; and a planning system that facilitates the release of land and the translation of permissions into completions. On the demand side: Germany has a more conservative mortgage market with greater restrictions on loan-to-value ratios; a tax system that favours long-term property ownership while discouraging speculation; and a combination of longer tenancies and more rent control, which together make private renting an attractive alternative to home ownership. Despite the many strong features of Germany’s housing market and wider policy framework, there remain areas in which policymakers in the UK can learn from Germany’s missteps. While Germany has managed to deliver more affordable homes in the last three decades, its model for delivering them, through the equivalent 20–30-year covenants, has led to a sharp drop in the availability of affordable rented homes. In addition, the private rented sector in Germany can be difficult to access, with lengthy property-search and application procedures, making entry difficult for prospective tenants and impacting on labour market flexibility. While the UK rental market may be too flexible, to the disadvantage of tenants, a full shift towards a German model could be problematic

Snowflake phononic topological insulator at the nanoscale

We show how the snowflake phononic crystal structure, which recently has been realized experimentally, can be turned into a topological insulator for mechanical waves. This idea, based purely on simple geometrical modifications, could be readily implemented on the nanoscale