185 research outputs found
Scattering theory of cooling and heating in optomechanical systems
No full textWe present a one-dimensional scattering theory which enables us to describe a wealth of effects arising from the coupling of the motional degree of freedom of scatterers to the electromagnetic field. Multiple scattering to all orders is taken into account. The theory is applied to describe the scheme of a Fabry-Perot resonator with one of its mirrors moving. The friction force, as well as the diffusion, acting on the moving mirror is derived. In the limit of a small reflection coefficient, the same model provides for the description of the mechanical effect of light on an atom moving in front of a mirror
Detecting topological invariants of quantum walks via weak measurements and losses
No full textTopological insulators have Hamiltonians with bulk topological invariants, which control the interesting processes at the surface of the system, but are hard to measure directly. We have found a way to measure the bulk topological invariant (winding number) of one-dimensional topological insulator Hamiltonians and quantum walks with chiral symmetry: it is given by the displacement of a single particle, observed via losses [1]. Losses represent the effect of repeated weak measurements on one sublattice only, which interrupt the dynamics periodically. When these do not detect the particle, they realize negative measurements. In our repeated measurement scheme these losses occur at the end of every timestep. In the limit of rapidly repeated, vanishingly weak measurements, this corresponds to non-Hermitian Hamiltonians, such as the lossy Su-Schrieffer-Heeger model [2]. Contrary to intuition, the time needed to detect the winding number can be made shorter by decreasing the efficiency of the measurement. Our scheme has since been used to measure the bulk topological invariants of a discrete-time quantum walk on photons [3].
References
[1] T Rakovszky, JK Asboth, and A Alberti, Phys. Rev. B 95, 201407 (2017).
[2] MS Rudner and LS Levitov, Phys. Rev. Lett. 102, 065703 (2009).
[3] X Zhan et al, Phys. Rev. Lett. 119, 130501 (2017).Non UBCUnreviewedAuthor affiliation: Wigner Research Centre for Physics HungaryResearche
Intelligence Explosion Episode 6: The Digital Underworld: Dr. Janos Mark Szakolczai on Onlife Crime, Digital Harm and Tech Colonialism
No full textIn this episode, I sit down with Dr. Janos Mark Szakolczai, author of the forthcoming book Onlife Criminology: Virtual Crimes and Real Harms (Bristol University Press, 2025).
Together we unpack:
• How the digital and physical worlds have collapsed into one.
• New forms of crime, harm, and hidden architectures of power.
• Big Tech’s models of surveillance, addiction, and inequality.
• Digital colonialism in the Global South.
• The erosion of trust through deepfakes and conspiracies.
• Dr. Janos’s call for an “Offlife” future to reclaim autonomy.
It’s a provocative conversation about the future of digital freedoms and the urgent choices societies must make
Topological bound states of a quantum walk with cold atoms
No full textWe suggest a method for engineering a quantum walk, with cold atoms as walkers, which presents topologically nontrivial properties. We derive the phase diagram, and show that we are able to produce a boundary between topologically distinct phases using the finite beam width of the applied lasers. A topologically protected bound state can then be observed, which is pinned to the interface and is robust to perturbations. We show that it is possible to identify this bound state by averaging over spin sensitive measures of the atom's position, based on the spin distribution that these states display. Interestingly, there exists a parameter regime in which our system maps on to the Creutz ladder
This, too, I blame on Hitler
No full textWhat becomes of those who survive? This collection of personal essays uses humor and reflection to explore the themes of inherited trauma and bicultural identity, finding sanctity in the unlikeliest of sources: irreverence. Whether reporting on Syrian refugees at the Hungarian-Serbian border, reflecting on his experimentations with sadomasochism, recounting a botched haircut at the hands of his six-year-old brother, or translating a musical written by his grandfather in a Soviet Gulag, the author grapples with the question: how does one discover nuance in personal heritage under the Manichean weight of a global atrocity like the Holocaust?M.F.A.by Adam Jano
Colour number, capacity and perfectness of directed graphs.
No full textWe introduce a new concept of chromatic number for directed graphs, called the colour number and use it to upper bound the transitive clique number and the Sperner capacity of arbitrary directed graphs. Our results represent a common generalization of previous bounds of Alon and the second author and lead to a concept of perfectness for directed graphs
Topological insulators: how the deep controls the superficial
No full textTopological insulators prevent current from passing through, but also ensure that it flows unimpeded from contact to contact on their surface. The combination of ideas from topology and band theory to explain and generalize this peculiar behaviour has had a transformative effect on condensed matter research
Bulk-boundary correspondence for chiral symmetric quantum walks
No full textDiscrete-time quantum walks (DTQW) have topological phases that are richer than those of time-independent lattice Hamiltonians. Even the basic symmetries, on which the standard classification of topological insulators hinges, have not yet been properly defined for quantum walks. We introduce the key tool of time frames, i.e., we describe a DTQW by the ensemble of time-shifted unitary time-step operators belonging to the walk. This gives us a way to consistently define chiral symmetry (CS) for DTQW's. We show that CS can be ensured by using an "inversion symmetric" pulse sequence. For one-dimensional DTQW's with CS, we identify the bulk Z x Z topological invariant that controls the number of topologically protected 0 and pi energy edge states at the interfaces between different domains, and give simple formulas for these invariants. We illustrate this bulk-boundary correspondence for DTQW's on the example of the "4-step quantum walk," where tuning CS and particle-hole symmetry realizes edge states in various symmetry classes
Topological delocalization in the completely disordered two-dimensional quantum walk
No full textWe investigate numerically and theoretically the effect of spatial disorder on two-dimensional split-step discrete-time quantum walks with two internal "coin" states. Spatial disorder can lead to Anderson localization, inhibiting the spread of quantum walks, putting them at a disadvantage against their diffusively spreading classical counterparts. We find that spatial disorder of the most general type, i.e., position-dependent Haar random coin operators, does not lead to Anderson localization but to a diffusive spread instead. This is a delocalization, which happens because disorder places the quantum walk to a critical point between different anomalous Floquet-Anderson insulating topological phases. We base this explanation on the relationship of this general quantum walk to a simpler case more studied in the literature and for which disorder-induced delocalization of a topological origin has been observed. We review topological delocalization for the simpler quantum walk, using time evolution of the wave functions and level spacing statistics. We apply scattering theory to two-dimensional quantum walks and thus calculate the topological invariants of disordered quantum walks, substantiating the topological interpretation of the delocalization and finding signatures of the delocalization in the finite-size scaling of transmission. We show criticality of the Haar random quantum walk by calculating the critical exponent eta in three different ways and find eta approximate to 0.52 as in the integer quantum Hall effect. Our results showcase how theoretical ideas and numerical tools from solid-state physics can help us understand spatially random quantum walks.11Nsciescopu
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