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Publication Open Access A photonic Carnot engine powered by a spin-star network(European Physical Society (EPS), 2017) Türkpençe, Deniz; Paternostro, Mauro; Department of Physics; Altıntaş, Ferdi; Müstecaplıoğlu, Özgür Esat; Researcher; Faculty Member; Department of Physics; College of Sciences; N/A; 1674We propose a spin-star network, where a central spin-(1/2), acting as a quantum fuel, is coupled to N outer spin-(1/2) particles. If the network is in thermal equilibrium with a heat bath, the central spin can have an effective temperature, higher than that of the bath, scaling nonlinearly with N. Such temperature can be tuned with the anisotropy parameter of the coupling. Using a beam of such central spins to pump a micromaser cavity, we determine the dynamics of the cavity field using a coarse-grained master equation. We find that the central-spin beam effectively acts as a hot reservoir to the cavity field and brings it to a thermal steady state whose temperature benefits from the same nonlinear enhancement with N and results in a highly efficient photonic Carnot engine. The validity of our conclusions is tested against the presence of atomic and cavity damping using a microscopic master equation method for typical microwave cavity-QED parameters. The role played by quantum coherence and correlations on the scaling effect is pointed out. An alternative scheme where the spin-(1/2) is coupled to a macroscopic spin-(N/2) particle is also discussed. Copyright (C) EPLA, 2017Publication Open Access All optical control of magnetization in quantum confined ultrathin magnetic metals(Nature Publishing Group (NPG), 2021) Department of Physics; Department of Electrical and Electronics Engineering; N/A; Müstecaplıoğlu, Özgür Esat; Onbaşlı, Mehmet Cengiz; Naseem, Muhammad Tahir; Zanjani, Saeedeh Mokarian; Faculty Member; Faculty Member; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; College of Engineering; Graduate School of Sciences and Engineering; 1674; 258783; N/A; N/AAll-optical control dynamics of magnetization in sub-10 nm metallic thin films are investigated, as these films with quantum confinement undergo unique interactions with femtosecond laser pulses. Our theoretical analysis based on the free electron model shows that the density of states at Fermi level (DOSF) and electron-phonon coupling coefficients (G(ep)) in ultrathin metals have very high sensitivity to film thickness within a few angstroms. We show that completely different magnetization dynamics characteristics emerge if DOSF and G(ep) depend on thickness compared with bulk metals. Our model suggests highly efficient energy transfer from femtosecond laser photons to spin waves due to minimal energy absorption by phonons. This sensitivity to the thickness and efficient energy transfer offers an opportunity to obtain ultrafast on-chip magnetization dynamics.Publication Open Access Applications of augmented reality in ophthalmology [invited](Optical Society of America (OSA), 2021) Artal, Pablo; Department of Physics; Department of Electrical and Electronics Engineering; Aydındoğan, Güneş; Kavaklı, Koray; Ürey, Hakan; Şahin, Afsun; Faculty Member; Faculty Member; Department of Physics; Department of Electrical and Electronics Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; College of Engineering; School of Medicine; N/A; N/A; 8579; 171267Throughout the last decade, augmented reality (AR) head-mounted displays (HMDs) have gradually become a substantial part of modern life, with increasing applications ranging from gaming and driver assistance to medical training. Owing to the tremendous progress in miniaturized displays, cameras, and sensors, HMDs are now used for the diagnosis, treatment, and follow-up of several eye diseases. In this review, we discuss the current state-of-the-art as well as potential uses of AR in ophthalmology. This review includes the following topics: (i) underlying optical technologies, displays and trackers, holography, and adaptive optics; (ii) accommodation, 3D vision, and related problems such as presbyopia, amblyopia, strabismus, and refractive errors; (iii) AR technologies in lens and corneal disorders, in particular cataract and keratoconus; (iv) AR technologies in retinal disorders including age-related macular degeneration (AMD), glaucoma, color blindness, and vision simulators developed for other types of low-vision patients.Publication Open Access Artificial gauge fields for the Bose-Hubbard model on a checkerboard superlattice and extended Bose-Hubbard model(Springer, 2012) Department of Physics; Işkın, Menderes; Faculty Member; Department of Physics; College of Sciences; 29659We study the effects of an artificial gauge field on the ground-state phases of the Bose-Hubbard model on a checkerboard superlattice in two dimensions, including the superfluid phase and the Mott and alternating Mott insulators. First, we discuss the single-particle Hofstadter problem, and show that the presence of a checkerboard superlattice gives rise to a magnetic flux-independent energy gap in the excitation spectrum. Then, we consider the many-particle problem, and derive an analytical mean-field expression for the superfluid-Mott and superfluid-alternating-Mott insulator phase transition boundaries. Finally, since the phase diagram of the Bose-Hubbard model on a checkerboard superlattice is in many ways similar to that of the extended Bose-Hubbard model, we comment on the effects of magnetic field on the latter model, and derive an analytical mean-field expression for the superfluid-insulator phase transition boundaries as well.Publication Open Access Biophotonic sensor applications based on photonic atoms - art. no. 60990Q(Society of Photo-optical Instrumentation Engineers (SPIE), 2006) Demir, Abdullah; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Microsphere resonators, i.e., photonic atoms, have found wide area of application in optical spectroscopy, quantum optics, cavity QED, switching, and sensing. Photonic atoms have unique optical properties such as high quality factor (Q-factor) morphology dependent resonances (MDR's), and relatively small volumes. High-Q MDR's are very sensitive to the refractive index change and microsphere uniformity. These tiny optical cavities, whose diameters vary from a few to several hundred micrometers, have resonances with reported Q-factors as large as 3x10(9). Due to their sensitivity, MDR's are also considered for biosensor applications. Binding of a protein or other biomolecules can be monitored by observing the wavelength shift of MDR's. A biosensor, based on this optical phenomenon, can even detect a single molecule, depending on the quality of the system. In this work, elastic scattering spectra from photonic atoms of different materials are experimentally obtained and MDR'S are observed. Preliminary results of unspecific binding of biomolecules are presented. Elastic light scattering spectra of MDR's for biosensor applications are calculated numerically for biomolecules such as Bovine Serum Albumin (BSA) and for Deoxyribo Nucleic Acid (DNA).Publication Open Access Black holes, gravitational waves and fundamental physics: a roadmap(Institute of Physics (IOP) Publishing, 2019) Barack, Leor; Cardoso, Vitor; Nissanke, Samaya; Sotiriou, Thomas P.; Askar, Abbas; Belczynski, Chris; Bertone, Gianfranco; Bon, Edi; Blas, Diego; Brito, Richard; Bulik, Tomasz; Burrage, Clare; Byrnes, Christian T.; Caprini, Chiara; Chernyakova, Masha; Chrusciel, Piotr; Colpi, Monica; Ferrari, Valeria; Gaggero, Daniele; Gair, Jonathan; Garcia-Bellido, Juan; Hassan, S. F.; Heisenberg, Lavinia; Hendry, Martin; Heng, Ik Siong; Herdeiro, Carlos; Hinderer, Tanja; Horesh, Assaf; Kavanagh, Bradley J.; Kocsis, Bence; Kramer, Michael; Le Tiec, Alexandre; Mingarelli, Chiara; Nardini, Germano; Nelemans, Gijs; Palenzuela, Carlos; Pani, Paolo; Perego, Albino; Porter, Edward K.; Rossi, Elena M.; Schmidt, Patricia; Sesana, Alberto; Sperhake, Ulrich; Stamerra, Antonio; Stein, Leo C.; Tamanini, Nicola; Tauris, Thomas M.; Arturo Urena-Lopez, L.; Vincent, Frederic; Volonteri, Marta; Wardell, Barry; Wex, Norbert; Yagi, Kent; Abdelsalhin, Tiziano; Angel Aloy, Miguel; Amaro-Seoane, Pau; Annulli, Lorenzo; Arca-Sedda, Manuel; Bah, Ibrahima; Barausse, Enrico; Barakovic, Elvis; Benkel, Robert; Bennett, Charles L.; Bernard, Laura; Bernuzzi, Sebastiano; Berry, Christopher P. L.; Berti, Emanuele; Bezares, Miguel; Juan Blanco-Pillado, Jose; Blazquez-Salcedo, Jose Luis; Bonetti, Matteo; Boskovic, Mateja; Bosnjak, Zeljka; Bricman, Katja; Bruegmann, Bernd; Capelo, Pedro R.; Carloni, Sante; Cerda-Duran, Pablo; Charmousis, Christos; Chaty, Sylvain; Clerici, Aurora; Coates, Andrew; Colleoni, Marta; Collodel, Lucas G.; Compere, Geoffrey; Cook, William; Cordero-Carrion, Isabel; Correia, Miguel; de la Cruz-Dombriz, Alvaro; Czinner, Viktor G.; Destounis, Kyriakos; Dialektopoulos, Kostas; Doneva, Daniela; Dotti, Massimo; Drew, Amelia; Eckner, Christopher; Edholm, James; Emparan, Roberto; Erdem, Recai; Ferreira, Miguel; Ferreira, Pedro G.; Finch, Andrew; Font, Jose A.; Franchini, Nicola; Fransen, Kwinten; Gal'tsov, Dmitry; Ganguly, Apratim; Gerosa, Davide; Glampedakis, Kostas; Gomboc, Andreja; Goobar, Ariel; Gualtieri, Leonardo; Guendelman, Eduardo; Haardt, Francesco; Harmark, Troels; Hejda, Filip; Hertog, Thomas; Hopper, Seth; Husa, Sascha; Ihanec, Nada; Ikeda, Taishi; Jaodand, Amruta; Jetzer, Philippe; Jimenez-Forteza, Xisco; Kamionkowski, Marc; Kaplan, David E.; Kazantzidis, Stelios; Kimura, Masashi; Kobayashi, Shiho; Kokkotas, Kostas; Krolik, Julian; Kunz, Jutta; Laemmerzahl, Claus; Lasky, Paul; Lemos, Jose P. S.; Said, Jackson Levi; Liberati, Stefano; Lopes, Jorge; Luna, Raimon; Ma, Yin-Zhe; Maggio, Elisa; Mangiagli, Alberto; Montero, Marina Martinez; Maselli, Andrea; Mayer, Lucio; Mazumdar, Anupam; Messenger, Christopher; Menard, Brice; Minamitsuji, Masato; Moore, Christopher J.; Mota, David; Nampalliwar, Sourabh; Nerozzi, Andrea; Nichols, David; Nissimov, Emil; Obergaulinger, Martin; Obers, Niels A.; Oliveri, Roberto; Pappas, George; Pasic, Vedad; Peiris, Hiranya; Petrushevska, Tanja; Pollney, Denis; Pratten, Geraint; Rakic, Nemanja; Racz, Istvan; Radia, Miren; Ramazanoglu, Fethi M.; Ramos-Buades, Antoni; Raposo, Guilherme; Rogatko, Marek; Rosca-Mead, Roxana; Rosinska, Dorota; Rosswog, Stephan; Ruiz-Morales, Ester; Sakellariadou, Mairi; Sanchis-Gual, Nicolas; Salafia, Om Sharan; Samajdar, Anuradha; Sintes, Alicia; Smole, Majda; Sopuerta, Carlos; Souza-Lima, Rafael; Stalevski, Marko; Stergioulas, Nikolaos; Stevens, Chris; Tamfal, Tomas; Torres-Forne, Alejandro; Tsygankov, Sergey; Valiante, Rosa; van de Meent, Maarten; Velhinho, Jose; Verbin, Yosef; Vercnocke, Bert; Vernieri, Daniele; Vicente, Rodrigo; Vitagliano, Vincenzo; Weltman, Amanda; Whiting, Bernard; Williamson, Andrew; Witek, Helvi; Wojnar, Aneta; Yakut, Kadri; Yan, Haopeng; Yazadjiev, Stoycho; Zaharijas, Gabrijela; Zilhao, Miguel; Department of Physics; Ramazanoğlu, Fethi Mübin; Ünlütürk, Kıvanç İbrahim; Faculty Member; PhD Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 254225; N/AThe grand challenges of contemporary fundamental physics dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions.The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature.The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.Publication Open Access Cadmium-free and efficient Type-II InP/ZnO/ZnS quantum dots and their application for LEDs(American Chemical Society (ACS), 2021) Ritter, Maximilian; Şahin, Mehmet; Ow-Yang, Cleva W.; Lechner, Rainer T.; Department of Electrical and Electronics Engineering; Department of Physics; N/A; N/A; Nizamoğlu, Sedat; Sennaroğlu, Alphan; Eren, Güncem Özgün; Sadeghi, Sadra; Jalali, Houman Bahmani; Han, Mertcan; Toker, Işınsu Baylam; Melikov, Rustamzhon; Önal, Asım; Öz, Fatma; Faculty Member; Faculty Member; PhD Student; PhD Student; Master Student; PhD Student; Department of Electrical and Electronics Engineering; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); College of Engineering; College of Sciences; Graduate School of Sciences and Engineering; 130295; 23851; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/AIt is a generally accepted perspective that type-II nanocrystal quantum dots (QDs) have low quantum yield due to the separation of the electron and hole wavefunctions. Recently, high quantum yield levels were reported for cadmium-based typeII QDs. Hence, the quest for finding non-toxic and efficient type-II QDs is continuing. Herein, we demonstrate environmentally benign type-II InP/ZnO/ZnS core/shell/shell QDs that reach a high quantum yield of similar to 91%. For this, ZnO layer was grown on core InP QDs by thermal decomposition, which was followed by a ZnS layer via successive ionic layer adsorption. The small-angle Xray scattering shows that spherical InP core and InP/ZnO core/ shell QDs turn into elliptical particles with the growth of the ZnS shell. To conserve the quantum efficiency of QDs in device architectures, InP/ZnO/ZnS QDs were integrated in the liquid state on blue light-emitting diodes (LEDs) as down-converters that led to an external quantum efficiency of 9.4% and a power conversion efficiency of 6.8%, respectively, which is the most efficient QD-LED using type-II QDs. This study pointed out that cadmium-free type-II QDs can reach high efficiency levels, which can stimulate novel forms of devices and nanomaterials for bioimaging, display, and lighting.Publication Open Access Collectively enhanced thermalization via multiqubit collisions(American Physical Society (APS), 2019) Niedenzu, Wolfgang; Kurizki, Gershon; Department of Physics; Müstecaplıoğlu, Özgür Esat; Roman-Ancheyta, Ricardo; Çakmak, Barış; Manatuly, A.; Faculty Member; Researcher; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 1674; N/A; 252838; N/AWe investigate the evolution of a target qubit caused by its multiple random collisions with N-qubit clusters. Depending on the cluster state, the evolution of the target qubit may correspond to its effective interaction with a thermal bath, a coherent (laser) drive, or a squeezed bath. In cases where the target qubit relaxes to a thermal state, its dynamics can exhibit a quantum advantage, whereby the target-qubit temperature can be scaled up proportionally to N-2 and the thermalization time can be shortened by a similar factor, provided the appropriate coherence in the cluster is prepared by nonthermal means. We dub these effects quantum superthermalization because of the analogies to superradiance. Experimental realizations of these effects are suggested.Publication Open Access Collisional unfolding of quantum Darwinism(American Physical Society (APS), 2019) Campbell, Steve; Paternostro, Mauro; Vacchini, Bassano; Department of Physics; Müstecaplıoğlu, Özgür Esat; Çakmak, Barış; Faculty Member; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 1674; 252838We examine the emergence of objectivity via quantum Darwinism through the use of a collision model, i.e., where the dynamics is modeled through sequences of unitary interactions between the system and the individual constituents of the environment, termed "ancillas." By exploiting versatility of this framework, we show that one can transition from a "Darwinistic" to an "encoding" environment by simply tuning their interaction. Furthermore, we establish that in order for a setting to exhibit quantum Darwinism we require a mutual decoherence to occur between the system and environmental ancillas, thus showing that system decoherence alone is not sufficient. Finally, we demonstrate that the observation of quantum Darwinism is sensitive to a nonuniform system-environment interaction.Publication Open Access Continuous dynamical decoupling and decoherence-free subspaces for qubits with tunable interaction(Springer, 2019) Yalçınkaya, İskender; Karpat, Göktuğ; Fanchini, F. F.; Department of Physics; Çakmak, Barış; Department of Physics; College of Sciences; 252838Protecting quantum states from the decohering effects of the environment is of great importance for the development of quantum computation devices and quantum simulators. Here, we introduce a continuous dynamical decoupling protocol that enables us to protect the entangling gate operation between two qubits from the environmental noise. We present a simple model that involves two qubits which interact with each other with a strength that depends on their mutual distance and generates the entanglement among them, as well as in contact with an environment. The nature of the environment, that is, whether it acts as an individual or common bath to the qubits, is also controlled by the effective distance of qubits. Our results indicate that the introduced continuous dynamical decoupling scheme works well in protecting the entangling operation. Furthermore, under certain circumstances, the dynamics of the qubits naturally led them into a decoherence-free subspace which can be used complimentary to the continuous dynamical decoupling.