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  • 1.
    Cattaneo, Roger
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Borodianskyi, Evgenii A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kalenyuk, Aleksey A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. National Academy of Sciences of Ukraine, Ukraine.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Moscow Institute of Physics and Technology, Russia.
    Superconducting Terahertz Sources with 12% Power Efficiency2021Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 16, nr 6, artikkel-id L061001Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Low power efficiency is one of the main problems of terahertz (THz) sources, colloquially known as “the THz gap.” In this work we present prototypes of THz devices based on whisker crystals of a high-temperature superconductor Bi2Sr2CaCu2O8+δ with a record high-radiation power efficiency of 12% at a frequency of approximately 4 THz. We employ various on- and off-chip detection techniques and, in particular, use the radiative cooling phenomenon for accurate evaluation of the emission power. We conclude that such devices can be used for creation of tunable, monochromatic, cw, compact, and power-efficient THz sources.

  • 2. Galin, M. A.
    et al.
    Rudau, F.
    Borodianskyi, Evgenii A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kurin, V. V.
    Koelle, D.
    Kleiner, R.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Moscow Institute of Physics and Technology, Russia.
    Klushin, A. M.
    Direct Visualization of Phase-Locking of Large Josephson Junction Arrays by Surface Electromagnetic Waves2020Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 14, nr 2, artikkel-id 024051Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Phase-locking of oscillators leads to super-radiant amplification of the emission power. This is particularly important for development of terahertz sources, which suffer from low emission efficiency. In this work we study large Josephson junction arrays containing several thousand Nb-based junctions. Using low-temperature scanning laser microscopy, we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting themselves as global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon-type electromagnetic waves propagating at the electrode-substrate interface. Thus, we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.

  • 3.
    Galin, Mikhail A.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Institute for Physics of Microstructures RAS, Russia.
    Borodianskyi, Evgenii A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kurin, V. V.
    Shereshevskiy, I. A.
    Vdovicheva, N. K.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Klushin, A. M.
    Synchronization of Large Josephson-Junction Arrays by Traveling Electromagnetic Waves2018Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 9, nr 5, artikkel-id 054032Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

  • 4.
    Golod, Taras
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kapran, Olena M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Moscow Institute of Physics and Technology, State University, Russia.
    Planar Superconductor-Ferromagnet-Superconductor Josephson Junctions as Scanning-Probe Sensors2019Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 11, nr 1, artikkel-id 014062Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We propose a magnetic scanning-probe sensor based on a single-planar Josephson junction with a magnetic barrier. The planar geometry together with the high magnetic permeability of the barrier facilitates a double flux-focusing effect, which helps to guide magnetic flux into the junction and thus enhances field sensitivity of the sensor. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic Cu−Ni and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nanometer scale and facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feedback operation over a broad dynamic range. We argue that the proposed sensor can outperform equally sized superconducting quantum-interference devices (SQUIDs) both in terms of magnetic-field sensitivity and spatial resolution, which makes it advantageous for scanning-probe microscopy.

  • 5.
    Grebenchuk, S. Yu
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Institute for Physics of Microstructures RAS, Russia.
    Cattaneo, Roger
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nonlocal Long-Range Synchronization of Planar Josephson-Junction Arrays2022Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 17, nr 6, artikkel-id 064032Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study arrays of planar Nb Josephson junctions with contacts to intermediate electrodes, which allow measurements of individual junctions and, thus, provide an insight into intricate array dynamics. We observe strong indications for array phase locking, despite a significant interjunction separation. Several unusual phenomena are reported, such as a bistable critical current with reentrant superconductivity upon switching of nearby junctions; and “incorrect” Shapiro steps, occurring at mixing frequencies between the external rf radiation and the internal Josephson frequency in nearby junctions. Our results reveal a surprisingly strong and long-range interjunction interaction, which is attributed to nonlocality of planar-junction electrodynamics, caused by the long-range spreading of stray electromagnetic fields. The nonlocality greatly enhances the high-frequency interjunction coupling and enables large-scale synchronization. Therefore, we conclude that planar geometry is advantageous for the realization of coherent Josephson electronics.

  • 6.
    Hovhannisyan, Razmik A.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Golod, Taras
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Superresolution magnetic imaging by a Josephson junction via holographic reconstruction of I c ( H ) modulation2023Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 20, nr 6, artikkel-id 064012Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work provides a proof -of -concept for superresolution magnetic imaging using a single Josephson junction. The technique resembles digital holography: magnetic patterns are obtained via an inverseproblem solution from diffractionlike modulation of the junction's critical current, I c (H) . We demonstrate numerical reconstruction of complex two-dimensional patterns, verify the technique experimentally using Nb-based planar junctions, and fabricate an operational sensor on a cantilever. Our results show that Josephson holography allows for both high spatial resolution (approximately 20 nm) and high field sensitivity (approximately 10 - 11 T R root Hz), thus resolving the trade-off problem between resolution and sensitivity in magnetic scanning probe imaging.

  • 7. Hu, Chang-Kang
    et al.
    Yuan, Jiahao
    Veloso, Bruno A.
    Qiu, Jiawei
    Zhou, Yuxuan
    Zhang, Libo
    Chu, Ji
    Nurbolat, Orkesh
    Hu, Ling
    Li, Jian
    Xu, Yuan
    Zhong, Youpeng
    Liu, Song
    Yan, Fei
    Tan, Dian
    Bachelard, R.
    Santos, Alan C.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Universidade Federal de São Carlos, São Carlos, Brazil.
    Villas-Boas, C. J.
    Yu, Dapeng
    Native Conditional iSWAP Operation with Superconducting Artificial Atoms2023Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 20, nr 3, artikkel-id 034072Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Controlling the flow of quantum information is a fundamental task for quantum computers, which is unfeasible to realize on classical devices. Coherent devices, which can process quantum states are thus required to route the quantum states that encode information. In this paper we demonstrate experimentally the smallest quantum transistor with a superconducting quantum processor, which is composed of a collector qubit, an emitter qubit, and a coupler (transistor gate). The interaction strength between the collector and emitter qubits is controlled by the frequency and state of the coupler, effectively implementing a quantum switch. Through the coupler-state-dependent Heisenberg (inherent) interaction between the qubits, a single-step (native) conditional iSWAP operation can be applied. To this end, we find that it is useful to take into consideration the higher-energy level for achieving a native and high-fidelity transistor operation. By reconstructing the quantum process tomography, we obtain an operation fidelity of 92.36% when the transistor gate is open (iSWAP implementation) and 95.23% in the case of closed gate (identity gate implementation). The architecture has strong potential in quantum information processing applications with superconducting qubits.

  • 8.
    Piveteau, Amélie
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Abbott, Alastair A.
    Sadiq, Muhammad
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bourennane, Mohamed
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Tavakoli, Armin
    Weak entanglement improves quantum communication using only product measurements2024Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 21, nr 3, artikkel-id 034053Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show that weakly entangled states can improve communication over a qubit channel using only separate, interference-free, measurements of individual photons. We introduce a communication task corresponding to the cryptographic primitive known as secret sharing and show that all steerable two-qubit isotropic states provide a quantum advantage in the success rate using only product measurements. Furthermore, we show that such measurements can even reveal communication advantages from noisy partially entangled states that admit no quantum steering. We then go further and consider a stochastic variant of secret sharing based on more-sophisticated, yet standard, partial Bell-state analyzers, and show that this reveals advantages also for a range of unsteerable isotropic states. By preparing polarization qubits in unsteerable states, we experimentally demonstrate increased success rates of both secret-sharing tasks beyond the best entanglement-unassisted qubit protocol. Our results reveal the capability of simple and scalable measurements in entanglement-assisted quantum communication to overcome large amounts of noise.

  • 9. Tsuneto, Makoto
    et al.
    Jing, Ran
    Chen, Xinzhong
    Kaushik, Sahal
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Stony Brook University, USA.
    Yao, Juntao
    Kharzeev, Dmitri E.
    Du, Xu
    Li, Qiang
    Liu, Mengkun
    Anomalous thermal effect in ZrTe5 observed via photothermal measurements2024Inngår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 21, nr 3, artikkel-id 034001Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, we explore the magnetothermoelectric power (MTP) of ZrTe5, a canonical Dirac semimetal, through a photothermal technique. Unlike conventional thermoelectric studies that rely on onchip heaters and are limited by fabrication processes, especially for stress -sensitive materials, our approach utilizes photothermal effects to induce temperature gradients. Our experiments, applying a magnetic field approximately parallel and transverse to the photocurrent detection direction, reveal that the photothermal method efficiently and reliably extracts both diagonal and off -diagonal components of the thermoelectric coefficient of ZrTe5. We observe that the longitudinal MTP reproduces features previously reported in thermal transport studies, while the photoinduced transverse MTP confirms the anomalous Nernst effect. This photothermal measurement technique opens avenues for investigating transport properties in a wide range of quantum materials, in both three-dimensional and two-dimensional systems.

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