### Browsing by Author "Autti, S."

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Item AC Josephson effect between two superfluid time crystals(Nature Publishing Group, 2021-02) Autti, S.; Heikkinen, P. J.; Mäkinen, J. T.; Volovik, G. E.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsQuantum time crystals are systems characterized by spontaneously emerging periodic order in the time domain(1). While originally a phase of broken time translation symmetry was a mere speculation(2), a wide range of time crystals has been reported(3-5). However, the dynamics and interactions between such systems have not been investigated experimentally. Here we study two adjacent quantum time crystals realized by two magnon condensates in superfluid(3)He-B. We observe an exchange of magnons between the time crystals leading to opposite-phase oscillations in their populations-a signature of the AC Josephson effect(6)-while the defining periodic motion remains phase coherent throughout the experiment. Our results demonstrate that time crystals obey the general dynamics of quantum mechanics and offer a basis to further investigate the fundamental properties of these phases, opening pathways for possible applications in developing fields, such as quantum information processing. Two adjacent quantum time crystals implemented by two magnon condensates in the superfluid B-phase of helium-3 are observed to coherently exchange magnons as a manifestation of the AC Josephson effect, offering insights on the dynamics and interactions between these phases of matter.Item Bose-Einstein Condensation of Magnons and Spin Superfluidity in the Polar Phase of HE 3(2018-07-12) Autti, S.; Dmitriev, V. V.; Mäkinen, J. T.; Rysti, J.; Soldatov, A. A.; Volovik, G. E.; Yudin, A. N.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Russian Academy of SciencesThe polar phase of He3, which is topological spin-triplet superfluid with the Dirac nodal line in the spectrum of Bogoliubov quasiparticles, has been recently stabilized in a nanoconfined geometry. We pump magnetic excitations (magnons) into the sample of polar phase and observe how they form a Bose-Einstein condensate, revealed by coherent precession of the magnetization of the sample. Spin superfluidity, which supports this coherence, is associated with the spontaneous breaking of U(1) symmetry by the phase of precession. We observe the corresponding Nambu-Goldstone boson and measure its mass emerging when applied rf field violates the U(1) symmetry explicitly. We suggest that the magnon BEC in the polar phase is a powerful probe for topological objects such as vortices and solitons and topological nodes in the fermionic spectrum.Item Light Higgs channel of the resonant decay of magnon condensate in superfluid He-3-B(2016-01) Zavyalov, Vladislav; Autti, S.; Eltsov, V. B.; Heikkinen, P. J.; Volovik, G. E.; Department of Applied Physics; Topological Quantum FluidsIn superfluids the order parameter, which describes spontaneous symmetry breaking, is an analogue of the Higgs field in the Standard Model of particle physics. Oscillations of the field amplitude are massive Higgs bosons, while oscillations of the orientation are massless Nambu-Goldstone bosons. The 125 GeV Higgs boson, discovered at Large Hadron Collider, is light compared with electroweak energy scale. Here, we show that such light Higgs exists in superfluid He-3-B, where one of three Nambu-Goldstone spin-wave modes acquires small mass due to the spin-orbit interaction. Other modes become optical and acoustic magnons. We observe parametric decay of Bose-Einstein condensate of optical magnons to light Higgs modes and decay of optical to acoustic magnons. Formation of a light Higgs from a Nambu-Goldstone mode observed in He-3-B opens a possibility that such scenario can be realized in other systems, where violation of some hidden symmetry is possible, including the Standard Model.Item Magnon Bose-Einstein condensates : From time crystals and quantum chromodynamics to vortex sensing and cosmology(American Institute of Physics, 2024-03-04) Mäkinen, J. T.; Autti, S.; Eltsov, V. B.; Department of Applied Physics; OtaNano; Topological Quantum FluidsUnder suitable experimental conditions, collective spin-wave excitations, magnons, form a Bose-Einstein condensate (BEC), where the spins precess with a globally coherent phase. Bose-Einstein condensation of magnons has been reported in a few systems, including superfluid phases of 3He, solid state systems, such as yttrium-iron-garnet films, and cold atomic gases. The superfluid phases of 3He provide a nearly ideal test bench for coherent magnon physics owing to experimentally proven spin superfluidity, the long lifetime of the magnon condensate, and the versatility of the accessible phenomena. We first briefly recap the properties of the different magnon BEC systems, with focus on superfluid 3He. The main body of this review summarizes recent advances in the application of magnon BEC as a laboratory to study basic physical phenomena connecting to diverse areas from particle physics and cosmology to vortex dynamics and new phases of condensed matter. This line of research complements the ongoing efforts to utilize magnon BECs as probes and components for potentially room-temperature quantum devices. In conclusion, we provide a roadmap for future directions in the field of applications of magnon BEC to fundamental research.Item Nonlinear two-level dynamics of quantum time crystals(Nature Publishing Group, 2022-06-02) Autti, S.; Heikkinen, P. J.; Nissinen, J.; Mäkinen, J. T.; Volovik, G. E.; Zavyalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Quantum Circuits and CorrelationsA time crystal is a macroscopic quantum system in periodic motion in its ground state. In our experiments, two coupled time crystals consisting of spin-wave quasiparticles (magnons) form a macroscopic two-level system. The two levels evolve in time as determined intrinsically by a nonlinear feedback, allowing us to construct spontaneous two-level dynamics. In the course of a level crossing, magnons move from the ground level to the excited level driven by the Landau-Zener effect, combined with Rabi population oscillations. We demonstrate that magnon time crystals allow access to every aspect and detail of quantum-coherent interactions in a single run of the experiment. Our work opens an outlook for the detection of surface-bound Majorana fermions in the underlying superfluid system, and invites technological exploitation of coherent magnon phenomena – potentially even at room temperature.Item Observation of a Time Quasicrystal and Its Transition to a Superfluid Time Crystal(2018-05-25) Autti, S.; Eltsov, V. B.; Volovik, G. E.; Department of Applied Physics; Topological Quantum FluidsWe report experimental realization of a quantum time quasicrystal and its transformation to a quantum time crystal. We study Bose-Einstein condensation of magnons, associated with coherent spin precession, created in a flexible trap in superfluid He3-B. Under a periodic drive with an oscillating magnetic field, the coherent spin precession is stabilized at a frequency smaller than that of the drive, demonstrating spontaneous breaking of discrete time translation symmetry. The induced precession frequency is incommensurate with the drive, and hence, the obtained state is a time quasicrystal. When the drive is turned off, the self-sustained coherent precession lives a macroscopically long time, now representing a time crystal with broken symmetry with respect to continuous time translations. Additionally, the magnon condensate manifests spin superfluidity, justifying calling the obtained state a time supersolid or a time supercrystal.Item Observation of Half-Quantum Vortices in Topological Superfluid 3He(2016-12-14) Autti, S.; Dmitriev, V. V.; Mäkinen, J. T.; Soldatov, A. A.; Volovik, G. E.; Yudin, A. N.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Russian Academy of Sciences; Moscow Institute of Physics and TechnologyOne of the most sought-after objects in topological quantum-matter systems is a vortex carrying half a quantum of circulation. They were originally predicted to exist in superfluid He3−A but have never been resolved there. Here we report an observation of half-quantum vortices (HQVs) in the polar phase of superfluid He3. The vortices are created with rotation or by the Kibble-Zurek mechanism and identified based on their nuclear magnetic resonance signature. This discovery provides a pathway for studies of unpaired Majorana modes bound to the HQV cores in the polar-distorted A phase.Item Propagation of self-localized Q-ball solitons in the 3He universe(2018-01-22) Autti, S.; Heikkinen, P. J.; Volovik, G. E.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsIn relativistic quantum field theories, compact objects of interacting bosons can become stable owing to conservation of an additive quantum number Q. Discovering such Q balls propagating in the universe would confirm supersymmetric extensions of the standard model and may shed light on the mysteries of dark matter, but no unambiguous experimental evidence exists. We have created long-lived Q-ball solitons in superfluid He3, where the role of the Q ball is played by a Bose-Einstein condensate of magnon quasiparticles. The principal qualitative attribute of a Q ball is observed experimentally: its propagation in space together with the self-created potential trap. Additionally, we show that this system allows for a quantitatively accurate representation of the Q-ball Hamiltonian. Our Q ball belongs to the class of the Friedberg-Lee-Sirlin Q balls with an additional neutral field ζ, which is provided by the orbital part of the Nambu-Goldstone mode. Multiple Q balls can be created in the experiment, and we have observed collisions between them. This set of features makes the magnon condensates in superfluid 3He a versatile platform for studies of Q-ball dynamics and interactions in three spatial dimensions.Item Rotating quantum wave turbulence(Nature Publishing Group, 2023-06) Mäkinen, J. T.; Autti, S.; Heikkinen, P. J.; Hosio, J. J.; Hänninen, R.; L’vov, V. S.; Walmsley, P. M.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; OtaNano; Topological Quantum Fluids; Weizmann Institute of Science; University of Manchester; Lancaster UniversityTurbulence under strong influence of rotation is described as an ensemble of interacting inertial waves across a wide range of length scales. In macroscopic quantum condensates, the quasiclassical turbulent dynamics at large scales is altered at small scales, where the quantization of vorticity is essential. The nature of this transition remains an unanswered question. Here we expand the concept of wave-driven turbulence to rotating quantum fluids where the spectrum of waves extends to microscopic scales as Kelvin waves on quantized vortices. We excite inertial waves at the largest scale by periodic modulation of the angular velocity and observe dissipation-independent transfer of energy to smaller scales and the eventual onset of the elusive Kelvin wave cascade at the lowest temperatures. We further find that energy is pumped to the system through a boundary layer distinct from the classical Ekman layer and support our observations with numerical simulations. Our experiments demonstrate a regime of turbulent motion in quantum fluids where the role of vortex reconnections can be neglected, thus stripping the transition between the classical and the quantum regimes of turbulence down to its constituent components.Item Self-trapping of magnon Bose-Einstein Condensates in the ground state and on excited levels: from harmonic to box confinement(2012) Autti, S.; Bunkov, Yu.M.; Eltsov, V.B.; Heikkinen, P.J.; Hosio, J.J.; Hunger, P.; Krusius, M.; Volovik, G.E.; O.V.Lounasmaa-laboratorioLong-lived coherent spin precession of 3He−B at low temperatures around 0.2Tc is a manifestation of Bose-Einstein condensation of spin-wave excitations or magnons in a magnetic trap which is formed by the order-parameter texture and can be manipulated experimentally. When the number of magnons increases, the orbital texture reorients under the influence of the spin-orbit interaction and the profile of the trap gradually changes from harmonic to a square well, with walls almost impenetrable to magnons. This is the first experimental example of Bose condensation in a box. By selective rf pumping the trap can be populated with a ground-state condensate or one at any of the excited energy levels. In the latter case the ground state is simultaneously populated by relaxation from the exited level, forming a system of two coexisting condensates.Item Suppressing the Kibble-Zurek Mechanism by a Symmetry-Violating Bias(American Physical Society, 2021-09-08) Rysti, J.; Mäkinen, J. T.; Autti, S.; Kamppinen, T.; Volovik, G. E.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsThe formation of topological defects in continuous phase transitions is driven by the Kibble-Zurek mechanism. Here we study the formation of single- and half-quantum vortices during transition to the polar phase of He3 in the presence of a symmetry-breaking bias provided by the applied magnetic field. We find that vortex formation is suppressed exponentially when the length scale associated with the bias field becomes smaller than the Kibble-Zurek length. We thus demonstrate an experimentally feasible shortcut to adiabaticity - an important aspect for further understanding of phase transitions as well as for engineering applications such as quantum computers or simulators.Item Vortex-mediated relaxation of magnon BEC into light Higgs quasiparticles(American Physical Society, 2021-09) Autti, S.; Heikkinen, P. J.; Laine, S. M.; Makinen, J. T.; Thuneberg, E.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Topological Quantum Fluids; Aalto University; University of Oulu; Yale University; Lancaster UniversityA magnon Bose-Einstein condensate (BEC) in superfluid He-3 is a fine instrument for studying the surrounding macroscopic quantum system. At zero temperature, the BEC is subject to a few distinct forms of decay into other collective excitations, owing to momentum and energy conservation in a quantum vacuum. We study the vortex-Higgs mechanism: The vortices relax the requirement for momentum conservation, allowing the optical magnons of the BEC to transform into light Higgs quasiparticles. This facilitates a direct measurement of the dimensions of the B-phase double-core vortex, providing experimental access to elusive phenomena, such as the Kelvin wave cascade and core-bound Majorana fermions. Our paper expands the spectrum of possible interactions between magnetic quasiparticles in He-3-B and lays the groundwork for building magnon-based quantum devices.