Participants

Universidad Rey Juan Carlos, Madrid, Spain

Progress in the control of a levitating sphere in superfluid He

Lancaster University, Lancaster, UK

Thermal dynamics in nanoelectronic devices cooled by on-chip magnetic refrigeration

On-chip demagnetization refrigeration has recently emerged as a powerful tool for reaching microkelvin electron temperatures in nanoscale structures and devices, but the relative importance of cooling on-chip and off-chip components remains an open question, and the intrinsic thermal transport dynamics of the materials involved are yet to be analyzed in this novel temperature range. Here we study demagnetization cooling of a Coulomb blockade thermometer with on-chip copper refrigerant only; the chip substrate and electrical connections remain at an elevated temperature. Comparing our experimental results with numerical simulations, we show that dynamics in this device are captured by a simple first-principles model of thermal transport between the subsystems. We use this model to show that the electron base temperature is only limited by heating from substrate lattice vibrations, outlining a recipe for a low-investment microkelvin platform for quantum technologies and fundamental nanoscience.

Heidelberg University, Heidelberg, Germany

Influence of the Host Material on the Calorimetrically Measured Be-7 Electron Capture Spectrum

In electron capture processes, an electron of the parent atom is captured by the nucleus and an electron neutrino is emitted, leaving the daughter atom in an excited state. Precise calculations of the atomic de-excitation processes can be performed for isolated atoms. In practice, radioactive decay occurs with the atom in a medium. We present the possibility to study effects the environment has on the Be-7 electron capture spectrum. Be-7 is the lightest nuclide to undergo electron capture, with a half-life of about 53 days and a Q-value of about 862 keV. For this study, Be-7 is ion-implanted into the three host materials gold, silver, and aluminum, which are each deposited onto the absorbers of low temperature metallic magnetic calorimeters. Our goal is to investigate the half-life, L/K ratio, and the energy distribution of the nuclear recoil for different host materials. The measured spectra will be compared to spectra derived by ab initio calculations. A deeper understanding of environment-induced effects on a decaying atom and a better model for nuclear recoils in a crystal will yield important results for experiments using Ho-163 for the determination of the neutrino mass scale, for the detection of coherent neutrino-nucleus scattering, and for the direct detection of dark matter.

Uzhhorod National University, Uzhorod, Ukraine

Relaxation phenomena in amorphous chalcogenide semiconductors

TU Wien, Vienna,  Austria

Quantum criticality in heavy fermion compounds

Royal Holloway University of London, London, UK

QUEST-DMC: Quantum enhanced superfluid technologies for Dark Matter and Cosmology

The QUEST-DMC project aims to address two fundamental questions in cosmology – what is the nature of dark matter and how did the early universe evolve. The two questions are linked through the requirement of beyond-standard model physics and the experimental approach of combining quantum sensors with ³He at ultralow temperatures. Here we report the progress towards these dual aims. Gravitational waves originating from a early universe first order electroweak-scale phase transition are predicted in many extensions of the Standard Model of particle physics. The space based interferometer LISA, planned for launch in 2037, has the potential to observe these waves. Numerical simulations of these phase transitions are reliant on nucleation theory and out of equilibrium dynamics that we will test in a quantum analogue system, the AB phase transition in superfluid ³He. Through precise control of the phase of the helium, by both nanoscale confinement and the creation of a bulk bubble with magnetic field, using quantum sensors to probe the helium we aim to resolve the nucleation puzzle in ³He. Simulation confronted with experiment will help to refine the models needed to interpret and predict the gravitational wave signatures for LISA. For models in which dark matter behaves as a particle, the possible mass range for a dark matter candidate spans many orders of magnitude. There exists a very well theoretically motivated sub GeV mass regime, that has been largely inaccessible to traditional dark matter searches. Using superfluid ³He at ultralow temperatures as a dark matter collision target we aim to improve on the sensitivity to spin-dependent interactions. The target cell will be instrumented with nanomechanical resonators to detect the thermal energy deposited by the collision. Andreev scattering around the moving nanobeam results in an enhancement in the the damping term of the resonator. Energy in the ionisation channel, leading to scintillation will be detected using transition edge sensors mounted around the cell.

Northwestern University, Evanston, Illinois, USA

Multiterminal superconducting proximity effect Josephson junctions

Institute of Experimental Physics SAS, Košice, Slovakia

Temperature calibration of resistive thermometers below 1 K in high magnetic field  using quartz tuning forks

We present a procedure how to perform a temperature calibration of the resistive thermometers (RuO₂ thermometers in our case) in high magnetic fields (up to 5 T) and temperature range below 1 K by means of a commercial tuning fork. Using a SQUID noise thermometer and a fix-point device, we performed a temperature calibration of the tuning fork resonance frequency and resistive thermometers in zero magnetic field. Then, we measured the temperature and field dependences of the resistive thermometers and tuning fork resonance frequencies. Applying known physical property of the tuning fork, in particular, that a scaled/normalized temperature dependence of the tuning fork’s resonance frequency is universal and magnetic field independent we performed the temperature calibration of the resistive thermometers in high magnetic fields and temperatures below 1 K.

University of Prešov, Prešov, Slovakia

Budapest Universty of Technology and Economics, Budapest, Hungary

Signatures of superconducting gating in epitaxial nanowires

Understanding the microscopic origin of the gate-controlled supercurrent (GCS) in superconducting nanobridges would play an important role in engineering convenient superconducting switches for various applications. The origin of this effect is not settled yet, therefore further measurements are needed. We have carefully investigated the GCS in a Ta and Al layers epitaxially grown on top of InAs nanowires. In this talk we will show thorough characterization of this effect and detailed switching current distribution measurements that can help to reveal the origin of GCS.

Aalto University, Helsinki, Finland

Nanoelectromechanical devices as probes of topological superfluid ³He

B.Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine (ILTPE), Kharkiv, Ukraine

Magnetic ordering in layered double hydroxides (LDH) via a low-temperature heat capacity and magnetization studies

Institute of Experimental Physics SAS, Košice, Slovakia

Investigation of quantum magnets in metallic tetraborides

Rare earth tetraborides crystallize in the Shastry-Sutherland lattice (SSL) which consists of orthogonal dimers. Such dimer quantum states are for more than 20 years under discussion for new information technologies [1] and the rare earth tetraborides can be a potential realization. The SSL in principle is a two-dimensional structure and one of the few cases where the phase diagram has been exactly solved [2]. Surprisingly, the first compound showing this structure, SrCu₂(BO₃)₂ exhibited exotic quantum states in form of fractional magnetization plateaus that have been explained in terms of topological physics [3]. Tetraborides with Ho and Tm ions show similar phenomena and are in contrast to SrCu₂(BO₃)₂ with critical field and temperature in a well accessible range (T_c = 6 – 15 K, B_c = 4 – 5 T, depending on the RE ion). Bulk properties of HoB₄, ErB₄ and TmB₄ are investigated in great detail (see e.g. [4, 5]). The zero-field ordering of all three compounds can be described by the dimer spin configuration, for HoB₄ and ErB₄ they are antiparallel, the dimers in TmB₄ are ferromagnetic with antiferromagnetic nearest neighbour dimers. In a magnetic field these structures break up into fractionalized  magnetization states with magnetization plateaus at 1/3, 4/9, 3/5 in HoB₄ and 1/7, 1/8, 1/9 in TmB₄. For TmB₄ the corresponding structure is fully solved by neutron diffraction [4], in the fractional phase one observes stripe like structures with q-spacings of 1/7, 1/8, unit cells that resemble domains walls, but strictly ordered. In higher field always a 1/2 plateau is reached, and the magnetic ordering again extends over 8 unit cells in TmB₄. The large, commensurate magnetic unit cells are difficult to explain with near neighbour interactions and not understood to date.

In comparison to the Heisenberg like Cu – spins in SrCu₂(BO₃)₂ the RE borides show crystal field anisotropy with preferred directions in [111] for HoB₄, whereas TmB₄ is a strong Ising system with preferred direction [001]. This renders the TmB₄ dimers as interesting qubit objects because Ising spins remain stable – there are no spin waves.

Results from bulk measurements show exotic behavior that despite numerous theoretical approaches to date is unexplained and it appears that new theoretical directions are necessary there. On the experimental side, one shortcoming so far is that all results are received from bulk samples, mostly single crystals. To move the experimental insight forward and, most important, to find a way to functionalize tetraborides, it is essential to study the 2D behavior. In an ideal world, one would study thin films created by epitaxial or other sputtering methods. To start such investigations, however, the use of clean, cleaved surfaces from bulk samples with surface sensitive methods like STM methods appears as a more realistic starting point.

[1] E. Shawish et al., Physical Review B75, 205442 (2007).
[2] B. Shastry, B. Sutherland, Physica B 108, 1069 (1981).
[3] S. E. Sebastian et al., Proceedings of the National Academy of Sciences 105, 20157 (2009).
[4] K. Siemensmeyer et al., Physical Review Letters 101, 177201 (2008).
[5] S. Gabáni et al., Journal of Alloys and Compounds 821, 153201 (2020).

University Bonn, Bonn, Germany

Quantum oscillations and signatures of electron interactions in the Dirac semi-metals ZrTe₅ and HfTe₅

University of Genova, Genova, Italy

Synthesis and crystal chemistry of YbCu_5-xM_x systems: the new member M = Zn

CNRS – Intitut Néel, Grenoble, France

Stochastic thermodynamics of a single nano-mechanical mode

Heidelberg University, Heidelberg, Germany

Investigating the Non-equilibrium Dynamics of two-level systems at Low Temperatures

The dielectric loss of amorphous materials along with noise and decoherence is the major limiting factor in many applications like superconducting circuits, Josephson junctions and quantum computing. It is mainly determined by atomic tunneling systems described by quantum mechanical two-level systems (TLS) which are broadly distributed low-energy excitations in the sample. The spontaneous phonon emission of an excited TLS gives rise to a relaxation time T1 and the interaction between TLSs with their thermally excited surrounding induces a dephasing timescale T2. These effects mainly determine the measurable dielectric loss in the observed material, which we ascertain by measuring the quality factor of a bridge type superconducting LC-resonator. The dielectric medium in between the capacitor plates is a sputter deposited a-SiO2 film. A variation of the Rabi-frequency through the electric field strength of the drive can thereby change the transition probability of the TLSs and thus the influence of loss generating effects. The setup shows a unique property when two off-resonant pump tones are applied symmetrically. In this limit, the resonator is emitting at the intermediate frequency of the driving fields. The underlying mechanism can therefore be explained by a nonlinear interaction of the rf-field with the TLSs and the resonator which is creating additional lines in the frequency spectrum. We present first measurements at a frequency of 1GHz performed with a micro-fabricated superconducting resonator.

Aalto University, Helsinki, Finland

Multipartite continuous-variable entanglement generation using Josephson metamaterials

Generation of quantum resources, most notably quantum entanglement, is an essential task for the new emerging industry employing quantum technologies. While entanglement in discrete variables represents the standard approach for quantum computing, continuous variable entanglement between microwave photons is a cornerstone for more robust quantum computing, sensing and communication schemes.

We have developed a low-loss Josephson metamaterial comprising superconducting, non-linear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 Giga entangled bits per second spanning over 4 GHz bandwidth. The device is operated as a travelling wave parametric amplifier under Kerr-relieving biasing conditions that allow us to generate microwave entanglement over previously inaccessible bandwidth. Furthermore, we successfully demonstrate single-mode squeezing in such devices, -3.1 dB below the zero-point level at half of the modulation frequency.

As we demonstrated, the broadband features of the TWPA allow operation over a few gigahertz bandwidth, and in combination with multiple pumps, pave the way toward the generation of “frequency-based” multimode entanglement. We propose a method for high-quality generation and control of entanglement between microwaves in multiple frequency ranges. Using the developed scheme, we present the first demonstration of an on-demand tunable entangled 3-partite and 4-partite states in a lumped-element Josephson parametric amplifier [1].

Multimode schemes can be employed for various quantum applications, such as CV computing with cluster states, secure and robust communications, distributed quantum-limited sensing, and search for dark matter [2]. We envision that generated quantum resources offer enhanced prospects for quantum data processing using parametric microwave cavities [3].

[1] K. Petrovnin et al., arXiv:2203.09247 (2022)

[2] M. Perelshtein, et al., arXiv:2111.06145 (2021)

[3] T. Elo et al., Appl. Phys. Lett. 114, 152601 (2019)

Heidelberg University, Heidelberg, Germany

Magnetic 1/f Noise in Superconducting Microstructures and the Fluctuation-Dissipation Theorem

The performance of superconducting devices like particle detectors, SQUIDs, and qubits is often limited by 1/f-noise and finite coherence times. Various types of slow fluctuators in the Josephson junctions and in the passive parts of these superconducting circuits can cause such noise, and devices usually suffer from a combination of different noise sources, which are hard to disentangle and therefore hard to eliminate. Magnetic flux noise caused by fluctuating magnetic moments of magnetic impurities or dangling bonds in superconducting inductances, surface oxides, insulating oxide layers, and adsorbates are a likely contribution in many cases. We present an experimental setup to measure both the complex impedance of superconducting microstructures, as well as the magnetic flux noise that is picked up by these structures. This allows for important sanity checks by connecting both quantities via the fluctuation-dissipation theorem. In order to allow for state-of-the-art sensitivity in both experiments, the structures under investigation are part of a Wheatstone-like bridge, read out by two cross-correlated independent dc-SQUID readout chains. We present measurements of the insulating SiO₂ layers of our devices, the superconducting structures themselves, and magnetically doped noble metal layers in the vicinity of the pickup coils at T = 20mK – 800mK and f = 100mHz – 100kHz.

Lancaster University, Lancaster, UK

Experiments with an individual quantum vortexes at mK temperatures

We are going to present our recent experiments with a single quantum vortexes in the helium-4 at mK temperatures. We demonstrate the real-time detection of quantum vortices by a nanoscale resonant beam in superfluid ⁴He at 10 mK. Essentially, we trap a single  vortex along the length of a nanobeam and observe the transitions as a vortex is either trapped or released, detected through the shift in the beam resonant frequency. By exciting a tuning fork, which is playing a role of the source of the turbulence, we control the ambient vortex density and follow its influence on the vortex capture and release rates demonstrating that these devices are capable of probing turbulence on the micron scale.

Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany

Royal Holloway University of London, London, UK

Electro-nuclear transition in YbRh₂Si₂; evidence for a spatially modulated electronic magnetism

Institute of Physics, Faculty of Sciences, P. J. Šafárik University, Košice, Slovakia

Interaction of electron beam with amorphous semiconductors

In my talk i will describe and analyze various phenomena occuring during interaction of chalcogenide amorphous semiconductors with electron beam with energy up to 30 keV. Among them electron induced material flow is one of the most peculiar ones. Various techniques such as synchrotron X-rays spectroscopy, Kelvin force microscopy and Atomic force microscopy were employed by us to develop theoretical models describing accumulation and dissipation of charge as well as structural changes in these materials during e-beam irradiation.

Lancaster University, Lancaster, UK

Superconducting quantum technology and the search for dark matter

Royal Holloway University of London, London, UK

Opening Microkelvin Regime to Quantum Materials and Quantum Devices

Refrigeration to ultra-low temperatures of order 1 mK and below has long been firmly established, underpinning the groundbreaking studies of superfluid ³He. However for many other strongly-correlated quantum systems achieving such temperatures is challenging due to large thermal resistance between the cooling stage and the sample and across the sample. We demonstrate cooling of such diverse systems as electrons in a 2-dimensional electron gas in a semiconductor heterostructure and both electrons and nuclear spins in intermetallic compounds YbRh₂Si₂ and PrOs₄Sn₁₂. The strategy involves engineering and characterising low-heat-leak rf-shielded environments; identifying the active degrees of freedom and addressing their cooling; and developing ultra-sensitive low-dissipation measurement techniques. In contrast to the in-situ coolers, we utilise a large remote nuclear demagnetisation stage, and establish strong thermal links to samples using high-conductivity metals and cryoliquids. This flexible approach allows independent control of sample temperature and field, opening the door to new many-body ground states in quantum materials and enhanced performance of quantum devices.

VTT Technical Research Centre of Finland Ltd, Finland

Coulomb blockade thermometers for microkelvin regime

We discuss our latest efforts to implement Coulomb blockade thermometers (CBT) for microkelvin regime. The fabrication relies on wafer-scale process flow based on SWAPS junction technology and electroplating of copper or gold with thicknesses up to tens of micrometers. We exploit new materials that may allow the absence of external magnetic field in primary thermometry in the desired temperature range. The process flow together with new CBT designs are presented.

CNRS – Intitut Néel, Grenoble, France

Charles University, Prague, Czech Republic

Locally probed quantum turbulence in superfluid ⁴He using vibrating micro-wire resonators

Oscillating structures such as vibrating wires or tuning forks have become established and traditional tools in research of cryogenic fluid dynamics and quantum turbulence. A significant body of literature exists on turbulence generated by these devices, with only a few works related to detection of externally applied turbulent flows. In view of recent developments of our field, where nanoscopic devices found use as sensitive detectors of quantized vortices, it is important to understand how mechanical resonators, in general, interact with external flows and what are the benefits and expected limitations of such measurements. While measurements with nanoresonators are extremely well-suited to study interactions with a small number of vortices, or possibly a single vortex pinned to the device, a micro-scale device is useful to investigate the effects of a highly turbulent vortex tangle.

In our work, we present an application of a vibrating NbTi wire loop as a detector of quantized vortices generated in thermal counterflow of He II. Second sound attenuation is used simultaneously to determine vortex line density, allowing for an in situ calibration of the mechanical resonator. Devices of this type, or their downscaled versions, can subsequently be used as local probes of dense vortex tangles in various flows of superfluid helium.

Department of Experimental Physics, Comenius University,  Bratislava, Slovakia

Superconducting resonant traveling wave parametric amplifiers

Royal Holloway University of London, London, UK

 High performance rapid turn-around cryogen-free microkelvin platform

We report the design and performance of a microkelvin platform based on a nuclear demagnetization stage, engineered and optimized for operation on a standard cryogen-free dilution refrigerator. PrNi₅ is used as the dominant refrigerant.

The platform provides a large area for mounting experiments in an ultralow temperature, low electromagnetic noise environment. The performance is characterized using current sensing noise thermometry. Temperatures as low as 395 μK have been reached, and a protocol has been established in which it is possible to operate experiments below 1 mK for 95% of the time, providing an efficient cryogen-free microkelvin environment for a wide range of science applications.

Charles University, Prague, Czech Republic

Pressure-induced superconductivity in non-centrosymmetric CeCuAl₃

Technische Universität München, München, Germany

Magnetic susceptibility measurements on YbRh₂Si₂ at ultralow temperatures and fields

Charles University, Prague, Czech Republic

Institute of Experimental Physics SAS, Košice, Slovakia

Superfluid ³He-B as a model system for qu-bits

There exist a few states with coherent spin precession in superfluid ³He-B, which are cosidered as the Bose-Einstein condensates of magnons i.e. the systems of excitations being in one quantum coherent state. Aim of this talk is to present a physical idea how one of these states, namely homogeneously precessing domain (HPD) can be used as a model system to study the properties of qu-bits.

Aalto University, Helsinki, Finland

Thermal self-oscillations in monolayer graphene coupled to a superconducting microwave cavity

We couple a monolayer graphene flake to a Molybdenum-Rhenium superconducting microwave resonator. The graphene forms a SINIS junction with a strong temperature dependent resistance. At certain conditions of pump power and frequency this nonlinearity leads to thermal self-oscillations appearing as sidebands in the cavity transmission response with strong temperature and gate tunability. When sending a probe signal at the frequency of the thermal oscillation side band, low noise amplification with a gain of more than 20dB is observed. Our experimental observations fit with a theoretical model based on thermal runaway and thermally modulated dissipation.

Lancaster University, Lancaster, UK

Role of surface layer in cooling of superfluid ³He in a demagnetization cryostat

We study a simple system with copper plates covered with sintered silver powder and immersed in liquid 3He. We can apply magnetic field up to 8T, measure temperature of liquid helium with vibrating wire, and apply heat to the liquid with another vibrating wire. By demagnetizing copper we are able to cool helium down to 0.12 mK. At this temperature heat capacity of surface layer of solid ³He can be larger than heat capacity of bulk superfluid. In the experiment we observe a complicated thermalization process with at three different time scales. This shows that in addition to liquid helium and copper we have at least two other thermal reservoirs which are probably located on the surface. We also see that these surface systems have field-dependent entropy and take part in the demagnetization process.

Lancaster University, Lancaster, UK

NbTi nanowires with circular cross-section

We report on a significant advance in implementing vibrating nanowire resonators as mechanical probes in helium superfluids as bolometers, thermometers and detectors of turbulence. The wires are made by drawing multifilament superconducting cables through a series of diamond dies to the desired diameter. The filaments of the cable are isolated by etching the copper matrix and then transported to a desired location to form an electro-mechanical resonator. Here we characterise superconducting nano-mechanical vibrating wire resonators with diameters in the range from 200 nm to 1 μm and lengths of up to 2 mm. The nanowires have been tested in superfluid helium-4, superfluid helium-3 and helium-3 gas. The devices have resonant frequencies in the range of 1 – 20 kHz. With such low frequencies, the devices are unaffected by damping from sound emission. We successfully describe the damping in the regimes where the viscous penetration depth is both larger than and smaller than the diameter of the wire. In the ballistic regime of ³He-B, lower than expected damping has also been observed. This is possibly due to a weaker Andreev reflection of quasiparticles as the wire diameter approaches the coherence length. Additionally, in superfluid helium-4, the wires are able to generate quantum turbulence and detect turbulence generated by a larger vibrating wire up to 5 mm away at 1.5 K. We have measured critical temperatures and critical currents for varying diameters of the wires. The contact resistance between a nanowire and an aluminium bond wire has been measured to be 30 mOhm.