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Strong light-matter coupling has been realized at the level of single atoms and photons throughout most of the electromagnetic spectrum, except for the THz range. Here, the authors report a THz-scale transport gap, induced by vacuum fluctuations in carbon nanotube quantum dot through the deep strong coupling of a single electron to a THz resonator.

Abstract  We have studied the elasticity of solid 4He in relation with its possible supersolidity. For this we have measured acoustic resonance frequencies in a 1 cm3 cell filled either with polycrystals or with single crystals of 4He. We have observed a large stiffening at low temperature as first observed by Day and Beamish in polycrystals. The 3He impurity content has been varied from 300 ppb to 0.4 ppb. When kept in equilibrium with liquid helium, single crystals should be impurity free. In these crystals, a large stiffening is observed, which should not be the result of the pinning of dislocations by impurities. [ABSTRACT FROM AUTHOR]

Abstract   We have recently found experimental evidence for the homogeneous nucleation of crystals in metastable liquid 4He at high pressure. For this we combined the focusing of a high intensity spherical acoustic wave with a simple light scattering technique. We discuss the analysis used to distinguish between nucleation of bubbles in the negative pressure swings of the wave from nucleation of crystals in the positive swings. We also discuss the interest of our results and future developments of our experiment in the general context of the study of nucleation and instability limits in phase diagrams. [ABSTRACT FROM AUTHOR]

Abstract   We have looked for dc mass transport through solid 4He in a simple experiment with two communicating vessels filled with solid 4He in equilibrium with liquid 4He. Through good quality crystals, we have observed no mass transport, in contradiction with the hypothesis of a Bose–Einstein condensation of vacancies. Through crystals containing grain boundaries, we have found superfluid flow along these grain boundaries. We discuss these results in the context of other experiments on supersolidity. [ABSTRACT FROM AUTHOR]

The control of light-matter interaction at the most elementary level has become an important resource for quantum technologies. Implementing such interfaces in the THz range remains an outstanding problem. Here, we couple a single electron trapped in a carbon nanotube quantum dot to a THz resonator. The resulting light-matter interaction reaches the deep strong coupling regime that induces a THz energy gap in the carbon nanotube solely by the vacuum fluctuations of the THz resonator. This is directly confirmed by transport measurements. Such a phenomenon which is the exact counterpart of inhibition of spontaneous emission in atomic physics opens the path to the readout of non-classical states of light using electrical current. This would be a particularly useful resource and perspective for THz quantum optics., (© 2021. The Author(s).)

Graphene quantum dots (GQDs) have recently attracted considerable attention, with appealing properties for terahertz (THz) technology. This includes the demonstration of large thermal bolometric effects in GQDs when illuminated by THz radiation. However, the interaction of THz photons with GQDs in the Coulomb blockade regime, i.e., single electron transport regime, remains unexplored. Here, we demonstrate the ultrasensitive photoresponse to THz radiation (from <0.1 to 10 THz) of a hBN-encapsulated GQD in the Coulomb blockade regime at low temperature (170 mK). We show that THz radiation of ∼10 pW provides a photocurrent response in the nanoampere range, resulting from a renormalization of the chemical potential of the GQD of ∼0.15 meV. We attribute this photoresponse to an interfacial photogating effect. Furthermore, our analysis reveals the absence of thermal effects, opening new directions in the study of coherent quantum effects at THz frequencies in GQDs.

Helium is recognized as a model system for the study of phase transitions. Of particular interest is the superfluid phase in two dimensions. We report measurements on superfluid helium films adsorbed on the surface of a suspended carbon nanotube. We measure the mechanical vibrations of the nanotube to probe the adsorbed helium film. We demonstrate the formation of helium layers up to five atoms thickness. Upon increasing the vapor pressure, we observe layer-by-layer growth with discontinuities in both the number of adsorbed atoms and the speed of the third sound in the adsorbed film. These hitherto unobserved discontinuities point to a series of first-order layering transitions. Our results show that helium multilayers adsorbed on a nanotube are of unprecedented quality compared to previous works. They pave the way to new studies of quantized superfluid vortex dynamics on cylindrical surfaces, of the Berezinskii-Kosterlitz-Thouless phase transition in this new geometry, and perhaps also to supersolidity in crystalline single layers as predicted in quantum Monte Carlo calculations.

We report the results of flow experiments in which two chambers containing solid ^{4}He are connected by a superfluid Vycor channel. At low temperatures and pressures, mechanically squeezing the solid in one chamber produced a pressure increase in the second chamber, a measure of mass transport through our solid-superfluid-solid junction. This pressure response is very similar to the flow seen in recent experiments at the University of Massachusetts: it began around 600 mK, increased as the temperature was reduced, then decreased dramatically at a temperature, T&#95;{d}, which depended on the ^{3}He impurity concentration. Our experiments indicate that the flow is limited by mass transfer across the solid-liquid interface near the Vycor ends, where the ^{3}He collects at low temperature, rather than by flow paths within the solid ^{4}He.

The giant plasticity of [Formula: see text]He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996-1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm[Formula: see text], sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10[Formula: see text]-10[Formula: see text] per cm[Formula: see text], that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.

When submitted to large stresses at high temperature, usual crystals may irreversibly deform. This phenomenon is known as plasticity and it is due to the motion of crystal defects such as dislocations. We have discovered that, in the absence of impurities and in the zero temperature limit, helium 4 crystals present a giant plasticity that is anisotropic and reversible. Direct measurements on oriented single crystals show that their resistance to shear nearly vanishes in one particular direction because dislocations glide freely parallel to the basal planes of the hexagonal structure. This plasticity disappears as soon as traces of helium 3 impurities bind to the dislocations or if their motion is damped by collisions with thermal phonons.

We report on the simultaneous measurements of the speed of sound and the density in liquid water under negative pressure. Application of a focused acoustic wave to the bulk liquid is able to generate negative pressures before nucleation of the vapor phase occurs. A method for time-resolved Brillouin scattering is developed to measure the speed of sound during the passage of a 1 MHz ultrasonic wave. This is coupled with a fiber optic probe hydrophone which allows the determination of the density. Together, these methods give an ambient temperature equation of state of metastable liquid water down to the acoustic cavitation threshold. Empirical equations of state of water are based on experimental data at positive pressure; the validity of their extrapolation to negative pressures had been tested only indirectly or with very weakly metastable liquid. We provide thermodynamic data that prove the fidelity of recent equations of state down to -26 MPa. However, this raises questions regarding the nature of the cavitation threshold observed in acoustic experiments, which is far less negative than expected.

We have measured the elasticity of high quality ultrapure ⁴He single crystals in the low temperature region where supersolidity is supposed to occur. At 20 mK, our results are consistent with elastic coefficients previously measured at 1.2 K. As the temperature increases from 20 to 100 mK, a large softening occurs because dislocations unpin from ³He impurities. In the absence of ³He impurities, dislocations are free to move down to 20 mK; the crystals are soft. The large magnitude of this anomalous softening shows that dislocations form a mobile mosaic structure. It illustrates the remarkable quantum plasticity of ⁴He crystals.

A 'supersolid' is a quantum solid in which a fraction of the mass is superfluid. As a remarkable consequence, it is rigid, but part of its mass is able to flow owing to quantum physical processes. This paradoxical state of matter was considered as a theoretical possibility as early as 1969, but its existence was discovered only in 2004, in (4)He. Since then, intense experimental and theoretical efforts have been made to explain the origins of this exotic state of matter. It now seems that its physical interpretation is more complicated than originally thought.

We have observed boundaries between hcp 4He crystal grains in equilibrium with liquid 4He. We have found that, when emerging at the liquid-solid interface, a grain boundary makes a groove whose dihedral angle 2theta is nonzero. This measurement shows that grain boundaries are not completely wet by the liquid phase, in agreement with recent Monte Carlo simulations. Depending on the value of theta, the contact line of a grain boundary with a solid wall may be wet by the liquid. In this case, the line is a thin channel with a curved triangular cross section, whose measured width agrees with predictions from a simple model. We discuss these measurements in the context of grain boundary premelting and for a future understanding of the possible supersolidity of solid 4He.

Dripping is usually associated with fluid motion, but here we describe the analogous phenomenon of a 3He crystal growing and melting under the influence of surface tension and gravity. The pinch-off of the crystal is described by a purely geometric equation of motion, viscous dissipation or inertia being negligible. In analogy to fluid pinch-off, the minimum neck radius R{n} goes to zero like a power law, but with a new scaling exponent of 12 . However, for a significant part of the neck's macroscopic evolution the scaling exponent is found to be much closer to 13 . This observation may be consistent with simulations and theoretical results showing a very slow approach to the asymptotic pinch solution, making the "critical region" very small, both in time and space. After pinch-off, we observe a similar 13 -scaling for the recoil of a crystal tip, both in simulation and experiment. For very early times our experiments are consistent with an approximate theory predicting an asymptotic regime with exponent 12 . Future experiments must show whether the transient 13 scaling is a universal feature of crystal melting, or perhaps an artifact of our experimental setup.

We investigate the limiting mechanical tension (negative pressure) that liquid water can sustain before cavitation occurs. The temperature dependence of this quantity is of special interest for water, where it can be used as a probe of a postulated anomaly of its equation of state. After a brief review of previous experiments on cavitation, we describe our method which consists in focusing a high amplitude sound wave in the bulk liquid, away from any walls. We obtain highly reproducible results, allowing us to study in detail the statistics of cavitation, and to give an accurate definition of the cavitation threshold. Two independent pressure calibrations are performed. The cavitation pressure is found to increase monotonically from -26 MPa at 0 degrees C to -17 MPa at 80 degrees C. While these values lie among the most negative pressures reported in water, they are still far away from the cavitation pressure expected theoretically and reached in the experiment by Angell and his group [Zheng, Science 254, 829 (1991)] (around -120 MPa at 40 degrees C). Possible reasons for this discrepancy are considered.

When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid (4)He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, we detect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries.

We present the first experimental analysis of drop coalescence in a case where the dynamics is not governed by viscous dissipation in the bulk nor by the inertia of the fluid flow, only by the geometry and mobility of surfaces. We found such a situation in the physics of 3He crystals near 0.32 K where the latent heat of crystallization vanishes. Two crystalline drops of 3He coalesce if their crystalline orientations are identical: a neck forms after the contact at time t=0, and the shape evolves towards that of one convex crystal by local growth and melting in a fraction of a second. We have found that the neck radius initially increases as t(1/3), as predicted by Maris. This behavior is also expected for superfluid drops. It is clearly distinguished from the logarithmic behavior and from the t(1/2) power law which have been predicted by Eggers et al. in more usual situations.

We have measured the contact angle of the interface of phase-separated 3He-4He mixtures against a sapphire window. We have found that this angle is finite and does not tend to zero when the temperature approaches T(t), the temperature of the tricritical point. On the contrary, it increases with temperature. This behavior is a remarkable exception to what is generally observed near critical points, i.e., "critical point wetting." We propose that it is a consequence of the "critical Casimir effect" which leads to an effective attraction of the 3He-4He interface by the sapphire near T(t).

We propose that the liquid-gas spinodal line of 3He reaches a minimum at 0.4 K. This feature is supported by our cavitation measurements. We also show that it is consistent with extrapolations of sound-velocity measurements. Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar behavior for the spinodal of water and related it to a change in sign of the expansion coefficient alpha, i.e., a line of density maxima. 3He exhibits such a line at positive pressure. We consider its extrapolation to negative pressure. Our discussion raises fundamental questions about the sign of alpha in a Fermi liquid along its spinodal.

By focusing a high-intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around P(m) + 4.7 bars ( P(m) = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by 2 to 3 orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki, and Sasaki that, in all previous experiments, solid helium nucleated on impurities.