Lev V. Levitin

In this paper we present a new evaluation of the thermodynamic status of The Provisional Low Temperature Scale from 0.9 mK to 1 K (PLTS-2000) based on comparison measurements with different primary thermometers. The evaluation includes new experimental data for the ultra-low temperature range down to 0.9 mK as well as data already published for the higher temperature range up to 0.7 K. In our data the PLTS-2000 agrees with thermodynamic temperature better than 0.33% from 1 K down to about 3 mK. Below 3 mK, PLTS-2000 is systematically higher than thermodynamic temperature with a deviation of 1% at 1.72 mK, which grows up to about 3.84% at 0.902 mK. A polynomial fit is given for the deviation of PLTS-2000 from thermodynamic temperature for the whole temperature range of the PLTS-2000. All data on which the current evaluation is based are provided in appendices. As the PLTS-2000 is thermodynamically correct better than 1% over almost its whole definition range, it is justified to consider the PLTS-2000 as a validated temperature scale and remove its provisional status.

Topological superconductors, characterized by spin-triplet Cooper pairing, are important for exploring unconventional pairing mechanisms and protected quantum states. Yet experimentally established odd-parity, spin-triplet superconductors remain scarce. Here we demonstrate that the heavy-fermion compound YbRh ₂ Si ₂ hosts distinct magnetic-field-tuned superconducting states, both Pauli limited and beyond this limit, revealed by high-resolution measurements of the complex electrical impedance. We also find that superconductivity is abruptly suppressed at the critical field associated with the primary antiferromagnetic transition of this compound. The onset of electro-nuclear spin density wave order enhances the superconductivity. We propose that this behaviour can be explained by the formation of a pair density wave that boosts a selected spin-triplet superconducting order parameter. Together, our findings indicate odd-parity superconductivity in YbRh ₂ Si ₂ and point to one of the superconducting states being the topological helical phase.

Anisotropic pair breaking close to surfaces favors the chiral A phase of the superfluid He 3 over the time-reversal invariant B phase. Confining the superfluid He 3 into a cavity of height D of the order of the Cooper pair size characterized by the coherence length ξ 0 —ranging between 16 nm (34 bar) and 77 nm (0 bar)—extends the surface effects over the whole sample volume, thus allowing stabilization of the A phase at pressures P and temperatures T where otherwise the B phase would be stable. In this Letter, the surfaces of such a confined sample are covered with a superfluid He 4 film to create specular quasiparticle scattering boundary conditions, preventing the suppression of the superfluid order parameter. We show that the chiral A phase is the stable superfluid phase under strong confinement over the full P − T phase diagram down to a quasi-two-dimensional limit D / ξ 0 = 1 , where D = 80 nm . The planar phase, which is degenerate with the chiral A phase in the weak-coupling limit, is not observed. The gap inferred from measurements over the wide pressure range from 0.2 to 21.0 bar leads to an empirical ansatz for temperature-dependent strong-coupling effects. We discuss how these results pave the way for the realization of the fully gapped two-dimensional p x + i p y superfluid under more extreme confinement.

Topological superconductors are essential elements of the periodic table of topological quantum matter. However, the relevant odd-parity spin-triplet superconductors are rare. We report high-resolution measurements of the complex electrical impedance of YbRh$_2$Si$_2$ down to 0.4 mK, that reveal the presence of several superconducting states, suppressed differently by magnetic field, both Pauli-limited and beyond the Pauli limit. Superconductivity is abruptly switched off at the critical field of the primary antiferromagnetic order. The onset of electro-nuclear spin density wave order enhances the superconductivity, which we account for by the simultaneous formation of a spin-triplet pair density wave. Together these observations provide compelling evidence for odd-parity superconductivity, and its underpinning by antiferromagnetism, and allow us to identify the topological helical state.

Quantum circuits interact with the environment via several temperature-dependent degrees of freedom. Multiple experiments to-date have shown that most properties of superconducting devices appear to plateau out at T ≈ 50 mK – far above the refrigerator base temperature. This is for example reflected in the thermal state population of qubits, in excess numbers of quasiparticles, and polarisation of surface spins – factors contributing to reduced coherence. We demonstrate how to remove this thermal constraint by operating a circuit immersed in liquid ³He. This allows to efficiently cool the decohering environment of a superconducting resonator, and we see a continuous change in measured physical quantities down to previously unexplored sub-mK temperatures. The ³He acts as a heat sink which increases the energy relaxation rate of the quantum bath coupled to the circuit a thousand times, yet the suppressed bath does not introduce additional circuit losses or noise. Such quantum bath suppression can reduce decoherence in quantum circuits and opens a route for both thermal and coherence management in quantum processors.