All-Microwave Manipulation of Superconducting Qubits with a Fixed-Frequency Transmon Coupler.

All-microwave control of fixed-frequency superconducting quantum computing circuits is advantageous for minimizing the noise channels and wiring costs. Here we introduce a swap interaction between two data transmons assisted by the third-order nonlinearity of a coupler transmon under a microwave drive. We model the interaction analytically and numerically and use it to implement an all-microwave controlled-Z gate. The gate based on the coupler-assisted swap transition maintains high drive efficiency and small residual interaction over a wide range of detuning between the data transmons.

Two superconducting states with broken time-reversal symmetry in FeSe1-xSx.

Iron-chalcogenide superconductors FeSe1-xSx possess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (µSR) measurements in FeSe1-xSx superconductors for 0 ≤ x ≤ 0.22 covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperature Tc for all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-field µSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x > 0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe1-xSx, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity.

Pure nematic quantum critical point accompanied by a superconducting dome.

SignificanceThe notion of the quantum critical point (QCP) is at the core of modern condensed matter physics. Near a QCP of the symmetry-breaking order, associated quantum-mechanical fluctuations are intensified, which can lead to unconventional superconductivity. Indeed, dome-shaped superconducting phases are often observed near the magnetic QCPs, which supports the spin fluctuation-driven superconductivity. However, the fundamental question remains as to whether a nonmagnetic QCP of electronic nematic order characterized by spontaneous rotational symmetry breaking can promote superconductivity in real materials. Here, we provide an experimental demonstration that a pure nematic QCP exists near the center of a superconducting dome in nonmagnetic FeSe[Formula: see text] Tex. This result evidences that nematic fluctuations enhanced around the nematic QCP can boost superconductivity.

Bose-Einstein condensation superconductivity induced by disappearance of the nematic state.

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 - x S x from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

Nematic quantum critical point without magnetism in FeSe1-xSx superconductors.

In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near [Formula: see text], the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

Heterotopic cervical pregnancy with preservation of the intrauterine gestation.

We present a case of heterotopic cervical pregnancy where intrauterine gestation was preserved successfully. A 40-year-old Japanese woman presented with heterotopic cervical pregnancy after intracytoplasmic sperm injection with elongated spermatid. An ultrasound examination showed heterotopic cervical pregnancy. Intrauterine gestation was preserved successfully with the use of local vasopressin injection, followed by curettage of cervical implantation and local methotrexate injection. The cervical pregnancy resolved, and a healthy full-term infant was delivered. When early diagnosis of a heterotropic cervical pregnancy is made, selective reduction of a cervical implantation might allow preservation of the intrauterine gestation, without hysterectomy. (Reprod Med Biol 2005; 4: 221-223).