Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.

The stochastic evolution of quantum systems during measurement is arguably the most enigmatic feature of quantum mechanics. Measuring a quantum system typically steers it towards a classical state, destroying the coherence of an initial quantum superposition and the entanglement with other quantum systems. Remarkably, the measurement of a shared property between non-interacting quantum systems can generate entanglement, starting from an uncorrelated state. Of special interest in quantum computing is the parity measurement, which projects the state of multiple qubits (quantum bits) to a state with an even or odd number of excited qubits. A parity meter must discern the two qubit-excitation parities with high fidelity while preserving coherence between same-parity states. Despite numerous proposals for atomic, semiconducting and superconducting qubits, realizing a parity meter that creates entanglement for both even and odd measurement results has remained an outstanding challenge. Here we perform a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a three-dimensional circuit quantum electrodynamics architecture and phase-sensitive parametric amplification. Using postselection, we produce entanglement by parity measurement reaching 88 per cent fidelity to the closest Bell state. Incorporating the parity meter in a feedback-control loop, we transform the entanglement generation from probabilistic to fully deterministic, achieving 66 per cent fidelity to a target Bell state on demand. These realizations of a parity meter and a feedback-enabled deterministic measurement protocol provide key ingredients for active quantum error correction in the solid state.

We use Superconducting QUantum Interference Device (SQUID) microscopy to characterize the current-phase relation (CPR) of Josephson Junctions from 3-dimentional topological insulator HgTe (3D-HgTe). We find clear skewness in the CPRs of HgTe junctions ranging in length from 200 nm to 600 nm. The skewness indicates that the Josephson current is predominantly carried by Andreev bound states with high transmittance, and the fact that the skewness persists in junctions that are longer than the mean free path suggests that the effect may be related to the helical nature of the Andreev bound states in the surface of HgTe.Topological insulators (TI) have a special band structure with important consequences for proximity-induced superconductivity. In 3-dimentional topological insulators (3D-TI), the inversion of the conduction and valence bands leads to conducting 2D surface states with energies that are linearly proportional to their momenta [1][2][3][4][5]. Spinmomentum locking protects the charge carriers at the surface against elastic backscattering [6,7]. These special properties are reflected in the superconducting proximity effect in an S/3D-TI bilayer or an S/TI/S junction, which may host Majorana fermions in a quasi-1D channel or vortex core [8][9][10]. Most previous works characterized current-voltage characteristics to determine the critical current's dependence on temperature, gate voltage, or magnetic field [11][12][13][14][15][16][17][18][19][20][21][22], while a few studies characterized the CPR [23,24].Here, we use a scanning SQUID microscope to perform contactless measurements of the diamagnetic response of Nb/HgTe bilayers and of the CPR of Nb/HgTe/Nb junctions. In contrast to previous CPR results [23,24], we find no evidence for bulk states, 2 and we observe that the CPRs of many junctions of different sizes consistently exhibit forward skewness.The CPR in an S/TI/S junction is a key diagnostic [8,[25][26][27][28][29][30][31][32]. Weak disorder in the TI far from the superconducting contacts theoretically does not affect the induced superconducting state [33,34]; therefore, Andreev bound states should form in hightransmittance surface channels [8,26,27,29,31]. A CPR with forward skewness -that is, a deviation from a perfect sinusoidal form -is a signature of such high-transmittance Andreev bound states [35][36][37].To our knowledge, there have not been direct observations of forward skewed CPRs in topological insulators [23,24], although the skewness has been indirectly inferred [24] from the Fraunhofer interference pattern. Previous CPR experiments in topological insulators [23,24] were complicated in part by bulk states, self-inductance effects, and bias voltage, factors that are eliminated in this work.Moreover, a skewed CPR can also result from ballistic transport [35]. Measurements in metallic break junctions showed that the CPR approaches the predictions for quantum point contacts in the ballistic limit [38]. In metallic atomic point contacts, the CPR was significantly skewed only in contacts wi...

About one out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet (Sanchis-Ojeda et al. 2014;Winn et al. 2018). All of the previously known ultrashort-period planets are either hot Jupiters, with sizes above 10 Earth radii (R ⊕ ), or apparently rocky planets smaller than 2 R ⊕ . Such lack of planets of intermediate size (the "hot Neptune desert") has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here, we report the discovery of an ultra-short-period planet with a radius of 4.6 R ⊕ and a mass of 29 M ⊕ , firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite (Ricker et al. 2015) revealed transits of the bright Sun-like star LTT 9779 every 0.79 days. The planet's mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0 +2.7 −2.9 % of the total mass. With an equilibrium temperature around 2000 K, it is unclear how this "ultra-hot Neptune" managed to retain such an envelope. Follow-up observations of the planet's atmosphere to better understand its origin and physical nature will be facilitated by the star's brightness (V mag = 9.8).