The layered semimetal WTe2 has recently been found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report here that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage. This discovery offers new possibilities for gatecontrolled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection. Main text:Many of the most important, and fascinating, phenomena in condensed matter emerge from the quantum mechanics of electrons in a lattice. The periodic potential of the lattice gives rise to Bloch energy bands, and so to the physics of semiconductors that underlies all modern-day electronics. On the more exotic side, electrons in a lattice can pair up to act as bosons and condense into a macroscopic quantum state conducting electricity with zero resistance. More recently, it was realized that Bloch wavefunctions can have a non-trivial topology, incorporating twists analogous to a Möbius strip. This led to the discovery of topological insulators-materials that are electrically insulating in their interior but have conducting boundary modes that result from the topological discontinuity between inside and outside(1). The first of these to be studied was the so-called 2D topological insulator (2D TI), in which the one-dimensional helical edge modes (spin locked to momentum) give rise to the quantum spin Hall effect(2-4).Materials that combine non-trivial topology with superconductivity have been the subject of active investigation in recent years. For example, hybrid structures that couple an s-wave superconductor to a 2D TI have also been proposed as platform for Majorana modes(5), whose non-abelian exchange properties might be harnessed for qubits(6) with coherence times far longer than those built on conventional platforms. There are also topological superconductors, in which vortices or boundaries can host Majorana modes(7).Here we report the remarkable finding that monolayer WTe2, recently shown(8-13) to be an intrinsic 2D TI, itself turns superconducting under moderate electrostatic gating. Several other non-topological layered materials superconduct in the monolayer limit, either intrinsically or under heavy doping using ionic liquid gates(14-22). However, the present case constitutes the first instance of a phase transition from a 2D topological insulator to a superconductor, which moreover is readily controlled by a gate voltage. The discovery creates new opportunities for gateable superconducting circuitry, and offers the potential to develop topological superconducting devices in a single material, as opposed to the hybrid constructions currently required.

The entropy of an electronic system offers important insights into the nature of its quantum mechanical ground state. This is particularly valuable in cases where the state is difficult to identify by conventional experimental probes, such as conductance. Traditionally, entropy measurements are based on bulk properties, such as heat capacity, that are easily observed in macroscopic samples but are unmeasurably small in systems that consist of only a few particles [1, 2]. In this work, we develop a mesoscopic circuit to directly measure the entropy of just a few electrons, and demonstrate its efficacy using the well understood spin statistics of the first, second, and third electron ground states in a GaAs quantum dot [3][4][5][6][7][8]. The precision of this technique, quantifying the entropy of a single spin-1 2 to within 5% of the expected value of k B ln 2, shows its potential for probing more exotic systems. For example, entangled states or those with non-Abelian statistics could be clearly distinguished by their low-temperature entropy [9][10][11][12][13].Our approach is analogous to the milestone of spin-tocharge conversion achieved over a decade ago, in which the infinitesimal magnetic moments of a single spin were detected by transforming them into the presence or absence of an electron charge [14,15]. Following this example, we perform an entropy-to-charge conversion, making use of the Maxwell relationthat connects changes in entropy, particle number, and temperature (S, N , and T , respectively) to changes in the chemical potential, µ, a quantity that is simple to measure and control. The Maxwell relation in Eq. 1 forms the basis of two theoretical proposals to measure non-Abelian exchange of Moore-Read quasiparticles in the ν = 5 2 state via their entropy [9,10]. Reference 10 proposes a strategy by which quasiparticle entropy could be deduced from a V m id V p G sens N − 1 N ∂S/∂N = 0 b V m id V m id V p N − 1 N ∂S/∂N > 0 c Vp δGsens Vp δGsens I heat I sens V sens V p G sens δG sens AC DC 500nm FIG. 1.Measurement protocol (a) Scanning electron micrograph of a device similar to the one measured. Electrostatic gates (gold) define the circuit in a 2D electron gas (2DEG), with grey gates grounded. Squares indicate ohmic contacts to the 2DEG. The temperature of the electron reservoir in the middle (red) is oscillated using AC current, I heat , at frequency f heat through the quantum point contact (QPC) on the left. A portion of the 5 µm-wide reservoir has been removed here for clarity. The occupation of the quantum dot, tunnel coupled to the right side the reservoir, is tuned by Vp and monitored by Isens through the charge sensor QPC. Isens is split into DC and AC components, the latter being measured by a lock-in amplifier at 2f heat . (b) and (c) Simulated DC charge sensor signal, Gsens, for a transition from N − 1 → N electrons at two temperatures (T Red > T Blue ), showing two possible cases for ∂S ∂N . Insets show the corresponding difference, δGsens, between hot and cold curves.the temperature-depende...