The capability to switch electrically between superconducting and insulating states of matter represents a novel paradigm in the state-of-the-art engineering of correlated electronic systems. An exciting possibility is to turn on superconductivity in a topologically non-trivial insulator, which provides a route to search for non-Abelian topological states. However, existing demonstrations of superconductor-insulator switches have involved only topologically trivial systems, and even those are rare due to the stringent requirement to tune the carrier density over a wide range. Here we report reversible, in-situ electrostatic on/off switching of superconductivity in a recently established quantum spin Hall insulator, namely monolayer tungsten ditelluride (WTe2). Fabricated into a van der Waals field effect transistor, the monolayer's ground state can be continuously gate-tuned from the topological insulating to the superconducting state, with critical temperatures Tc up to ~ 1 Kelvin. The critical density for the onset of superconductivity is estimated to be ~ 5×10 12 cm -2 , among the lowest for two-dimensional (2D) superconductors. Our results establish monolayer WTe2 as a material platform for engineering novel superconducting nanodevices and topological phases of matter. One Sentence Summary: Monolayer WTe2, a quantum spin Hall insulator, exhibits superconductivity at very low electron densities.A field effect switch between superconducting and insulating states allows, in-principle, tremendous possibilities for engineering novel superconducting devices. However, materials allowing for this switch reversibly and in situ are rare, and it remains difficult to implement such a device into existing technologies (1). An area where a suitable material may find unique application is at the intersection of superconductivity and topological insulators, which hosts a fertile landscape of interesting quantum phenomena, including non-Abelian topological excitations (2-4). Some topological insulators have been turned into superconductors via chemical doping (5, 6), application of pressure (7-9), or via the proximity effect (10, 11), which are either Electrically Tunable Low Density Superconductivity in a Monolayer Topological Insulator

Abstract:Coherent control of quantum states has been demonstrated in a variety of superconducting devices. In all these devices, the variables that are manipulated are collective electromagnetic degrees of freedom: charge, superconducting phase, or flux. Here, we demonstrate the coherent manipulation of a quantum system based on Andreev bound states, which are microscopic quasiparticle states inherent to superconducting weak links. Using a circuit quantum electrodynamics setup we perform single-shot readout of this "Andreev qubit". We determine its excited state lifetime and coherence time to be in the microsecond range.Quantum jumps and parity switchings are observed in continuous measurements. In addition to possible quantum information applications, such Andreev qubits are a testbed for the physics of single elementary excitations in superconductors. 2The ground state of a uniform superconductor is a many-body coherent state. Microscopic excitations of this superconducting condensate, which can be created for example by the absorption of photons of high enough energy, are delocalized and incoherent because they have energies in a continuum of states. Localized states arise in situations where the superconducting gap Δ or the superconducting phase undergo strong spatial variations: examples include Shiba states around magnetic impurities (1), Andreev states in vortices (2) or in weak links between two superconductors (3). Because they have discrete energies within the gap, Andreev states are expected to be amenable to coherent manipulation (4,5,6,7,8). In the simplest weak link, a single conduction channel shorter than the superconducting coherence length  , there are only two Andreev levels, governed by the transmission probability  of electrons through the channel and the phase difference  between the two superconducting condensates (3). Despite the absence of actual barriers, quasiparticles (bogoliubons) occupying these Andreev levels are localized over a distance  around the weak link by the gradient of the superconducting phase, and the system can be considered an "Andreev quantum dot " (5,6). Figure 1 EE (13,14). The e state can also be reached directly from g by absorption of a photon of energy 2.A EHere we demonstrate experimentally the manipulation of coherent superpositions of states g and , e even if parasitic transitions to o are also observed.3 Atomic-size contacts are suitable systems to address the Andreev physics because they accommodate a small number of short conduction channels (15). We create them using the microfabricated break-junction technique (16). (Fig. 3D). The analysis (23) of this real-time trace yields a parity switching rate of 50kHz (20). 5The coherent manipulation at   of the two-level system formed by g and e is illustrated in Fig. 4. Figure 4A shows the Rabi oscillations between g and e obtained by varying the duration of a driving pulse at frequency 1 ( , )A ff   (Movie S1). Figure 4B shows how the populations of g and e change when the driving pulse frequency ...

The Josephson effect describes the flow of supercurrent in a weak link-such as a tunnel junction, nanowire or molecule-between two superconductors. It is the basis for a variety of circuits and devices, with applications ranging from medicine to quantum information. Experiments using Josephson circuits that behave like artificial atoms are now revolutionizing the way we probe and exploit the laws of quantum physics. Microscopically, the supercurrent is carried by Andreev pair states, which are localized at the weak link. These states come in doublets and have energies inside the superconducting gap. Existing Josephson circuits are based on properties of just the ground state of each doublet, and so far the excited states have not been directly detected. Here we establish their existence through spectroscopic measurements of superconducting atomic contacts. The spectra, which depend on the atomic configuration and on the phase difference between the superconductors, are in complete agreement with theory. Andreev doublets could be exploited to encode information in novel types of superconducting qubits.

In apparent contradiction to the laws of thermodynamics, Maxwell's demon is able to cyclically extract work from a system in contact with a thermal bath, exploiting the information about its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. Here, we realize a Maxwell demon experiment that tracks the state of each constituent in both the classical and quantum regimes. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon's memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.quantum thermodynamics | superconducting circuits | quantum information I n 1867, pondering the newly developed thermodynamic laws, Maxwell came to the disturbing conclusion that a "demon" can extract work cyclically from a thermodynamic system beyond the limits set by the second law when acting upon the information it obtains about the system (1). This paradox was resolved a century later when Landauer realized that information processing has an entropic cost and Bennett argued that the demon's memory must take full part in the thermodynamic cycle (2). Recent experiments have realized classical versions of elementary Maxwell demons in various physical systems (3-8). Although quantum versions have long been investigated theoretically (9-13), experimental realizations are in their infancy (7,8), and a full characterization is still missing. Using superconducting circuits, we reveal the inner mechanics of a quantum Maxwell demon that is able to extract work from a quantum system. Importantly, we are able to directly probe the extracted work by measuring the output power emitted by the system through stimulated emission, without inferring it from system trajectories (3-6, 14). We are thus able to demonstrate how the information stored in the demon's memory affects the extracted work. To make the characterization complete, we also measure the entropy and energy of the system and the demon. Superconducting circuits thus reveal themselves as a suitable experimental testbed for the blooming field of quantum thermodynamics of information (15)(16)(17)(18)(19).In the experiment, the system S is a transmon superconducting qubit (20) with energy difference hfS = h × 7.09 GHz between its ground |g and excited |e states. It is embedded in a microwave cavity that resonates at fD = 7.91 GHz and plays the role of the demon's memory D. The dispersive Hamiltonian reads H = hfS |e e|S + hfD d † d − hχd † d |e e|S , where d is the annihilation operator of a photon in the cavity. The last term induces a frequency shift of the cavity by −χ = −33 MHz when the qubit is excited. Reciprocally, the qubit frequency is shif...

We have observed that the supercurrent across phase-biased, highly transmitting atomic size contacts is strongly reduced within a broad phase interval around π. We attribute this effect to quasiparticle trapping in one of the discrete subgap Andreev bound states formed at the contact. Trapping occurs essentially when the Andreev energy is smaller than half the superconducting gap Δ, a situation in which the lifetime of trapped quasiparticles is found to exceed 100 μs. The origin of this sharp energy threshold is presently not understood.

The fluorescence of a resonantly driven superconducting qubit is measured in the time domain, providing a weak probe of the qubit dynamics. Prior preparation and final, single-shot measurement of the qubit allows to average fluorescence records conditionally on past and future knowledge. The resulting interferences reveal purely quantum features characteristic of weak values. We demonstrate conditional averages that go beyond classical boundaries and probe directly the jump operator associated with relaxation. The experimental results are remarkably captured by a recent theory, which generalizes quantum mechanics to open quantum systems whose past and future are known.In quantum physics, measurement results are random but their statistics can be predicted assuming some knowledge about the system in the past. Additional knowledge from a future measurement [1] deeply changes the statistics in the present and leads to purely quantum features [2,3]. In particular conditioned average outcomes of a weak measurement, revealing the so-called weak values, were shown to go beyond the classically allowed range and give a way to directly measure complex quantities [4]. Recently, these concepts have been considered in the general case of open quantum systems where decoherence occurs [5][6][7]. Then, what are the properties of weak values for the unavoidable measurement associated to decoherence, the one performed by the environment? Here, we answer this question in the simplest open quantum system: a quantum bit in presence of a relaxation channel. We continuously monitor the fluorescence emitted by a superconducting qubit driven at resonance. Conditioned on initial preparation and final single shot measurement outcome of the qubit state, we probe weak values displaying non-classical properties. The fluorescence signal exhibits interferences between oscillations associated to past and future quantum states [5][6][7]. The measured data are in complete agreement with theory.A two-level system irradiated at resonance undergoes Rabi oscillations between ground state |g and excited state |e . Conversely, these oscillations leave a footprint in the emitted fluorescence field. In the spectral domain, two side peaks appear around resonance frequency, constituting the Mollow triplet [8]. They were first observed in quantum optics and more recently in the microwave range [9]. If the detection setup allows monitoring fluorescence in the time domain, one gets a weak probe of the qubit. To access weak values of the associated qubit operator, one additionally needs to post-select the experiments depending on qubit state, which therefore needs to * These two authors contributed equally to this work be measured in a single-shot manner. Superconducting qubits in cavity are fit for this task [10][11][12][13]. The principle of our experiment is described in Fig. 1. A transmon qubit with frequency ν q = 5.19 GHz is enclosed in a nonresonant superconducting 3D cavity [14], connected to two transmission lines. Line a is coupled as weakly as the i...

A qubit can relax by fluorescence, which prompts the release of a photon into its electromagnetic environment. By counting the emitted photons, discrete quantum jumps of the qubit state can be observed. The succession of states occupied by the qubit in a single experiment, its quantum trajectory, depends in fact on the kind of detector. How are the quantum trajectories modified if one measures continuously the amplitude of the fluorescence field instead? Using a superconducting parametric amplifier, we perform heterodyne detection of the fluorescence of a superconducting qubit. For each realization of the measurement record, we can reconstruct a different quantum trajectory for the qubit. The observed evolution obeys quantum state diffusion, which is characteristic of quantum measurements subject to zeropoint fluctuations. Independent projective measurements of the qubit at various times provide a quantitative verification of the reconstructed trajectories. By exploring the statistics of quantum trajectories, we demonstrate that the qubit states span a deterministic surface in the Bloch sphere at each time in the evolution. Additionally, we show that when monitoring fluorescence field quadratures, coherent superpositions are generated during the decay from excited to ground state. Counterintuitively, measuring light emitted during relaxation can give rise to trajectories with increased excitation probability.

Quantum coherence and control is foundational to the science and engineering of quantum systems 1,2 . In van der Waals (vdW) materials, the collective coherent behavior of carriers has been probed successfully by transport measurements 3-6 . However, temporal coherence and control, as exemplified by manipulating a single quantum degree of freedom, remains to be verified. Here we demonstrate such coherence and control of a superconducting circuit incorporating graphene-based Josephson junctions. Furthermore, we show that this device can be operated as a voltage-tunable transmon qubit 7-9 , whose spectrum reflects the electronic properties of massless Dirac fermions traveling ballistically 4,5 . In addition to the potential for advancing extensible quantum computing technology, our results represent a new approach to studying vdW materials using microwave photons in coherent quantum circuits.