Sensitivity and Linearity of Superconducting Radio-Frequency Single-Electron Transistors: Effects of Quantum Charge Fluctuations

We have investigated the effects of quantum fluctuations of quasiparticles on the operation of superconducting radio-frequency single-electron transistors (RF-SETs) for large values of the quasiparticle cotunneling parameter $\alpha=8E_{J}/E_{c}$, where $E_{J}$ and $E_{c}$ are the Josephson and charging energies. We find that for $\alpha>1$, subgap RF-SET operation is still feasible despite quantum fluctuations that renormalize the SET charging energy and wash out quasiparticle tunneling thresholds. Surprisingly, such RF-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity.Comment: Submitted to Phys. Rev. Let

Transient vortex dynamics and evolution of Bose metal from a 2D superconductor on MoS2_2

The true character of physical phenomena is thought to be reinforced as the system becomes disorder-free. In contrast, the two-dimensional (2D) superconductor is predicted to turn fragile and resistive away from the limit I -> 0, B -> 0, in the pinning-free regime. It is intriguing to note that the very vortices responsible for achieving superconductivity by pairing, condensation, and, thereby reducing the classical dissipation, render the state resistive driven by quantum fluctuations in the T -> 0. While cleaner systems are being explored for technological improvements, the 2D superconductor turning resistive when influenced by weak electric and magnetic fields has profound consequences for quantum technologies. A metallic ground state in 2D is beyond the consensus of both Bosonic and Fermionic systems, and its origin and nature warrant a comprehensive theoretical understanding supplemented by in-depth experiments. A real-time observation of the influence of vortex dynamics on transport properties so far has been elusive. We explore the nature and fate of a low-viscous, clean, 2D superconducting state formed on an ionic-liquid gated few-layered MoS$_2$ sample. The vortex-core being dissipative, the elastic depinning, intervortex interaction, and the subsequent dynamics of the vortex-lattice cause the system to behave like an overdamped harmonic oscillator, leaving transient signatures in the transport characteristics. The temperature and magnetic field dependence of the transient nature and the noise characteristics of the magnetoresistance confirm that quantum fluctuations are solely responsible for the Bose metal state and the fragility of the superconducting state

Non-abelian fractional quantum hall effect for fault-resistant topological quantum computation.

Topological quantum computation (TQC) has emerged as one of the most promising approaches to quantum computation. Under this approach, the topological properties of a non-Abelian quantum system, which are insensitive to local perturbations, are utilized to process and transport quantum information. The encoded information can be protected and rendered immune from nearly all environmental decoherence processes without additional error-correction. It is believed that the low energy excitations of the so-called=5/2 fractional quantum Hall (FQH) state may obey non-Abelian statistics. Our goal is to explore this novel FQH state and to understand and create a scientific foundation of this quantum matter state for the emerging TQC technology. We present in this report the results from a coherent study that focused on obtaining a knowledge base of the physics that underpins TQC. We first present the results of bulk transport properties, including the nature of disorder on the 5/2 state and spin transitions in the second Landau level. We then describe the development and application of edge tunneling techniques to quantify and understand the quasiparticle physics of the 5/2 state

Single-electron quantum dot in Si/SiGe with integrated charge-sensing

Single-electron occupation is an essential component to measurement and manipulation of spin in quantum dots, capabilities that are important for quantum information processing. Si/SiGe is of interest for semiconductor spin qubits, but single-electron quantum dots have not yet been achieved in this system. We report the fabrication and measurement of a top-gated quantum dot occupied by a single electron in a Si/SiGe heterostructure. Transport through the quantum dot is directly correlated with charge-sensing from an integrated quantum point contact, and this charge-sensing is used to confirm single-electron occupancy in the quantum dot.Comment: 3 pages, 3 figures, accepted version, to appear in Applied Physics Letter