We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations. DOI: 10.1103/PhysRevLett.116.110402 Gated semiconductor quantum dots are a leading candidate for quantum information processing due to their high speed, density, and compatibility with mature fabrication technologies [1,2]. Quantum dots are formed by spatially confining individual electrons using a combination of material interfaces and nanoscale metallic gates. Although several quantized degrees of freedom are available [3][4][5], the electron spin is often employed as a qubit due to its long coherence time [6,7]. Spin-spin coupling may be controlled via the kinetic exchange interaction, which has the benefit of short range and electrical controllability. Numerous qubit proposals use exchange, including as a two-qubit gate between ESR-addressed spins [8], a single axis of control in a two dot system also employing gradient magnetic fields [9] or spin-orbit couplings [10], or as a means of full qubit control on a restricted subspace of at least three coupled spins [11][12][13]. However, since exchange relies on electron motion, it is susceptible to electric field fluctuations, or charge noise. Limiting the consequence of this noise is critical to attaining performance of exchange-based qubits adequate for quantum information processing.Charge noise in semiconductor quantum dots may originate from a variety of sources including electric defects at interfaces and in dielectrics [14]. These defects typically result in electric fields that exhibit an approximate 1=f noise spectral density. Conventional routes for reducing charge noise include improving materials and interfaces [15] and dynamical decoupling [16][17][18][19]. In this Letter, rather than addressing the microscopic origins or detailed spectrum of charge noise, we introduce a "symmetric" mode of operation where the exchange interaction is less susceptible to that noise. This is done by biasing the device to a regime where the strength of the exchange interaction is first-order insensitive to dot chemical potential fluctuations but is still controllable by modulating the interdot tunnel barrier. This dramatically reduces the effects of charge noise.The principle of symmetric operation can be understood by treating charge noise as equivalent to voltage fluctuations on confinement gates. This approximation is valid when materi...

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time.

We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures.Comment: 3 pages, 3 figure

We report on an experimental demonstration of graphene-metal ohmic contacts with contact resistance below 100 Ω µm. These have been fabricated on graphene wafers, both with and without hydrogen intercalation, and measured using the transmission line method. Specific contact resistivities of 3 × 10−7 to 1.2 × 10−8 Ω cm2 have been obtained. The ultra-low contact resistance yielded short-channel (source-drain distance of 0.45 µm) HfO2/graphene field effect transistors (FETs) with a low on-resistance (Ron) of 550 Ω µm and a high current density of 1.7 A/mm at a source-drain voltage of 1 V. These values represent state-of-the-art (SOA) performance in graphene-metal contacts and graphene FETs. This ohmic contact resistance is comparable to that of SOA high-speed III–V high electron mobility transistors.

We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function -control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the (0, 2) ↔ (1, 1) charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.Silicon-based quantum devices hold great promise for realizing spin qubits.1 The ability to isotopically purify silicon has resulted in the demonstration of extremely long spin-coherence times in donor-based silicon devices 1,2 and in recent years a series of results have demonstrated many fundamental properties of electrostatically-defined silicon-based quantum devices. Measurements of T 1 , 3 valley splitting, 4,5 and Pauli blockade 5,6 were made using doped depletion-mode SiGe devices. Improved device performance was achieved by eliminating the intentional dopants in the SiGe heterostructure, a major source of noise and instability, making necessary the use of a global field gate to accumulate electrons. This allowed for demonstrations of high mobility two-dimensional electron gases (2DEGs), 7 Coulomb blockade, 8 valley splitting, 9 Pauli blockade, 10 and T 2 and Rabi measurements. 11The ideal realization of electrostatically-defined quantum dot devices would have independent control of the charge occupancy of each dot and its associated exchange couplings. A promising approach is to utilize an accumulation-based design in which independent localized gates are used to create electron baths, create and control quantum dot occupancy, and modulate tunnel barriers between them. A Si metal-oxide-semiconductor based design has shown great promise along these lines, demonstrating in quick succession charge sensing, 12,13valley splitting, 14 and well-controlled double-dot behavior including spin blockade.15 However better isolation from residual disorder due to gate oxide charges can potentially be achieved using a SiGe heterostructure.In this Letter we report on a double quantum dot device with an integrated dot charge sensor based on a synthesis of the improved gated control of the accumulationmode designs 15 with the lower disorder of field-gated SiGe heterostructure designs.10 This approach builds on our previous experience with quantum devices made using single gates in accumulation mode 16,17 and achieves the goal of complete gated control over a set of quantum dots and inter-dot couplings, dominating over the effects of disorder. A nominally undoped epitaxial heterostructure similar...