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 demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.Comment: 4 pages, 4 figure

By analyzing the temperature ͑T͒ and density ͑n͒ dependence of the measured conductivity ͑͒ of twodimensional ͑2D͒ electrons in the low-density ͑ϳ10 11 cm −2 ͒ and temperature ͑0.02-10 K͒ regimes of highmobility ͑1.0 and 1.5ϫ 10 4 cm 2 / Vs͒ Si metal-oxide-semiconductor field-effect transistors, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity-driven percolation transition where the density-dependent conductivity vanishes as ͑n͒ ϰ ͑n − n p ͒ p , with the exponent p ϳ 1.2 being consistent with a percolation transition. The "metallic" behavior of ͑T͒ for n Ͼ n p is shown to be well described by a semiclassical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.The so-called two-dimensional ͑2D͒ metal-insulator transition ͑MIT͒ has been a subject 1,2 of intense activity and considerable controversy ever since the pioneering experimental discovery 3 of the 2D MIT phenomenon in Si metaloxide-semiconductor field-effect transistors ͑MOSFETs͒ by Kravchenko and Pudalov some 15 years ago. The apparent MIT has now been observed in almost all existing 2D semiconductor structures, including Si MOSFETs, 3,4 electrons, 5-7 and holes [8][9][10][11] in GaAs/AlGaAs, and electrons in Si/SiGe ͑Refs. 12 and 13͒ systems. The basic phenomenon refers to the observation of a carrier density-induced qualitative change in the temperature dependence of the resistivity ͑n , T͒, where n c is a critical density separating an effective "metallic" phase ͑n Ͼ n c ͒ from an "insulating" phase ͑n Ͻ n c ͒, exhibiting d / dT Ͼ 0͑Ͻ0͒ behavior typical of a metal ͑insulator͒.The high-density metallic behavior ͑n Ͼ n c ͒ often manifests in a large ͑by 25% for electrons in GaAs/AlGaAs heterostructures to factors of 2-3 in Si MOSFETs͒ increase in resistivity with increasing temperature in the lowtemperature ͑0.05-5 K͒ regime where phonons should not play much of a role in resistive scattering. The insulating regime, at least for very low ͑n Ӷ n c ͒ densities and temperatures, seems to be the conventional activated transport regime of a strongly localized system. The 2D MIT phenomenon occurs in relatively high-mobility systems, although the mobility values range from 10 4 cm 2 / Vs ͑Si MOSFET͒ to 10 7 cm 2 / Vs͑GaAs/ AlGaAs͒ depending on the 2D system under consideration. The 2D MIT phenomenon is also considered to be a low-density phenomenon although, depending on the 2D system under consideration, the critical density n c differs by 2 orders of magnitude ͑n c ϳ 10 11 cm −2 in 2D Si and ϳ10 9 cm −2 in high-mobility GaAs/AlGaAs heterostructures͒. The universal features of the 2D MIT phenomenon are ͑1͒ the existence of a critical density n c distinguishing an effective high-density metallic ͑d / dT Ͼ 0 for n Ͼ n c ͒ phase from an effective low-density insulating ͑d / dT Ͻ 0 for n Ͻ n c ͒ phase, and ͑2͒ while the insulating phase for n Ͻ n c seems mostly to manifest the conventional activated transport be...

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