Reduced Sensitivity to Charge Noise in Semiconductor Spin Qubits via Symmetric Operation

Abstract

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...

Quantum simulation of a Fermi–Hubbard model using a semiconductor quantum dot array

“…Coherent evolution of excitations in charge and spin can span many sites, as, contrary to what might be expected, dissipation and decoherence rates induced by electromagnetic noise can be made > 20 times smaller than the relevant coupling energies [20,22,23]. Furthermore, local control and read-out of both charge and spin degrees of freedom have become matured areas of research, given the large ongoing effort of using quantum dots as a platform for quantum information processing [17,18,19,20,21,22,23]. In particular, excellent control of small on-site energy differences [20,28] or tunnel couplings [22,23] has been shown at specific values of electron filling and tuning.…”

“…As such, pure state initialization of highly-entangled states is possible even without the use of adiabatic initialization schemes [27]. Coherent evolution of excitations in charge and spin can span many sites, as, contrary to what might be expected, dissipation and decoherence rates induced by electromagnetic noise can be made > 20 times smaller than the relevant coupling energies [20,22,23]. Furthermore, local control and read-out of both charge and spin degrees of freedom have become matured areas of research, given the large ongoing effort of using quantum dots as a platform for quantum information processing [17,18,19,20,21,22,23].…”

“…Furthermore, local control and read-out of both charge and spin degrees of freedom have become matured areas of research, given the large ongoing effort of using quantum dots as a platform for quantum information processing [17,18,19,20,21,22,23]. In particular, excellent control of small on-site energy differences [20,28] or tunnel couplings [22,23] has been shown at specific values of electron filling and tuning. Quantum simulation experiments can leverage many of these developments, trading off some of the experimental difficulties involved in full coherent control for ease of scaling.…”

“…Seminal efforts are underway in the control of artificial quantum systems, that can be made to emulate the underlying Fermi-Hubbard models [5, 6, 7, 8,9,10,11]. Electrostatically confined conduction band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical pure-state initialisation and readily adhere to an engineerable Fermi-Hubbard Hamiltonian [12,13,14,15,16,17,18,19,20,21,22,23]. Until now, however, the substantial electrostatic disorder inherent to solid state has made attempts at emulating Fermi-Hubbard physics on solid-state platforms few and far between [24,25].…”

Charge-noise-insensitive gate operations for always-on, exchange-only qubits

“…This is possible since the sweet spot is independent of the tunneling elements. The idea of tuning the tunneling barrier directly to control the exchange interaction (as opposed to changing the relative energy-level detuning between two dots), which was the original idea of QD spin qubits [1], has been successfully demonstrated in recent experiments in Si/SiGe [24] and GaAs/AlGaAs [26] QD devices. The exchange operation via gate-tuning the tunneling barrier near the symmetric operating point (SOP) of the detuning leads to much higher fidelity during the exchange operations than the conventional control of the exchange interaction by tilting (detuning) the double dot system.…”