Screening of charged impurities with multielectron singlet-triplet spin qubits in quantum dots

Abstract

Charged impurities in semiconductor quantum dots comprise one of the main obstacles to achieving scalable fabrication and manipulation of singlet-triplet spin qubits. We theoretically show that using dots that contain several electrons each can help to overcome this problem through the screening of the rough and noisy impurity potential by the excess electrons. We demonstrate how the desired screening properties turn on as the number of electrons is increased, and we characterize the properties of a double quantum dot singlet-triplet qubit for small odd numbers of electrons per dot. We show that the sensitivity of the multi-electron qubit to charge noise may be an order of magnitude smaller than that of the two-electron qubit.One of the most promising paths to scalable quantum computation is to use laterally defined double quantum dots (DQDs) in semiconductor heterostructures. The qubit is formed by the spin states of the two-electron DQD with total spin projection zero along the z axis. 1,2 Such a qubit is insensitive to spatially uniform magnetic field fluctuations, and, most importantly, amenable to fast electrical manipulation. 2,3 Recent experiments have made tremendous advances along these lines, demonstrating single-qubit initialization, arbitrary manipulation, and single-shot readout, all within a fraction of the coherence time of the qubit. 3-7 Preliminary steps toward an entangling two-qubit gate have also been reported. 8 In principle, successful completion of that program leaves the (admittedly enormous) challenge of scaling up to large numbers of qubits as the last remaining hurdle in the fabrication of a practical quantum computer.However, a practical issue has emerged which threatens to be a crippling impediment to continued rapid progress. The semiconductor samples used to create quantum dots invariably contain a number of charge impurity centers, perhaps 10 10 cm −2 in GaAs systems. 9 Even if the charge on these centers can be frozen to avoid switching noise, their presence inhibits access to the oneelectron-per-dot regime since the lowest energy states of the dot may be fragmented due to the roughened potential landscape. [10][11][12][13] This makes it difficult to find samples suitable for spin qubit realization. Furthermore, typically the impurities do introduce some switching noise, [14][15][16][17] so that even in samples in which the impurities are all far enough from the DQD that a two-electron singlettriplet qubit can be accessed, the interdot exchange energy is still subject to random fluctuation, leading to gate errors and decoherence. [18][19][20] This necessitates operating in a parameter regime such that the sensitivity of the exchange energy to the charge noise is minimized, a so-called "sweet spot". 21 In general, the charge noise problem is even more pernicious when performing twoqubit operations directly mediated by the Coulomb interaction, and one must again seek a sweet spot. 22,23 However, in practice, this strategy may not be sufficient since one typically cannot optimize ov...

Coherent Operations and Screening in Multielectron Spin Qubits

“…We find significantly improved coherence in the multi-electron case, consistent with expectations of screening by core electrons [7,22]. By analyzing the dephasing during the exchange-gate operation, we characterize the electrical noise environment for each occupancy.…”

“…In single-electron dots, both nuclear [18,19] and electrical [20,21] dephasing have been characterized, with electrical noise modeled as a fluctuating detuning between double-dot levels. Multi-electron quantum dots have also received theoretical attention due to ease of realization as well as possibly improved performance [7,9,[22][23][24].In this Letter, we investigate coherent exchange oscillations in coupled multi-electron GaAs quantum dots-this operation was specifically chosen to be sensitive to electrical noiseand compare results to oscillations in the same device operated with single-electron dots. We find significantly improved coherence in the multi-electron case, consistent with expectations of screening by core electrons [7,22].…”

“…Multi-electron quantum dots have also received theoretical attention due to ease of realization as well as possibly improved performance [7,9,[22][23][24].…”

“…In single-electron dots, both nuclear [18,19] and electrical [20,21] dephasing have been characterized, with electrical noise modeled as a fluctuating detuning between double-dot levels. Multi-electron quantum dots have also received theoretical attention due to ease of realization as well as possibly improved performance [7,9,[22][23][24].…”

Tunable Spin-Qubit Coupling Mediated by a Multielectron Quantum Dot

“…Thus, setting the quantum dot level splitting and impurity-dot detunings to values within an optimal range in principle enables highfidelity exchange gates. Finally, studies of exchange in multi-electron quantum dots [51][52][53] suggest that exchange coupling of the type discussed in the present work, which is derived from tunneling via an excited orbital of a multi-level quantum dot with lower-energy orbitals filled by electron pairs, may exhibit increased robustness against fluctuations caused by charge noise due to screening of the Coulomb interaction by the paired "core" electrons already present in the dot. Varying the number of electrons in the dot changes the spacing between the outermost levels [3] and consequently ∆ M , so that J may be tuned in discrete steps.…”