High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits in GaAs

Abstract

Single-qubit operations on singlet-triplet qubits in GaAs double quantum dots have not yet reached the fidelities required for fault-tolerant quantum information processing. Considering experimentally important constraints and using measured noise spectra, we numerically minimize the effect of decoherence (including high-frequency non-Markovian noise) and show theoretically that quantum gates with fidelities higher than 99.9% are achievable. We also present a self-consistent tuning protocol which should allow the elimination of individual systematic gate errors directly in an experiment.One well-established possibility to realize a qubit with electron spins in a semiconductor is to use the m s = 0 spin singlet and triplet states of two electrons as computational basis states [1]. In contrast to single electron spins this encoding allows for all-electrical qubit control. Very long coherence times of up to 200 µs [2], all aspects of single-qubit operation (e.g. initialization [3] and single-shot readout [4]) and a first two-qubit gate [5] have been demonstrated experimentally for such singlettriplet (ST) qubits in GaAs quantum dots. Universal single-qubit control was also shown [6] but subject to large uncharacterized errors. Limiting control error rates to ∼ 10 −3 is a crucial requirement for fault tolerant quantum computing with quantum error correction (QEC) [7][8][9]. Estimates based on coherence time measurements [2,10] indicate that very high gate fidelities should be possible for GaAs-based two-electron spin qubits. However, nonlinearities in the electric control and experimental constraints make the direct application of control methods such as Rabi driving difficult.Previous theoretical work has shown how universal control on the single-and two-qubit level can be achieved in the face of limited dynamic control range [11]. Additionally, gating sequences which are insensitive to slow (quasistatic) control fluctuations have been proposed for this qubit system [12][13][14][15]. While these proposals provide very useful conceptual guidance, a direct implementation will be impeded by experimental constraints such as finite pulse rise times and sampling rate of voltage pulses. Likewise, decoherence effects caused by charge noise [10] and nuclear spin fluctuations [16] have a significant effect.In this letter we use numerical pulse optimization to address the problems posed by systematic inaccuracies and decoherence. Pulse optimization is common in NMR [17] and is also receiving increasing attention in quantum information [12,[18][19][20][21][22][23][24]. In contrast to these previous approaches, our optimization is specifically tailored to the ST-qubit system and includes not only the relevant physical effects but also the most important hardware constraints and the effect of high-frequency nonMarkovian noise. We use experimentally determined parameters and noise spectra [10,16,25] to compute expected gate fidelities and find implementations with no systematic errors and optimized robustness to both slow and fas...

Noise-compensating pulses for electrostatically controlled silicon spin qubits

“…21,25,26 While a vast literature on dynamically corrected gates (DCGs) has existed long before the conception of spin qubits, it was only recently realized that the special constraints imposed by solid state spin systems often require brand-new approaches to developing DCGs. [27][28][29][30][31][32] While one of these approaches, known as supcode, 27 has already been experimentally demonstrated for nitrogen-vacancy centers in diamond 33 and is also applicable to single donor spins in Si, it is particularly suitable for singlet-triplet qubits subject to Overhauser and charge noise because it is designed to yield comparatively simple pulse sequences that cancel both while respecting all experimental constraints, including the restriction to positive, single-axis control. In the case of isotope-enriched Si, however, canceling Overhauser noise is no longer necessary, leaving open the possibility that robust quantum operations on Si devices might be realized with even simpler and faster control sequences.…”

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

“…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.…”

Non-Gaussian signatures and collective effects in charge noise affecting a dynamically decoupled qubit

“…1 Studies of exchange-coupled electron spin qubits in GaAs QDs, in particular, have shifted their attention from the nuclear to the charge environment, as the important role of the latter has been identified. [2][3][4] Recent works include design and implementation of exchange-only three-spin qubits in a triple QD that have better immunity against low-frequency electrical noise, 5 multielectron spin qubits with demonstrated reduced exchange noise, 6 and self-calibrated, optimized pulse sequence 7 and asymmetric double dot geometry 8 , both tailored to mitigate charge noise for high-fidelity single-qubit gates in singlet-triplet (S − T 0 ) spin qubits. Charge noise was also shown to cause relaxation in a single electron spin qubit, through the spin-orbit interaction.…”