Self-consistent measurement and state tomography of an exchange-only spin qubit

Abstract

Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements and non-orthogonal control axes.

Characterizing gate operations near the sweet spot of an exchange-only qubit

“…(Charge noise in the tunnel coupling 11 is not suppressed at this point, but is not thought to be a dominant noise source. 12 ) As consistent with recent experiments, 13 Z-rotations are performed at the sweet spot, while Xrotations are obtained by pulsing away from this point. In principle, the different rotations can be turned on and off independently.…”

“…For GaAs-based devices, most leakage channels can be suppressed by enforcing sizeable energy splittings. 15 As consistent with recent experiments, 13 we therefore consider the energy hierarchy gµ B B J gµ B ∆B > 0, where J is the exchange interaction generated by the tunnel couplings. (In Appendix F, we also consider 28 Si-based devices, which do not require such an energy heirarchy, due to the absence of nuclear spins.)…”

“…14 In certain regimes we find that the main limit on the gate fidelities arises from the Overhauser fields, as consistent with experimental observations. 13 However, when the gates are properly optimized, we predict that charge noise should determine the upper bound on gate fidelities. After optimization, we find that gate fidelities at the sweet spot are typically 20 times better than away from the sweet spot.…”

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

“…Advances in device size, connectivity and homogeneity are underway as well in the pursuit of scalable quantum computing, the results of which can be directly leveraged. Examples include scalable gate layouts for 1D arrays [37,38] as well as square [39] and triangular [40] geometries, industrial-grade fabrication processes [41] and magnetically quiet 28 Si substrates [42], that open up further possibilities for quantum simulation experiments with quantum dots.…”

Field-effect-induced two-dimensional electron gas utilizing modulation-doped ohmic contacts

“…A high quality two-dimensional electron gas is induced by a field-effect and is tunable over a wide range of density. Device design, fabrication, and low temperature (T= 0.3K) transport data are reported.Nanostructures such as quantum point contacts and quantum dots fabricated on modulation-doped AlGaAs/GaAs heterostructures are widely used to explore nanoscale electron transport and are utilized extensively in spin-based approaches to quantum computing [1][2][3][4][5][6][7][8]. Modulation-doped GaAs/AlGaAs heterostructures possess several desirable attributes including very high mobility of the underlying two-dimensional electron gas (2DEG) and the relative ease of nanostructure fabrication.…”