High-fidelity entangling gate for double-quantum-dot spin qubits

Abstract

Electron spins in semiconductors are promising qubits because their long coherence times enable nearly 10 9 coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. Here, we demonstrate an entangling gate between two double-quantum-dot spin qubits in GaAs by using a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking, we measure single-qubit gate fidelities of~99%, and through self-consistent quantum measurement, state, and process tomography, we measure an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault-tolerant quantum information processing.npj Quantum Information (2017) 3:3 ; doi:10.1038/s41534-016-0003-1 INTRODUCTIONThe quantum phase coherence of isolated spins in semiconductors 1-7 can persist for long times, reaching tens of milliseconds for electron spins 8 and tens of minutes for nuclear spins. 9 Such long coherence times enable single-qubit gate fidelities exceeding the threshold for fault-tolerant quantum computing 8 and make spins promising qubits. However, entangling spins is difficult because magnetic interactions between spins are weak. For electrons, this challenge can be met by exploiting the charge of the electron for electric-dipole 10 or gate-controlled exchange coupling 2 between spins. In these methods, however, the qubit energy depends on electric fields, and charge noise in the host material limits singlequbit coherence.11 Charge noise also affects other qubit platforms. For example, heating due to charge noise is a limiting factor in the coherence of trapped ion qubits, 12 and the transmon superconducting qubit was designed to suppress noise from charge fluctuations in superconducting islands.13 Strategies such as composite pulses, 14, 15 dynamical decoupling, 11 and sweet-spot operation [16][17][18] have been developed to mitigate the effects of charge noise.In this work, we present a technique to suppress decoherence caused by charge noise. The key idea is to apply a large transverse qubit energy splitting that does not depend on electric fields and, therefore, suppresses the effects of charge fluctuations. We implement this scheme with two singlet-triplet qubits, each of which consists of two electrons in a double-quantum-dot.

Fast pulse sequences for dynamically corrected gates in singlet-triplet qubits

“…Several different implementations of qubits using such electronic spins include the single-spin Loss-DiVincenzo qubit [2][3][4][5][6][7][8][9] , the singlet-triplet qubit [10][11][12][13][14][15][16]18,19,33 , the tripledot exchange-only qubit [20][21][22][23][24] , and "hybrid" qubits with three electrons in two dots [25][26][27] . While there has been much experimental progress on improving the fidelity of gate operations 3,13,15,28 , with fidelities as high as about 99% for single-qubit gates and about 90% for two-qubit gates having been demonstrated 29 , more work must still be done to comfortably exceed the 99% surface-code threshold for all gate operations.…”

“…In this work, we focus on singlet-triplet qubits, which have been used to experimentally demonstrate some of the highest single-and two-qubit gate fidelities achieved so far in a spin-qubit system 29 . Such a qubit consists of two electrons occupying a double quantum dot, with an electrostatically controlled Heisenberg exchange interaction between them and a magnetic field gradient across the two dots, the latter typically produced with either a micromagnet or by polarizing the nuclear spins 12,30,[49][50][51] .…”

Crosstalk error correction through dynamical decoupling of single-qubit gates in capacitively coupled singlet-triplet semiconductor spin qubits

“…This form of coupling, which is used extensively for inter-qubit coupling in singlet-triplet qubits 15,28 , is simpler to treat than a Heisenberg exchange coupling, since capacitive coupling, as we will see, cannot cause leakage out of the logical subspace. We also assume that the field and charge noise in our system is quasistatic.…”

“…However, noise-induced error has been a formidable challenge adversely affecting experimental progress in spin qubits. Fortunately, considerable progress has been achieved recently, with singlequbit gate fidelities of 99% and two-qubit gate fidelities of 90% having been reported in singlet-triplet double-dot qubits 28 . Several methods for reducing error have been developed for these platforms.…”

High-Fidelity Single-Shot Singlet-Triplet Readout of Precision-Placed Donors in Silicon

“…Furthermore, ST readout can be used to measure single-electron spin qubits [13] at lower magnetic fields and higher temperatures easing the constraints on microwave electronics and cryogenic cooling [38]. Finally, encoding qubits using ST states [21,39,40] allows for an all electrical approach for control; in particular, multiple qubits can be coupled by utilizing the inherent electric dipole coupling given by the (1,1)-(2,0) charge configurations [41,42]. The results obtained herein, in addition to the reduced complexity of electron confinement in donors, make a compelling case for further research on the scaling of multiple forms of donor-based quantum computing architectures.…”