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

Abstract

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true "sweet spot" to fluctuations of the quantum dot energy levels. Both single-and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a new resource for quantum computation and communication.Semiconductor qubits [1,2] are a leading candidate technology for quantum information processing [3]. Spins can have extremely long quantum coherence due to a decoupling of spin information from charge noise in many materials, and they are small, enabling high density. But these strengths pose a challenge for control as microwave pulses generally result in slow gates with significant potential for crosstalk to nearby qubits. The exchange interaction on the other hand provides a natural and fast method for entangling semiconductor qubits: it can be used to perform two-spin entangling operations with a finite-length voltage pulse or to couple spins with a constant interaction. Exchange also provides a solution to the control problem by allowing a two-level system to be encoded into the greater Hilbert space of multiple physical spins. Following work on decoherence free subspaces and subsystems (DFS) [4][5][6], many multi-spinbased qubits have been proposed and demonstrated with various desirable properties for quantum computing: as examples, 2-DFS (a.k.a. "singlet-triplet") [7][8][9][10], 3-DFS (a.k.a. "exchange-only") [11][12][13][14], or 4-DFS qubits [15] of various implementations are possible. The DFS gives some immunity to global field fluctuations, but more importantly it allows for gate operations via a sequence of pair-wise exchange interactions between spins with fast, baseband voltage control on the metallic top-gates, obviating the need for RF pulses. However, charge noise likely limits gate fidelity [16,17] as charge and spin are coupled while spins undergo exchange.The effects of charge (or other) noise on memory or gate fidelity can be suppressed to a certain extent by taking advantage of natural or engineered "sweet" spots: a spot in parameter space where critical system properties are minimally effected by certain environmental changes. Sweet spots have been an effective tool to increase the coherence of superconducting qubits [18,19] and more recently have been applied to exchange-only qubits [20][21][22][23][24]. For example, in the "resonant exchange" (RX) qubit-an encoded qubit made out of 3 quantum dot qubits with "always-on" exchange interactions and a much higher chemical potential for the middle dot than the outer dots-a partial sweet spot is maintained while microwave control allows for single qubit operations [20,21] "resonant" with the gap of the 3-spin system. The first derivative of the RX qubit frequency vanishes for one of the two detuning parameters that are affected by charge noise. For two qubit gates, the RX qubit offers a relatively large transition dipole...

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

“…One platform that is particularly promising is that of electron spins in semiconductor quantum dots, due to their compatibility with the existing semiconductor industry, as well as to the fast electrical control and long coherence times available in these systems. 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.…”

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

“…We then determine the parameters needed to make the sum of the errors from the "naïve" sequence and the "identity" zero. We present the parameters that we extract from this procedure for all 24 Clifford gates and present an example, a rotation by π about theˆ x +ˆ y axis. Overall, the resulting pulse sequences consist of more "blocks" (up to nine) than those used to correct only noise-induced error for a single qubit in isolation.…”

“…Several different platforms for realizing qubits exist, but our focus in this work will be on electronic spins in semiconductor quantum dots. Several different types of semiconductor quantum dot electron spin qubits exist, such as the single-spin exchange qubit [2][3][4][5][6][7][8][9] , the singlet-triplet twoelectron double-dot qubit [10][11][12][13][14][15][16][17][18][19] , the exchange-only threeelectron triple-dot qubit [20][21][22][23][24] , and the "hybrid" threeelectron double-dot qubit [25][26][27] . The semiconductor spin qubit platform has the advantages of being compatible with the existing semiconductor electronics industry as well as the ability to perform gates more quickly (using fast electrical pulses) than other platforms, such as superconducting and ion trap qubits.…”

Transport-induced suppression of nuclear field fluctuations in multi-quantum-dot systems

“…Its two main sources are (i) hyperfine coupling of the electronic spins to the randomly fluctuating nuclear spin baths in the quantum dots [17][18][19][20] and (ii) charge fluctuations in the environment that interfere with exchange-based qubit control [21,22]. The latter could be mitigated by enhancing device quality or operating the qubit at a (higher-order) sweet spot [23][24][25][26][27], which leaves the nuclear spin noise as an important intrinsic obstacle for further progress.…”