Fast and high-fidelity state preparation and measurement in triple-quantum-dot spin qubits

Abstract

We demonstrate rapid, high-fidelity state preparation and measurement in exchange-only Si/SiGe triple-quantum-dot qubits. Fast measurement integration (980 ns) and initialization (≈300 ns) operations are performed with all-electrical, baseband control. We emphasize a leakage-sensitive joint initialization and measurement metric, developed in the context of exchange-only qubits but applicable more broadly, and report an infidelity of 2.5±0.5×10 −3 . This result is enabled by a highvalley-splitting heterostructure, initialization at the 2-to-3 electron charge boundary, and careful assessment and mitigation of T1 during spin-to-charge conversion. The ultimate fidelity is limited by a number of comparably-important factors, and we identify clear paths towards further improved fidelity and speed. Along with an observed single-qubit randomized benchmarking error rate of 1.7×10 −3 , this work demonstrates initialization, control, and measurement of Si/SiGe triple-dot qubits at fidelities and durations which are promising for scalable quantum information processing.

Universal logic with encoded spin qubits in silicon

“…Fabrication of larger arrays should also follow yield-enhancement trajectories established by the silicon-based classical microelectronics industry as designs are improved [6,7]. Demonstrations with silicon qubits include single-spin control via electron-dipole-spin resonance [8], initialization and read-out of entangled spin states [9], entangling operations between two single-spin qubits using exchange [10][11][12], and charge shuttling in arrays as large as nine dots [13,14]. However, many of these results have employed control methods which may present challenges for scaling.…”

“…Though EO control was proposed over twenty years ago [16], experimental demonstration of encoded entangling operations has waited for the availability of fullyfunctional six-dot devices. Three recent advances enabled the result we report on here: the Single Layer Etch-Defined Gate Electrode (SLEDGE) process [18] for making high-yielding devices, the use of narrow Si/Si 0.3 Ge 0.7 quantum wells to increase the probability that dots have sufficiently large valley energy for measurement and initialization [9], and improved control software to manage the complexity of these larger quantum manipulations [1]. Using these, we demonstrate an average 2-qubit Clifford fidelity of 97.1 ± 0.2% and the FW controlled-NOT gate [17,19] (FW-CNOT) with 96.3 ± 0.7% fidelity, both limited by well-understood sources of magnetic noise [20], and an encoded SWAP operation with 99.3 ± 0.5% fidelity.…”

“…The encoded |0 state corresponds to two-spin singlet (S 12 = 0) and encoded |1 corresponds to two-spin triplet (S 12 = 1). Qubit states are measured by mapping their spin state to charge configuration with Pauli spin blockade [3,9]. A similar physical mechanism is used to initialize qubits into singlet states [9].…”

“…Qubit states are measured by mapping their spin state to charge configuration with Pauli spin blockade [3,9]. A similar physical mechanism is used to initialize qubits into singlet states [9]. We prepare and measure spin states on the outermost dot pairs (P1/P2 and P5/P6), with the X3 gate connecting the innermost electron spins in dots P3 and P4.…”

“…The m 123 degree of freedom is called the gauge, and it is unaffected by exchange operation between any of the three spins. The Pauli spin blockade process on dots 1 and 2 provides initialization and measurement only of the S 12 quantum number and has no impact on m 123 [9]. Singlet measurements on spins 1 and 2 therefore measure the probability of |0, 1/2; m 123 independent of m 123 , and provide no information to distinguish encoded triplet |1, 1/2; m 123 states from leaked states |1, 3/2; m 123 .…”