We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs. tunneling time ( 50 ≈ )and singlet-triplet splitting vs. thermal energy ( 20 ≈ ) allow energy-selective tunneling-based spin-to-charge conversion with readout visibility 92.6 % ≈ . Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs.Further, we discuss the generality of the method for use in other materials, including silicon.

Fast and high-fidelity quantum state detection is essential for building robust spin-based quantum information processing platforms in semiconductors. The Pauli spin blockade (PSB)-based spin-to-charge conversion and its variants are widely used for the spin state discrimination of two-electron singlet-triplet (ST 0) qubits; however, the single-shot measurement fidelity is limited by either the low signal contrast, or the short lifetime of the triplet state at the PSB energy detuning, especially due to strong mixing with singlet states at large magnetic field gradients. Ultimately, the limited single-shot measurement fidelity leads to low visibility of quantum operations. Here, we demonstrate an alternative method to achieve spin-to-charge conversion of ST 0 qubit states using energy-selective tunneling between doubly occupied quantum dots (QDs) and electron reservoirs. We demonstrate a single-shot measurement fidelity of 90% and an S-T 0 oscillation visibility of 81% at a field gradient of 100 mT (~500 MHz h (g*•μ B) −1); this allows single-shot readout with full electron charge signal contrast and, at the same time, long and tunable measurement time with negligible effect of relaxation even at strong magnetic field gradients. Using an rf-sensor positioned opposite to the QD array, we apply this method to two ST 0 qubits and show high-visibility readout of two individual single-qubit gate operations is possible with a single rf single-electron transistor sensor. We expect our measurement scheme for two-electron spin states can be applied to various hosting materials and provides a simplified and complementary route for multiple qubit state detection with high accuracy in QD-based quantum computing platforms.

Electron pairing due to a repulsive Coulomb interaction in a triple quantum dot (TQD) is experimentally studied. It is found that electron pairing in two dots of a TQD is mediated by the third dot, when the third dot strongly couples with the other two via Coulomb repulsion so that the TQD is in the twofold degenerate ground states of (1, 0, 0) and (0, 1, 1) charge configurations. Using the transport spectroscopy that monitors electron transport through each individual dot of a TQD, we analyze how to achieve the degeneracy in experiments, how the degeneracy is related to electron pairing, and the resulting nontrivial behavior of electron transport. Our findings may be used to design a system with nontrivial electron correlations and functionalities.

We report individual confinement and two-axis qubit operations of two electron spin qubits in GaAs gate-defined sextuple quantum dot array with integrated micro-magnet. As a first step toward multiple qubit operations, we demonstrate coherent manipulations of three singlet-triplet qubits showing underdamped Larmor and Ramsey oscillations in all double dot sites. We provide an accurate measure of site-dependent field gradients and rms electric and magnetic noise, and we discuss the adequacy of simple rectangular micro-magnet for practical use in multiple quantum dot arrays. We also discuss current limitations and possible strategies for realizing simultaneous multi-qubit operations in extended linear arrays.Fabrication of large array of qubits and demonstration of coherent multi-qubit operations are necessary steps for realizing scalable quantum processing unit 1-4 . Harnessing spin degree of freedom in gate defined quantum dots (QDs) has attracted significant interests [5][6][7][8][9][10][11][12][13][14] owing to spin's long coherence time in solid state system and potential scalability including well-established fabrication technology 2,15,16 . Depending on the degree of spin-charge mixing, elementary quantum operations of Loss-DiVincenzo (single electron) 12,14,17,18 , singlet-triplet (ST0, two electron) 5,10,11,[19][20][21] , and various three electron spin qubits [22][23][24] have been demonstrated. Electron loading and charge state manipulation are also performed in ~ 10 coupled linear dots 3,25 and ~ 4 site two dimensional arrays 1,4 . Moreover, QD Hamiltonian parameters have shown to be widely tunable 19,[26][27][28] . With possibility of efficient electrical control, QD systems are also emerging as promising quantum simulators 26,29,30 .Toward realizing multi-qubit operations in semiconductor quantum chips, coherent qubit addressing beyond ground state property tuning is important. In this work, we demonstrate individual operations of three ST0 qubits formed in different double dot sites in a sextuple linear quantum dot array with rf-single-electron transistor (rf-set) sensors. Following pioneering works such as recent demonstration of four individual Loss-DiVincenzo qubit operations in GaAs 31 , we employed proximal rectangular micro-magnet enabling sizable magnetic field gradient 32 and two-axis control for all ST0 qubits in our device. This work addresses spatial range of experimentally usable field gradient by simple micro-magnet using qubit oscillations as sensitive magnetic probes, and we show that magnitude of electric and magnetic noise of all qubits are comparable to the previous works. Moreover, we discuss examples of adverse effects of surface micro-magnet structure fabricated on the device without interposing dielectrics, which calls for further optimization of sensor and magnet position to enable simultaneous qubit and sensor operations in similar device geometries.

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