  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 $MHz\cdot h \cdot(g^{*}\cdot \mu_B)^{-1}$);
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.
