Integrated silicon qubit platform with single-spin addressability, exchange control and single-shot singlet-triplet readout

ARTICLE DOI: 10.1038/s41467-018-06039-x OPEN Integrated silicon qubit platform with single-spin addressability, exchange control and single-shot singlet-triplet readout 1234567890():,; M. A. Fogarty1,6, K. W. Chan 1, B. Hensen1, W. Huang1, T. Tanttu1, C. H. Yang1, A. Laucht1, M. Veldhorst2, F. E. Hudson1, K. M. Itoh3, D. Culcer 4, T. D. Ladd5, A. Morello 1 & A. S. Dzurak1 Silicon quantum dot spin qubits provide a promising platform for large-scale quantum computation because of their compatibility with conventional CMOS manufacturing and the long coherence times accessible using 28Si enriched material. A scalable error-corrected quantum processor, however, will require control of many qubits in parallel, while performing error detection across the constituent qubits. Spin resonance techniques are a convenient path to parallel two-axis control, while Pauli spin blockade can be used to realize local parity measurements for error detection. Despite this, silicon qubit implementations have so far focused on either single-spin resonance control, or control and measurement via voltage-pulse detuning in the two-spin singlet–triplet basis, but not both simultaneously. Here, we demonstrate an integrated device platform incorporating a silicon metal-oxidesemiconductor double quantum dot that is capable of single-spin addressing and control via electron spin resonance, combined with high-fidelity spin readout in the singlet-triplet basis. 1 Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, NSW 2052, Australia. 2 QuTech and Kavli Institute of Nanoscience, TU Delft, Lorentzweg 1, 2628CJ Delft, The Netherlands. 3 School of Fundamental Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. 4 School of Physics, The University of New South Wales, Sydney 2052, Australia. 5 HRL Laboratories, LLC, 3011 Malibu Canyon Rd., Malibu, CA 90265, USA. 6Present address: London Centre for Nanotechnology, UCL, 17-19 Gordon St, London WC1H 0AH, UK. Correspondence and requests for materials should be addressed to M.A.F. (email: ) or to B.H. (email: ) or to A.S.D. (email: ) NATURE COMMUNICATIONS | (2018)9:4370 | DOI: 10.1038/s41467-018-06039-x | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-06039-x T he manipulation of single-spin qubits in silicon, using either ac magnetic1,2 or electric3–6 fields at microwave frequencies, has been a powerful driver of progress in the field of solid state qubit development, in part due to the sophistication of microwave technology which allows convenient twoaxis control of the qubit via simple phase adjustment, and the generation of complex pulse sequences for dynamical decoupling. This has resulted in high-fidelity single-qubit gates2,4–7 and initial two-qubit gates now realised in a variety of structures8–10. To date, all demonstrations of single-shot readout in silicon systems employing spin resonance1–4,6 have utilized single-spin selective tunnelling to a reservoir11. While convenient, this reservoir-based readout approach is not well suited to gate-based dispersive sensing12, which has significant advantages in terms of minimizing electrode overheads for large-scale qubit architectures. In contrast, readout based on Pauli spin blockade13 in the singlet–triplet basis of a double QD14 is compatible with dispersive sensing and, when combined with an ancilla qubit, can be used for parity readout in quantum error detection and correction codes15–17. Moreover, because singlet–triplet readout can provide high-fidelity spin readout at much lower magnetic fields than single-spin reservoir-based readout11, it allows spin-resonance control to be performed at lower microwave frequencies, which will benefit scalability. Qubits based on singlet–triplet spin states were first demonstrated in GaAs heterostructures14,18 and have now been operated in a variety of silicon-based structures19–23. High-fidelity singleshot singlet-triplet readout has also recently been demonstrated in various silicon systems22,24,25. Here, in order to combine the ability to address individual spin qubits using ESR with the voltage-pulse-based detuning control and high-fidelity readout of pairs of spins in the singlet-triplet basis, we employ a 28Si metal-oxide-semiconductor (SiMOS) double quantum dot device26,27 (Fig. 1a, b) with a microwave transmission line that can be used to supply ESR pulses, similar to one previously used for demonstration of a two-qubit logic gate8. The device also includes an integrated single-electron-transistor (SET) sensor to achieve the single-charge sensitivity required for singlet–triplet readout. Electrons are populated into the two quantum dots (QD1 and QD2) with occupancy (N1, N2) using positive voltages on gates G1 and G2. An electron reservoir is induced beneath the Si–SiO2 interface via a positive bias on gate ST, which also serves as the SET top gate. The reservoir is isolated from QD1 and QD2 by a barrier gate B (see Fig. 1a, b). Results Single-shot singlet–triplet readout. Figure 1c shows the stability diagram of the double QD system in the charge regions (N1, N2) where we operate the device. When two electrons occupy a double quantum dot, the exchange interaction results in an energy splitting between the singlet (S) and triplet (T−,T0, T+) spin states. The exchange interaction can be controlled by electrical pulsing on nearby gates, providing a means to initialize, control and read out the singlet and triplet states14. At the core of singlet–triplet spin readout is the observation of Pauli spinblockade (PSB)19,28–31. When pulsing from the (1, 1)→ to (0, 2) charge configurations, the QD1 electron tunnels to QD2 only when the two spatially separated electrons were initially in the singlet spin configuration. The triplet states are blockaded from tunnelling due to the large exchange interaction in the (0, 2) charge configuration. The blockade is made observable on the stability diagram by applying a pulse sequence19,28 to gates G1 and G2 as depicted in Fig. 1c. After first flushing the system of a QD1 electron to create the (0, 1) state at A, a (1, 1) state at B loads a randomly configured mixture of singlet and triplet states (solid 2 arrow in Fig. 1c). The current through the nearby single-electrontransistor (SET) is recorded at this position, tuned to be at the half-maximum point of a Coulomb peak. The system is then ramped to a variable measurement point (dashed arrows in Fig. 1c, d) where the SET current is measured again. A map of the comparison current ΔISET between these two points is created, where the derivative in sweep direction d(ΔISET)/d(ΔVG1) (Fig. 1c) decorrelates the capacitive coupling of the control gates to the SET island. A change in the charge configuration marks a shift in the SET current, clearly observed as bright/dark bands. The bright band in the (...truncated)