G-factor of electrons in gate-defined quantum dots in a strong in-plane magnetic field

We analyze orbital effects of an in-plane magnetic field on the spin structure of states of a gated quantum dot based in a two-dimensional electron gas. Starting with a $k \cdot p$ Hamiltonian, we perturbatively calculate these effects for the conduction band of GaAs, up to the third power of the magnetic field. We quantify several corrections to the g-tensor and reveal their relative importance. We find that for typical parameters, the Rashba spin-orbit term and the isotropic term, $H_{43} \propto {\bf P}^2 {\bf B} \cdot \boldsymbol{\sigma}$, give the largest contributions in magnitude. The in-plane anisotropy of the g-factor is, on the other hand, dominated by the Dresselhaus spin-orbit term. At zero magnetic field, the total correction to the g-factor is typically 5-10% of its bulk value. In strong in-plane magnetic fields, the corrections are modified appreciably.Comment: 24 pages, 8 figures; v2 is in content identical to the version published in PRB. Compared to v1, the minor changes adopted in v2 are reflecting the PRB referees' suggestion

Spectroscopy of Quantum-Dot Orbitals with In-Plane Magnetic Fields

We show that in-plane-magnetic-field assisted spectroscopy allows extraction of the in-plane orientation and full 3D shape of the quantum mechanical orbitals of a single electron GaAs lateral quantum dot with sub-nm precision. The method is based on measuring orbital energies in a magnetic field with various strengths and orientations in the plane of the 2D electron gas. As a result, we deduce the microscopic quantum dot confinement potential landscape, and quantify the degree by which it differs from a harmonic oscillator potential. The spectroscopy is used to validate shape manipulation with gate voltages, agreeing with expectations from the gate layout. Our measurements demonstrate a versatile tool for quantum dots with one dominant axis of strong confinement.Comment: 4 pages, 3 color figures, including supplementary on arXi

A spin qubit in a fin field-effect transistor

Quantum computing's greatest challenge is scaling up. Several decades ago, classical computers faced the same problem and a single solution emerged: very-large-scale integration using silicon. Today's silicon chips consist of billions of field-effect transistors (FinFETs) in which current flow along the fin-shaped channel is controlled by wrap-around gates. The semiconductor industry currently employs fins of sub-10$\,$nm width, small enough for quantum applications: at low temperature, an electron or hole can be trapped under the gate and serve as a spin qubit. An attractive benefit of silicon's advantageous scaling properties is that quantum hardware and its classical control circuitry can be integrated in the same package. This, however, requires qubit operation at temperatures greater than 1$\,$K where the cooling is sufficient to overcome the heat dissipation. Here, we demonstrate that a silicon FinFET is an excellent host for spin qubits that operate even above 4$\,$K. We achieve fast electrical control of hole spins with driving frequencies up to 150$\,$MHz and single-qubit gate fidelities at the fault-tolerance threshold. The number of spin rotations before coherence is lost at these "hot" temperatures already matches or exceeds values on hole spin qubits at mK temperatures. While our devices feature both industry compatibility and quality, they are fabricated in a flexible and agile way to accelerate their development. This work paves the way towards large-scale integration of all-electrical and ultrafast spin qubits

Hyperfine-phonon spin relaxation in a single-electron GaAs quantum dot

Understanding and control of the spin relaxation time T-1 is among the key challenges for spinbased qubits. A larger T-1 is generally favored, setting the fundamental upper limit to the qubit coherence and spin readout fidelity. In GaAs quantum dots at low temperatures and high inplane magnetic fields B, the spin relaxation relies on phonon emission and spin-orbit coupling. The characteristic dependence T-1 alpha B-5 and pronounced B-field anisotropy were already confirmed experimentally. However, it has also been predicted 15 years ago that at low enough fields, the spin-orbit interaction is replaced by the coupling to the nuclear spins, where the relaxation becomes isotropic, and the scaling changes to T-1 alpha B-3. Here, we establish these predictions experimentally, by measuring T-1 over an unprecedented range of magnetic fields-made possible by lower temperature-and report a maximum T-1 = 57 +/- 15 s at the lowest fields, setting a record electron spin lifetime in a nanostructure

Rapid single-shot parity spin readout in a silicon double quantum dot with fidelity exceeding 99 %

Silicon-based spin qubits offer a potential pathway toward realizing a scalable quantum computer owing to their compatibility with semiconductor manufacturing technologies. Recent experiments in this system have demonstrated crucial technologies, including high-fidelity quantum gates and multiqubit operation. However, the realization of a fault-tolerant quantum computer requires a high-fidelity spin measurement faster than decoherence. To address this challenge, we characterize and optimize the initialization and measurement procedures using the parity-mode Pauli spin blockade technique. Here, we demonstrate a rapid (with a duration of a few us) and accurate (with >99% fidelity) parity spin measurement in a silicon double quantum dot. These results represent a significant step forward toward implementing measurement-based quantum error correction in silicon

Silicon quantum dot devices with a self-aligned second gate layer

We implement silicon quantum dot devices with two layers of gate electrodes using a self-alignment technique, which allows for ultra-small gate lengths and intrinsically perfect layer-to-layer alignment. In a double quantum dot system, we investigate hole transport and observe current rectification due to Pauli spin blockade. Magnetic field measurements indicate that hole spin relaxation is dominated by spin-orbit interaction, and enable us to determine the effective hole $g$-factor $\simeq1.6$. From an avoided singlet-triplet crossing, occurring at high magnetic field, the spin-orbit coupling strength $\simeq0.27$meV is obtained, promising fast and all-electrical spin control

Hamiltonian Phase Error in Resonantly Driven CNOT Gate Above the Fault-Tolerant Threshold

Because of their long coherence time and compatibility with industrial foundry processes, electron spin qubits are a promising platform for scalable quantum processors. A full-fledged quantum computer will need quantum error correction, which requires high-fidelity quantum gates. Analyzing and mitigating the gate errors are useful to improve the gate fidelity. Here, we demonstrate a simple yet reliable calibration procedure for a high-fidelity controlled-rotation gate in an exchange-always-on Silicon quantum processor allowing operation above the fault-tolerance threshold of quantum error correction. We find that the fidelity of our uncalibrated controlled-rotation gate is limited by coherent errors in the form of controlled-phases and present a method to measure and correct these phase errors. We then verify the improvement in our gate fidelities by randomized benchmark and gate-set tomography protocols. Finally, we use our phase correction protocol to implement a virtual, high-fidelity controlled-phase gate.Comment: Main article: 22 pages, 4 figures; Supplementary material: 6 pages, 5 figures, 1 tabl

Ultrafast Hole Spin Qubit with Gate-Tunable Spin-Orbit Switch

A key challenge in quantum computation is the implementation of fast and local qubit control while simultaneously maintaining coherence. Qubits based on hole spins offer, through their strong spin-orbit interaction, a way to implement fast quantum gates. Strikingly, for hole spins in one-dimensional germanium and silicon devices, the spin-orbit interaction has been predicted to be exceptionally strong yet highly tunable with gate voltages. Such electrical control would make it possible to switch on demand between qubit idling and manipulation modes. Here, we demonstrate ultrafast and universal quantum control of a hole spin qubit in a germanium/silicon core/shell nanowire, with Rabi frequencies of several hundreds of megahertz, corresponding to spin-flipping times as short as ~1 ns - a new record for a single-spin qubit. Next, we show a large degree of electrical control over the Rabi frequency, Zeeman energy, and coherence time - thus implementing a switch toggling from a rapid qubit manipulation mode to a more coherent idling mode. We identify an exceptionally strong but gate-tunable spin-orbit interaction as the underlying mechanism, with a short associated spin-orbit length that can be tuned over a large range down to 3 nm for holes of heavy-hole mass. Our work demonstrates a spin-orbit qubit switch and establishes hole spin qubits defined in one-dimensional germanium/silicon nanostructures as a fast and highly tunable platform for quantum computation

Long-term Safety and Efficacy of New-Generation Drug-Eluting Stents inWomenWith AcuteMyocardial Infarction From theWomen in Innovation and Drug-Eluting Stents (WIN-DES) Collaboration

Importance Women with acute myocardial infarction (MI) undergoing mechanical reperfusion remain at increased risk of adverse cardiac events and mortality compared with their male counterparts. Whether the benefits of new-generation drug-eluting stents (DES) are preserved in women with acute MI remains unclear. Objective To investigate the long-term safety and efficacy of new-generation DES vs early-generation DES in women with acute MI. Design, Setting, and Participants Collaborative, international, individual patient-level data of women enrolled in 26 randomized clinical trials of DES were analyzed between July and December 2016. Only women presenting with an acute coronary syndrome were included. Study population was categorized according to presentation with unstable angina (UA) vs acute MI. Acute MI included non–ST-segment elevation MI (NSTEMI) or ST-segment elevation MI (STEMI). Interventions Randomization to early- (sirolimus- or paclitaxel-eluting stents) vs new-generation (everolimus-, zotarolimus-, or biolimus-eluting stents) DES. Main Outcomes and Measures Composite of death, MI or target lesion revascularization, and definite or probable stent thrombosis at 3-year follow-up. Results Overall, the mean age of participants was 66.8 years. Of 11 577 women included in the pooled data set, 4373 (37.8%) had an acute coronary syndrome as clinical presentation. Of these 4373 women, 2176 (49.8%) presented with an acute MI. In women with acute MI, new-generation DES were associated with lower risk of death, MI or target lesion revascularization (14.9% vs 18.4%; absolute risk difference, −3.5%; number needed to treat [NNT], 29; adjusted hazard ratio, 0.78; 95% CI, 0.61-0.99), and definite or probable stent thrombosis (1.4% vs 4.0%; absolute risk difference, −2.6%; NNT, 46; adjusted hazard ratio, 0.36; 95% CI, 0.19-0.69) without evidence of interaction for both end points compared with women without acute MI (P for interaction = .59 and P for interaction = .31, respectively). A graded absolute benefit with use of new-generation DES was observed in the transition from UA, to NSTEMI, and to STEMI (for death, MI, or target lesion revascularization: UA, −0.5% [NNT, 222]; NSTEMI, −3.1% [NNT, 33]; STEMI, −4.0% [NNT, 25] and for definite or probable ST: UA, −0.4% [NNT, 278]; NSTEMI, −2.2% [NNT, 46]; STEMI, −4.0% [NNT, 25]). Conclusions and Relevance New-generation DES are associated with consistent and durable benefits over 3 years in women presenting with acute MI. The magnitude of these benefits appeared to be greater per increase in severity of acute coronary syndrome