Session J29: Semiconductor Qubits II

Live

Quantum computing promises to tackle exciting and computationally difficult problems. Intel is leveraging 50 years of experience in semiconductor manufacturing to develop silicon-based spin qubit devices. A key challenge in this endeavor is reducing the effect of noise on the spin qubits to an acceptable level. The effects of noise can be reduced by lowering its strength or the sensitivity of the qubits to the noise. For spin qubits, charge noise is an important contributor, limiting coherence and gate fidelity. The impact of charge noise can be mitigated with fabrication changes, so it is important to acquire statistically relevant data in a systematic way. We find that the level of charge noise can vary considerably depending on the configuration of the quantum dot that hosts the spin qubit. Therefore, we measure the noise over large parameter ranges on many quantum dots. Using fast noise measurements, we find two-level systems that increase charge noise excessively. Further, we confirm the linear dependence of charge noise on temperature. Using a cryogenic wafer probing tool that enables efficient 300-mm-wafer-scale measurements at 1.6 K, we can inform fabrication decisions with statistically relevant data. We also highlight the expected impact of noise on system performance.   

Live

Quantum dot spin qubits are a promising platform for large-scale quantum computers. Their inherent compatibility with semiconductor fabrication technology promises the ability to scale up to large numbers of qubits. However, all prior experiments are limited to two-qubit logic.

Here, we go beyond these demonstrations and operate a four-qubit quantum processor. Furthermore, we define the quantum dots in a two-by-two grid and thereby realize the first two-dimensional qubit array with semiconductor qubits, a crucial step toward quantum error correction and practical quantum algorithms. We achieve these results by defining qubits based on hole states in strained planar germanium quantum wells, enabling a high degree of control, well defined qubit states, and fast, all-electrical qubit driving.

We perform one, two, three, and four qubit logic for all qubit combinations, realizing a compact and high-connectivity circuit. Furthermore, we show that the hole coherence can be extended up to 100 ms using refocusing pulses and employ this to perform a quantum circuit executed on the full four-qubit system. These results mark an important step for scaling up spin qubits in two dimensions and position planar germanium as a prime candidate for practical quantum applications.   

Live

Spin qubits are considered to be among the most promising candidates for building a quantum processor [1] with group IV semiconductors standing out for their long coherence times and CMOS integrability. Recently, holes in Ge have proven scale-up beyond two qubits with a 4 Loss-DiVincenzo qubit device realized [2]. Here we demonstrate a spin qubit in a Ge/SiGe heterostructure realized with a single layer of confinement gates. We observe fast electrically controlled X and Z-rotations exceeding 100 MHz with dephasing times of 1 μs which we extend beyond 10 μs with echo techniques. The extracted noise spectral density suggests that the dephasing time is limited by low frequency fluctuations which we attribute to nuclear spins. The reported results demonstrate that holes in Ge are a viable candidate for integration into a large scale quantum processor.

[1] Vandersypen, L. M. K. et al. Interfacing spin qubits in quantum dots and donors-hot, dense, and coherent. npj Quantum Information 3 (2017).
[2] Hendrickx, N. W. et al. A four-qubit germanium quantum processor. arXiv: 2009.04268 [cond-mat] (2020).   

Live

The pseudospin of heavy-holes confined in a semiconductor quantum dot represents a promising candidate for a fast and robust qubit. While hole spin manipulation by a classical electric field utilizing the Dresselhaus spin-orbit interaction has been demonstrated, our work explores cavity-based qubit manipulation and coupling schemes for inversion-symmetric crystals forming a planar heavy-hole quantum dot. Choosing the exemplary material Germanium, we derive an effective cavity-mediated ground state spin coupling that harnesses the cubic Rashba spin-orbit interaction. In addition, we propose an optimal set of parameters which allows for Rabi frequencies in the MHz range, thus entering the strong coupling regime of cavity quantum electrodynamics.   

Live

Quantum computers promise an exponential speedup for certain computational tasks. Silicon-based spin qubits are among the prime candidates for implementing large-scale quantum circuits by leveraging widely deployed industrial CMOS technology. Furthermore, qubit operation at temperatures above 1K allows for on-chip integration of classical control electronics [Petit et al. & Yang et al., Nature 580, 350&355 (2020)].
Here, we demonstrate a hole spin qubit integrated in a silicon fin-field effect transistor (FinFET)[Geyer et al., arXiv:2007.15400 (2020)] operating at temperatures as high as 4.2K. Spin-orbit mediated electric-dipole spin resonance is used for fast (150MHz) all-electrical qubit control. A spin dephasing time of T₂*=150ns is measured which increases by a factor of 2 when cooling down to 1.5K. The qubit can be decoupled from low-frequency noise using dynamical decoupling sequences to increase the coherence time by a factor of 10. Randomized benchmarking using a series of Clifford gates allows to determine a single-gate fidelity of 98.9%(97.9%) at 1.5K(4.2K).
These results establish high-temperature hole spin qubits in silicon FinFETs as a potential platform for scalable quantum computing.   

Live

We have calculated [1] the electric-dipole (pseudospin-electric) coupling between heavy and light holes in GaAs from first principles. We find a transition dipole of 0.5 debye, a significant fraction of that for the hydrogen-atom 1s→2p transition. In addition, we derive the Dresselhaus spin-orbit coupling that is generated by this transition dipole for heavy holes in a triangular quantum well. A quantitative microscopic description of this pseudospin-electric coupling may be important for understanding the origin of spin splitting in quantum wells, spin coherence/relaxation (T₂^*/T₁) times, spin-electric coupling for cavity-QED, electric-dipole spin resonance, and spin non-conserving tunneling in double quantum dot systems. Related results for the first-principles calculation of hyperfine couplings [2] for holes in GaAs, silicon, and germanium will also be discussed.

[1] P. Philippopoulos, S. Chesi, D. Culcer, and W. A. Coish, Phys. Rev. B 102, 075310 (2020).

[2] P. Philippopoulos, S. Chesi, W. A. Coish, Phys. Rev. B 101, 115302 (2020).   

Live

The spin state of an elementary charge is a well-established candidate for quantum information processing. Multiple semiconductor platforms are presently under considerable study to determine their viability as host materials for spin qubits, in particular their suitability as hosts of scalable qubit unit cells.
Germanium quantum wells in planar heterostrucutres (Ge/SiGe) have recently emerged as a highly promising platform in which to host heavy-hole based spin qubits. Demonstrations of the first quantum dots, two qubit logic, fast single qubit operations with fault tolerant fidelities, and even the operation of two dimensional arrays have occurred within the relatively short time span of 3 years.
Here, we build on this momentum, reporting hole spin qubits with the longest spin relaxation times and dephasing times ever measured in the platform, and investigate their magnetic field dependence, concluding that spin decoherence does not constitute a road block for quantum information processing with hole spins in germanium. We also present measurements in coherent control of hole spin qubits in a 2x2 quantum dot array, constituting a milestone in semiconductor based quantum computing.   

Live

Lithographically patterned quantum dots in strained Ge/SiGe have become promising candidates for quantum computing, with a quick progression to qubit logic demonstrations. Here we present an experimental measurement of the out-of-plane g-factor in this material for a single hole confined to a quantum dot, which avoids the strong orbital effects that can occur in this configuration. Strong asymmetry of the g-factor between the in- and out-of-plane directions is seen. These results are in agreement with calculations using the Luttinger Hamiltonian and suggest dramatic tunability through both the B-field and the charge state.   

Live

Si-compatible low-dimensional systems have been exploiting either tensile strained Si or compressively strained Germanium (Ge) quantum wells (QWs). For quantum information, the latter has been explored in new schemes for hole spin qubits. Hole spins have attracted a great deal of attention. Nevertheless, most of experimental investigations on 2D gas systems have so far focused on heavy-hole states (HH) due to the compressively strained heterostructure currently exploited, where the valence band degeneracy is lifted and leaves HH states energetically above the light-hole (LH) states. However, the ability to exploit LH states will be a powerful paradigm beneficial for quantum information technologies. To harness these largely unexplored advantages of LH states, we present a new low-dimensional system consisting of highly tensile strained Ge quantum well grown on Si wafers using GeSn as barriers. Several spectroscopic techniques were used to identify the LH confined states in the Ge well. The obtained heterostructure shows optical transitions that are modulated in the midinfrared range. This ability to engineer quantum structure where LH is the ground state in an optically active group IV platform lays the groundwork for a new class of Si-compatible quantum technologies.   

Live

The spin splitting is a fundamental property of an electron confined in a semiconductor in an external magnetic field and sets the qubit energy – a key parameter for quantum computation. Here, we experimentally determine isotropic and anisotropic g-factor corrections in lateral GaAs single-electron quantum dots (Camenzind, Svab et al., arXiv:2010.11185 (2020)). We extract Zeeman splittings by measuring tunnel rates into individual spin states of an empty quantum dot for various in-plane magnetic field strengths/directions. We find an anisotropic correction of ≈7% of the average g-factor, in good agreement with recent theory (Stano et al. PRB98, 195314 (2018)), where this is due to Dresselhaus spin-orbit interaction (SOI) using a coefficient of 10.6 eVų. The isotropic correction is measured to reduce the average g-factor 10-15% below the bulk value |g|=0.44, due to Rashba SOI and an additional ''43-term'' SOI. These corrections are predicted to depend strongly on the thickness of the wave function in the z-direction perpendicular to the 2D gas. These findings provide the fundamental physics of the single-electron spin splitting and as such are at the heart of GaAs spin qubits.   

Live

Hole spins in InAs/GaAs quantum dot molecules (QDMs) show promise as semiconductor spin qubits, as hole states are weakly coupled to nuclear spins. QDMs are desired over single dots for the tunability of molecular states, formed by QD states brought to resonance with applied electric fields. Near resonance, the Zeeman splitting of these molecular hole states can be enhanced or suppressed, providing electrical tuning of the hole g-tensor. The formation and tunability of these states is well observed experimentally. Here, we provide an atomistic tight-binding theory for this tuning of Zeeman splitting to elucidate the mechanism for this enhancement and to identify ways to further manipulate the hole g-tensor. We find that the g-tensor tunability near resonance is explained by a strong enhancement or suppression of coupling between the spatial motion of the hole state and the magnetic field, as described in our theory by a Peierls term. In contrast, there is no change in the hole spin or leakage into the interdot barrier near resonance. Phenomenological modeling suggests that spin-dependent interdot tunneling is the mechanism for the g-tensor tunability. We use our atomistic theory to explore the spatial implications of spin-dependent tunneling to critically assess this mechanism.   

Live

We theoretically consider a 3-qubit system of electron spins in a silicon triple quantum dot with always-on exchange coupling. We have shown how to perform local spin rotations and 2-qubit entangling gates between neighboring qubits in the absence of noise, these form a universal set. However, in practice, low fidelity due to noise is an important limitation of quantum devices, so it is necessary to create gates robust against noise. In this talk we will show how to make a CZ gate robust against charge noise, the most important noise in the silicon 3-qubit device. Using the fact that the hamiltonian accrues error in two separately controlled and commuting SU(2) subspaces we apply the pulse shaping method of Ref.[1] simultaneously in both spaces. Matching the pulse time in both spaces leads to a pulse that is robust to both fluctuations in exchange coupling and the electron g-factor induced by charge noise. The robust pulse maintains a fidelity of 10^-4 at 3.5% fluctuations in exchange or g-factor, an improvement over the 1% for the naive pulse.
[1]Barnes, et al., 2015. Robust quantum control using smooth pulses and topological winding. Scientific Reports,5(1).   

Live

Over the last fifty years, the CMOS (Complementary-Metal-Oxide-Semiconductor) electronics industry has been continuously scaling down transistors in size, to increase performance and reduce power consumption. Nowadays, the smallest transistors in industry achieve 5nm features. As a result, those silicon structures tend to exhibit undesirable quantum effects for a classical transistor which appear to be new research opportunities for quantum information processing.
In particular, it is nowadays possible to trap single electron spins in silicon quantum dots and perform high fidelity quantum gates. These demonstrations combined with the intrinsic properties of the silicon lattice (low spin orbit and hyperfine interaction) make CMOS device an excellent candidate for scalable quantum architectures.
In this presentation, we will show how we can detect a single spin in a CMOS device thanks to an original approach which combines gate-based dispersive charge sensing and a latched Pauli spin blockade mechanism. This scalable method allows us to read out a single spin with a fidelity above 98% for 0.5 ms integration time. Moreover, we show that the demonstrated high read-out fidelity is fully preserved up to 0.5 K. Finally, we will show how these results holds particular relevance for the future co-integration of spin qubits and classical control electronics.