Gate-defined Quantum Dots Research Articles

Coherence of a field gradient driven singlet-triplet qubit coupled to multielectron spin states in 28Si/SiGe

Engineered spin-electric coupling enables spin qubits in semiconductor nanostructures to be manipulated efficiently and addressed individually. While synthetic spin-orbit coupling using a micromagnet is widely investigated for driving and entangling qubits based on single spins in silicon, the baseband control of encoded spin qubits with a micromagnet in isotopically purified silicon has been less well investigated. Here, we demonstrate fast singlet-triplet qubit oscillation (~100 MHz) in a gate-defined double quantum dot in 28Si/SiGe with an on-chip micromagnet with which we show the oscillation quality factor of an encoded spin qubit exceeding 580. The coherence time T2* is analyzed as a function of potential detuning and an external magnetic field. In weak magnetic fields, the coherence is limited by frequency-independent noise whose time scale is faster than the typical data acquisition time of ~100 ms, which limits the T2* below 1 μs in the ergodic limit. We present evidence of sizable and coherent coupling of the qubit with the spin states of a nearby quantum dot, demonstrating that appropriate spin-electric coupling may enable a charge-based two-qubit gate in a (1,1) charge configuration.

Quantum Interference and Coherent Population Trapping in a Double Quantum Dot.

Quantum interference is a natural consequence of wave-particle duality in quantum mechanics, and is widely observed at the atomic scale. One interesting manifestation of quantum interference is coherent population trapping (CPT), first proposed in three-level driven atomic systems and observed in quantum optical experiments. Here, we demonstrate CPT in a gate-defined semiconductor double quantum dot (DQD), with some unique twists as compared to the atomic systems. Specifically, we observe CPT in both driven and nondriven situations. We further show that CPT in a driven DQD could be used to generate adiabatic state transfer. Moreover, our experiment reveals a nontrivial modulation to the CPT caused by the longitudinal driving field, yielding an odd-even effect and a tunable CPT. Our results broaden the field of CPT, and open up the possibility of quantum simulation and quantum computation based on adiabatic passage in quantum dot systems.

SI/GE Quantum Dot Channel FETs for Multi-Bit Computing

This paper presents quantum dot channel (QDC) FETs in quantum wire and coupled quantum dot configurations for cryogenic operation with multi-state operation. It also describes gate-all-around (GAA) quantum dot channel (QDC) FETs that exhibit potential multi-state characteristics at room temperature. FETs with cladded Si and Ge quantum dot layers as a transport channel have been fabricated. The formation of a quantum dot superlattice (QDSL) when SiOx-cladded Si and/or GeOx-cladded Ge quantum dots (QD) are assembled results in mini-energy sub-bands in the conduction and valence band. The intra-mini-energy band transitions results in significant changes in the drain current when gate and/or drain voltages are varied. This novel feature provides a pathway for 16-/32-state logic in CMOS-X configuration. The gate-defined Si quantum dot FETs, comprising of tunnel barrier coupled, have been reported for quantum computing at cryogenic temperatures.

Coupled vertical double quantum dots at single-hole occupancy

Gate-defined quantum dots define an attractive platform for quantum computation and have been used to confine individual charges in a planar array. Here, we demonstrate control over vertical double quantum dots confined in a strained germanium double quantum well. We sense individual charge transitions with a single-hole transistor. The vertical separation between the quantum wells provides a sufficient difference in capacitive coupling to distinguish quantum dots located in the top and bottom quantum wells. Tuning the vertical double quantum dot to the (1,1) charge state confines a single-hole in each quantum well beneath a single plunger gate. By simultaneously accumulating holes under two neighboring plunger gates, we are able to tune to the (1,1,1,1) charge state. These results motivate quantum dot systems that exploit the third dimension, opening new opportunities for quantum simulation and quantum computing.

Prospects of silicide contacts for silicon quantum electronic devices

Metal contacts in semiconductor quantum electronic devices can offer advantages over doped contacts, primarily due to their reduced fabrication complexity and lower temperature requirements during processing. Some metals can also facilitate ambipolar device operation or form superconducting contacts. Furthermore, a sharp metal–semiconductor interface allows for contact placement in close proximity to the active device area avoiding damage caused by dopant implantation. However, in the case of gate-defined quantum dots in intrinsic silicon, the formation of a Schottky barrier at the silicon–metal interface can lead to large, nonlinear contact resistances at cryogenic temperatures. We investigate this issue by examining hole transport through metal oxide-semiconductor transistors with platinum silicide contacts on intrinsic silicon substrates. We extract the contact and channel resistances as a function of temperature and improve the cryogenic conductance of the device by more than an order of magnitude by implementing meander-shaped contacts. In addition, we observe signatures of enhanced transport through localized defect states, which we attribute to platinum clusters in the depletion region of the Schottky contacts that form during the silicidation process. These results showcase the prospects of silicide contacts in the context of cryogenic quantum devices and address associated challenges.

Non-symmetric Pauli spin blockade in a silicon double quantum dot

Spin qubits in gate-defined silicon quantum dots are receiving increased attention thanks to their potential for large-scale quantum computing. Readout of such spin qubits is done most accurately and scalably via Pauli spin blockade (PSB), however, various mechanisms may lift PSB and complicate readout. In this work, we present an experimental study of PSB in a multi-electron low-symmetry double quantum dot (DQD) in silicon nanowires. We report on the observation of non-symmetric PSB, manifesting as blockaded tunneling when the spin is projected to one QD of the pair but as allowed tunneling when the projection is done into the other. By analyzing the interaction of the DQD with a readout resonator, we find that PSB lifting is caused by a large coupling between the different electron spin manifolds of 7.90 μeV and that tunneling is incoherent. Further, magnetospectroscopy of the DQD in 16 charge configurations, enables reconstructing the energy spectrum of the DQD and reveals the lifting mechanism is energy-level selective. Our results indicate enhanced spin-orbit coupling which may enable all-electrical qubit control of electron spins in silicon nanowires.

Long-lived valley states in bilayer graphene quantum dots

Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanisms are unlikely to be at work for valleys, promising more robust qubits. The Berry curvature associated with valley states allows for electrical control of their energies, suggesting routes for coherent qubit manipulation. However, the relaxation time of valley states—which ultimately limits these qubits’ coherence properties and therefore their suitability as practical qubits—is not yet known. Here we measure the characteristic relaxation times of these spin and valley states in gate-defined bilayer graphene quantum dot devices. Different valley states can be distinguished from each other with a fidelity of over 99%. The relaxation time between valley triplets and singlets exceeds 500 ms and is more than one order of magnitude longer than for spin states. This work facilitates future measurements on valley-qubit coherence, demonstrating bilayer graphene as a practical platform hosting electrically controlled, long-lived valley qubits.

Vertical gate-defined double quantum dot in a strained germanium double quantum well

Vertical gate-defined double quantum dot in a strained germanium double quantum well

Numerical analysis of photon absorption of gate-defined quantum dots embedded in asymmetric bull’s-eye optical cavities

Improving the photon–spin conversion efficiency without polarization dependence is a major challenge in realizing quantum interfaces gate-defined quantum dots (QDs) for polarization-encoded photonic quantum network systems. Previously, we reported the design of an air-bridge bull’s-eye cavity that enhances the photon absorption efficiency of an embedded gate-defined QD regardless of the photon polarization. Here, we numerically demonstrate that a further 1.6 times improvement in efficiency is possible by simply adjusting the distance of the substrate from the semiconductor slab where the bull’s-eye structure is formed. Our analysis clarifies that the upward-preferred coupling and narrow far-field emission pattern realized by substrate-induced asymmetry enable the improvement.

Correlations of spin splitting and orbital fluctuations due to 1/f charge noise in the Si/SiGe quantum dot

Fluctuations in electric fields can change the position of a gate-defined quantum dot (QD) in a semiconductor heterostructure. In the presence of magnetic field gradient, these stochastic shifts of electron's wavefunction lead to fluctuations of electron's spin splitting. The resulting spin dephasing due to charge noise limits the coherence times of spin qubits in isotopically purified Si/SiGe quantum dots. We investigate the spin splitting noise caused by such a process due to microscopic motion of charges at the semiconductor-oxide interface. We compare effects of isotropic and planar displacement of the charges and estimate their densities and typical displacement magnitudes that can reproduce experimentally observed spin splitting noise spectra. We predict that for a defect density of 1010 cm−2, visible correlations between noises in spin splitting and in energy of electron's ground state in the quantum dot are expected.

A vertical gate-defined double quantum dot in a strained germanium double quantum well

A vertical gate-defined double quantum dot in a strained germanium double quantum well

Collective Microwave Response for Multiple Gate-Defined Double Quantum Dots.

We fabricate and characterize a hybrid quantum device that consists of five gate-defined double quantum dots (DQDs) and a high-impedance NbTiN transmission resonator. The controllable interactions between DQDs and the resonator are spectroscopically explored by measuring the microwave transmission through the resonator in the detuning parameter space. Utilizing the high tunability of the system parameters and the high cooperativity (Ctotal > 17.6) interaction between the qubit ensemble and the resonator, we tune the charge-photon coupling and observe the collective microwave response changing from linear to nonlinear. Our results present the maximum number of DQDs coupled to a resonator and manifest a potential platform for scaling up qubits and studying collective quantum effects in semiconductor-superconductor hybrid cavity quantum electrodynamics systems.

Full Tunability and Quantum Coherent Dynamics of a Driven Multilevel System

Tunability of an artificial quantum system is crucial to its capability to process quantum information. However, tunability usually poses significant demand on the design and fabrication of a device. In this work, we demonstrate that Floquet engineering based on longitudinal driving provides distinct possibilities in enhancing the tunability of a quantum system without needing additional resources. In particular, we study a multilevel model based on gate-defined double quantum dots, where coherent interference occurs when the system is driven longitudinally. We develop an effective model to describe the driven dynamics of this multilevel system, and show that it is highly tunable via the driving field. We then illustrate the versatility and rich physics of a driven multilevel system by exploring phenomena such as driving modulation of resonances, adiabatic state transfer, and dark state. In the context of qubit control, we propose noise-resistant quantum gates based on adiabatic passage. The theoretical consideration we present here is rather general, and is in principle valid for other multilevel quantum systems.

Reducing charge noise in quantum dots by using thin silicon quantum wells

Charge noise in the host semiconductor degrades the performance of spin-qubits and poses an obstacle to control large quantum processors. However, it is challenging to engineer the heterogeneous material stack of gate-defined quantum dots to improve charge noise systematically. Here, we address the semiconductor-dielectric interface and the buried quantum well of a 28Si/SiGe heterostructure and show the connection between charge noise, measured locally in quantum dots, and global disorder in the host semiconductor, measured with macroscopic Hall bars. In 5 nm thick 28Si quantum wells, we find that improvements in the scattering properties and uniformity of the two-dimensional electron gas over a 100 mm wafer correspond to a significant reduction in charge noise, with a minimum value of 0.29 ± 0.02 μeV/Hz½ at 1 Hz averaged over several quantum dots. We extrapolate the measured charge noise to simulated dephasing times to CZ-gate fidelities that improve nearly one order of magnitude. These results point to a clean and quiet crystalline environment for integrating long-lived and high-fidelity spin qubits into a larger system.

Single-Shot Readout of Multiple Donor Electron Spins with a Gate-Based Sensor

Proposals for large-scale semiconductor spin-based quantum computers require high-fidelity single-shot qubit readout to perform error correction and read out qubit registers at the end of a computation. However, as devices scale to larger qubit numbers integrating readout sensors into densely packed qubit chips is a critical challenge. Two promising approaches are minimizing the footprint of the sensors, and extending the range of each sensor to read more qubits. Here we show high-fidelity single-shot electron spin readout using a nanoscale single-lead quantum dot (SLQD) sensor that is both compact and capable of reading multiple qubits. Our gate-based SLQD sensor is deployed in an all-epitaxial silicon donor spin-qubit device, and we demonstrate single-shot readout of three 31P donor quantum dot electron spins with a maximum fidelity of 95%. Importantly, in our device the quantum dot confinement potentials are provided inherently by the donors, removing the need for additional metallic confinement gates and resulting in strong capacitive interactions between sensor and donor quantum dots. Our results are consistent with a 1/d1.4 scaling of the capacitive coupling between sensor and 31P dots (where d is the sensor-dot distance), compared to 1/d2.5−3.0 in gate-defined quantum dot devices. Due to the small qubit size and strong capacitive interactions in all-epitaxial donor devices, we estimate a single sensor can achieve single-shot readout of approximately 15 qubits in a linear array, compared to 3–4 qubits for a similar sensor in a gate-defined quantum dot device. Our results highlight the potential for spin-qubit devices with significantly reduced sensor densities.Received 19 October 2021Revised 19 August 2022Accepted 4 January 2023DOI:https://doi.org/10.1103/PRXQuantum.4.010319Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum computationQuantum information with solid state qubitsQuantum metrologyQuantum Information

Gate-controlled hysteresis curves and dual-channel conductivity in an undoped Si/SiGe 2DEG structure

Exploring the cryogenic transport properties of two-dimensional electron gas in semiconductor heterostructures is always a focus of fundamental research on Si-based gate-controlled quantum devices. In this work, based on the electrical and magnetic transport characteristics of Si/SiGe heterostructure Hall bar-shaped field effect transistors (FETs) at 10 K and 1.6 K, we study the effects of electron tunneling, which occurs in the heterostructure and populates the oxide/semiconductor interface, on its transport properties. The initial position of dual-channel conduction is determined by the gate-controlled electrical hysteresis curves. Furthermore, we discover that there exist different tunneling mechanisms of electrons in a Si quantum well under the action of gate voltage, and the electron tunneling can well explain the two drain current plateaus appearing in direct-current transfer characteristics. Combining the power-law exponent of electron mobility versus density curve and the gate-related Dingle ratio, we clarify the dominant scattering mechanism, and the result can be supported by different tunneling mechanisms. Our work demonstrates the gate-dependent electronic transport performance in undoped Si/SiGe heterostructure FETs, which has an implication for the development of Si/SiGe heterostructure gate-defined quantum dot quantum computation.

Polarization-independent enhancement of optical absorption in a GaAs quantum well embedded in an air-bridge bull’s-eye cavity with metal electrodes

Electron spins in gate-defined quantum dots (QDs) formed in semiconductor quantum wells (QWs) are promising stationary qubits for implementing large-scale quantum networks in a scalable manner. One key ingredient for such a network is an efficient photon–spin interface that converts any polarization state of a flying photonic qubit to the corresponding spins state of the electron in gate-defined QDs. A bull’s-eye cavity is an optical cavity structure that can enhance the photon absorption of an embedded gate-defined QD without polarization dependence. In this paper, we report the successful fabrication of air-bridge bull’s-eye cavities with metal electrodes and demonstrate the nearly polarization-independent optical absorption of a GaAs QW embedded in the cavities. This work marks an important step toward realizing an efficient photon–spin interface using gate-defined QDs.

On Noise-Sensitive Automatic Tuning of Gate-Defined Sensor Dots

In gate-defined quantum dot systems, the conductance change of electrostatically coupled sensor dots allows the observation of the quantum dots' charge and spin states. Therefore, the sensor dots must be optimally sensitive to changes in its electrostatic environment. A series of conductance measurements varying the two sensor-dot-forming barrier gate voltages serve to tune the dot into a corresponding operating regime. In this paper, we analyze the noise characteristics of the measured data and define a criterion to identify continuous regions with a sufficient signal-gradient-to-noise ratio. Hence, accurate noise estimation is required when identifying the optimal operating regime. Therefore, we evaluate several existing noise estimators, modify them for 1D data, optimize their parameters, and analyze their quality based on simulated data. The estimator of Chen et al. [1] turns out to be best suited for our application concerning minimally scattering results. Furthermore, using this estimator in an algorithm for flank-of-interest classification in measured data shows the relevance and applicability of our approach.

State initialization of a hot spin qubit in a double quantum dot by measurement-based quantum feedback control

A measurement-based quantum feedback protocol is developed for spin-state initialization in a gate-defined double quantum dot spin qubit coupled to a superconducting cavity. The protocol improves qubit state initialization as it is able to robustly prepare the spin in shorter time and reach a higher fidelity, which can be preset. Being able to preset the fidelity aimed at is a highly desired feature enabling qubit initialization to be more deterministic. The protocol developed herein is also effective at high temperatures, which is critical for the current efforts toward scaling up the number of qubits in quantum computers.

Zoo of silicon-based quantum bits

Zoo of silicon-based quantum bits