arXiv subjects

Xuedong Hu

Publications and source records attributed to Xuedong Hu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Fast spin-valley-based quantum gates in Si with micromagnets

An electron spin qubit in silicon quantum dots holds promise for quantum information processing due to the scalability and long coherence. An essential ingredient to recent progress is the employment of micromagnets. They generate a synthetic spin-orbit coupling (SOC), which allows high-fidelity spin manipulation and strong interaction between an electron spin and cavity photons. To scaled-up quantum computing, multiple technical challenges remain to be overcome, including controlling the valley degree of freedom, which is usually considered detrimental to a spin qubit. Here, we show that it is possible to significantly enhance the electrical manipulation of a spin qubit through the effect of constructive interference and the large spin-valley mixing. To characterize the quality of spin control, we also studied spin dephasing due to charge noise through spin-valley mixing. The competition between the increased control strength and spin dephasing produces two sweet-spots, where the quality factor of the spin qubit can be high. Finally, we reveal that the synthetic SOC leads to distinctive spin relaxation in silicon, which explains recent experiments.

cond-mat.mes-hall↗

arXiv2021

Impact of the valley orbit coupling on exchange gate for spin qubits in silicon quantum dots

The presence of degenerate conduction band valleys and how they are mixed by interfaces play critical roles in determining electron interaction and spectrum in a silicon nanostructure. Here we investigate how the valley phases affect the exchange interaction in a symmetric two-electron silicon double quantum dot. Through a configuration interaction calculation, we find that exchange splitting is suppressed at a finite value of valley phase difference between the two dots, and reaches its minimum value ({\sim} 0) when the phase difference is π. Such a suppression can be explained using the Hubbard model, through the valley-phase-dependent dressing by the doubly occupied states on the ground singlet and triplet states. The contributions of the higher orbital states also play a vital role in determining the value of the exchange energy in general, which is a crucial parameter for applications such as exchange gates for spin qubits.

cond-mat.mes-hall↗

arXiv2021

Floquet state depletion in ac-driven circuit QED

We perform Floquet spectroscopy in a GaAs double quantum dot system coupled to a high-impedance superconducting resonator. By applying microwave induced consecutive passages under a double resonance condition, we observe novel Landau-Zener-Stückelberg-Majorana interference patterns that stem from a cavity-assisted interference pattern modified by the depletion of the ground state. Our experimental results reveal the stationary state behavior of a strongly driven two-level system, and are consistent with the simulations based on our theoretical model. This study provides an excellent platform for investigating the dynamics of Floquet states in the presence of strong driving.

cond-mat.mes-hall↗

arXiv2021

Anisotropic g-Factor and Spin-Orbit Field in a Ge Hut Wire Double Quantum Dot

Holes in nanowires have drawn significant attention in recent years because of the strong spin-orbit interaction, which plays an important role in constructing Majorana zero modes and manipulating spin-orbit qubits. Here, from the strongly anisotropic leakage current in the spin blockade regime for a double dot, we extract the full g-tensor and find that the spin-orbit field is in plane with an azimuthal angle of 59° to the axis of the nanowire. The direction of the spin-orbit field indicates a strong spin-orbit interaction along the nanowire, which may have originated from the interface inversion asymmetry in Ge hut wires. We also demonstrate two different spin relaxation mechanisms for the holes in the Ge hut wire double dot: spin-flip cotunneling to the leads, and spin-orbit interaction within the double dot. These results help establish feasibility of a Ge-based quantum processor.

cond-mat.mes-hall↗

arXiv2021

Controlling Synthetic Spin-Orbit Coupling in a Silicon Quantum Dot with Magnetic Field

Tunable synthetic spin-orbit coupling (s-SOC) is one of the key challenges in various quantum systems, such as ultracold atomic gases, topological superconductors, and semiconductor quantum dots. Here we experimentally demonstrate controlling the s-SOC by investigating the anisotropy of spin-valley resonance in a silicon quantum dot. As we rotate the applied magnetic field in-plane, we find a striking nonsinusoidal behavior of resonance amplitude that distinguishes s-SOC from the intrinsic spin-orbit coupling (i-SOC), and associate this behavior with the previously overlooked in-plane transverse magnetic field gradient. Moreover, by theoretically analyzing the experimentally measured s-SOC field, we predict the quality factor of the spin qubit could be optimized if the orientation of the in-plane magnetic field is rotated away from the traditional working point.

cond-mat.mes-hall↗

arXiv2021

Decoherence of Coupled Flip-Flop Qubits Due to Charge Noise

We study the decoherence effect of charge noise on a single flip-flop qubit and two dipole-coupled qubits. We find that a single flip-flop qubit is highly resistant to charge noise at its sweet spots. However, due to the proximity of the charge excited states to the flip-flop logical states, the presence of charge noise greatly reduces the fidelity of two-qubit operations. We identify leakage from the qubit Hilbert space as the main culprit for the reduced gate fidelity. We also explore different bias conditions to mitigate this decoherence channel.

quant-ph↗

arXiv2021

Measurement of tunnel coupling in a Si double quantum dot based on charge sensing

In Si quantum dots, valley degree of freedom, in particular the generally small valley splitting and the dot-dependent valley-orbit phase, adds complexities to the low-energy electron dynamics and the associated spin qubit manipulation. Here we propose a four-level model to extract tunnel coupling information for a Si double quantum dot (DQD). This scheme is based on a charge sensing measurement on the ground state as proposed in the widely used protocol for a GaAs double dot [DiCarlo et. al., PRL 92. 226801]. Our theory can help determine both intra- and inter-valley tunnel coupling with high accuracy, and is robust against system parameters such as valley splittings in the individual quantum dots.

cond-mat.mes-hall↗

arXiv2021

Electric-dipole-induced resonance and decoherence of a dressed spin in a quantum dot

Electron spin qubit in a quantum dot has been studied extensively for scalable quantum information processing over the past two decades. Recently, high-fidelity and fast single-spin control and strong spin-photon coupling have been demonstrated using a synthetic spin-orbit coupling created by a micromagnet. Such strong electrical driving suggests a strongly coupled spin-photon system. Here we study the relaxation, pure dephasing, and electrical manipulation of a dressed-spin qubit based on a driven single electron in a quantum dot. We find that the pure dephasing of a dressed qubit due to charge noise can be suppressed substantially at a sweet spot of the dressed qubit. The relaxation at low magnetic fields exhibits non-monotonic behavior due to the energy compensation from the driving field. Moreover, the longitudinal component of the synthetic spin-orbit field could provide fast electric-dipole-induced dressed-spin resonance (EDDSR). Based on the slow dephasing and fast EDDSR, we further propose a scheme for dressed-spin-based semiconductor quantum computing.

cond-mat.mes-hall↗

arXiv2021

Impact of $\mathcal{T}$-symmetry on spin decoherence and control in a synthetic spin-orbit field

The electrical control of a spin qubit in a quantum dot relies on spin-orbit coupling (SOC), which could be either intrinsic to the underlying crystal lattice or heterostructure, or extrinsic via, for example, a micro-magnet. Here we show that a key difference between the intrinsic SOC and the synthetic SOC introduced by a micro-magnet is their symmetry under time reversal. Specifically, the time-reversal symmetry ($\mathcal{T}$-symmetry) of the intrinsic SOC leads to not only the traditional van Vleck cancellation known for spin relaxation, but also vanishing spin dephasing to the lowest order of SOC, which we term as "longitudinal spin-orbit field cancellation". On the other hand, the synthetic SOC from a micro-magnet breaks the $\mathcal{T}$-symmetry, therefore eliminates both the "van Vleck cancellation" and the "longitudinal spin-orbit field cancellation". In other words, the effective field $\vecΩ$ experienced by the spin qubit does not depend on the quantization magnetic field anymore, and a longitudinal component is allowed for $\vecΩ$ to the first order of SOC. Consequently, spin relaxation and dephasing are qualitatively modified compared with the case of the intrinsic SOC. Furthermore, the fidelity of electric-dipole spin resonance based on $\vecΩ$ could be optimized, with potential applications in spin-based quantum computing.

cond-mat.mes-hall↗

arXiv2021

Theory of Hole-Spin Qubits in Strained Germanium Quantum Dots

We theoretically investigate the properties of holes in a Si$_{x}$Ge$_{1-x}$/Ge/ Si$_{x}$Ge$_{1-x}$ quantum well in a perpendicular magnetic field that make them advantageous as qubits, including a large ($>$100~meV) intrinsic splitting between the light and heavy hole bands, a very light ($\sim$0.05$\, m_0$) in-plane effective mass, consistent with higher mobilities and tunnel rates, and larger dot sizes that could ameliorate constraints on device fabrication. Compared to electrons in quantum dots, hole qubits do not suffer from the presence of nearby quantum levels (e.g., valley states) that can compete with spins as qubits. The strong spin-orbit coupling in Ge quantum wells may be harnessed to implement electric-dipole spin resonance, leading to gate times of several nanoseconds for single-qubit rotations. The microscopic mechanism of this spin-orbit coupling is discussed, along with its implications for quantum gates based on electric-dipole spin resonance, stressing the importance of coupling terms that arise from the underlying cubic crystal field. Our results provide a theoretical foundation for recent experimental advances in Ge hole-spin qubits.

cond-mat.mes-hall↗

arXiv2021

Long-Distance Superexchange between Semiconductor Quantum-Dot Electron Spins

Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin state of the chain, we observe oscillations between the distant end spins. We resolve the dynamics of both the end spins and the chain itself, and our measurements agree with simulations. Superexchange is a promising technique to create long-distance coupling between quantum-dot spin qubits.

cond-mat.mes-hall↗

arXiv2020

Correlated spectrum of distant semiconductor qubits coupled by microwave photons

We develop a new spectroscopic method to quickly and intuitively characterize the coupling of two microwave-photon-coupled semiconductor qubits via a high-impedance resonator. Highly distinctive and unique geometric patterns are revealed as we tune the qubit tunnel couplings relative to the frequency of the mediating photons. These patterns are in excellent agreement with a simulation using the Tavis-Cummings model, and allow us to readily identify different parameter regimes for both qubits in the detuning space. This method could potentially be an important component in the overall spectroscopic toolbox for quickly characterizing certain collective properties of multiple cavity QED coupled qubits.

cond-mat.mes-hall↗

arXiv2020

Giant anisotropy of spin relaxation and spin-valley mixing in a silicon quantum dot

In silicon quantum dots (QDs), at a certain magnetic field commonly referred to as the "hot spot", the electron spin relaxation rate (T_1^(-1)) can be drastically enhanced due to strong spin-valley mixing. Here, we experimentally find that with a valley splitting of 78.2 ${\pm}$ 1.6 $μ$eV, this hot spot in spin relaxation can be suppressed by more than 2 orders of magnitude when the in-plane magnetic field is oriented at an optimal angle, about 9° from the [100] sample plane. This directional anisotropy exhibits a sinusoidal modulation with a 180° periodicity. We explain the magnitude and phase of this modulation using a model that accounts for both spin-valley mixing and intravalley spin-orbit mixing. The generality of this phenomenon is also confirmed by tuning the electric field and the valley splitting up to 268.2 ${\pm}$ 0.7 $μ$eV.

cond-mat.mes-hall↗

arXiv2020

Classical route to quantum chaotic motions

We extract the information of a quantum motion and decode it into a certain orbit via a single measurable quantity. Such that a quantum chaotic system can be reconstructed as a chaotic attractor. Two configurations for reconstructing this certain orbit are illustrated, which interpret quantum chaotic motions from the perspectives of probabilistic nature and the uncertainty principle, respectively. We further present a strategy to import classical chaos to a quantum system, revealing a connection between the classical and quantum worlds.

quant-ph↗

arXiv2020

Lifting of Spin Blockade by Charged Impurities in Si-MOS Double Quantum Dot Devices

One obstacle that has slowed the development of electrically gated metal-oxide-semiconductor (MOS) singlet-triplet qubits is the frequent lack of observed spin blockade, even in samples with large singlet-triplet energy splittings. We present theoretical and experimental evidence that the cause of this problem in MOS double quantum dots is the stray positive charges in the oxide inducing accidental levels near the device's active region that allow spin blockade lifting. We also present evidence that these effects can be mitigated by device design modifications, such as overlapping gates.

cond-mat.mes-hall↗

arXiv2019

Coherent electron transport in silicon quantum dots

With silicon being the go-to material for spin qubits, and motivated by the demand of a scalable quantum computer architecture for fast and reliable quantum information transfer on-chip, we study coherent electron transport in a silicon double quantum dot. We first examine the valley-orbital dynamics in a silicon double dot, and discuss how to properly measure the tunnel couplings as well as the valley phase difference between two quantum dots. We then focus on possible phase and spin flip errors during spin transport across a silicon double dot. In particular, we clarify correction on the effective $g$-factor for the electron spin from the double dot confinement potential, and quantify the resulting phase error. We then study spin fidelity loss due to spin-valley mixing, which is a unique feature of silicon quantum dots. We show that a small phase correction between valleys can cause a significant coherence loss. We also investigate spin flip errors caused by either an external inhomogeneous magnetic field or the intrinsic spin-orbit coupling. We show that the presence of valleys makes it possible to have much broader (in terms of interdot detuning) level anti-crossings compared to typical anti-crossings in, for example, a GaAs double dot, and such broad anti-crossings lead to amplification of spin flip errors. Lastly, we design a pulse sequence to suppress various possible spin flip errors by taking advantage of the multiple level anti-crossings in a silicon double dot and employing Landau-Zener transitions.

cond-mat.mes-hall↗

arXiv2019

Effects of Interface Steps on the Valley Orbit coupling in a Si/SiGe quantum dot

Valley-orbit coupling is a key parameter for a silicon quantum dot in determining its suitability for applications in quantum information processing. In this paper we study the effect of interface steps on the magnitude and phase of valley-orbit coupling for an electron in a silicon quantum dot. Within the effective mass approximation, we find that the location of a step on the interface is important in determining both the magnitude and the phase of the valley-orbit coupling in a Si/SiGe quantum dot. Specifically, our numerical results show that the magnitude of valley orbit coupling can be suppressed up to 75\% by a step of one atomic monolayer, and its phase can change by almost $π$. When two steps are present, the minimum value of the valley-orbit coupling can even approach zero. We also clarify the effects of an applied external magnetic field and the higher orbital states on the valley-orbit coupling. Overall, our results illustrate how interface roughness affect the valley-orbit coupling in silicon, and how spin qubits in silicon may be affected.

cond-mat.mes-hall↗

arXiv2018

Quantum memory and gates using a Lambda-type quantum emitter coupled to a chiral waveguide

By coupling a $Λ$-type quantum emitter to a chiral waveguide, in which the polarization of a photon is locked to its propagation direction, we propose a controllable photon-emitter interface for quantum networks. We show that this chiral system enables the SWAP gate and a hybrid-entangling gate between the emitter and a flying single photon. It also allows deterministic storage and retrieval of single-photon states with high fidelities and efficiencies. In short, this chirally coupled emitter-photon interface can be a critical building block toward a large-scale quantum network.

quant-ph↗