Kaiyu Yang, Shi-Liang Zhu, Z. D. Wang
A large-scalable quantum computer model, whose qubits are represented by the subspace subtended by the ground state and the single exciton state on semiconductor quantum dots, is proposed. A universal set of quantum gates in this system may be achieved by a mixed approach, composed of dynamic evolution and nonadibatic geometric phase.
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We propose a scheme for realizing quantum controlled phase gates with two nonidentical quantum dots trapped in two coupled photonic crystal cavities and driven by classical laser fields under the condition of non-small hopping limit. During the gate operation, neither the quantum dots are excited, while the system can acquire different phases conditional upon the different states of the quantum dots. Along with single-qubit operations, a two-qubit controlled phase gate can be achieved.
We propose a scheme for realizing two-qubit quantum phase gates with two nonidentical quantum dots trapped in two coupled photonic crystal cavities and driven by classical laser fields. During the gate operation, neither the cavity modes nor the quantum dots are excited, so the decoherence can be suppressed. The system can acquire a phase conditional upon the states of the quantum dots, which can be used to realize the controlled phase gate.
A quantum computer based on an asymmetric coupled dot system has been proposed and shown to operate as the controlled-NOT-gate. The basic idea is (1) the electron is localized in one of the asymmetric coupled dots. (2)The electron transfer takes place from one dot to the other when the energy-levels of the coupled dots are set close. (3)The Coulomb interaction between the coupled dots mutually affects the energy levels of the other coupled dots. The decoherence time of the quantum computation and the measurement time are estimated. The proposed system can be realized by developing the technolo…
We propose a scheme for realizing a two-qubit controlled phase gate via an unconventional geometric phase with two nonresonant quantum dots trapped in a photonic crystal cavity. In this system, the quantum dots simultaneously interact with a large detuned cavity mode and strong driving classical light fields. During the gate operation, the quantum dots undergo no transitions, while the cavity mode is displaced along a closed path in the phase space. In this way, the system can acquire geometric phases conditional upon the states of the quantum dots. After implementing single-qubit operations, …
We investigated the quantum gates of coupled quantum dots, theoretically, when charging effects can be observed. We have shown that the charged states in the qubits can be observed by the channel current of the MOSFET structure.
We propose a resonator-assisted entangling gate for spin qubits with high fidelity. Each spin qubit corresponds to two electrons in a nanowire double quantum dot, with the singlet and one of the triplets as the logical qubit states. The gate is effected by virtual charge dipole transitions. We include noise in our model to show feasibility of the scheme under current experimental conditions.
We present some deterministic schemes to construct universal quantum gates, that is, controlled- NOT, three-qubit Toffoli, and Fredkin gates, between flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. The control qubit of our gates is encoded on the polarization of the moving single photon and the target qubits are encoded on the confined electron spins in quantum dots inside optical microcavities. Our schemes for these universal quantum gates on a hybrid system have some advantages. First, all the gates are accomplished…
We present an idealized model involving interacting quantum dots that can support both the dynamical and geometrical forms of quantum computation. We show that by employing a structure similar to the one used in the Aharonov-Bohm effect we can construct a topological two-qubit phase-gate that is to a large degree independent of the exact values of the control parameters and therefore resilient to control errors. The main components of the setup are realizable with present technology.
Variational quantum compiling for three-qubit gates design in quantum dots
针对可扩展量子计算中量子逻辑门设计需求，本工作利用变分量子编译算法，基于纯物理相互作用项的时间无关哈密顿量设计出对多种噪声鲁棒的高保真三量子比特门，且方法适用于多类量子系统，助力构建更优量子计算架构。
A single electron spin in a double quantum dot in a magnetic field is considered in terms of a four-level system. By describing the electron motion between the potential minima by spin-conserving tunneling and spin flip caused by a spin-orbit coupling, we inversely engineer faster-than-adiabatic state manipulation operations based on the geometry of four-dimensional (4D) rotations. In particular, we show how to transport a qubit among the quantum dots performing simultaneously a required spin rotation.
Geometric two-qubit gates in silicon-based double quantum dots
本文针对硅双量子点自旋量子比特受电荷噪声影响的问题，提出在局域磁场梯度远大于交换相互作用的区域，通过解析与数值方法实现纠缠和非纠缠几何两比特门，保真度超过99%并优于动力学操作。
Geometric manipulation of a quantum system offers a method for fast, universal, and robust quantum information processing. Here, we propose a scheme for universal all-geometric quantum computation using non-adiabatic quantum holonomies. We propose three different realizations of the scheme based on an unconventional use of quantum dot and single-molecule magnet devices, which offer promising scalability and robust efficiency.
We present some compact quantum circuits for a deterministic quantum computing on electron-spin qubits assisted by quantum dots inside single-side optical microcavities, including the CNOT, Toffoli, and Fredkin gates. They are constructed by exploiting the giant optical Faraday rotation induced by a single-electron spin in a quantum dot inside a single-side optical microcavity as a result of cavity quantum electrodynamics. Our universal quantum gates have some advantages. First, all the gates are accomplished with a success probability of 100% in principle. Second, our schemes require no addit…
Using electrostatic gates to control the electron positions, we present a new controlled-NOT gate based on quantum dots. The qubit states are chosen to be the spin states of an excess conductor electron in the quantum dot; and the main ingredients of our scheme are the superpositions of space-time paths of electrons and the effect of Coulomb blockade. All operations are performed only on individual quantum dots and are based on fundamental interactions. Without resorting to spin-spin terms or other assumed interactions, the scheme can be realized with a dedicated circuit and a necessary number…
We propose a new scheme for solid-state photonic quantum computation in which trapped photons in optical cavities are taken as a quantum bit. Quantum gates can be realized by coupling the cavities with quantum dots through waveguides. The proposed scheme allows programmable and deterministic gate operations and the system can be scaled up to many quantum bits.
We present Monte Carlo wavefunction simulations for quantum computations employing an exchange-coupled array of quantum dots. Employing a combination of experimentally and theoretically available parameters, we find that gate fidelities greater than 98 % may be obtained with current experimental and technological capabilities. Application to an encoded 3 qubit (nine physical qubits) Deutsch-Josza computation indicates that the algorithmic fidelity is more a question of the total time to implement the gates than of the physical complexity of those gates.
We discuss the configuration and acoustic phonon effect on the conditional phase gate using self assembled quantum dots. As an example, we discuss the simplest three dots conditional phase gate, and we found that the fidelity of conditional phase gate depends strongly on the configuration and temperature. Numerical simulation shows that line-configuration resonant with lower single exciton energy level performs better during the gate operation.
In this paper we study the implementation of non-adiabatic geometrical quantum gates with in semiconductor quantum dots. Different quantum information enconding/manipulation schemes exploiting excitonic degrees of freedom are discussed. By means of the Aharanov-Anandan geometrical phase one can avoid the limitations of adiabatic schemes relying on adiabatic Berry phase; fast geometrical quantum gates can be in principle implemented
We propose a scheme for a two-qubit conditional phase gate by quantum Zeno effect with semiconductor quantum dots. The system consists of two charged dots and one ancillary dot that can perform Rabi oscillations under a resonant laser pulse. The quantum Zeno effect is induced by phonon-assisted exciton relaxation between the ancillary dot and the charged dots, which is equivalent to a continuous measurement. We solve analytically the master equation and simulate the dynamics of the system using a realistic set of parameters. In contrast to standard schemes, larger phonon relaxation rates incre…
Two-qubit interactions are at the heart of quantum information processing. For single-spin qubits in semiconductor quantum dots, the exchange gate has always been considered the natural two-qubit gate. The recent integration of magnetic field or g-factor gradients in coupled quantum dot systems allows for a one-step, robust realization of the controlled phase (C-Phase) gate instead. We analyze the C-Phase gate durations and fidelities that can be obtained under realistic conditions, including the effects of charge and nuclear field fluctuations, and find gate error probabilities of below 10-4,…
We present a feasible scheme for performing an optically controlled phase gate between two conduction electron spin qubits in adjacent self assembled quantum dots. Interaction between the dots is mediated by the tunneling of the valence hole state which is activated only by applying a laser pulse of the right polarization and frequency. Combining the hole tunneling with the Pauli blocking effect, we obtain conditional dynamics for the two quantum dots, which is the essence of our gating operations. Our results are of explicit relevance to the recent generation of vertically stacked self-assemb…
Based on an idea that spatial separation of charge states can enhance quantum coherence, we propose a scheme for quantum computation with quantum bit (qubit) constructed from two coupled quantum dots. Quantum information is stored in electron-hole pair state with the electron and hole locating in different dots, which enables the qubit state being very long-lived. Universal quantum gates involving any pair of qubits are realized by coupling the quantum dots through cavity photon which is a hopeful candidate to transfer long-range information. Operation analysis is carried out by estimating the…
We show a method for implementing universal quantum computing using of a singlet and triplets of nanowire double quantum dots coupled to a one-dimensional transmission line resonator. This method is attractive for both quantum computing and quantum control with inhibition of spontaneous emission, enhanced spin qubit lifetime, strong coupling and quantum nondemolition measurements of spin qubits. We analyze the performance and stability of all required operations and emphasize that all techniques are feasible with current experimental technology.
A potential scheme is proposed for realizing a two-qubit quantum gate in semiconductor quantum dots. Information is encoded in the spin degrees of freedom of one excess conduction electron of each quantum dot. We propose to use two lasers, radiation two neighboring QDs, and tuned to blue detuning with respect to the resonant frequencies of individual excitons. The two-qubit phase gate can be achieved by means of both Pauli-blocking effect and dipole-dipole coupling between intermediate excitonic states.
Motivated by recent experiments of Zajac et al. [arXiv:1708.03530], we theoretically describe high-fidelity two-qubit gates using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We use a combination of analytical calculations and numerical simulations to provide the optimal pulse sequences and parameter settings for the gate operation. We present a novel synchronization method which avoids detrimental spin flips during the gate operation and provide details about phase mismatches accumulated during the two-qubit gates which occur due…
We present an all-optical implementation of quantum computation using semiconductor quantum dots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read-out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantum dots: exciton decay, hole mixing and phonon decoherence. We introduce a…
Achieving low-error, exchange-interaction operations in quantum dots for quantum computing imposes simultaneous requirements on the exchange energy's dependence on applied voltages. A double quantum dot (DQD) qubit, approximated with a quadratic potential, is solved using a full configuration interaction method. This method is more accurate than Heitler-London and Hund-Mulliken approaches and captures new and significant qualitative behavior. We show that multiple regimes can be found in which the exchange energy's dependence on the bias voltage between the dots is compatible with current quan…
We present a solid-state implementation of ultrafast conditional quantum gates. Our proposal for a quantum-computing device is based on the spin degrees of freedom of electrons confined in semiconductor quantum dots, thus benefiting from relatively long decoherence times. More specifically, combining Pauli blocking effects with properly tailored ultrafast laser pulses, we are able to obtain sub-picosecond spin-dependent switching of the Coulomb interaction, which is the essence of our conditional phase-gate proposal. This allows us to realize {\it a fast two qubit gate which does not translate…
We give an elementary introduction to the notion of quantum entanglement between distinguishable parties and review a recent proposal about solid state quantum computation with spin-qubits in quantum dots. The indistinguishable character of the electrons whose spins realize the qubits gives rise to further entanglement-like quantum correlations. We summarize recent results concerning this type of quantum correlations of indistinguishable particles.
The universal quantum computation is obtained when there exists asymmetric anisotropic exchange between electron spins in coupled semiconductor quantum dots. The asymmetric Heisenberg model can be transformed into the isotropic model through the control of two local unitary rotations for the realization of essential quantum gates. The rotations on each qubit are symmetrical and depend on the strength and orientation of asymmetric exchange. The implementation of the axially symmetric local magnetic fields can assist the construction of quantum logic gates in anisotropic coupled quantum dots. Th…
Controlled phase gate in exchange coupled quantum dots affected by quasistatic charge noise
针对半导体量子比特中电荷噪声影响两比特门保真度的问题，研究了准静态失谐噪声对双量子点自旋比特受控相位门的作用，推导了弱噪声下相关解析表达式并验证了序列保真度指数衰减规律仍成立。
We demonstrate a reconfigurable quantum dot gate architecture that incorporates two interchangeable transport channels. One channel is used to form quantum dots and the other is used for charge sensing. The quantum dot transport channel can support either a single or a double quantum dot. We demonstrate few-electron occupation in a single quantum dot and extract charging energies as large as 6.6 meV. Magnetospectroscopy is used to measure valley splittings in the range of 35-70 microeV. By energizing two additional gates we form a few-electron double quantum dot and demonstrate tunable tunnel …
Implementing high-fidelity two-qubit gates in single-electron spin qubits in silicon double quantum dots is still a major challenge. In this work, we employ analytical methods to design control pulses that generate high-fidelity entangling gates for quantum computers based on this platform. Using realistic parameters and initially assuming a noise-free environment, we present simple control pulses that generate CNOT, CPHASE, and CZ gates with average fidelities greater than 99.99\% and gate times as short as 45 ns. Moreover, using the local invariants of the system's evolution operator, we sho…
Implementations for quantum computing require fast single- and multi-qubit quantum gate operations. In the case of optically controlled quantum dot qubits theoretical designs for long-range two- or multi-qubit operations satisfying all the requirements in quantum computing are not yet available. We have developed a design for a fast, long-range two-qubit gate mediated by a photonic microcavity mode using excited states of the quantum dot-cavity system that addresses these needs. This design does not require identical qubits, it is compatible with available optically induced single qubit operat…
Spin qubit gates via phonon buses in electron nanowires
研究利用电子纳米线实现量子点间自旋-自旋耦合，实验参数下耦合强度超过30MHz，为可扩展量子计算架构提供新方法。
Quantum gates in coupled quantum dots controlled by coupling modulation
通过调制耦合实现量子门控制，用于量子计算中的单量子位和纠缠门操作。
Multi-qubit DC gates over an inhomogeneous array of quantum dots
本研究提出在非均匀量子点阵列上通过直流演化实现多量子比特门，发展了基于微扰处理的门分解与误差优化形式体系，展示了其在量子纠错和算法中的加速潜力。
Addressability of spin qubits in a silicon double quantum dot setup in the (1,1) charge configuration relies on having a large difference between the Zeeman splittings of the electrons. When the difference is not sufficiently large, the rotating wave approximation becomes inaccurate. We consider a device working in this regime, with always-on exchange coupling, and describe how a CZ gate and arbitrary one-qubit gates which are robust against charge noise can be implemented by smoothly pulsing the microwave source, while eliminating the crosstalk. We find that the most significant deviations fr…
We study the effect of the spin-orbit interaction on quantum gate operations based on the spin exchange coupling where the qubit is represented by the electron spin in a quantum dot or a similar nanostructure. Our main result is the exact cancellation of the spin-orbit effects in the sequence producing the quantum XOR gate for the ideal case where the pulse shapes of the exchange and spin-orbit interactions are identical. For the non-ideal case, the two pulse shapes can be made almost identical and the gate error is strongly suppressed by two small parameters, the spin-orbit constant and the d…
Measuring pulse heating in Si quantum dots with individual two-level fluctuators
本研究通过测量Si量子点中带电双能级波动子的响应，揭示了电压脉冲导致的加热机制，并提出减少门面积以缓解加热的可能方法。
We present a theory for understanding the exchange interaction between electron spins in neighboring quantum dots, either by changing the detuning of the two quantum dots or independently tuning the tunneling barrier between quantum dots. The Hubbard model and a more realistic confining-potential model are used to investigate how the tilting and barrier control affect the effective exchange coupling and thus the gate fidelity in both the detuning and symmetric regimes. We show that the exchange coupling is less sensitive to the charge noise through tunnel barrier control (while allowing for ex…
We consider a new quantum gate mechanism based on electron spins in coupled semiconductor quantum dots. Such gates provide a general source of spin entanglement and can be used for quantum computers. We determine the exchange coupling J in the effective Heisenberg model as a function of magnetic (B) and electric fields, and of the inter-dot distance (a) within the Heitler-London approximation of molecular physics. This result is refined by using sp-hybridization, and by the Hund-Mulliken molecular-orbit approach which leads to an extended Hubbard description for the two-dot system that shows a…
Manipulative Properties of the Asymmetry Double Quantum Dots via Laser and Gate Voltage
本文采用密度矩阵方法研究非对称双量子点系统，发现激光脉冲或门电压可操控其增益、吸收、色散及隧穿转移的态布居，为量子系统电光操控提供可能。
In this paper, we present a machine learning framework to design high-fidelity multi-qubit gates for quantum processors based on quantum dots in silicon, with qubits encoded in the spin of single electrons. In this hardware architecture, the control landscape is vast and complex, so we use the deep reinforcement learning method to design optimal control pulses to achieve high fidelity multi-qubit gates. In our learning model, a simulator models the physical system of quantum dots and performs the time evolution of the system, and a deep neural network serves as the function approximator to lea…
We show that the g-factor and the spin-flip time T_{1} of a heterojunction quantum dot is very sensitive to the band-bending interface electric field even in the absence of wave function penetration into the barrier. When this electric field is of the order of 10^{5} V/cm, g and T_{1} show high sensitivity to dot radius and magnetic field arising from the interplay between Rashba and Dresselhaus spin-orbit interactions. This result opens new possibilities for the design of a quantum dot spin quantum computer where g-factor and T_{1} can be engineered by manipulating the spin-orbit coupling thr…
A spin qubit in semiconductor quantum dots holds promise for quantum information processing for scalability and long coherence time. An important semiconductor qubit system is a double quantum dot trapping two electrons or holes, whose spin states encode either a singlet-triplet qubit or two single-spin qubits coupled by exchange interaction. In this article, we report progress on spin dephasing of two exchange-coupled spins in a double quantum dot. We first discuss the schemes of two-qubit gates and qubit encodings in gate-defined quantum dots or donor atoms based on the exchange interaction.…
Gated InAs quantum dots embedded in surface acoustic wave cavities for low-noise optomechanics
针对自组装InAs量子点光力学应用需调控电荷态、降低电荷噪声的需求，通过分子束外延与纳米加工集成栅控InAs量子点与声表面波腔，在保留各子系统最优性能的同时实现低噪声光力学，为微波-光学量子转换的高效系统研究奠定关键基础。
In this theoretical manuscript I propose a scheme for entangling a single electron semiconductor spin qubit with a single electron semiconductor charge qubit in a triangular triple quantum dot configuration. Two out of three quantum dots are used to define a single electron semiconductor charge qubit. Furthermore, the spin qubit is embedded in the Zeeman sub-levels of the third quantum dot. Combining single qubit gates with entangling CNOT gates allows one to construct a SWAP gate, and therefore to use the semiconductor spin qubit as a long-lived memory for the semiconductor charge qubit.
How NOT to build control-target gates in semiconductor quantum dots and beyond
该研究针对半导体量子点器件架构无法高效实现CNOT等非对称控制-目标门的问题，提出异质双量子点器件蓝图，可在同位素纯化硅中实现单脉冲容错的100纳秒CNOT门。
We demonstrate a hybrid device architecture where the charge states in a double quantum dot (DQD) formed in a Si/SiGe heterostructure are read out using an on-chip superconducting microwave cavity. A quality factor Q = 5,400 is achieved by selectively etching away regions of the quantum well and by reducing photon losses through low-pass filtering of the gate bias lines. Homodyne measurements of the cavity transmission reveal DQD charge stability diagrams and a charge-cavity coupling rate g_c/2pi = 23 MHz. These measurements indicate that electrons trapped in a Si DQD can be effectively couple…
We present an improved fabrication process for overlapping aluminum gate quantum dot devices on Si/SiGe heterostructures that incorporates low-temperature inter-gate oxidation, thermal annealing of gate oxide, on-chip electrostatic discharge (ESD) protection, and an optimized interconnect process for thermal budget considerations. This process reduces gate-to-gate leakage, damage from ESD, dewetting of aluminum, and formation of undesired alloys in device interconnects. Additionally, cross-sectional scanning transmission electron microscopy (STEM) images elucidate gate electrode morphology in …
A diverse set of two-qubit gates for spin qubits in semiconductor quantum dots
针对半导体量子点自旋量子比特，提出一种基于海森堡相互作用的快速复合双量子比特门方案，扩展了可用门类型并降低器件要求，实验验证与模拟高度吻合，助力NISQ计算。
Defining quantum dots in semiconductor based heterostructures is an essential step in initializing solid-state qubits. With growing device complexity and increasing number of functional devices required for measurements, a manual approach to finding suitable gate voltages to confine electrons electrostatically is impractical. Here, we implement a two-stage device characterization and dot-tuning process which first determines whether devices are functional and then attempts to tune the functional devices to the single or double quantum dot regime. We show that automating well established manual…
A vertical gate-defined double quantum dot in a strained germanium double quantum well
本文提出利用多量子阱异质结构超越平面系统，在应变锗双量子阱中实现了栅极定义垂直双量子点的运行，通过量子输运测量验证了其垂直堆叠结构，并讨论了在量子计算与模拟中的应用前景。
Two-dimensional (2D) materials are a family of layered materials exhibiting rich exotic phenomena, such as valley-contrasting physics. Down to single-particle level, unraveling fundamental physics and potential applications including quantum information processing in these materials attracts significant research interests. To unlock these great potentials, gate-controlled quantum dot architectures have been applied in 2D materials and their heterostructures. Such systems provide the possibility of electrical confinement, control, and manipulation of single carriers in these materials. In this …
We investigate a hybrid quantum system involving spin qubits, based on the spins of electrons confined in quantum dots, and topological qubits, based on Majorana fermions. In such a system, gated control of the charge on the quantum dots allows transfer of quantum information between the spin and topological qubits, and the topological system can be used to facilitate transfer of spin qubits between spatially separated quantum dots and to initialize entangled spin-qubit pairs. Here, we show that the coupling to the topological system also makes it possible to perform entangling two-qubit gates…
On Noise-Sensitive Automatic Tuning of Gate-Defined Sensor Dots
本文研究了门定义量子点传感器点的自动调谐问题，通过分析测量数据的噪声特性并评估改进多种噪声估计器，提出利用Chen等人的估计器识别最佳工作区间，实现了对感兴趣侧翼的有效分类。
Hole spin qubits in planar Ge heterostructures are one of the frontrunner platforms for scalable quantum computers. In these systems, the spin-orbit interactions permit efficient all-electric qubit control. We propose a minimal design modification of planar devices that enhances these interactions by orders of magnitude and enables low power ultrafast qubit operations in the GHz range. Our approach is based on an asymmetric potential that strongly squeezes the quantum dot in one direction. This confinement-induced spin-orbit interaction does not rely on microscopic details of the device such a…
Remote Entangling Gates for Spin Qubits in Quantum Dots using a Charge-Sensitive Superconducting Coupler
本文提出利用电荷敏感超导耦合器对量子点中远程自旋量子比特施加微波激活CZ门，通过两种脉冲方案和含1/f^β电荷噪声的非马尔可夫模拟，证明在现实条件下门保真度可超过90%。
We propose two schemes to coherently transfer arbitrary quantum states of the two-electron singlet-triplet qubit across a chain of 3 quantum dots. The schemes are based on electrical control over the detuning energy of the quantum dots. The first is a pulse-gated scheme, requiring dc pulses and engineering of inter- and intra-dot Coulomb energies. The second scheme is based on the adiabatic theorem, requiring time-dependent control of the detuning energy through avoided crossings at a rate that the system remains in the ground state. We simulate the transfer fidelity using typical experimental…
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