arXiv subjects

Xiao-Feng Shi

Publications and source records attributed to Xiao-Feng Shi.

35 records · Page 2Linked to original sources

arXiv2021

Rydberg quantum computation with nuclear spins in two-electron neutral atoms

Alkaline-earth-like~(AEL) atoms with two valence electrons and a nonzero nuclear spin can be excited to Rydberg state for quantum computing. Typical AEL ground states possess no hyperfine splitting, but unfortunately a GHz-scale splitting seems necessary for Rydberg excitation. Though strong magnetic fields can induce a GHz-scale splitting, weak fields are desirable to avoid noise in experiments. Here, we provide two solutions to this outstanding challenge with realistic data of well-studied AEL isotopes. In the first theory, the two nuclear spin qubit states $|0\rangle$ and $|1\rangle$ are excited to Rydberg states $|r\rangle$ with detuning $Δ$ and 0, respectively, where a MHz-scale detuning $Δ$ arises from a weak magnetic field on the order of 1~G. With a proper ratio between $Δ$ and $Ω$, the qubit state $|1\rangle$ can be fully excited to the Rydberg state while $|0\rangle$ remains there. In the second theory, we show that by choosing appropriate intermediate states a two-photon Rydberg excitation can proceed with only one nuclear spin qubit state. The second theory is applicable whatever the magnitude of the magnetic field is. These theories bring a versatile means for quantum computation by combining the broad applicability of Rydberg blockade and the incomparable advantages of nuclear-spin quantum memory in two-electron neutral atoms.

physics.atom-ph↗

arXiv2021

Single-site Rydberg addressing in 3D atomic arrays for quantum computing with neutral atoms

Neutral atom arrays are promising for large-scale quantum computing especially because it is possible to prepare large-scale qubit arrays. An unsolved issue is how to selectively excite one qubit deep in a 3D atomic array to Rydberg states. In this work, we show two methods for this purpose. The first method relies on a well-known result: in a dipole transition between two quantum states driven by two off-resonant fields of equal strength but opposite detunings $\pmΔ$, the transition is characterized by two counter-rotating Rabi frequencies $Ωe^{\pm iΔt}$~[or $\pmΩe^{\pm iΔt}$ if the two fields have a $π$-phase difference]. This pair of detuned fields lead to a time-dependent Rabi frequency $2Ω\cos(Δt)$~[or $2iΩ\sin(Δt)$], so that a full transition between the two levels is recovered. We show that when the two detuned fields are sent in different directions, one atom in a 3D optical lattice can be selectively addressed for Rydberg excitation, and when its state is restored, the state of any nontarget atoms irradiated in the light path is also restored. Moreover, we find that the Rydberg excitation by this method can significantly suppress the fundamental blockade error of a Rydberg gate, paving the way for a high-fidelity entangling gate with commonly used quasi-rectangular pulse that is easily obtained by pulse pickers. Along the way, we find a second method for single-site Rydberg addressing in 3D, where a selected target atom can be excited to Rydberg state while preserving the state of any nontarget atom due to a spin echo sequence. The capability to selectively address a target atom in 3D atomic arrays for Rydberg excitation makes it possible to design large-scale neutral-atom information processor based on Rydberg blockade.

quant-ph↗

arXiv2021

Transition Slow-Down by Rydberg Interaction of Neutral Atoms and a Fast Controlled-NOT Quantum Gate

Exploring controllable interactions lies at the heart of quantum science. Neutral Rydberg atoms provide a versatile route toward flexible interactions between single quanta. Previous efforts mainly focused on the excitation annihilation~(EA) effect of the Rydberg blockade due to its robustness against interaction fluctuation. We study another effect of the Rydberg blockade, namely, the transition slow-down~(TSD). In TSD, a ground-Rydberg cycling in one atom slows down a Rydberg-involved state transition of a nearby atom, which is in contrast to EA that annihilates a presumed state transition. TSD can lead to an accurate controlled-{\footnotesize NOT}~({\footnotesize CNOT}) gate with a sub-$μ$s duration about $2π/Ω+ε$ by two pulses, where $ε$ is a negligible transient time to implement a phase change in the pulse and $Ω$ is the Rydberg Rabi frequency. The speedy and accurate TSD-based {\footnotesize CNOT} makes neutral atoms comparable~(superior) to superconducting~(ion-trap) systems.

quant-ph↗

arXiv2021

Reconstruction and interpretation of photon Doppler velocimetry spectrum for ejecta particles from shock-loaded sample in vacuum

The photon Doppler velocimetry (PDV) spectrum is investigated in an attempt to reveal the particle parameters of ejecta from shock-loaded samples in a vacuum. A GPU-accelerated Monte-Carlo algorithm, which considers the multiple-scattering effects of light, is applied to reconstruct the light field of the ejecta and simulate the corresponding PDV spectrum. The influence of the velocity profile, total area mass, and particle size of the ejecta on the simulated spectra is discussed qualitatively. To facilitate a quantitative discussion, a novel theoretical optical model is proposed in which the single-scattering assumption is applied. With this model, the relationships between the particle parameters of ejecta and the peak information of the PDV spectrum are derived, enabling direct extraction of the particle parameters from the PDV spectrum. The values of the ejecta parameters estimated from the experimental spectrum are in good agreement with those measured by a piezoelectric probe.

physics.app-ph↗

arXiv2019

Suppress motional dephasing of ground-Rydberg transition for high-fidelity quantum control with neutral atoms

The performance of many control tasks with Rydberg atoms can be improved via suppression of the motion-induced dephasing between ground and Rydberg states of neutral atoms. The dephasing often occurs during the {\it gap} time when the atom is shelved in a Rydberg state before its deexcitation. By using laser fields to induce specific extra phase change to the Rydberg state during the gap time, it is possible to faithfully transfer the Rydberg state back to the ground state after the gap. Although the Rydberg state transitions back and forth between different eigenstates during the gap time, it preserves the blockade interaction between the atom of interest and a nearby Rydberg excitation. This simple method of suppressing the motional dephasing of a flying Rydberg atom can be used in a broad range of quantum control over neutral atoms.

quant-ph↗

arXiv2019

Fast, Accurate, and Realizable Two-Qubit Entangling Gates by Quantum Interference in Detuned Rabi Cycles of Rydberg Atoms

High-fidelity entangling quantum gates based on Rydberg interactions are required for scalable quantum computing with neutral atoms. Their realization, however, meets a major stumbling block -- the motion-induced dephasing of the transition between the ground and Rydberg states. By using quantum interference between different detuned Rabi oscillations, we propose a practical scheme to realize a class of accurate entangling Rydberg quantum gates subject to a minimal dephasing error. We show two types of such gates, $U_{1}$ and $U_{2}$, in the form of $\text{diag}\{1, e^{iα}, e^{iγ}, e^{iβ}\}$, where $α, γ$, and $β$ are determined by the parameters of lasers and the Rydberg blockade $V$. $U_{1}$ is realized by sending to the two qubits a single off-resonant laser pulse, while $U_{2}$ is realized by individually applying one pulse of detuned laser to each qubit. Our method has several advantages. First, the gates are accurate because the fidelity of $U_k$ is limited only by a rotation error below $10^{-5}$ and the Rydberg-state decay. Decay error on the order of $10^{-5}$ can be easily obtained because all transitions are detuned, resulting in small population in Rydberg state. Second, the motion-induced dephasing is minimized because there is no gap time in which a population is left in the Rydberg shelving states of either qubit. Third, the gate is resilient to the variation of $V$. This is because among the three phases $α,γ$, and $β$, only the last has a (partial) dependence on $V$. Fourth, the Rabi frequency and $V$ in our scheme are of similar magnitude, which permits a fast implementation of the gate when both of them are of the feasible magnitude of several megahertz. The rapidity, accuracy, and feasibility of this interference method can lay the foundation for entangling gates in universal quantum computing with neutral atoms.

quant-ph↗

arXiv2018

Accurate Quantum Logic Gates by Spin Echo in Rydberg Atoms

Scalable quantum computing is based on realizable accurate quantum gates. For neutral atoms, it is an outstanding challenge to design a high-fidelity two-qubit entangling gate without resorting to difficult techniques like shaping laser pulses or cooling atoms to motional ground states. By using spin echo to suppress the blockade error, we propose an easily realizable controlled-phase Rydberg quantum gate of high intrinsic fidelity. In the context of spin echo, we show that the fundamental blockade error of the traditional Rydberg gate, on the order of $ε\sim10^{-3}$, actually results from two `clockwise' rotations of Rabi frequencies $\barΩ_\pm=V\pm \sqrt{V^2+Ω^2}$. In our `echo' sequence, such an error can be suppressed to the order of $ε^2$ by adding two `anticlockwise' rotations with frequencies $-\barΩ_\pm$. With the blockade error effectively removed, the error caused by Rydberg state decay becomes the final fundamental limit to the gate accuracy, which in principle, can be reduced beyond the level of $10^{-5}$. Furthermore, due to the small population $ε$ involved in the `echo' process, the spin-echo gate is robust against the variation of Rydberg blockade caused by the drift of the qubits, so that it can still be much more accurate than that of a traditional Rydberg gate even for qubits cooled only to the sub-mK regime.

quant-ph↗

arXiv2018

Deutsch, Toffoli, and CNOT Gates via Rydberg Blockade of Neutral Atoms

Universal quantum gates and quantum error correction~(QEC) lie in the heart of quantum information science. Large-scale quantum computing depends on a universal set of quantum gates, in which some gates may be easily carried out, while others are hard with a certain physical system. There is a unique three-qubit quantum gate called the Deutsch gate~[$\mathbb{D}(θ)$], from which alone a circuit can be constructed so that any feasible quantum computing is attainable. As far as we know, however, $\mathbb{D}(θ)$ has not been demonstrated. Here we design an easily realizable $\mathbb{D}(θ)$ by using Rydberg blockade of neutral atoms, where $θ$ can be tuned to any value in $[0,π]$ by adjusting the strengths of external control fields. Using similar protocols, we further show that both the Toffoli and CNOT gates can be achieved with only three laser pulses. The Toffoli gate, being universal for classical reversible computing, is also useful for QEC that plays an important role in quantum communication and fault-tolerant quantum computation. The possibility and briefness to realize these gates shed new light on the study of quantum information with neutral atoms.

quant-ph↗

arXiv2018

Universal Barenco quantum gates via a tunable non-collinear interaction

The Barenco gate~($\mathbb{B}$) is a type of two-qubit quantum gate based on which alone universal quantum computation can be achieved. Each $\mathbb{B}$ is characterized by three angles ($α,θ$, and $ϕ$) though it works in a two-qubit Hilbert space. Here we design $\mathbb{B}$ via a non-collinear interaction $V|r_1r_2\rangle\langle r_1r_3|+$H.c., where $|r_i\rangle$ is a state that can be excited from a qubit state and $V$ is adjustable. We present two protocols of $\mathbb{B}$. The first~(second) protocol consists of two~(six) pulses and one~(two) wait period(s), where the former causes rotations between the qubit states and excited states, and the latter induces gate transformation via the non-collinear interaction. In the first protocol, the variable $ϕ$ can be tuned by varying phases of external controls, and the other two variables $α$ and $θ$, tunable via adjusting the wait duration, have a linear dependence upon each other. Meanwhile, the first protocol can give rise to the CNOT and Controlled-Y gates. In the second protocol, $α,θ$, and $ϕ$ can be varied by changing the interaction amplitudes and wait durations, and the latter two are dependent on $α$ non-linearly. Both protocols can also lead to another universal gate when $\{α,ϕ\}=\{1/4,1/2\}π$ with appropriate parameters. Implementation of these universal gates is analyzed based on the van der Waals interaction of neutral Rydberg atoms.

quant-ph↗

arXiv2017

Rydberg Quantum Gates Free from Blockade Error

Accurate quantum gates are basic elements for building quantum computers. There has been great interest in designing quantum logic gates by using blockade effect of Rydberg atoms recently. The fidelity and operation speed of these gates, however, are fundamentally limited by an intrinsic blockade error. Here we propose a type of quantum gates, which are based on Rydberg blockade effect, yet free from any blockade error. In contrast to the "blocking" method in previous schemes, we use Rydberg energy shift to realize a rational generalized Rabi frequency so that a $π$ phase for one input state of the gate emerges. This leads to an accurate Rydberg quantum logic gate that can operate on a 0.1-$μs$ timescale or faster because it works by a Rabi frequency which is comparable to the blockade shift.

quant-ph↗

arXiv2017

Annulled van der Waals interaction and fast Rydberg quantum gates

A pair of neutral atoms separated by several microns and prepared in identical s-states of large principal quantum number experience a van der Waals interaction. If microwave fields are used to generate a superposition of s-states with different principal quantum numbers, a null point may be found at which a specific superposition state experiences no van der Waals interaction. An application of this novel Rydberg state in a quantum controlled-Z gate is proposed, which takes advantage of GHz rate transitions to nearby Rydberg states. A gate operation time in the tens of nanoseconds is predicted.

physics.atom-ph↗

arXiv2014

Entanglement of neutral-atom chains by spin-exchange Rydberg interaction

Conditions to achieve an unusually strong Rydberg spin-exchange interaction are investigated and proposed as a means to generate pairwise entanglement and realize a SWAP-like quantum gate for neutral atoms. Ground-state entanglement is created by mapping entangled Rydberg states to ground states using optical techniques. A protocol involving SWAP gate and pairwise entanglement operations is predicted to create global entanglement of a chain of $N$ atoms in a time that is independent of $N$.

quant-ph↗

arXiv2013

Nuclear spin polarization in a single quantum dot pumped by two laser beams

We theoretically investigate dynamic nuclear spin polarization in a self-assembled quantum dot pumped optically by two laser beams. With the assumption that a noncollinear interaction between the hole spin and nuclear spins leads to nuclear spin polarization, we find that both weak and strong nuclear spin polarizations can arise, depending on the intensities and central frequencies of the lasers. For weak nuclear spin polarization, we use a perturbation method to show that the distribution of the nuclear spin Overhauser field may become significantly narrower. Using Monte Carlo simulations to study a single quantum dot, we find that strong nuclear spin polarization can also be generated via appropriate optical pumping.

cond-mat.mes-hall↗

arXiv2010

Exotic phase diagram of a topological quantum system

We study the quantum phase transitions (QPTs) in the Kitaev spin model on a triangle-honeycomb lattice. In addition to the ordinary topological QPTs between Abelian and non-Abelian phases, we find new QPTs which can occur between two phases belonging to the same topological class, namely, either two non-Abelian phases with the same Chern number or two Abelian phases with the same Chern number. Such QPTs result from the singular behaviors of the nonlocal spin-spin correlation functions at the critical points.

cond-mat.stat-mech↗

arXiv2009

Quantum emulation of a spin system with topologically protected ground states using superconducting quantum circuits

Using superconducting quantum circuit elements, we propose an approach to experimentally construct a Kitaev lattice, which is an anisotropic spin model on a honeycomb lattice with three types of nearest-neighbor interactions and having topologically protected ground states. We study two particular parameter regimes to demonstrate both vortex and bond-state excitations. Our proposal outlines an experimentally realizable artificial many-body system that exhibits exotic topological properties.

cond-mat.supr-con↗

arXiv2009

Topological quantum phase transition in an extended Kitaev spin model

We study the quantum phase transition between Abelian and non-Abelian phases in an extended Kitaev spin model on the honeycomb lattice, where the periodic boundary condition is applied by placing the lattice on a torus. Our analytical results show that this spin model exhibits a continuous quantum phase transition. Also, we reveal the relationship between bipartite entanglement and the ground-state energy. Our approach directly shows that both the entanglement and the ground-state energy can be used to characterize the topological quantum phase transition in the extended Kitaev spin model.

cond-mat.stat-mech↗

arXiv2008

Topological states and braiding statistics using quantum circuits

Using superconducting quantum circuits, we propose an approach to construct a Kitaev lattice, i.e., an anisotropic spin model on a honeycomb lattice with three types of nearest-neighbor interactions. We study two particular cases to demonstrate topological states (i.e., the vortex and bond states) and show how the braiding statistics can be revealed. Our approach provides an experimentally realizable many-body system for demonstrating exotic properties of topological phases.

cond-mat.mes-hall↗