arXiv subjects

Xiao-Feng Shi

Publications and source records attributed to Xiao-Feng Shi.

At least 19 recordsLinked to original sources

arXiv2026

Rydberg-Mediated Nonlinear Quantum Optics

Rydberg atoms have emerged as a versatile platform for quantum optics due to their exaggerated properties, particularly their strong long-range interactions, which enable a new regime of light-matter interaction. By mapping the interactions between Rydberg atoms onto photons, effective photon-photon interactions can be realized at the single-photon level, thereby overcoming the intrinsic weakness of conventional optical nonlinearities. In this review, we first introduce the fundamental physical principles of Rydberg-mediated quantum optics, and then discuss some key developments, including single-photon engineering, photonic quantum gates, contactless nonlinear optics, and quantum entanglement, providing a comprehensive overview of the current state and prospects of this rapidly developing field.

quant-ph↗

arXiv2026

Ancilla-assisted dark-state quantum gates in ultracold polar molecules

High-fidelity entangling gates are useful for circuit-based quantum computing, yet their realization in a scalable platform is challenging. Optically trapped ultracold polar molecules offer a potentially scalable platform. Here, we introduce a two-qubit molecular entangling gate via a novel ancilla-assisted dark-state mechanism, where high fidelity is achievable. In particular, we find that by using appropriate qubit states for the ancilla and the data qubits, dark states can appear, and entanglement can emerge by driving the ancilla only. It is also feasible to extend the two-qubit gate to a multi-qubit gate via the same dark-state mechanism.

quant-ph↗

arXiv2026

Fast measurement-based generation of large-scale Greenberger-Horne-Zeilinger state with atomic nuclear-spin qubits

Large-scale Greenberger-Horne-Zeilinger~(GHZ) state is useful for quantum technologies but difficult to be prepared. Here, we propose fast measurement-based preparation of large-scale GHZ states by a four-qubit quantum phase gate with nuclear-spin qubits of alkaline-earth-like atoms, which is named as quantum ferromagnetic gate~(QFG) due to its analogy to the alignment of molecular magnetic moments in a classical magnet. A high-fidelity Rydberg-mediated QFG can be realized in a time of $6π/Ω_{\text{m}}$ with $Ω_{\text{m}}$ the maximal Rydberg Rabi frequency. From a product state of three data atom and one ancilla atom, a gluing circuit with one QFG, two single-qubit gates, and a projective measurement of the ancilla can generate a 3-qubit GHZ state, and repetition of this gluing circuit can lead to 9, 27, 81, 243, $\cdots$-qubit GHZ states. Analyses based on currently available techniques show that a 243-qubit GHZ state is realizable, and more qubits can be entangled with higher detection fidelity.

quant-ph↗

arXiv2026

Coherence enhancement of Rydberg polaritons

Quantum nonlinear optics by Rydberg polaritons can enable single-photon transistor and switch, single-photon source, and deterministic quantum information processing. A major hindrance in this study is the fast motional decoherence. Here, we devise a scheme to significantly enhance the coherence of Rydberg polariton by letting the atoms {\it remember} their velocities, or, alternatively, by {\it changing} the phase of Rydberg polariton according to its storage time. After the Rydberg polariton is prepared with a Rydberg state $|r_1\rangle$, i.e., during the storage time, two laser fields induce a transition between $|r_1\rangle$ and a nearby Rydberg state $|r_2\rangle$ via a low-lying intermediate state $\lvert f\rangle$ which is largely detuned. In particular, we find that either a $2π\mathbb{N}$ protocol, a $π$-wait-$π$ protocol, or a wait-$π$ protocol, along with an appropriate choice of $\lvert f\rangle$ can lead to a phase-coherent Rydberg polariton upon its retrieval. Importantly, the coherent transition between $|r_1\rangle$ and $|r_2\rangle$ ensures that the Rydberg polariton can block the Rydberg excitation of nearby atoms as in usual applications of Rydberg polaritons. Numerics show that the theory can nearly completely eliminate the motional dephasing, leaving Rydberg-state decay as the only fundamental channel of decoherence. This sheds light on a broad application of Rydberg-mediated quantum nonlinear optics.

physics.atom-ph↗

arXiv2026

Barium-based Rydberg atom quantum technologies with long Rydberg coherence

The short Doppler-limited coherence time of the laser-excited Rydberg state, usually orders of magnitude shorter than the lifetime of the Rydberg state, hinders the Rydberg-mediated quantum technologies. Here, we show that a 649~nm~$-$~658~nm two-photon excitation of the $6sng~^1G_4$ Rydberg state from a long-lived d-orbital clock state of barium can be achieved with a two-photon wavevector that is tiny, which effectively removes the Doppler-limited decoherence. Moreover, the $6sng~^1G_4$ Rydberg state has strong dipole-dipole interaction due to small Förster defect with nearby Rydberg states and possesses long radiative lifetime. These can benefit quantum computing based on individually trapped neutral atoms, and can enable long-lived Rydberg polaritons in atomic media, which brings fresh opportunities in all-optical quantum information.

quant-ph↗

arXiv2026

Macroscopic position-position entanglement by photon recoil in Rydberg atoms

Entanglement between two spatially separate matter particles can be generated via many means and often resides in the internal states of particles. Here, via Rydberg blockade in two spatially separate neutral atoms, we find that the photon recoil in Rydberg excitation can push one atom microns away provided the other atom exerts a state-dependent Rydberg-mediated blockade. When the atoms are recaptured by optical traps, a position-position entangled state between two spatially separate atoms can emerge. This realizes a Bell state of two atoms, where the entanglement exists in the position of each atom and the distance between the two possible locations of each atom can be in the hundred-micron regime.

quant-ph↗

arXiv2026

Asymmetric quantum Rabi model, trap-dipole resonance, and quantum gates with optically trapped ultracold polar molecules

Optically trapped ultracold polar molecules can have multiple long-lived states for coding quantum information, and can exhibit electric dipole-dipole interactions~(DDI) which enables entanglement generation. The general understanding on the quantized motion~(QM) of molecules in the traps is that it causes fluctuation of DDI. Here, we find that the molecular QM can realize an asymmetric quantum Rabi model, which is of specific importance in the study of fundamental physics. The molecular QM can also lead to an exotic trap-dipole resonance, resulting in excess population loss to uncoupled motional states, and, hence, should be avoided in a general quantum control over polar molecules. To examine the impact of QM on quantum computing based on polar molecules, we introduce two gate protocols, a fast iSWAP gate which can be realized by a global microwave pulse of pulse area smaller than $2π$, and a controlled-phase gate with an arbitrary controlled phase, and find that both gates can attain a high fidelity.

quant-ph↗

arXiv2026

High-fidelity molecular quantum logic gates resilient to interaction fluctuation

Optically trapped polar molecules are promising for quantum information processing, yet the accuracy of an entangling molecular gate is limited by the uncertainty of dipole-dipole interactions~(DDI) from the molecular motion in traps. We show that two $π$ pulses of global microwave excitation can yield a high-fidelity controlled-phase gate when assisted by two single-qubit gates. The gate is resilient to the uncertainty of DDI because it does not rely on populating DDI-coupled states. Further, the controlled phase is fully tunable by varying the relative phase of the two global microwave pulses, and, hence, the gate can find applications in a wide range of quantum algorithms involving quantum Fourier transform. Moreover, we introduce a motional-mode separation technique to quantum mechanically study the influence of the molecular motion, which shows that the gate fidelity can be over 0.9999 with typical experimental conditions.

quant-ph↗

arXiv2025

Direction switchable single-photon emitter using a Rydberg polariton

All-optical redirection or routing of single photons is essential for quantum networks. Although studied in various systems both in theory and experiment, the redirection of single photons with many output ports, compatible with large-scale photonic circuits, still needs to be explored. Here, we demonstrate a direction switchable single-photon emitter using a Rydberg polariton. The Rydberg component of the stored photon is changed using a stimulated Raman transition with a specific intermediate state. By adjusting the direction of the retrieval laser, we can redirect the emitted photon into a rich variety of alternative modes. Building upon this scheme, we propose a quantum routing of single photons with \textit{N} output channels and unity routing efficiency. In addition, the protocol reduces the effect of motional dephasing increasing the photon lifetime to $>10~μ$s ($>20$ times photon processing time), enabling functional quantum devices based on Rydberg polaritons.

quant-ph↗

arXiv2023

Coherence-preserving cooling of nuclear spin qubits in a weak magnetic field

Nuclear spin memories of divalent neutral atoms can allow spin-preserving resolved-sideband cooling in a strong magnetic field [Phys. Rev. Lett. 99, 123001 (2007)]. We present a theory for cooling $^{87}$Sr nuclear-spin qubits in a weak magnetic field. The theory depends on laser excitation of $5s5p~^1P_1$ to a nearby state which results in $m_J$-dependent AC Stark shifts large compared to the hyperfine interaction. This effectively suppresses the nuclear-spin mixing due to the hyperfine interaction. Sideband cooling via the clock state quenched by the AC Stark-shifted $^1P_1$ state leads to nuclear-spin-preserving spontaneous emission back to the ground state. More than being compatible with low magnetic fields, the theory is applicable when the nuclear spin qubits are defined by the two lowest Zeeman substates.

quant-ph↗

arXiv2023

Fast nuclear-spin entangling gates compatible with large-scale atomic arrays

Nuclear-spin entangling gates with divalent atoms can be executed by one global laser pulse when $Δ_{\text{Z}}<Ω$, where $Δ_{\text{Z}}$ is the Zeeman-splitting-dominated frequency difference for the clock-Rydberg transitions of the two nuclear-spin qubit states and $Ω$ is the maximal Rabi frequency. Concerning the sensitivity of Rydberg-state energy to magnetic fluctuation, the gate is compatible with large-scale atomic arrays for weaker magnetic field is suitable for ensuring uniform field in a large qubit array. The gate can have a high fidelity because the relaxation and dephasing of Rydberg states, which limit the fidelity and grow with $1/Ω$, can be mitigated with easily attainable large $Ω$.

quant-ph↗

arXiv2022

Quantum logic and entanglement by neutral Rydberg atoms: methods and fidelity

Quantum gates and entanglement based on dipole-dipole interactions of neutral Rydberg atoms are relevant to both fundamental physics and quantum information science. The precision and robustness of the Rydberg-mediated entanglement protocols are the key factors limiting their applicability in experiments and near-future industry. There are various methods for generating entangling gates by exploring the Rydberg interactions of neutral atoms, each equipped with its own strengths and weaknesses. The basics and tricks in these protocols are reviewed, with specific attention paid to the achievable fidelity and the robustness to the technical issues and detrimental innate factors.

quant-ph↗

arXiv2022

Quantum gates with weak van der Waals interactions of neutral Rydberg atoms

Neutral atoms are promising for large-scale quantum computing, but accurate neutral-atom entanglement depends on large Rydberg interactions which strongly limit the interatomic distances. Via a phase accumulation in detuned Rabi cycles enabled by a Rydberg interaction of similar magnitude to the Rydberg Rabi frequency, we study a controlled-phase gate with an arbitrary phase and extend it to the controlled-NOT gate. The gates need only three steps for coupling one Rydberg state, depend on easily accessible van der Waals interaction that naturally arises between distant atoms, and have no rotation error in the weak interaction regime. Importantly, they can work with very weak interactions so that well-separated qubits can be entangled. The gates are sensitive to the irremovable fluctuation of Rydberg interactions, but can still have a fidelity over 98\% with realistic position fluctuation of qubits separated over 20~$μ$m.

quant-ph↗

arXiv2022

Hyperentanglement of divalent neutral atoms by Rydberg blockade

Hyperentanglement~(HE), the simultaneous entanglement between two particles in more than one degrees of freedom, is relevant to both fundamental physics and quantum technology. Previous study on HE has been focusing on photons. Here, we study HE in individual neutral atoms. In most alkaline-earth-like atoms with two valence electrons and a nonzero nuclear spin, there are two stable electronic states, the ground state and the long-lived clock state, which can define an electronic qubit. Meanwhile, their nuclear spin states can define a nuclear qubit. By the Rydberg blockade effect, we show that the controlled-Z~(C$_{\text{Z}}$) operation can be generated in the electronic qubits of two nearby atoms, and simultaneously in their nuclear qubits as well, leading to a C$_{\text{Z}}\otimes$C$_{\text{Z}}$ operation which is capable to induce HE. The possibility to induce HE in individual neutral atoms offers new opportunities to study quantum science and technology based on neutral atoms.

quant-ph↗

arXiv2022

Fast atom-photon entangling gates with a superconducting coplanar waveguide

Entanglement between atoms and microwave photons in a superconducting coplanar waveguide~(SCW) can enable hybrid quantum devices and interface static and flying qubits. We study a one-step controlled-Z~(C$_{\text{Z}}$) gate between a neutral atom trapped near a SCW and a microwave mode in the SCW, which is an extension of the gate proposed in [J. D. Pritchard, et.al., Phys. Rev. A 89, 010301(R) (2014)]. The gate protocol is simple and requires one laser pulse for exciting a transition between the ground and Rydberg states of the neutral atom.

quant-ph↗

arXiv2022

Fast nuclear-spin gates and electrons-nuclei entanglement of neutral atoms in weak magnetic fields

We present fast Rydberg-mediated entanglement involving nuclear spins of divalent atoms with $^{171}$Yb as an example. First, we show a nuclear-spin controlled phase gate of an arbitrary phase realizable either with two laser pulses when assisted by Stark shifts, or with three pulses. Second, we propose to create a state $(\lvert\text{cc}\rangle_{\text{e}} \otimes \lvertΦ\rangle_{\text{n}} + \lvertΦ\rangle_{\text{e}} \otimes \lvertΨ\rangle_{\text{n}} )/\sqrt{2}$ entangled between the electrons~(e) and nuclear spins~(n) of two atoms, where $\lvertΦ\rangle$ and $\lvertΨ\rangle$ are two orthogonal Bell states and $\lvert \text{c}\rangle_{\text{e}}$ denotes the clock state. For want of a better term, it is called a Super Bell State for it mimics a ``large'' Bell state incorporating three ``smaller'' Bell states. Third, we show a protocol to create a three-atom state $(\sqrt{3}\lvert\text{ccc}\rangle_{\text{e}} \otimes \lvertΛ\rangle_{\text{n}} + \lvert \text{W}\rangle_{\text{e}} \otimes \lvert \text{GHZ}\rangle_{\text{n}} )/2$, where $\lvertΛ\rangle_{\text{n}}$ is a nuclear-spin state, $\lvert \text{W}\rangle_{\text{e}}$ is a W state in the ground-clock state space, and $\lvert \text{GHZ}\rangle_{\text{n}}$ is the Greenberger-Horne-Zeilinger~(GHZ) state in the nuclear-spin state space. The four protocols possess high intrinsic fidelities, do not require single-site Rydberg addressing, and can be executed with large $Ω_{\text{m}}$ in a weak, Gauss-scale magnetic field for they involve Rydberg excitation of both nuclear-spin qubit states in each atom. The latter two protocols can enable measurement-based preparation of Bell, hyperentangled, and GHZ states.

quant-ph↗

arXiv2022

The fractional Chern insulator with Rydberg-dressed neutral atoms

Topological nontrivial bands can be realized via Rydberg-dressed neutral atoms. We propose a two-dimensional hard-core boson model with a topological ground enrgy at band on a honeycomb lattice, where the particle hopping is realized via van der Waals interaction that exchanges the Rydberg states of two interacting atoms, while nonzero phases associated with hopping is created by transferring the optical phase of laser fields to the atomic pair wave function. Using exactly diagonalization and infinite density matrix renormalization group simulation, we find in the system a fractional Chern insulator phase with a Chern number C = 1/2, which can persist in the presence of weak many-body interactions. Our studies indicate that fractional Chern insulators can be studied with neutral-atom arrays.

cond-mat.quant-gas↗

arXiv2022

Rydberg wire gates for universal quantum computation

Rydberg atom arrays offer flexible geometries of strongly-interacting neutral atoms, which are useful for many quantum applications such as quantum simulation and quantum computation. Here we consider a gate-based quantum computing scheme for a Rydberg-atom array. We utilize auxiliary atoms which are used as a quantum wire to mediate controllable interactions among data-qubit atoms. We construct universal quantum gates for the data atoms, by using single-atom addressing operations. Standard one-, two-, and multi-qubit solutions are explicitly obtained as respective sequences of pulsed operations acting on individual data and wire atoms. A detailed resource estimate is provided for an experimental implementation of this scheme in a Rydberg quantum simulator.

quant-ph↗