arXiv subjects

Guo-Zhu Song

Publications and source records attributed to Guo-Zhu Song.

10 recordsLinked to original sources

arXiv2026

Entanglement concentration of high-dimensional unknown partially entangled state

High-dimensional quantum systems offer a number of advantages in larger information capacity, stronger noise resiliency, higher improved efficiency and accuracy over the qubit systems. In quantum communication the maximally entangled states will inevitably become mixed states or less-entangled pure states by the channel noise during the practical transmission or storage. We propose a universal scheme to concentrate nonlocal high-dimensional generalized Bell states with unknown parameters. After the cross-Kerr nonlinearities, $X$-quadrature homodyne measurements, and single-partite projection measurements are performed only at Bob's site, a two-qutrit maximally entangled Bell state can be distilled, while previous entanglement concentration protocols (ECPs) mostly focused on two-level qubit systems. The concentrated partially entangled qubit states, reserved as the by-product are the fascinating resources for some quantum information processing tasks. Moreover, single-qutrit projection measurement, the key ingredient for our ECP with unknown parameters, are completed by using linear optical elements. Additionally, linear optical high-dimensional ECP with known parameters are also designed.

quant-ph↗

arXiv2026

The simplified quantum circuits for implementing quantum teleportation

It is crucial to design quantum circuits as small as possible and as shallow as possible for quantum information processing tasks. We design quantum circuits with simplified gate-count, cost, and depth for implementing quantum teleportation among various entangled channels. Here the gate-count/cost/depth of the Greenberger-Horne-Zeilinger-based quantum teleportation is reduced from 10/6/8 to 9/4/6, the two-qubit-cluster-based quantum teleportation is reduced from 9/4/5 to 6/3/5, the three-qubit-cluster-based quantum teleportation is reduced from 12/6/7 to 8/4/5, the Brown-based quantum teleportation is reduced from 25/15/17 to 18/8/7, the Borras-based quantum teleportation is reduced from 36/25/20 to 15/8/11, and the entanglement-swapping-based quantum teleportation is reduced from 13/8/8 to 10/5/5. Note that, no feed-forward recover operation is required in the simplified schemes. Moreover, the experimentally demonstrations on IBM quantum computer indicate that our simplified and compressed schemes can be realized with good fidelity.

quant-ph↗

arXiv2026

Improved entanglement-based high-dimensional optical quantum computation with linear optics

Quantum gates are the essential block for quantum computer. High-dimensional quantum gates exhibit remarkable advantages over their two-dimensional counterparts for some quantum information processing tasks. Here we present a family of entanglement-based optical controlled-SWAP gates on $\mathbb{C}^{2}\otimes \mathbb{C}^{d}\otimes \mathbb{C}^{d}$. With the hybrid encoding, we encode the control qubits and target qudits in photonic polarization and spatial degrees of freedom, respectively. The circuit is constructed using only $(2+3d)$ ($d\geq 2$) linear optics, beating an earlier result of 14 linear optics with $d=2$. The circuit depth 5 is much lower than an earlier result of 11 with $d=2$. Besides, the fidelity of the presented circuit can reach 99.4\%, and it is higher than the previous counterpart with $d=2$. Our scheme are constructed in a deterministic way without any borrowed ancillary photons or measurement-induced nonlinearities. Moreover, our approach allows $d>2$.

quant-ph↗

arXiv2025

Tunable photon scattering by an atom dimer coupled to a band edge of a photonic crystal waveguide

Quantum emitters trapped near photonic crystal waveguides have recently emerged as an exciting platform for realizing novel quantum matter-light interfaces. Here we study tunable photon scattering in a photonic crystal waveguide coupled to an atom dimer with an arbitrary spatial separation. In the weak-excitation regime, we give the energy levels and their decay rates into the waveguide modes in the dressed basis, which both depend on the distance between the two atoms. We focus on the Bragg case and anti-Bragg case, where subradiant and superradiant states are produced and perfect transmission with a $π$ phase shift may occur on resonance. We observe quantum beats in the photon-photon correlation function of the reflected field in the anti-Bragg case. Moreover, the frequencies of quantum beats can be controlled due to the tunability of the bound states via the dispersion engineering of the structure. We also observe directional photon emission in the anti-Bragg case and give the dynamic mechanism of the perfect transmission. We quantify the effects of the system imperfections, including the deviation in the distance between the two atoms and the asymmetry in the atomic decay rates into the waveguide modes. With recent experimental advances in the superconducting microwave transmission lines, our results should soon be realizable.

quant-ph↗

arXiv2022

Synthesis and upper bound of Schmidt rank of the bipartite controlled-unitary gates

Quantum circuit model is the most popular paradigm for implementing complex quantum computation. Based on Cartan decomposition, we show that $2(N-1)$ generalized controlled-$X$ (GCX) gates, $6$ single-qubit rotations about the $y$- and $z$-axes, and $N+5$ single-partite $y$- and $z$-rotation-types which are defined in this paper are sufficient to simulate a controlled-unitary gate $\mathcal{U}_{cu(2\otimes N)}$ with $A$ controlling on $\mathbb{C}^2\otimes \mathbb{C}^N$. In the scenario of the unitary gate $\mathcal{U}_{cd(M\otimes N)}$ with $M\geq3$ that is locally equivalent to a diagonal unitary on $\mathbb{C}^M\otimes \mathbb{C}^N$, $2M(N-1)$ GCX gates and $2M(N-1)+10$ single-partite $y$- and $z$-rotation-types are required to simulate it. The quantum circuit for implementing $\mathcal{U}_{cu(2\otimes N)}$ and $\mathcal{U}_{cd(M\otimes N)}$ are presented. Furthermore, we find $\mathcal{U}_{cu(2\otimes2)}$ with $A$ controlling has Schmidt rank two, and in other cases the diagonalized form of the target unitaries can be expanded in terms of specific simple types of product unitary operators.

quant-ph↗

arXiv2021

Optical properties of a waveguide-mediated chain of randomly positioned atoms

We theoretically study the optical properties of an ensemble of two-level atoms coupled to a one-dimensional waveguide. In our model, the atoms are randomly located in the lattice sites along the one-dimensional waveguide. The results reveal that the optical transport properties of the atomic ensemble are influenced by the lattice constant and the filling factor of the lattice sites. We also focus on the atomic mirror configuration and quantify the effect of the inhomogeneous broadening in atomic resonant transition on the scattering spectrum. Furthermore, we find that initial bunching and persistent quantum beats appear in photon-photon correlation function of the transmitted field, which are significantly changed by filling factor of the lattice sites. With great progress to interface quantum emitters with nanophotonics, our results should be experimentally realizable in the near future.

quant-ph↗

arXiv2019

Microwave transmission through an artificial atomic chain coupled to a superconducting photonic crystal

Emitters strongly coupled to a photonic crystal provide a powerful platform for realizing novel quantum light-matter interactions. Here we study the optical properties of a three-level artificial atomic chain coupled to a one-dimensional superconducting microwave photonic crystal. A sharp minimum-energy dip appears in the transmission spectrum of a weak input field, which reveals rich behavior of the long-range interactions arising from localized bound states. We find that the dip frequency scales linearly with both the number of the artificial atoms and the characteristic strength of the long-range interactions when the localization length of the bound state is sufficiently large. Motivated by this observation, we present a simple model to calculate the dip frequency with system parameters, which agrees well with the results from exact numerics for large localization lengths. We observe oscillation between bunching and antibunching in photon-photon correlation function of the output field. Furthermore, we find that the model remains valid even though the coupling strengths between the photonic crystal and artificial atoms are not exactly equal and the phases of external driving fields for the artificial atoms are different. Thus, we may infer valuable system parameters from the dip location in the transmission spectrum, which provides an important measuring tool for the superconducting microwave photonic crystal systems in experiment. With remarkable advances to couple artificial atoms with microwave photonic crystals, our proposal may be experimentally realized in currently available superconducting circuits.

quant-ph↗

arXiv2018

Photon transport mediated by an atomic chain trapped along a photonic crystal waveguide

We theoretically investigate the transport properties of a weak coherent input field scattered by an ensemble of $Λ$-type atoms coupled to a one-dimensional photonic crystal waveguide. In our model, the atoms are randomly located in the lattice along the crystal axis. We analyze the transmission spectrum mediated by the tunable long-range atomic interactions, and observe the highest-energy dip. The results show that the highest-energy dip location is associated with the number of the atoms, which provides an accurate measuring tool for the emitter-waveguide system. We also quantify the influence of a Gaussian inhomogeneous broadening and the dephasing on the transmission spectrum, concluding that the highest-energy dip is immune to both the inhomogeneous broadening and the dephasing. Furthermore, we study photon-photon correlations of the reflected field and observe quantum beats. With tremendous progress in coupling atoms to photonic crystal waveguides, our results may be experimentally realizable in the near future.

quant-ph↗

arXiv2017

Heralded quantum repeater based on the scattering of photons off single emitters in weak-coupling regime

We propose a heralded quantum repeater based on the scattering of photons off single emitters in one-dimensional waveguides. We show the details by implementing nonlocal entanglement generation, entanglement swapping, and entanglement purification modules with atoms in waveguides, and discuss the feasibility of the repeater with currently achievable technology. In our scheme, the faulty events can be discarded by detecting the polarization of the photons. That is, our protocols are accomplished with fidelity of 100% in principle, which is advantageous for implementing realistic long-distance quantum communication. Moreover, additional atomic qubits are not required, but only a single-photon medium. Our scheme is scalable and attractive since it can be realized in solid-state quantum systems. With the great progress on controlling atom-waveguide systems, the repeater may be very useful in quantum information processing in the future.

quant-ph↗

arXiv2017

Photon scattering by an atomic ensemble coupled to a one-dimensional nanophotonic waveguide

We theoretically investigate the quantum scattering of a single-photon pulse interacting with an ensemble of $Λ$-type three-level atoms coupled to a one-dimensional waveguide. With an effective non-Hermitian Hamiltonian, we study the collective interaction between the atoms mediated by the waveguide mode. In our scheme, the atoms are randomly placed in the lattice along the axis of the one-dimensional waveguide, which closely corresponds to the practical condition that the atomic positions can not be controlled precisely in experiment. Many interesting optical properties occur in our waveguide-atom system, such as electromagnetically induced transparency (EIT) and optical depth. Moreover, we observe that strong photon-photon correlation with quantum beats can be generated in the off-resonant case, which provides an effective candidate for producing non-classical light in experiment. With remarkable progress in waveguide-emitter system, our scheme may be feasible in the near future.

quant-ph↗