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A scheme is proposed for achieving robust population inversion in five‐level systems by means of composite pulses. An example of such a system consists of the magnetic sublevels with angular momenta Jg=2$J_g=2$ and Je=2$J_e=2$. Through elaborately constructing the relative phases of pulse pairs, the composite sequences perform well in suppressing the uncorrelated pulse area errors. In particular, the five pulse‐pair sequence possesses good robustness and a short evolution time. The composite sequences are further designed to compensate for a single type of pulse area errors to any desired order. This work provides a high‐efficiency way for robust quantum state manipulation in five‐level systems. [ABSTRACT FROM AUTHOR]

An extended Su–Schrieffer–Heeger (SSH) model consisting of an SSH chain with an embedded quantum ring (QR) is investigated. In the case of two SSH chains with a symmetric distribution with respect to QR, by calculating the real‐space winding number and energy spectrum, the hopping amplitude‐induced topological phase is discovered. The probability distributions of gap states and the relationship between energy and magnetic flux prove the existence of the Aharonov–Bohm effect in the QR. Moreover, in the case of asymmetric distribution, the model possesses a zero‐energy mode within the energy gap, and it is found that it can realize quantum state transfer. By adjusting the connection site between the right SSH chain and QR, the direction of the output port can be flexibly engineered. Furthermore, it is shown that high‐fidelity quantum state transfer can still be achieved with the increasing of the system size. The tunable quantum state transfer based on the zero‐energy mode can be equivalent to a topological tunable directional switch with properties of non‐directional transfer. This work provides an approach for studying topological phase transition and tunable topological devices in an SSH chain with an embedded QR. [ABSTRACT FROM AUTHOR]

This paper discusses continuous-time quantum walks and asymptotic state transfer in graphs with an involution. By providing quantitative bounds on the components of the eigenvectors of the Hamiltonian, it provides an approach to achieving high-fidelity state transfer by strategically selecting energy potentials based on the maximum degrees of the graphs. The study also involves an analysis of the time necessary for quantum transfer to occur. [ABSTRACT FROM AUTHOR]

Spin qubits in semiconductor quantum dots are an attractive candidate for scalable quantum information processing. Reliable quantum state transfer and entanglement between spatially separated spin qubits is a highly desirable but challenging goal. Here, we propose a fast and high-fidelity quantum state transfer scheme for two spin qubits mediated by virtual microwave photons. Our general strategy involves using a superadiabatic pulse to eliminate non-adiabatic transitions, without the need for increased control complexity. We show that arbitrary quantum state transfer can be achieved with a fidelity of 95.1% within a 60 ns short time under realistic parameter conditions. We also demonstrate the robustness of this scheme to experimental imperfections and environmental noises. Furthermore, this scheme can be directly applied to the generation of a remote Bell entangled state with a fidelity as high as 97.6%. These results pave the way for fault-tolerant quantum computation on spin quantum network architecture platforms. [ABSTRACT FROM AUTHOR]

We analytically investigate the one-excitation spin dynamics in a homogeneous closed spin-1/2 chain via diagonalization of the one-excitation block of the XX-Hamiltonian, which allows to derive the analytical expressions for probability amplitudes describing state transfers between any two spins of a chain. We analytically investigate the M-neighbor approximation ( M ≥ 1 ) of spin dynamics with arbitrary initial state and analyze its accuracy using special integral characteristics defined in terms of the above probability amplitudes. We find M providing the required accuracy of evolution approximation for chains of different lengths. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the transfer of an unknown single-qubit state from Alice to Bob via a two-step discrete-time quantum random walk on a cycle with four vertices on a four-qubit nuclear magnetic resonance quantum processor. The qubits with Alice and Bob are used as coin qubits and the walk is carried out on in a two-qubit 'Gaming Arena'. In this scheme, the required entangled state is generated naturally via conditional shift operators during the quantum walk, instead of being prepared in advance. We implement controlled operators at Bob's end, which are controlled by Alice's coin qubit and arena qubits, in order to reconstruct Alice's randomly generated state at Bob's end. To characterize the state transfer process, we perform quantum process tomography by repeating the experiment for a set of input states { | 0 ⟩ , | 1 ⟩ , | + ⟩ , | - ⟩ } . Using an entanglement witness, we certify that the quantum walk generates a genuine quadripartite entangled state of all four qubits. To evaluate the efficacy of the transfer scheme, we use quantum state tomography to reconstruct the transferred state by calculating the projection of the experimentally reconstructed four-qubit density matrix onto three-qubit basis states. Our results demonstrate that the quantum circuit is able to perform quantum state transfer via the two-step quantum random walk with high fidelity. [ABSTRACT FROM AUTHOR]

We present a feasible scheme to implement a planar and tunable quantum state transfer (QST) via topologically protected zero-energy mode in a splicing Y-junction Su–Schrieffer–Heeger (SSH) chain. The introduction of the elaborate nearest-neighbor (NN) hopping enables one to generate a topological interface at the central site of the Y-junction. By modulating the NN hopping adiabatically in the chain, the quantum state initially prepared at the central site can be simultaneously transferred to the three endpoints of the Y-junction with the equal/unequal probabilities. The planar distribution of QST is expected to realize a quantum router, whose function is to make the quantum information on the central site (input port) appear equally/unequally at the three endpoints (output ports) with different directions. Moreover, the numerical simulations demonstrate that the scheme possesses the robustness on the fluctuations of the NN hopping and the on-site potential in the system. Furthermore, we show that the number of the output ports with different directions can be flexibly increased in an extended X-junction SSH chain, and the experimental feasibility for implementing special QST in a superconducting qubit-resonator system is briefly discussed. Our work extends the space distribution of QST from linear distribution to planar distribution and promotes the construction of large-scale quantum networks. [ABSTRACT FROM AUTHOR]

Twin vertices in simple unweighted graphs are vertices that have the same neighbours, and, in the case of weighted graphs with possible loops, the corresponding incident edges have equal weights. In this paper, we explore the role of twin vertices in quantum state transfer. In particular, we provide characterizations of periodicity, perfect state transfer, and pretty good state transfer between twin vertices in a weighted graph with respect to its adjacency, Laplacian and signless Laplacian matrices. As an application, we provide characterizations of all simple unweighted double cones on regular graphs that exhibit periodicity, perfect state transfer, and pretty good state transfer. [ABSTRACT FROM AUTHOR]

The Su–Schrieffer–Heeger (SSH) model, commonly used for robust state transfers through topologically protected edge pumping, has been generalized and exploited to engineer diverse functional quantum devices. Here, we propose to realize a fast topological beam splitter based on a generalized SSH model by accelerating the quantum state transfer (QST) process essentially limited by adiabatic requirements. The scheme involves delicate orchestration of the instantaneous energy spectrum through exponential modulation of nearest neighbor coupling strengths and onsite energies, yielding a significantly accelerated beam splitting process. Due to properties of topological pumping and accelerated QST, the beam splitter exhibits strong robustness against parameter disorders and losses of system. In addition, the model demonstrates good scalability and can be extended to two-dimensional crossed-chain structures to realize a topological router with variable numbers of output ports. Our work provides practical prospects for fast and robust topological QST in feasible quantum devices in large-scale quantum information processing. [ABSTRACT FROM AUTHOR]

Spin qubits in semiconductor quantum dots are an attractive candidate for scalable quantum information processing. Reliable quantum state transfer and entanglement between spatially separated spin qubits is a highly desirable but challenging goal. Here, we propose a fast and high-fidelity quantum state transfer scheme for two spin qubits mediated by virtual microwave photons. Our general strategy involves using a superadiabatic pulse to eliminate non-adiabatic transitions, without the need for increased control complexity. We show that arbitrary quantum state transfer can be achieved with a fidelity of 95.1% within a 60 ns short time under realistic parameter conditions. We also demonstrate the robustness of this scheme to experimental imperfections and environmental noises. Furthermore, this scheme can be directly applied to the generation of a remote Bell entangled state with a fidelity as high as 97.6%. These results pave the way for fault-tolerant quantum computation on spin quantum network architecture platforms.

The geometrical significance of winding number is used to study the topological characteristics of a single-excitation Heisenberg XY spin model represented by the Su–Schrieffer–Heeger (SSH) model. Due to the presence of topological edge states, the quantum state may be conveyed from the first site to the last site with a high degree of fidelity. Also examined is a Heisenberg XY model on the kagome lattice. In the topological phase of the kagome lattice, the three corner states are not affected by the Dzyaloshinskii–Moriya (DM) interactions. For the kagome lattice with and without the DM interactions, the state of a spin on one corner site can be transferred to the other two corner sites in various ways during the topological phase. The quantum state transfer (QST) for the XY model on the kagome lattice without the DM interactions is symmetrical and can only produce a three-body entangled state. In contrast, the QST for the XY model on the kagome lattice with the DM interactions is in an asymmetrical form, similar to one-way propagation, and it can produce both product states and three-body entangled states.

We investigate a quantum-state transfer protocol through a quasi-1D spin-1/2 channel defined on a hexagonal layer featuring hopping disorder. In the absence of disorder, the quantum channel displays two flat bands hosting compact localized states. That is signalled by the appearance of singularities in the density of states. We show that an arbitrary qubit state can be transmitted with high fidelity between the two ends even in the presence of noise as long as they are not in resonance with energy levels surrounding the flat band. This higher sensitivity against disorder is due to the residual single-cell compact localized states accompanied by a degeneracy lifting. [ABSTRACT FROM AUTHOR]

Quantum networks consist of quantum nodes that are linked by entanglement, and quantum information can be transferred from one node to another. Operations can be applied to qubits of local nodes coordinated by classical communication to manipulate quantum states, and readout/measurement will be employed to obtain results. Here, we use quantum circuits to simulate quantum state transfer between two nodes connected in a linear geometry through other nodes. We explore the interplay between gate and readout errors on the performance of state transfer. We find that the nominal success probability is not necessarily a monotonic function of the two error rates and employ numerical simulations and analytic tools to understand their interplay. [ABSTRACT FROM AUTHOR]

Fenna-Mathews-Olson complexes participate in the photosynthetic process of Sulfur Green Bacteria. These biological subsystems exhibit quantum features which possibly are responsible for their high efficiency; the latter may comprise multipartite entanglement and the apparent tunnelling of the initial quantum state. At first, to study these aspects, a multidisciplinary approach including experimental biology, spectroscopy, physics, and math modelling is required. Then, a global computer modelling analysis is achieved in the computational biology domain. The current work implements the Hierarchical Equations of Motion to numerically solve the open quantum system problem regarding this complex. The time-evolved states obtained with this method are then analysed under several measures of entanglement, some of them already proposed in the literature. However, for the first time, the maximum overlap with respect to the closest separable state is employed. This authentic multipartite entanglement measure provides information on the correlations, not only based on the system bipartitions as in the usual analysis. Our study has led us to note a different view of FMO multipartite entanglement as tiny contributions to the global entanglement suggested by other more basic measurements. Additionally, in another related trend, the initial state, considered as a Förster Resonance Energy Transfer, is tracked using a novel approach, considering how it could be followed under the fidelity measure on all possible permutations of the FMO subsystems through its dynamical evolution by observing the tunnelling in the most probable locations. Both analyses demanded significant computational work, making for a clear example of the complexity required in computational biology. [ABSTRACT FROM AUTHOR]

High-fidelity quantum state transfer is critical for quantum communication and scalable quantum computation. Current quantum state transfer algorithms on the complete bipartite graph, which are based on discrete-time quantum walk search algorithms, suffer from low fidelity in some cases. To solve this problem, in this paper we propose a two-stage quantum state transfer algorithm on the complete bipartite graph. The algorithm is achieved by the generalized Grover walk with one marked vertex. The generalized Grover walk's coin operators and the query oracles are both parametric unitary matrices, which are designed flexibly based on the positions of the sender and receiver and the size of the complete bipartite graph. We prove that the fidelity of the algorithm is greater than 1 - 2 ϵ 1 - ϵ 2 - 2 2 ϵ 1 ϵ 2 or 1 - 2 + 2 2 ϵ 1 - ϵ 2 - 2 + 2 2 ϵ 1 ϵ 2 for any adjustable parameters ϵ 1 and ϵ 2 when the sender and receiver are in the same partition or different partitions of the complete bipartite graph. The algorithm provides a novel approach to achieve high-fidelity quantum state transfer on the complete bipartite graph in any case, which will offer potential applications for quantum information processing. [ABSTRACT FROM AUTHOR]

Quantum state transfer (QST) plays a central role in the field of quantum computation and communication, but its quality will be deteriorated by the ubiquitous variations and noise in quantum systems. Here we propose robust and nonadiabatic protocols for transmitting quantum state across a strongly coupled spin chain, especially in the presence of unwanted disorders in the couplings. To this end, we approximately map the low-energy subspaces of the odd-size Heisenberg chain to a two-level system, and derive the sensitivity of the final fidelity with respect to systematic deviations or time-varying fluctuations. Subsequently, leveraging the flexibility of the inverse-engineering technique, we optimize the state-transfer robustness concerning these perturbations individually. The resulting schemes allow for more stable QST than the original accelerated schemes and only require manipulating the two boundary couplings instead of the whole system, which open up the possibility of fast and robust information transfer in spin-based quantum systems.

We consider the dynamics of an ensemble of cold atoms coupled to light through radiation pressure in an optical lattice. We show that the steady-state displacement of the atomic ensemble shows bistable behavior which can be tuned by the position of the cold atoms in the Brillouin zone. Further, we describe a scheme to transfer the quantum state of the light field to the atomic field of two ensembles of cold atoms inside two independent optical cavities. This method leads to quantum entanglement which can be used in implementing quantum communication and computing. An Einstein-Podolsky-Rosen (EPR) state may be achieved with a judicious choice of parameters for our system. [ABSTRACT FROM AUTHOR]

Given two graphs G 1 of order n 1 and G 2 , the neighborhood corona of G 1 and G 2 , denoted by G 1 ★ G 2 , is the graph obtained by taking one copy of G 1 and taking n 1 copies of G 2 , in the meanwhile, linking all the neighbors of the i-th vertex of G 1 with all vertices of the i-th copy of G 2 . In our work, we give some conditions that G 1 ★ G 2 is not periodic. Furthermore, we demonstrate some sufficient conditions for G 1 ★ G 2 having no perfect state transfer. Some examples are provided to explain our results. In addition, for the reason that the graph admitting perfect state transfer is rare, we also consider pretty good state transfer on neighborhood corona of two graphs. We show some sufficient conditions for G 1 ★ G 2 admitting pretty good state transfer. [ABSTRACT FROM AUTHOR]

We put forward a proposal to construct a quantum network using a hybrid cavityâ€"magnonics system as with two nodes. At each node, a cascade of the quantum system consists of cavityâ€"magnonics and magnonicâ€"qubit interactions, and the quantum interface between the flying qubit and superconducting qubit is mediated by a magnon. Considering the phase resulting from the distance between the two nodes, we derive a master equation for two superconducting qubits and show that, by adiabatically controlling the cavityâ€"magnon coupling, perfect quantum state transfer between two qubits can be realized. We also consider the influence of intrinsic dissipation of the magnetic mode and the cavity mode. In an unideal case, the design time-dependent cavityâ€"magnonics couplings obtained in the ideal case are still employed. Our results show that low intrinsic loss in the magnetic mode and the cavity mode is still welcome for the high fidelity of state transfer. [ABSTRACT FROM AUTHOR]

Shortcuts to adiabaticity (STA) have been successfully applied both theoretically and experimentally to a wide variety of quantum control tasks. In previous work the authors have developed an analytic extension to STA, called enhanced shortcuts to adiabaticity (eSTA), that extends STA methods to systems where STA cannot be applied directly (2020 Phys. Rev. Res. 2 023360). Here we generalize this approach and construct an alternative eSTA method that takes advantage of higher order terms. We apply this eSTA method to the expansion of both a Gaussian trap and accordion lattice potential, demonstrating the improved fidelity and robustness of eSTA. [ABSTRACT FROM AUTHOR]

Fenna-Mathews-Olson complexes participate in the photosynthetic process of Sulfur Green Bacteria. These biological subsystems exhibit quantum features which possibly are responsible for their high efficiency; the latter may comprise multipartite entanglement and the apparent tunnelling of the initial quantum state. At first, to study these aspects, a multidisciplinary approach including experimental biology, spectroscopy, physics, and math modelling is required. Then, a global computer modelling analysis is achieved in the computational biology domain. The current work implements the Hierarchical Equations of Motion to numerically solve the open quantum system problem regarding this complex. The time-evolved states obtained with this method are then analysed under several measures of entanglement, some of them already proposed in the literature. However, for the first time, the maximum overlap with respect to the closest separable state is employed. This authentic multipartite entanglement measure provides information on the correlations, not only based on the system bipartitions as in the usual analysis. Our study has led us to note a different view of FMO multipartite entanglement as tiny contributions to the global entanglement suggested by other more basic measurements. Additionally, in another related trend, the initial state, considered as a Förster Resonance Energy Transfer, is tracked using a novel approach, considering how it could be followed under the fidelity measure on all possible permutations of the FMO subsystems through its dynamical evolution by observing the tunnelling in the most probable locations. Both analyses demanded significant computational work, making for a clear example of the complexity required in computational biology.

For the past few years, shortcuts to adiabaticity have attracted great attention since they can avoid the necessity of slow driving for adiabatic method while inheriting the partial robustness of the adiabatic dynamics. In this paper, a universal approach to implement quantum state transfer in a three‐state system via optimal transitionless quantum driving is presented. The fast path is optimized for great robustness against uncontrolled systematic error by selecting appropriate experimental parameters. The robustness of the protocol is discussed via numerical simulations. Compared with conventional transitionless quantum driving and adiabatic passage, the protocol via optimized transitionless quantum driving possesses more robustness against the systematic error. Furthermore, decoherence arising from atomic spontaneous emission and dephasing only has a slight impact on the fidelity of quantum state transfer. This work opens up a new avenue for high‐fidelity quantum state transfer in the case of the uncontrolled errors. [ABSTRACT FROM AUTHOR]

We consider the fidelity of state transfer on an unweighted path on n ≥ 3 vertices, where a loop of weight w has been appended at each of the end vertices. It is known that if w is transcendental, then there is pretty good state transfer from one end vertex to the other; we prove a companion result to that fact, namely that there is a dense subset of [ 1 , ∞) such that if w is in that subset, pretty good state transfer between end vertices is impossible. Under mild hypotheses on w and t, we derive upper and lower bounds on the fidelity of state transfer between end vertices at readout time t. Using those bounds, we localise the readout times for which that fidelity is close to 1. We also provide expressions for, and bounds on, the sensitivity of the fidelity of state transfer between end vertices, where the sensitivity is with respect to either the readout time or the weight w. Throughout, the results rely on detailed knowledge of the eigenvalues and eigenvectors of the associated adjacency matrix. [ABSTRACT FROM AUTHOR]

We investigate the transfer of arbitrary single qubit states through a rectangular 2D lattice hosting the isotropic Heisenberg model under a magnetic field along the z direction, where the spin-spin interaction strengths along the vertical sublattices, referred to as rungs, are much stronger than the interactions along other sublattices. Using the rung-rung interaction as perturbation, the model can be mapped to an effective 1D model, and the transfer of low-energy rung states from one rung to another can be represented by a transfer of an arbitrary single-qubit state from one lattice site to another on the 1D effective lattice defined by the effective 1D Hamiltonian. Exploiting this, we propose protocols for transferring arbitrary single-qubit states from one lattice site to another by using specific encoding (decoding) of the single-qubit state into (from) the rung states. We demonstrate that this protocol leads to a quantum advantage in the single-qubit state transfer. • Single-qubit state transfer via isotropic Heisenberg model on 2D zig-zag rectangular lattice in strong rung-coupling limit. • Low-energy effective Hamiltonian approach to quantum state transfer. • Utilizing time-evolutions generated by 1D Hamiltonians for state transfer through a 2D lattice. • Specific state encoding and decoding for systems with 2 and 4 qubits on the vertical sublattices. • Quantum advantage in single-qubit state transfer. [ABSTRACT FROM AUTHOR]

Perfect State Transfer is known to be possible on complex next-to-nearest-neighbor structures of qubits. The complexity of these networks coupled with the pervasive imperfections of the real world can lead to shortcomings in their practical construction. We analyze several realistic categories of flaws in the networks and show that they can have a harsh effect on the network’s performance. Dynamical decoupling is a known quantum error correction algorithm that can be used for partial or complete mitigation of undesired artefacts in quantum systems. We show here that dynamical decoupling can, in principle, also be used to stabilize the complex next-to-nearest-neighbor networks of qubits. We compare the proposed dynamical decoupling schemes with a baseline of dynamical decoupling scheme that eliminates all but one path to quantum state transfer on the network, thus effectively transforming it to a nearest-neighbor chain. [ABSTRACT FROM AUTHOR]

We present a feasible scheme to implement a planar and tunable quantum state transfer (QST) via topologically protected zero-energy mode in a splicing Y-junction Su–Schrieffer–Heeger (SSH) chain. The introduction of the elaborate nearest-neighbor (NN) hopping enables one to generate a topological interface at the central site of the Y-junction. By modulating the NN hopping adiabatically in the chain, the quantum state initially prepared at the central site can be simultaneously transferred to the three endpoints of the Y-junction with the equal/unequal probabilities. The planar distribution of QST is expected to realize a quantum router, whose function is to make the quantum information on the central site (input port) appear equally/unequally at the three endpoints (output ports) with different directions. Moreover, the numerical simulations demonstrate that the scheme possesses the robustness on the fluctuations of the NN hopping and the on-site potential in the system. Furthermore, we show that the number of the output ports with different directions can be flexibly increased in an extended X-junction SSH chain, and the experimental feasibility for implementing special QST in a superconducting qubit-resonator system is briefly discussed. Our work extends the space distribution of QST from linear distribution to planar distribution and promotes the construction of large-scale quantum networks.

Continuous U(1) gauge symmetry, which guarantees the conservation of total excitations in linear bosonic systems, will be broken when it comes to the strong-coupling regime where the rotation wave approximation (RWA) fails. Here we develop analytic solutions for multi-mode bosonic systems with XX-type couplings beyond RWA, and propose a novel scheme to implement high-fidelity quantum state transfer (QST) and entanglement preparation (EP) with high speed. The scheme can be realized with designated coupling strength and pulse duration with which the excitation number keeps unchanged regardless of the breakdown of the global U(1) symmetry. In QST tasks, we consider several typical quantum states and demonstrate that this method is robust against thermal noise and imperfections of experimental sequence. In EP tasks, the scheme is successfully implemented for the preparation of Bell states and W-type states, within a shortest preparation time.

The topological properties of a circuit‐QED lattice are studied and it is found that the system undergoes a topological phase transition accompanied by the changes of Chern numbers from 2 to −1. For different lattice sizes, the energy eigenvalue spectra and the probability distributions of edge states are shown. Moreover, it is revealed that, depending on the strictly adiabatic evolution of the time‐dependent Hamiltonian, several different topological quantum state transfers between the first two sites and the last two sites can be achieved by employing the topologically protected edge states as the transfer channels. Further, it is demonstrated that, by using resonator‐based input–output process, the photonic topological edge state can be detected by measuring the average photon number in the steady state. The scheme opens up a new prospect for investigating the potential application of topological matter in quantum information processing. [ABSTRACT FROM AUTHOR]

We simulate the evolution of three-node spin chain on the quantum processor of IBM Quantum Experience using the diagonalization of XX-Hamiltonian and representing the evolution operator in terms of CNOT operations and one-qubit rotations. We study the single excitation transfer from the first to the third node and show the significant difference between calculated and theoretical values of state transfer probability. Then, we propose a method reducing this difference by applying the two-parameter transformation including the shift and scale of the calculated probabilities. We demonstrate the universality of this transformation inside of the class of three-node evolutionary systems governed by the XX-Hamiltonian. [ABSTRACT FROM AUTHOR]

Cavity optomechanical systems converting quantum state between photons facilitate the development of scalable quantum information processors. The control of state transfer in such systems require producing fast and robust transfer to the target state with high fidelity. Shortcuts to adiabaticity (STA) has recently been proved a powerful method for performing fast quantum state conversion in optomechanical systems, which is, however, not robust enough against deviations in control parameters. Here a scheme to realize robust photon state transfer in a universal cavity optomechanical system by combining STA with optimal control technique (OCT) is proposed. Minimizing systematic error sensitivities with adjustable optimization parameters allows to control simultaneously the transfer speed, fidelity, and robustness against errors. Numerical results show that the optimized scheme is insensitive to deviations in optomechanical coupling and frequency detuning over a broad range. The effects of dissipation on the transfer fidelity are also examined for practical implementation of the scheme in realistic scenarios. [ABSTRACT FROM AUTHOR]

A theoretical investigation has been made of the magnetoplasmon excitations in a quasi-one-dimensional electron system composed of vertically stacked, self-assembled InAs/GaAs quantum dots. The smaller length scales involved in the experiments impel us to consider a perfectly periodic system of two-dimensionally confined InAs quantum dot layers separated by GaAs spacers. Subsequent system is subjected to a two-dimensional confining (harmonic) potential in the x – y plane and an applied magnetic field (B) in the symmetric gauge. This scheme defines virtually a system of quantum wire comprised of vertically stacked quantum dots (VSQD). We derive and discuss the Dyson equation, the generalized (nonlocal and dynamic) dielectric function, and the inverse dielectric function for investigating the single-particle and collective (magnetoplasmon) excitations within the framework of (full) random-phase approximation (RPA). As an application, we study the influence of the confinement potential and the magnetic field on the component eigenfunctions, the density of states (DOS), the Fermi energy, the collective excitations, and the inverse dielectric functions. How the B-dependence of DOS validate the VSQD mimicking the realistic quantum wires, the Fermi energy oscillates as a function of the Bloch vector, the intersubband single-particle continuum bifurcates at the origin, a collective excitation emerges and propagates within the gap of the split single-particle continuum, and the alteration in the well- and barrier-widths allows to customize the excitation spectrum in the desired energy range are some of the remarkable features of this investigation. These findings demonstrate, for the very first time, the significance of investigating the system of VSQD subjected to a quantizing magnetic field. Given the edge over the planar quantum dots and the foreseen applications in the single-electron devices and quantum computation, investigating the system of VSQD is deemed vital. The results suggest exploiting magnetoplasmon qubits to be a potential option for implementing the solemn idea of quantum state transfer in devising quantum gates for the quantum computation and quantum communication networks. [ABSTRACT FROM AUTHOR]

The main challenge in deterministic quantum state transfer (QST) between remote mechanical resonators is the local decoherence and the transmission losses in the communication channel. In the path of overcoming this limitation, here we employ a shortcut to adiabatic passage protocol to devise a fast and reliable evolution path between two remote mechanical modes in separate optomechanical systems (OMSs). A QST between the two nodes is conceived by engineering their coupling to an intermediate fiber optical channel. The coupling pulses are operated such that the dark eigenmode of the system is decoupled from the fiber modes and transitions to the bright modes are compensated for by counterdiabatic drives. We show that one obtains a QST with high fidelity for various Gaussian states. The efficiency is compared to that of adiabatic passage (AP) protocol in the presence of losses and noises. Our results show that while the AP protocol is very sensitive to the decoherence, the shortcut to adiabaticity provides a robust and fast QST even for small values of the coupling strength. The performance of both protocols are also investigated for the case of multimode fiber through numerical and an effective single-model model which is found by the elimination of off-resonant fiber modes. Our findings may pave the way for using OMSs in the realization of continuous-variable Gaussian QST.

In light of recent interest in Hadamard diagonalisable graphs (graphs whose Laplacian matrix is diagonalisable by a Hadamard matrix), we generalise this notion from real to complex Hadamard matrices. We give some basic properties and methods of constructing such graphs. We show that a large class of complex Hadamard diagonalisable graphs have vertex sets forming an equitable partition, and that the Laplacian eigenvalues must be even integers. We provide a number of examples and constructions of complex Hadamard diagonalisable graphs, including two special classes of graphs: the Cayley graphs over Z r d , and the non–complete extended p –sum (NEPS). We discuss necessary and sufficient conditions for (α , β) –Laplacian fractional revival and perfect state transfer on continuous–time quantum walks described by complex Hadamard diagonalisable graphs and provide examples of such quantum state transfer. [ABSTRACT FROM AUTHOR]

We study transfer of single photon in an one-dimensional finite Glauber-Fock cavity array whose coupling strengths satisfy a square root law. The evolved state in the array can be mapped to an upper truncated coherent state if the cavities are resonant. Appropriate choices of resonance frequencies provide perfect transfer of a photon between any two cavities in the array. Perfect transfer of the photon allows perfect quantum state transfer between the cavities. Our findings may help in realizing quantum communication and information processing in photonic lattices. [ABSTRACT FROM AUTHOR]

Perfect state transfer can be achieved between two marked vertices of graphs like a star graph, a complete graph with self-loops and a complete bipartite graph, and two-dimensional Lattice by means of discrete-time quantum walk. In this paper, we investigate the quality of quantum state transfer between two marked vertices of an unsymmetrical graph like the butterfly network. Our numerical results support the conjecture that the fidelity of state transfer depends on the quantum state to be transferred dynamically. The butterfly network is a typical example studied in networking coding. Therefore, these results can provide a clue to the construction of quantum network coding schemes. [ABSTRACT FROM AUTHOR]

We construct infinite families of graphs in which pretty good state transfer can be induced by adding a potential to the nodes of the graph (i.e. adding a number to a diagonal entry of the adjacency matrix). Indeed, we show that given any graph with a pair of cospectral nodes, a simple modification of the graph, along with a suitable potential, yields pretty good state transfer between the nodes. This generalizes previous work, concerning graphs with an involution, to asymmetric graphs. [ABSTRACT FROM AUTHOR]

Quantum state transfer between two distant parties is at the heart of quantum computation and quantum communication. Among the various protocols, the counterdiabatic driving (CD) method, by suppressing the unwanted transitions with an auxiliary Hamiltonian H cd (t) , offers a fast and robust strategy to transfer quantum states. However, H cd (t) term often takes a complicated form in higher-dimensional systems and is difficult to realize in experiment. Recently, the Floquet-engineered method was proposed to emulate the dynamics induced by H cd (t) without the need for complex interactions in multi-qubit systems, which can accelerate the adiabatic process through the fast-oscillating control in the original Hamiltonian H 0 (t). Here, we apply this method in the Heisenberg spin chains, with only control of the two marginal couplings, to achieve the fast, high-fidelity, and robust quantum state transfer. Then we report an experimental implementation of our scheme using a nuclear magnetic resonance simulator. The experimental results demonstrate the feasibility of this method in complex many-body system and thus provide a new alternative to realize the high-fidelity quantum state manipulation in practice. [ABSTRACT FROM AUTHOR]

• We propose a scheme for transfer of quantum continuous variable states such as coherent superposition state, two-mode entangled coherent state and the four-photon cluster state by using coupled resonator waveguide network. • The decoherence on the transfer of the four-photon cluster is discussed. • The fidelity for the transfer process of the four-photon cluster is close to 100%. A high fidelity quantum transfer scheme for continuous variable states based on coupled resonator waveguide network is presented. We first give the transfer scheme of an arbitrary superposition of coherent states by using single coupled resonator waveguide, next we give the transfer scheme of entangled coherent states by using two parallel coupled resonator waveguides, and finally give the transfer scheme of continuous variable cluster states by using four parallel coupled resonator waveguides. The fidelity of all schemes is up to 100%. [ABSTRACT FROM AUTHOR]

A simple method for transmitting quantum states within a quantum computer is via a quantum spin chain - that is, a path on n vertices. Unweighted paths are of limited use, and so a natural generalization is to consider weighted paths; this has been further generalized to allow for loops (potentials in the physics literature). We study the particularly important situation of perfect state transfer with respect to the corresponding adjacency matrix or Laplacian through the use of orthogonal polynomials. Low-dimensional examples are given in detail. Our main result is that PST with respect to the Laplacian matrix cannot occur for weighted paths on vertices nor can it occur for certain symmetric weighted trees. The methods used lead us to a conjecture directly linking the rationality of the weights of weighted paths on vertices, with or without loops, with the capacity for PST between the end vertices with respect to the adjacency matrix. [ABSTRACT FROM AUTHOR]

Abstract Quantum state transfer is one of the most prominent protocols in quantum information. In the context of optomechanics, it is still an important task, as it allows us to reliably convert an optical pulse into a mechanical excitation. In this paper, the quantum state transfer between a moveable mirror and cavity field is studied when the system is surrounded by non-Markovian environments, which has been recently realized in experiment. Utilizing the experimental spectrum density, we find that the transition from weak non-Markovianity to strong non-Markovianity happens when choosing a suitable optomechanical coupling strength. We also show that quantum state transfer can be implemented with high fidelity in the non-Markovian environment, and the non-Markovian memory effect is helpful for state transfer both in the short-time and long-time scales. [ABSTRACT FROM AUTHOR]

A protocol for fast and robust magnon transport in a one-dimensional spin chain is devised. Employing an approximate mapping between the chain and a single harmonically trapped particle, we exploit the known analytic control protocols for the latter and adopt them to achieve fast, high-fidelity transport in the chain. We compare the performance with finite time adiabatic protocols, showing that the designed scheme allows for significantly faster and more stable transport. Furthermore, we show that a sharp transition exists between regions in which the protocol is effective and when it breaks down, giving rise to a heuristic speed limit for the process.

The geometrical significance of winding number is used to study the topological characteristics of a single-excitation Heisenberg X Y spin model represented by the Su–Schrieffer–Heeger (SSH) model. Due to the presence of topological edge states, the quantum state may be conveyed from the first site to the last site with a high degree of fidelity. Also examined is a Heisenberg X Y model on the kagome lattice. In the topological phase of the kagome lattice, the three corner states are not affected by the Dzyaloshinskii–Moriya (DM) interactions. For the kagome lattice with and without the DM interactions, the state of a spin on one corner site can be transferred to the other two corner sites in various ways during the topological phase. The quantum state transfer (QST) for the X Y model on the kagome lattice without the DM interactions is symmetrical and can only produce a three-body entangled state. In contrast, the QST for the X Y model on the kagome lattice with the DM interactions is in an asymmetrical form, similar to one-way propagation, and it can produce both product states and three-body entangled states. • Winding number of a single-excitation 1D spin model with DM interactions is studied. • A one-way QST is realized on a kagome lattice with DM interactions. • The maximum fidelity is improved by breaking the symmetry of QST on a kagome lattice. [ABSTRACT FROM AUTHOR]