You are an expert at summarizing long articles. Proceed to summarize the following text: 
the system we analyze is an open spin - chain of @xmath0 elements , whose hamiltonian reads @xmath1+\sum^n_{i=1}b(t)\hat{z}_i.\ ] ] here , @xmath2 and @xmath3 are the coupling strengths of the pairwise interaction between adjacent spins and @xmath4 is a magnetic field . in our notation , @xmath5 and @xmath6 denote respectively the @xmath7 and @xmath8-pauli matrix . for the sake of simplicity , we consider @xmath0 as an odd number . however , all the results presented here can be straightforwardly adapted to the case of an even number of spins in the chain . physical units are chosen throughout the article so that @xmath9 . it is important to note that the inter - spin couplings and the amplitude of the magnetic field are site independent . this is a feature that differenciates the system at hand from a few previous proposals for perfect state transfer available in the literature  @xcite . although eq . ( [ modelloxy ] ) is spatially unmodulated ( in analogy with  @xcite ) , the price to pay in order to achieve unit transfer fidelity for any length of the chain is the time dependence of the interaction strengths . we will show later in this article that such a request can also be relaxed : perfect state transfer can be achieved even if only the magnetic field is time - dependent while any other term is constant . ( [ modelloxy ] ) does not preserve the number of spin - excitations as it does not commute with the total @xmath8-component of the spin of the system . frequently , when one faces the case of dynamics ruled by spin - preserving models , on the assumption of proper boundary conditions , it is convenient to diagonalize the coupling hamiltonian by means of a sequence of operation comprising wigner - jordan , fourier , and bogoliubov transforms  @xcite . in our case , due to the spin - non preserving nature of @xmath10 , the mutual coupling among sectors of the hilbert space labeled by different quantum spin numbers has to be considered . rather than applying techniques for the exact diagonalization of eq . ( [ modelloxy ] ) , here we tackle the evolution of the system by means of an _ information - flux approach _ , which is specifically designed for multi - spin interactions  @xcite . our method does not rely on the explicit analysis of the energy spectrum of the chain and allows us to gather an intuitive picture of the dynamics at hand . such an approach has already proved its flexibility and efficiency in identifying hamiltonian configurations suitable for state transfer and entanglement generation  @xcite . the information - flux approach requires the time - evolved form of specific operators @xmath11 in the heisenberg picture , which is given by @xmath12 with @xmath13 the time evolution operator , @xmath14 and @xmath15 . using this method , one is able to understand the dependence of @xmath16 on any @xmath17 and design the proper set of interactions in such a way that it becomes possible to _ drive _ a desired evolution by means of engineered quantum interference  @xcite . in our case , we will exploit the time dependence of the couplings in order to induce precisely this effect . our task here is the transmission of quantum information from the first spin to the last one in the chain . this requires the study of @xmath18 s , which can be decomposed into the operator basis built out of all possible tensor products of @xmath19 with @xmath20 the @xmath21 identity operator applied to spin @xmath22 . we have @xmath23 where @xmath24 is the initial state of the whole chain , the first spin being in @xmath25 and the rest of the chain in @xmath26 ( this definition can be easily generalized to the case of mixed states of the chain ) . the coefficient @xmath27 is defined as the information flux at time @xmath28 from @xmath29 to @xmath30  @xcite . if our system achieves @xmath31 when @xmath32 and for @xmath33 , we have perfect @xmath34 state transfer . we can determine which are the terms of the operator basis that are involved in the decomposition of @xmath18 s by representing such operators in an oriented weighted graph , whose construction is straightforward . each node of the graph corresponds to an operator involved in the decomposition . if the interest is focused on the evolution of @xmath30 , it should occupy the first node of the graph . any operator resulting from the commutators of @xmath30 with the hamiltonian should be included in the graph and linked to it with an oriented edge . these steps are repeated as necessary and the construction process ends when no new operator is created on commutation . a numerical weight , determined by the performed commutations and needed in order to calculate the information flux , is attached to each edge of the graph . finally , an outgoing ( incoming ) edge corresponds to a + ( - ) sign . due to the particular form of @xmath35 in eq . ( [ modelloxy ] ) , @xmath36 and @xmath37 will generate the oriented graph shown in fig . [ doublechain ] ( where the case @xmath38 has been considered ) . clearly , @xmath39 already appears in the graph corresponding to @xmath40 and we do not need to construct a dedicated graph for it .    for the sake of clarity , here we show how to evaluate the information flux for a time - independent hamiltonian such as eq . ( [ modelloxy ] ) for @xmath41 . the method can be explicitly extended to any time - dependent model . however , in our numerical analysis we divide the total time of the evolution in a series of short steps within which the hamiltonian is taken as constant . in this way , in the limit of a large number of steps and correspondingly short time steps , the flux of information can be accurately evaluated by means of only a light computational effort . a cut - off in the number of considered temporal steps is chosen so as to have no noticeable difference in the results if a larger number is taken . for a time - independent hamiltonian , @xmath42 and , by means of the operator expansion formula  @xcite , we have @xmath43 with @xmath44 the nested commutator of order @xmath45 between @xmath10 and @xmath46 and @xmath47 .    here , for the sake of a clear presentation of our proposal and without loss of generality , we focus on the case of @xmath38 ( the case of an arbitrary @xmath0 being easily extrapolated from our example ) and study the dynamics of @xmath48 under the model in eq . ( [ modelloxy ] ) . the terms in the operator expansion are deduced by @xmath49=-2i\,j_y\,\hat{y}_4\hat{z}_5 + 2i\,b\,\hat{y}_5,\\   \hat{\cal c}_2(\hat{x}_5)&=[{\hat{\cal h}},[{\hat{\cal h}},\hat{x}_5]]=2ib(2i\,j_x\,\hat{x}_4\hat{z}_5 - 2i\,b\,\hat{x}_5)\\ & -2i\,j_y(2i\,j_y\,\hat{x}_5 + 2i\,j_x\,\hat{x}_3\hat{z}_4\hat{z}_5 - 2i\,b\,\hat{x}_4\hat{z}_5),\\    & \vdots   \end{split } \label{commutators}\ ] ] clearly , the only operators involved in this iterative sequence are those in the graph of fig .  [ doublechain ] . therefore , it is possible to write the evolved operators @xmath50 as @xmath51 when the parameters @xmath52 can not be analytically evaluated due to the difficulties of the evolution , it is still possible to approximate them by means of recurrence formulas . we have @xmath53 where @xmath54 is a proper cut - off and @xmath55 with @xmath56 ( @xmath57 ) for @xmath58 ( @xmath59 ) . it is immediate to note that these recurrence formulas come directly from the commutation rules in eq . ( [ commutators ] ) . they can thus be easily derived from the graph in fig . [ doublechain ] . the same formulas , but with the initial conditions @xmath56 ( @xmath57 ) for @xmath60 ( @xmath61 ) , describe the evolution of the operator @xmath62 . the approach described so far constitutes the basis for the analysis of perfect quantum state transfer via temporally controlled information kicking , which is the focus of the next section . let us study the action of @xmath63 in eq . ( [ modelloxy ] ) when only one of the inter - spin coupling terms differs from zero . the graph in fig . [ doublechain ] shows that , in these conditions , every node is linked to just one nearest neighbor . in fact , the full oriented graph for the information flux analysis corresponding to eq . ( [ modelloxy ] ) can be seen as the juxtaposition of three mutually disconnected subgraphs , each associated with only one of the coupling terms in @xmath10 and represented by a different color in fig . [ doublechain ] . it is easy to track the operator dynamics associated with these configurations . let us consider , for instance , the case @xmath64 and @xmath65 . [ doublechain ] ( black color ) shows that @xmath48 and @xmath62 are mutually linked and , by using the formal quantitative analysis of the previous section , we have @xmath66\hat{x}_5+\sin[2\beta(t)]\hat{y}_5,\\ & \hat{\cal y}_5(t)=-\sin[2\beta(t)]\hat{x}_5+\cos[2\beta(t)]\hat{y}_5 \end{split}\ ] ] with @xmath67 . clearly , an isolated node in a graph corresponds to an operator that is constant in time [ for instance , @xmath68 when @xmath69 , @xmath70 , as in fig . [ doublechain ] ( light green ) ] . this result is key to our investigation . if we let the interaction terms be alternatively non - zero for a time window @xmath71 $ ] such that @xmath72 , the action of @xmath63 in eq . ( [ modelloxy ] ) will correspond to _ kicks of information _ between pairs of connected nodes of the graphs discussed above . in fig . [ doublechainjumps ] we show an example for @xmath73 being alternately non - zero for five time intervals such that @xmath74 for @xmath75 [ @xmath76 for @xmath77 , with @xmath78 [ @xmath79 whenever @xmath80 [ @xmath81 . the negligibility of the mutual overlaps of the coupling functions ensures that the information kicks occur according to the ordered scheme in fig .  [ doublechainjumps ] . a similar transport of information from the leftmost to the rightmost part of the graph is achieved by kicking information using a pattern given by fixing @xmath82 ( @xmath83 ) and alternately changing the amplitude of @xmath84 ( @xmath85 ) and @xmath86 in a way so as to satisfy the conditions stated above . after a sequence of @xmath0 ( for time - dependent @xmath84 and @xmath85 ) or @xmath87 kicks ( for time - dependent @xmath86 and either @xmath84 or @xmath85 ) , one obtains @xmath88 where @xmath89 is the total time of the evolution . by initially preparing the @xmath90 spin of the chain ( except the first ) in an eigenstate of @xmath91 , there is a complete end - to - end transport of information across the chain . this evolution interaction can thus be exploited for a perfect state transfer exactly as it happens for the hamiltonian model of ref . @xcite . quite interestingly , any other state - transfer protocol relying on the same information - flux structure , as those in refs . @xcite , can be mapped into the very same temporal pattern of information kicking illustrated here . this is the main result of our investigation , proving that an alternative to the standard architecture of properly arranged interaction - strength patterns for quantum state transfer is actually possible . the key is the introduction of an alternated series of non - overlapping temporal kicks of information . the equivalence of the two scenarios has here been proven in terms of an isomorphism of the respective information fluxes and is , as such , completely general . depending on the details of a specific practical realization of the intra - chain couplings , temporal control of the form highlighted here can be more convenient than pre - fabrication of a specific pattern of static coupling strengths . this is the case , for instance , for quasi - unidimensional optical lattices loaded with neutral atoms , where inter - site couplings are achievable in a time - controlled way via external optical potentials or cold atomic collisions . in perspective , this could also be a viable option in solid - state structures ( such as arrays of josephson junctions ) , where fabrication of exactly the pattern required for ideal state transfer would be demanding ( if not prohibitive ) already at moderate chain lengths . achieving control via proper voltage / magnetic pulses inducing time - controlled information kicks would be a possibility to exploit instead . in order to go beyond the idealization of the scheme assessed in the previous section and make our analysis closer to more realistic situations , the ideal conditions on the pattern of time - dependent couplings invoked before will be relaxed here . let us first consider @xmath92 with @xmath2 and @xmath4 following the behavior dictated by @xmath93^m,\\ & b(t)=b_{max}[\cos(t+\pi/4)]^m , \end{split } \label{sincos}\ ] ] where @xmath94 are the maximum values of @xmath2 and @xmath4 ( properly chosen in order to satisfy the integral condition ) and @xmath95 is an even number . pulse shapes of this form ( for moderate values of @xmath95 ) are routinely generated by commercial pulse generators . a large value of @xmath95 obviously implies small overlap between the two coupling functions . a period of @xmath96 will correspond to a `` horizontal '' and a `` vertical '' kick in the associated operator graph . for a numerical analysis of the information - flux behavior under the action of this time - dependent hamiltonian , we divided the total time interval @xmath97 $ ] in short steps . within each of them , both @xmath2 and @xmath4 are taken as constant and numerically equal to the average values they assume in the respective time window . in this way , the recurrence formulas presented in sec .  [ informationflux ] allow us to estimate the evolution of @xmath40 or @xmath39 for the duration of the step . clearly , the shorter is the time step , the more accurate is the estimate . the method is then iterated for each step and the total evolution of @xmath40 or @xmath39 is finally reconstructed . as the number of operators involved in each decomposition is equal to @xmath98 , a light computational power is required .    in order to evaluate the performance of our scheme , both the coefficient @xmath99 , attached to @xmath100 in the decomposition of @xmath101 , and @xmath102 , attached to @xmath103 in the decomposition of @xmath104 , should be studied . however , one can equally well characterize the whole state transfer process simply by considering @xmath105 only . the maximum value of @xmath105 for two values of @xmath95 is studied against @xmath0 in figs . [ plotpowers ] * ( a ) * and * ( b)*.    * ( a ) *    0.5 cm    it is matter of performing an explicit calculation to see that the maximum of @xmath102 follows similar behaviors . we then quantify the mean transmission fidelity @xmath106 by averaging the state fidelity @xmath107 between a pure input state @xmath108 to transfer and the state @xmath109 of spin @xmath0 at time @xmath28 . by assuming a uniform distribution of @xmath110 s  @xcite , we find @xmath111.\ ] ] for instance , if we take @xmath112 , we obtain for any length of the chain @xmath113 a value of @xmath114 . as @xmath115 , the average fidelity converges to @xmath57 , resulting in perfect state transfer .    the ability to temporally control both @xmath84 and @xmath86 can however be experimentally demanding . in particular , while the arrangement of external potentials with a desired time - dependence can be relatively straightforward , this might not be the case for @xmath116 s , which are in general determined by coupling mechanisms internal to the chain itself and hardly controllable with the degree of accuracy required by our scheme . we thus make our requests lighter by taking @xmath84 as constant and allowing a time dependence only for the external magnetic field . obviously , it is formally the same to take a constant @xmath86 with time - varying @xmath2 . however , pragmatically our choice seems to us to be the least demanding . we thus adapt the strategy assessed before to the case of @xmath117 . our quantitative study shows that a slight modification to the sinusoidal trend described before is sufficient to retain the efficiency of the protocol . we take well - spaced square - shaped pulses for @xmath4 as shown in fig . [ plotbandjbehavior ] . the parameter @xmath118 quantifies the width of magnetic field pulses . it can be roughly seen as half of the overall period divided by the time in which @xmath86 is close to its maximum value . clearly , a large value of @xmath118 corresponds to a more ideal case , as the overlap between the two functions is smaller , hence affecting the maximum value that @xmath105 can achieve and , in turn , the transfer fidelity . the dependence of @xmath119 on @xmath118 is shown in fig .  [ 5qubitsdelta5to20 ] for @xmath38 . 0.25 cm    the fidelity @xmath120 is already larger than @xmath121 for @xmath122 . of course , the effect of the overlap between the two functions depends on @xmath0 : the longer the chain , the lower the transmission fidelity . the dependence of the fidelity on the length of the chain is plotted in fig . [ 5to15qubitsdelta5to20 ] .    0.25 cm    we note that , for @xmath0 up to @xmath123 , a value of @xmath124 is large enough to obtain the maximum of @xmath125 @xmath126 ( corresponding to @xmath127 ) .    to give a complete overview of the protocol , one can finally analyze , for a fixed @xmath118 , the behavior of the maximum of @xmath125 against @xmath0 . the case of @xmath128 has been considered , and the results are shown in fig . [ 5to25qubitsdelta20 ] .    in these conditions , the corresponding transmission fidelity for a chain with @xmath129 spins is @xmath130 . we have also studied the case in which the non - zero interaction terms are @xmath84 and @xmath85 , with @xmath131 . the transmission fidelity is larger than the one obtained with non - zero @xmath84 ( @xmath85 ) and @xmath86 , as the number of kicks required is smaller and the effect of the overlap is thus reduced . however , we presented the results for non - zero @xmath84 ( @xmath85 ) and @xmath86 as we consider the choice of a time - dependent magnetic field the least demanding from an experimental point of view . in this section we discuss another interesting aspect of the proposed model , namely the possibility to generate genuine multi - partite entanglement . bi - partite as well as multi - partite entanglement creation in spin chains has been recently studied from different points of view . for instance , one can generate entanglement by means of a proper pattern of coupling parameters  @xcite , by driving the evolution through a resonant interaction  @xcite , by performing a global quench  @xcite , or by leaving two empty sites in a uniformly filled chain  @xcite . by assuming the ability to initialize the state of the chain before the application of the pulse sequence in the protocol proposed in this article ( we necessarily have more demanding requirements for this particular task ) , greenberger - horne - zeilinger ( ghz ) states can be generated  @xcite . such a resource is well - known to be useful for multi - agent protocols for distributed qip like quantum secret sharing , remote implementation of unknown operations and quantum average estimation  @xcite . let us consider again the hamiltonian @xmath10 in eq . ( [ modelloxy ] ) under the ideal conditions mentioned above . it is interesting to notice that , after the proper kicking of information , the evolution of _ all _ the spin operators associated with the elements of the chain have exactly the same form as those obtained by using the model proposed in refs . @xcite . we now want to evaluate the dynamics of a particular pure state of the chain under the action of this hamiltonian . we will need to abandon the assumption that at most a single spin excitation populates the system . it is straightforward to check that , whenever spin @xmath22 is in an eigenstate of @xmath132 , the corresponding symmetric spin @xmath133 will end up in the same state . whenever two or more spins are initially in eigenstates of @xmath134 or @xmath135 , the corresponding symmetric spins will end up in an entangled state . let us consider , for definiteness , an even number of spins . we call @xmath136 the indices corresponding to spins initially prepared in eigenstates @xmath137 of @xmath6 and @xmath138 those labeling spins initially prepared in eigenstates @xmath139 of @xmath134 ( the analysis can be straightforwardly generalized to the case of odd number of spins and eigenstates of @xmath135 ) . the initial state of the chain is thus @xmath140 it is matter of a straightforward calculation to see that , in this case , the final state corresponds to a mirror - inversion operation on the state @xmath141,\ ] ] with @xmath142 depending on the actual form of @xmath143 . clearly , @xmath144 is the tensor product of a separable state for spins @xmath136 and an @xmath95-particle ghz - like state for spins @xmath138 . by means of the information flux approach , we have presented a new strategy for obtaining perfect state transfer in a finite , open chain of spins . we have shown that the evolution of spin operators under the action of a site - dependent hamiltonian that is already known to allow perfect state transfer can be mimicked by the use of a homogeneous time - dependent hamiltonian . the engineering of the coupling strengths , necessary in the first scenario , is thus converted to a temporal arrangement of the interaction terms . we have discussed a particularly interesting case where such a time dependence can be reduced to the requirement of just a non - constant magnetic field affecting the spins of the chain . interestingly , this very same model can also be used in order to create a ghz - like entangled state shared by a sub - set of spins of our choice , leaving all the other spins in a separable state . we believe that our proposal to achieve perfect state transfer and quantum correlation sharing in many - body registers of interacting spins widens the possibilities for efficient and non - demanding short / medium - haul quantum communication . m. christandl , n. datta , a. ekert , and a. j. landahl , , 187902 ( 2004 ) ; c. albanese , m. christandl , n. datta , and a. ekert , , 230502 ( 2004 ) ; g. m. nikolopoulos , d. petrosyan , and p. lambropoulos , europhys . lett . * 65 * , 297 ( 2004 ) ; g. m. nikolopoulos , d. petrosyan , and p. lambropoulos , j. phys . : matter * 16 * , 4991 ( 2004 ) ; m. christandl , n. datta , t. c. dorlas , a. ekert , a. kay , and a. j. landahl , , 032312 ( 2005 ) . m. hillery , v. buek , and a. berthiaume , , 1829 ( 1999 ) ; c. schmid , p. trojek , m. bourennane , c. kurtsiefer , m. zukowski , and h. weinfurter , , 230505 ( 2005 ) ; a. m. wang , , 062323 ( 2007 ) ; c. di franco , m. paternostro , and m. s. kim , , 052316 ( 2007 ) .
we propose a strategy for perfect state transfer in spin chains based on the use of an unmodulated coupling hamiltonian whose coefficients are explicitly time dependent . 
 we show that , if specific and non - demanding conditions are satisfied by the temporal behavior of the coupling strengths , our model allows perfect state transfer . 
 the paradigma put forward by our proposal holds the promises to set an alternative standard to the use of clever encoding and coupling - strength engineering for perfect state transfer . 
 multiple - spin systems , in particular spin chains , have recently been the object of extensive studies . from the quantum information processing ( qip ) 
 viewpoint , such systems embody valuable media for quantum protocols . 
 in fact , it has been found that specific forms of built - in and permanent intra - register couplings , such as those typical of spin - chain models , could be used for the purposes of quantum computation  @xcite and communication in quantum networks . 
 this second possibility is particularly interesting . 
 in fact , while photons are ideal candidates for the long - haul transmission of information among different local nodes  @xcite , their use in hybrid architectures for quantum networks ( where the nodes are embodied by matter - like systems ) requires the use of quantum interfaces . despite the impressive experimental successes reported in this area , the realization of an interface is usually accompanied by technical problems and errors in transmission . on the other hand , for short - distance quantum communication , 
 an alternative arrangement where information carriers and processors are embodied by physical systems having the same nature could be more advantageous . in the seminal work in ref . 
 @xcite , the idea of using spin chains as _ 
 quantum wires _ has been proposed as a way to avoid the interfacing problems mentioned above . 
 the original idea has then been extended along various directions ( for more details , see ref . 
 @xcite ) . in particular , refs . 
 @xcite showed that , by properly engineering the strength of the couplings in the chain , perfect state transfer could be achieved . a proposal to bypass the initialization of the medium by means of local operations and measurements on the extremal spins of the chain 
 has been recently put forward  @xcite . 
 a similar result can be obtained by properly encoding the state to transmit in two spins at one end of the chain  @xcite .    here , an alternative strategy to the one proposed in ref . 
 @xcite is presented . instead of the pre - arrangement of the interaction strengths across the chain 
 , we consider a uniform distribution of time - dependent couplings . 
 we show that perfect transfer of information is achieved in our time - dependent architecture as well and that a formal mapping of the dynamics achieved in the two models is in order , thus letting our proposal emerge as an alternative paradigma for perfect quantum state transfer . 
 we also study a scenario where the coupling strengths of the hamiltonian are constant , while an external time - dependent magnetic field is used in order to provide the necessary temporal modulation required for information transmission . 
 we believe that the introduction of a time - dependent term in the intra - chain couplings allows to bypass the intrinsic rigidity of protocols based on unmodulated and pre - engineered spin media . 
 in fact , it is usually the case that spin - chain models are arranged so as to implement a specific communication or computational task and can not be  recycled " and used for a different one . 
 moreover , should an experimentally - prepared pattern of coupling strengths be found not accurate enough , the whole medium should be discarded ( unless a non - ideal performance of the quantum protocol could be tolerated ) . differently , by allowing the presence of time - dependent hamiltonian terms , a more dynamical  adjustment " of the medium performances would be possible by implementing a simple feedback loop : state transmission can be tested using a given functional form of the time - dependent terms of the hamiltonian and , according to the results , this can be tuned so as to converge toward better performances . a time - dependent scheme for quantum state transfer has been discussed by lyakhov and bruder in ref . 
 @xcite . 
 building up on general result described in ref . 
 @xcite , they only let the first and last spin - pair of a chain to experience time - dependent couplings . 
 our scheme , on the other hand , is different as we either consider a full set of time - dependent interaction strengths or a fully homogeneous pattern of constant couplings with a time - dependent external magnetic field . 
 the remainder of this article is organized as follows . in sec .  [ system ] 
 we describe the model used in our proposal . in sec . 
 [ informationflux ] we shortly review the main features of the information - flux approach , which is the main tool used in our investigation , and adapt it to our time - dependent analysis . 
 sec . 
 [ results ] illustrates the way we obtain perfect state transfer when the coupling strengths depend on the interaction time via sharply rising / lowering pulses , while sec . 
 [ realistic ] is dedicated to the investigation of a much more realistic setting that considers finite rising times and pulse duration . in sec . 
 [ multipartite ] , the proposed model is exploited in order to create a genuine multi - partite entangled state . 
 finally , sec . 
 [ remarks ] summarizes our results .