Driven Quantum Walks

Driven Quantum Walks Craig S. Hamilton,1, ∗ Regina Kruse,2 Linda Sansoni,2 Christine Silberhorn,2 and Igor Jex1 1FNSPE, Czech Technical University in Prague, Bˇrehová7, 115 19, Praha 1, Czech Republic 2Applied Physics, University of Paderborn, Warburger Straße 100, D-33098 Paderborn, Germany In this letter we introduce the concept of a driven quantum walk. This work is motivated by recent theoretical and experimental progress that combines quantum walks and parametric down- conversion, leading to fundamentally different phenomena. We compare these striking differences by relating the driven quantum walks to the original quantum walk. Next, we illustrate typical dynamics of such systems and show these walks can be controlled by various pump configurations and phase matchings. Finally, we end by proposing an application of this process based on a quantum search algorithm that performs faster than a classical search. PACS numbers: 05.45.Xt, 42.50.Gy, 03.67.Ac Quantum walks (QW) have become widely studied the- encoded into a matrix C which describes the connections oretically [1{3] and experimentally in a variety of dif- between the different modes of the system, as well as the ferent settings such as classical optics [4{7], photons in on-site terms. The CTQW has the generic Hamiltonian, waveguide arrays [8, 9] and trapped atoms [10, 11]. They ^ X y exhibit different behaviour than classical random walks HC = Cj;ka^ja^k + h.c.; (1) due to the interference of the quantum walker [12]. The j;k quantum walk paradigm has been used to demonstrate y wherea ^j; a^j are the bosonic creation and annihilation op- that they are capable of universal quantum computing erators respectively of the walker on the jth site of the [13, 14], including search algorithms [15] and more gen- graph and the evolution of the walk is given simply by eral quantum transport problems [16, 17]. The quantum the Schrödingerequation. Traditionally, the initial state walk comes in two varieties, the discrete-time (DTQW) y is localized on a single mode e.g. jφ(t = 0)i =a ^nj0i. The and continuous-time (CTQW); the latter case will be the key to our subsequent analysis is to use the set of eigen- focus of this letter. modes which diagonalize the original Hamiltonian (1), Recently an array of coupled waveguide channels with fA^kg. This will lead to (1) being written in the form a down-conversion term was theoretically studied [18] ^ P ^y ^ H = k ΩkAkAk, where the fΩkg are the eigenfrequen- and experimentally demonstrated [19, 20]. A classi- cies, which in 1D lines and 2D lattices can be considered cal pump drives a process creating down-converted light as a dispersion curve. The transformation from the orig- that then travels throughout the waveguide structure by inal, physical basis fa^kg to the eigenbasis is given by the evanescent coupling (the pump beam does not couple to matrix T, A = T a. For the PQW the number of walkers other channels). This can be modelled by adding an extra in the eigenmodes of the system does not change. Dur- term to the original CTQW Hamiltonian that converts ing the propagation, the phases between the eigenmodes two photons (walkers) from a single pump photon (the change, leading to the well-known QW properties. converse operation is also possible). During the driven QW the walkers are created and annihilated and this in We now add an extra term to this Hamiltonian that turn leads to very different dynamics when compared to creates and destroys photons i.e. we change the number the traditional passive QW (PQW) which is restricted of walkers during the QW. This term will take one of two by a constant number of walkers. forms, In this letter we investigate driven quantum walks. We ^ X y ∗ arXiv:1408.6647v1 [quant-ph] 28 Aug 2014 H = Γ (t)â + Γ (t)â ; (2) connect the physical properties of the non-linear waveg- L L;k k L;k k k uide arrays with the traditional quantum walk formalism. ^ X y 2 ∗ 2 Based upon the description of the system in the eigen- HS = ΓS;k(t)âk + ΓS;k(t)âk: (3) mode basis, we show, that any driven QW can be de- k composed into a PQW and an intricate input state. Fur- We call these terms lasing and squeezing respectively, thermore, we are able to selectively pump spatial eigen- as they are the Hamiltonians used for the generation modes of the system, allowing for in situ control of the of coherent states (created by a laser above threshold) QW properties. Finally, we take advantage of this unique and squeezed vacuum states [21]. The lasing term can property of driven QW to implement a search algorithm be realised in a DTQW with walkers added after each that demonstrates a quantum speed-up over a classical time step (to be studied later) and the second term was walker. recently studied [18{20]. We only include one growth The CTQW is defined by a graph of coupled modes, term at a time in our Hamiltonian and assume that these such as a 2D lattice, and this graph topology can be processes take place continually within the walk and are 2 driven by an undepleted classical pump (i.e. a large am- (a) Driven QW I plitude coherent state) with a vacuum input state. The in parameter Γ is the spatial pump shape and its time de- I pendence will only depend on the pump frequency. The out H^ second term S is a down-conversion process of two pho- n tons from a single pump photon. In the main text of the (b) Passive QW Iin letter we focus on the lasing QW. Similar results for the n squeezing QW may be found in the supplementary infor- mation [22]. Using the transformation T the complete Hamiltonian n H^C + H^L in the eigenbasis is, FIG. 1: Any driven QW (a) can be decomposed into an ^ X ^y ^ X ^y intricate multimode state followed by a passive QW (b). H = ΩkAkAk + Sk(t)Ak + h.c. (4) k k The output states of the two system will be the same. −1 −1 where SL = T ΓL (SS = T ΓST for the squeezing term as ΓS is a matrix ). We now move to the interaction picture of the dynamics using the transformation taining the same structure of state creation followed by ^ Q ^y ^ quantum walk. [21] U = k exp iΩkAkAkt which allows us to rewrite equation (4) as The ìnitial' state in (6) is determined by two factors, both contained within the matrix S. Firstly, the pump ^ X ^y iΩkt Hint = Sk(t)Ake + h.c. (5) shape which is simply the absolute value of the elements k of S, jSkj, (or jSk;k0 j for H^S) and determines which eigenmodes have a non-zero growth. The second factor is the Integrating over time, z = R dt0S(t0), and using [23] to phase-matching that occurs due to the time-dependence disentangle the Schrödingerevolution operator, when we of S, typically of the form e−i!pt, where ! is the pump convert back to the original operator basis, fa^ g this p k frequency. More complicated time dependencies would yields an output state of the form, not alter the interpretation presented here. When we 0 1 ! insert the pump shape into the interaction Hamiltonian, X y X 0 y jαL;outi = exp @−it Ck;k0 a^ a^k0 A exp −i zka^ j0i X y k k H^ = S exp (i(Ω − ! )t) A^ + h.c.; (7) k;k0 k int k k p k | {z } | {z } k jαL;ini U^P QW it becomes clear that we have to fulfil the eigenmode U^ jα i ; = P QW L;in phase-matching condition !p = Ωk for H^L (or !p = (6) Ωk + Ωk0 for H^S). When integrating over time phase- where U^P QW is the evolution operator of the PQW, 0 matched eigenmodes grow linearly in time whereas non- jαL;ini is the ìnitial state' and zk is the expression for z phase matched modes oscillate depending upon the size in the physical basis. This result is illustrated in figure 1. of the phase mismatch. These two factors give some con- We can decompose any driven QW in figure 1(a) into a trol over which eigenmodes are created during the QW. multimode coherent state that is then launched into the original CTQW with Hamiltonian evolution U^P QW , as The main difference between the driven quantum walk shown in figure 1(b), leading to the same output state. described here and the original CTQW can be easily seen A more detailed derivation may be found in the supple- in the nature of the eigenmodes. In the CTQW the mentary material [22]. amplitudes of the eigenmodes are fixed at the start of the walk and only the phases change in time. In our We can apply a similar theory to the other growth term driven QW we can choose the eigenmode(s) we want to H^ and arrive at an identically structured evolution of S create by changing the pump's spatial shape and fre- the walk i.e. first we create a multimode squeezed state quency to drive and phase-match combinations of eigen- then we evolve this state through the quantum walk. modes, thus the amplitudes (and phases) of the eigen- Note that the state creation and walk are not indepen- modes change. For longer walks phase-matching is the dent; both are a function of the overall time/length of more significant factor, where the majority of walkers the walk.

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