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Quantum mechanics of lattice gas automata. I. One particle plane waves and potentials

a r X i v :q u a n t -p h /9611005v 1 5 N o v 1996July 1996

revised October 1996

quant-ph/9611005

QUANTUM MECHANICS OF LATTICE GAS AUTOMATA I.ONE PARTICLE PLANE WAVES AND POTENTIALS David A.Meyer Institute for Physical Sciences and Project in Geometry and Physics Department of Mathematics University of California/San Diego La Jolla,CA 92093-0112dmeyer@https://www.doczj.com/doc/df11384199.html, ABSTRACT

Classical lattice gas automata e?ectively simulate physical processes such as di?usion and ?uid ?ow (in certain parameter regimes)despite their simplicity at the microscale.Mo-tivated by current interest in quantum computation we recently de?ned quantum lattice gas automata;in this paper we initiate a project to analyze which physical processes these models can e?ectively simulate.Studying the single particle sector of a one dimensional quantum lattice gas we ?nd discrete analogues of plane waves and wave packets,and then investigate their behaviour in the presence of inhomogeneous potentials.

PACS numbers:03.65.-w,02.70.-c,11.55.Fv,89.80.+h.

KEY WORDS:quantum lattice gas;quantum cellular automaton;quantum computation.

1.Introduction

The?rst quantum lattice gas automaton(QLGA)appeared as Feynman’s path integral for a relativistic particle in1+1dimensions[1];independently Riazanov constructed a2+1 dimensional QLGA as the path integral for the next higher dimensional Dirac equation[2]. In these formulations the quantum particle is conceptualized as evolving along spacetime trajectories,each of which is assigned a probability amplitude which is the product of a sequence of‘scattering’amplitudes describing the evolution of the particle during a single timestep.Thus these QLGA are discretizations of quantum mechanical processes.

Feynman’s path integral formulation of quantum mechanics reproduces the standard Schr¨o dinger formulation of wave functions obeying partial di?erential equations[1].These di?erential equations can be discretized directly,giving equations which Succi and Benzi naturally identify in the lattice gas paradigm as quantum lattice Boltzmann equations[3]. It is a familiar,although not often useful,observation that any numerical evolution of a discretized partial di?erential equation can be interpreted as the evolution of some cellular automaton(CA),if one allows the set of states to be R,or C,or Z N for some very large N.Taking this perspective,Bialynicki-Birula constructs a model for quantum evolution—a linear unitary CA[4]—which is essentially equivalent to,although derived independently of,Succi and Benzi’s equations.

The equivalence of a QLGA simulation and the evolution of a set of quantum lattice Boltzmann equations/unitary CA depends on the equivalence of the path integral and standard formulations of quantum mechanics in the continuum.Our recent work explaining the necessity of non-unitarity in earlier attempts of Gr¨o ssing,Zeilinger,et al.to construct homogeneous CA for quantum evolution[5]demonstrates this equivalence directly for the discrete models[6].We also note that,in contrast to simulation with deterministic or probabilistic LGA,simulation with a QLGA requires evolution along all possible spacetime trajectories.This may be achieved(slowly)by evolution of the quantum lattice Boltzmann equation on a classical computer or,at present hypothetically(but rapidly),by simulation on a quantum computer.

In fact,given the arguments that massive parallelism will optimize nanoscale quantum computer architecture[7],it is plausible that the?rst useful quantum computation[8]will implement a QLGA simulation of some quantum mechanical process.This provides two reasons to pursue the project described in this series of papers:we want to explore not only quantum mechanical phenomena which can be simulated e?ectively by QLGA,but also how well,as Feynman suggested[9],a quantum computer might simulate physics.In addition, we expect the quantum mechanics of LGA to have implications for discrete models of fundamental physics:we have already found remarkable consequences of unitarity in linear [6,10]and nonlinear[11]QCA.

We begin in the next section by recalling the model of[6]with which we will be working:the most general one dimensional homogeneous QLGA with a single particle of speed no more than1in lattice units.The local evolution rule for this model has two free

parameters:essentially the second measures the coupling between two copies of Feynman’s original QLGA in which the?rst measures the‘mass’of the particle.

This generalized QLGA is exactly solvable,just as is a single Feynman QLGA.In Section3we demonstrate this by?nding the discrete analogues of plane waves in,and the dispersion relation for,our QLGA.We also show the results of simulations of the former—on a deterministic computer.

We might imagine a one particle QLGA being simulated quantum mechanically by a ballistic electron in a solid state lattice[12]or as the‘low energy’sector of a line of dynamical quantum spins[9](‘low energy’meaning,e.g.,the con?gurations with one spin up and the rest down).In the former case[13],and certainly if our interest is in the QLGA as a discrete approximation to the Dirac equation[6],it is natural to investigate wave packets representing a semi-classical quantum particle.We do so in Section4.

In Section5we show how to introduce an inhomogeneous potential into the model. Concentrating on?nite square well potentials,we determine the dependence of the fre-quency/energy eigenvalues on the depth of the well and?nd that the eigenfunctions take the expected form.Finally,we utilize the results of Section4and show the results of simulations of a wave packet in a?nite square well.

We summarize our results in Section6and indicate the directions in which this re-search is continuing.

2.The one particle QLGA

A lattice gas automaton(LGA)should be envisioned as a collection of particles moving synchronously from vertex to vertex on a?xed graph(lattice)L:At the beginning of each timestep each particle is located at some vertex and is labelled with a‘velocity’indicating along which edge incident to that vertex it will move during the‘advection’half of the timestep.After moving along the designated edge to the next vertex,in the‘scattering’half of the timestep the particles at each vertex interact according to some rule which assigns new‘velocity’labels to each.For the purposes of this paper we will consider only one dimensional lattices L,isomorphic to the integer lattice Z or some periodic quotient thereof.In this case there are only two possible‘velocities’:left and right.We will further restrict our attention to LGA with only a single particle;for some preliminary work on QLGA with multiple particles,see[6,14,15].

A QLGA is a LGA for which the time evolution is unitary.To make this precise we must?rst identify the Hilbert space of the theory.For a one particle QLGA in one dimension an orthonormal basis for the Hilbert space H is given by|x,α (in the standard Dirac notation[16]),where x∈L denotes position andα∈{±1}denotes‘velocity’.At each time the state of the QLGA is described by a state vector in H:

Ψ(t)= x,αψα(t,x)|x,α ,(2.1)

where the amplitudesψα(t,x)∈C and the norm ofΨ(t),as measured by the inner product on H,is:

1= x,α

ψα(t,x)ψα(t,x)as the probability that the particle be in the state|x,α at time t[16,17].As usual,therefore,the basis state vectorsΨ=|x,α correspond to‘classical’states—with probability1there is a single particle at x with‘velocity’α—and a generic state vector(2.1)is a superposition of these ‘classical’states,each of which has integer values(one1,the rest0)for the number of particles at each lattice site.(In general,the basis vectors for the n particle subspace of the QLGA Hilbert space are exactly the possible states of a classical deterministic LGA with n particles[15].)

In order for the evolution to have the‘advection’interpretation described above,the basis vectors should evolve so that

x,α|U|y,β =0(2.3) only when x=y+β.This is equivalent to a condition on the amplitudes:

ψα(t+1,x)=fα ψ?1(t,x+1),ψ+1(t,x?1) ,

where taking fαto be independent of x means that the QLGA is homogeneous in space. As the notation suggests,it is convenient to combine the left and right moving amplitudes at x into a two component complex vectorψ(t,x):= ψ?1(t,x),ψ+1(t,x) so that a state vector is written

Ψ(t)= xψ(t,x)|x .

We showed in[6]that the most general unitary evolution for a one dimensional QLGA with parity invariance*is unitarily equivalent to

ψ(t+1,x)= 0i sinθ

0cosθ ψ(t,x?1)+ cosθ0

i sinθ0 ψ(t,x+1),(2.4)

up to some overall phase which has no physical e?ect.Here the parameterθ∈R,or more precisely,tanθ,plays a role something like‘mass’:whenθ=0the particle travels only on the lightcone;as tanθincreases its probability for moving more slowly does also.

Notice that just as in deterministic LGA in one dimension,the particle has a Z2valued ‘Lagrangian’conserved quantity measuring the parity of its?ducial space coordinate.This ‘spurious’conserved quantity partitions the set of particles in a deterministic LGA into two

decoupled gases[18]and in the QLGA de?ned by(2.4)it partitions the set of amplitudes ψ(t,x)into two independent sets according to x+t(mod2).This motivates consideration of the most general,no less local model which breaks this symmetry,namely

ψ(t+1,x)=w?1ψ(t,x?1)+w0ψ(t,x)+w+1ψ(t,x+1),(2.5) where w i∈M2(C)are2×2complex matrices.In terms of the basis vectors|x,α ,now (2.3)holds when x=y+βor x=y,i.e.,the particle can have nonzero amplitude to maintain its position.The condition that the global evolution,i.e.,the matrix U,be unitary is expressed in terms of the w i by the equations:

w?1w??1+w0w?0+w+1w?+1=I

w0w??1+w+1w?0=0(2.6)

w+1w??1=0,

together with their Hermitian conjugates[6].Imposing also the condition of parity(re-?ection)invariance on evolution of the form(2.5),we showed in[6]that the most general solution,up to unitary equivalence and an overall phase,is given by

w?1=cosρ 0i sinθ

0cosθ w+1=cosρ cosθ0 i sinθ0

w0=sinρ sinθ?i cosθ

?i cosθsinθ

.(2.7)

Hereρ∈R is a coupling parameter breaking the‘spurious’symmetry.Whenρ=0,(2.5) reduces to(2.4),the QLGA which is unitarily equivalent to the models of Feynman[1], Succi and Benzi[3],and Bialynicki-Birula[4].As tanρincreases,the relative weight of w0 increases and the particle has greater probability of maintaining its position,i.e.,having zero velocity.This is the?rst indication of a symmetry betweenθandρwhich will become more explicit as we investigate the general QLGA of(2.5)and(2.7).

3.Plane waves

The local evolution rule(2.5)is linear so we expect the model to be exactly solvable.In [6]we solved theρ=0case by counting spacetime lattice paths in order to compute the propagator Kαβ(t,x;0,0):= x,α|U t|0,β https://www.doczj.com/doc/df11384199.html,ttice paths are more di?cult to count when the particle has nonzero amplitudes for maintaining its position during each timestep.Avoiding this di?culty leads us to a more physical approach—?nding the discrete analogue of plane waves in a QLGA.

Recall that the QLGA is homogeneous,i.e.,U commutes with the translation(shift) operator T de?ned by(Tψ)(x):=ψ(x+1).Suppose L=Z N.Then the eigenvalues of T are e ik for wave numbers k=2πn/N,n∈{0,...,N?1},and the corresponding eigenvectorsΨ(k)satisfy:

ψ(k)(x+1)=e ikψ(k)(x).(3.1)

Figure1.Evolution of the n=1right moving plane wave on a periodic lattice withθ=π/3 andρ=π/4.Figure2.Evolution of the n=2right moving plane wave on a periodic lattice withθ=π/3 andρ=π/4.

Since[U,T]=0and U is unitary,theΨ(k)are also eigenvectors for U with

UΨ(k)=e?iωkΨ(k),(3.2) for some frequenciesωk∈R.The eigenvectorsΨ(k)are the discrete analogues of plane waves since they evolve simply by phase multiplication.

Since the action of U is de?ned by(2.5),(3.1)and(3.2)imply that

e?iωkψ(k)(x)=w?1ψ(k)(x?1)+w0ψ(k)(x)+w+1ψ(k)(x+1)

=(e?ik w?1+w0+e ik w+1)ψ(k)(x)

=:D(k)ψ(k)(x).

(3.3) Thus the e?iωk are eigenvalues of D(k)∈M2(C),i.e.,solutions of

det(D(k)?e?iωk I)=0,(3.4) where I is the2×2identity https://www.doczj.com/doc/df11384199.html,ing the parametrization(2.7)of the w i,(3.4) reduces to the condition

cosω=cos k cosθcosρ+sinθsinρ.(3.5) For a given wave number k,(3.5)determines two frequencies±ωk in terms of the rule parametersθandρ.Call the corresponding eigenvectors of D(k)(normalized to have length1/N)ψ(k,±1)(0)∈C2,so that the corresponding plane waves are de?ned by(3.1) to be

Ψ(k,?):= xψ(k,?)(0)e ikx|x .(3.6)

Figures1and2show the evolution of?=+1(right moving)plane waves for n=1,2. The probabilityψ?(t,x)ψ(t,x)(whereψ?(t,x):=t

?π/2π/2k ?π/2π/2ω

π?π?π/2π/2k

?π/2

π/2

ωπ?πFigure 3.The dispersion relation for θ=π/3

and ρ=π/4.π/12≤|ω|≤5π/12.Figure 4.The dispersion relation in the ‘mass-less’case θ=ρ=π/6.|ω|≤2π/3.

Notice that when the wave number k increases,so does the frequency ω—the time period is shorter in Figure 2than in Figure 1.Also the phase velocity ω/k decreases—the crest of the wave moves more slowly.In fact,(3.5)is the exact dispersion relation ,giving the frequency in terms of the wave number.Figure 3graphs the dispersion relation for the QLGA with the same rule parameters used in the simulations of Figures 1and 2.The graph has re?ection symmetry about both axes since (3.5)is invariant under both k →?k and ω→?ω.Each re?ection alone changes the direction of the plane wave.When k =0,ω=±(θ?ρ);when k =±π,ω=±(θ+ρ?π).These values exemplify another symmetry of the dispersion relation—invariance under θ←→ρ;this is a symmetry in the QLGA rule space which is not realizable by a local unitary transformation.Figure 4graphs the special case of equal rule parameters;here the dispersion relation passes through the origin.

By comparison with plane waves in continuum quantum mechanics [16,17],we know that ωand k should be interpreted as being proportional to energy and momentum ,respectively.Expanding the dispersion relation (3.5)around k =0and ω=0to second order,we ?nd ω2=k 2cos θcos ρ+2 1?cos(θ?ρ) .(3.7)For a relativistic particle in the continuum,

E 2=p 2c 2+m 2c 4.(3.8)

Comparing (3.7)and (3.8)suggests that the 1?cos(θ?ρ)=0case,i.e.,the θ=ρcase shown in Figure 4,corresponds to the particle being massless .

Not only do the plane wave parameters ωand k bear the interpretation of proportion-ality to the conserved quantities energy and momentum,but they also label a complete set of (nonlocal)conserved quantities for the QLGA.Since T is orthogonal its eigenvectors Ψ(k,?)are orthogonal for distinct wave numbers k .Furthermore,D (k )is unitary,so its

eigenvectorsψ(k,±1)(0)are orthogonal for each k and hence so are the plane wavesΨ(k,±1). Since we normalized the eigenvectors of D(k)to have length1/N,the plane waves(3.6) form an orthonormal basis for H which we denote by{|k,? }.Consider any state vector Ψ∈H:

Ψ= xψ(x)|x

= xψ(x) k,?|k,? k,?|x

= k,? x k,?|x ψ(x) |k,?

The parenthesized expression is the amplitude of|k,? in the new basis:

(k):= x k,?|x ψ(x)

?

= x yψ(k,?)(0)e iky|y ?|x ψ(x)

= ψ(k,?)(0) ? xψ(x)e?ikx(3.9)

=: ψ(k,?)(0) ??ψ(k),

where?ψ(k)is the discrete Fourier transform ofψ(x).The plane waves|k,? evolve by phase multiplication so the probabilities

Figure5.Evolution of the k0=π/4wave packet(4.1)with width s=32for rule parameters

θ=π/3,ρ=π/4.The probability peak moves from x=31at t=0to x=54at t=49;thus

the group velocity is approximately23/49≈0.47.

for even s≤N,where the inverse binomial coe?cient outside the sum is the requisite

normalization factor.This wave packet is localized around x0,having support on the interval[x0?s/2,x0+s/2].Figure5shows the evolution for wave number k0=π/4and width s=32on the lattice Z64.The rule parameters are the same as those used in the

simulations shown in Figures1and2.In contrast to those graphs,the vertical axis in Figure5shows the probability that the particle is in the state|x .

This simulation shows that the k0=π/4wave packet moves with well de?ned group

velocity to the right.The result is just what we would expect by analogy with the contin-uum situation and can be analyzed in the same way,by transforming to the|k,? basis. Using(3.9)we compute the amplitudes in this basis:

(k)= ψ(k,?)(0) ? 2s s ?1 xψ(k0,+1)(0)e ik0x s x?x0+s/2 e?ikx

?

= ψ(k,?)(0) ?ψ(k0,+1)(0) 2s s ?1 x s x?x0+s/2 e i(k0?k)x

= ψ(k,?)(0) ?ψ(k0,+1)(0) 2s s ?1e i(k0?k)(x0?s/2)(1+e i(k0?k))s

= ψ(k,?)(0) ?ψ(k0,+1)(0) 2s s ?1e i(k0?k)x02s cos s k0?k

Figure6.Evolution of the k0=π/4wave packet(4.1)with width s=8for rule parameters

θ=π/3,ρ=π/4.This wave packet disperses more rapidly than the one shown in Figure5:

The peak probability at the end of the simulation is less than half of the initial peak probability;

left moving ripples carrying o?some of the probability are also visible.

The amplitudes(4.2)give probabilities peaked around k=k0,so this is also a wave packet in momentum space.As usual,the group velocity is the slope of the dispersion relation (3.5),i.e.,dω/d k|k

,which is 6/4≈0.49for the values used in the simulation of 0

Figure5;this is in good agreement with the measured value of approximately0.47.

The width of the peak in(4.2)depends inversely on s:as s decreases,i.e.,the width of the wave packet in position space decreases,the width of the momentum peak increases. The simulation in Figure6shows the evolution of a wave packet with width s=8.We note that while the group velocity is the same as in Figure5,there is substantially more dispersion,indicating a greater interval of contributing wave numbers.This is a general result,not depending on the speci?c form of our wave packet;the reciprocity relation for the discrete Fourier transform has consequences similar to those of the uncertainty relation for the continuous Fourier transform[19].

Figure7shows a simulation of a wave packet built from the plane wave with smallest nonzero wave number on the Z64lattice:k0=π/32.The horizontal tangent to the graph of the dispersion relation at k=0,as shown in Figure3,indicates that the group velocity of this wave packet will be small.Furthermore,even with width s=32the wave number interval includes the left going modes whose presence is visible in Figure7;the consequence is an interference pattern and no very well de?ned group velocity.

Figure7.Evolution of the k0=π/32wave packet(4.1)with width s=32;the rule parameters are stillθ=π/3,ρ=π/4.This wave packet disperses even more rapidly than the one shown in Figure6:left moving waves carry o?some of the probability and an interference pattern is created.

Figure8.Evolution of the k0=π/32wave packet(4.1)with width s=32for rule parameters θ=π/6=ρ.This wave packet disperses very little and has group velocity close to1in lattice units.

Finally,Figure 8shows the evolution of the same wave packet but for the rule pa-rameters θ=π/6=ρwhose dispersion relation is graphed in Figure 4.Here the group velocity is close to 1in lattice units,even for k 0as small as π/32;the particle is indeed ‘massless’.There is almost no dispersion in this simulation;the probability contained in left going modes is nonzero,but too small by several orders of magnitude to be visible in Figure 8.

5.Potentials

The one particle QLGA described in Section 2is the most general homogeneous model for particle speeds no more than 1.To simulate physical systems (or to do useful computation),some inhomogeneity must be introduced.In each of the equations (2.6),which express the unitarity condition,all the w i correspond to the scattering/interaction at a single lattice point,as do all the w ?i .In the ?rst equation these are the same lattice point,while in the second and third they are di?erent.Thus if w i (x )=w i ,constants independent of x ,solve these equations,so do e ?iφ(x )w i .As observed already by Feynman [1]and Riazanov [2],such an x dependent phase realizes an inhomogeneous potential in the continuum limit of the discrete path sum for the Dirac equation.Here we investigate its e?ects on the quantum mechanics of our LGA,expecting them to be similar to those in the continuum limit.

For simplicity,we restrict our attention to a ?nite square well potential,i.e.,w i (x ):= e ?iφw i if N/4≤x <3N/4;w i otherwise,

where the w i are de?ned by (2.7).We begin by considering the e?ect of di?erent values for φ.?π/2π/2??π?π/2π/2πωπ?πFigure 9.The eigenvalues ωof U for a square well of depth φand width N/2on a lattice of size

N =8with θ=π/3and ρ=π/4.

Recall that the frequency (or energy)

eigenvalues ωare doubly degenerate ex-

cept for those with the largest and small-

est absolute value.(See Figure 3,where

each horizontal line intersecting the graph

of the dispersion relation does so at two

points except when tangent to the maxi-

mum or minimum of either branch of the

curve.)As with any perturbation to the

evolution,we expect the introduction of

an inhomogeneity in the potential to re-

solve the degenerate eigenvalues.Figure

9shows that this is indeed the case:as φ

increases away from 0the eigenvalues ωof

U increase and the degenerate ones split.The eigenvalues in Figure 9have been computed for only N =8;Figure 10shows

?π/2

π/2??π

?π/2π/2πω

π?π?π/2π/2?

?π?π/2π/2

πωπ?πFigure 10.The eigenvalues ωof U for a square

well of depth φand width N/2on a lattice of size

N =32with θ=π/3and ρ=π/4.Figure 11.The eigenvalues ωof U for a square well of depth φand width N/2on a lattice of size N =32in the ‘massless’case θ=π/6=ρ.

the results for N =32.On the larger lattice it is clear what happens:the horizontal bands of frequency/energy eigenvalues correspond to the eigenvalues of the unperturbed,homogeneous system,while the diagonal bands of eigenvalues correspond to the same ones,but shifted by the depth φof the square well.The periodicity along the frequency axis shown in these graphs is a symptom of the ambiguity in the de?nition of energy due to discrete time evolution [20].The graphs in Figures 9and 10have been computed for the QLGA with parameter values θ=π/3,ρ=π/4,the dispersion relation for which is shown in Figure 3.Repeating the calculations for the ‘massless’case,with dispersion relation shown in Figure 4,gives the frequency/energy eigenvalue plot shown in Figure 11.The degenerate levels still split,but much less than before for the same φvalues,and the part of the band structure resulting from the nonzero minimum positive frequency in the massive dispersion relation vanishes.

Now consider the eigenvectors of U ,namely the eigenfunctions for our discrete version of a ?nite square well.Since U is no longer translation invariant we do not have an equation like (3.3)to solve for the eigenfunctions analytically.Rather than developing a cross boundary matching method as is used in the continuum problem for a periodic square well potential [21],here we simply ?nd the eigenvectors of U numerically.Figure 12shows the eigenfunctions corresponding to the three smallest positive eigenvalues for the QLGA with θ=π/3and ρ=π/4.The depth of the square well is φ=π/24.We see exactly the lowest modes we would expect from our experience with such a potential in the continuum.As the energy of the eigenfunction increases there is greater probability that the particle is outside the well—in the region of higher potential.Figure 13shows an eigenfunction which is approximately a plane wave in both regions:it has larger wave number in the well than outside it.For analytic results on the closely related problem of a step potential,and some discussion of their consequences for the physical interpretation of QLGA,see [15].

64128192256x

Re(ψ

-1

)

064128192256x

Re(ψ

-1

) 0

Figure12.The three eigenfunctions of U with smallest positive eigenvalues:π/12<0.2622< 0.2634<0.2653,for a square well of depthπ/24 and width N/2on a lattice of size N=256with θ=π/3andρ=π/4.Figure13.An eigenfunction for a particle with eigenvalue0.3985<5π/12large enough not to be con?ned completely to the square well of the previous?gure.Outside the square well the wave number decreases and the probability increases.

Finally,suppose we prepare one of the semiclassical wave packets studied in Section 4in a?nite square https://www.doczj.com/doc/df11384199.html,ing the dispersion relation(3.5),we?nd that the k0=π/4, width s=32wave packet(4.1)of Figure5has peak frequencyω0=Cos?1 (1+√

Figure14.Evolution of the k0=π/4wave packet(4.1)with width s=32for rule parameters

θ=π/3,ρ=π/4in a square well of depthφ=π/6.There is very little re?ection as the wave

packet passes the right wall of the square well at x=3N/4=48.

whenρ=0.In this paper we have begun to investigate the quantum mechanics of the gen-eral two parameter rule.We?nd that even without going to a continuum limit the QLGA reproduces the quantum mechanical phenomena of plane waves and wave packets obeying a dispersion relation(3.5).Furthermore,the model straightforwardly accommodates the inclusion of inhomogeneous potentials.The eigenvectors of the evolution matrix give the quantum mechanical eigenfunctions for the lattice gas particle,and simulations exhibit the semi-classical evolution of a wave packet,in the presence of a square well potential.

Taking a QLGA seriously as a possible model for quantum computation by,for ex-ample,ballistic electrons in a lattice of solid state nanostructures,raises many additional questions,some of which will be addressed in subsequent papers in this series:Inhomo-geneity of the substrate can be incorporated in the model by varying the rule parameters while maintaining global unitarity.Finite,non-periodic,boundary conditions can be im-posed similarly[22].Higher dimensional[2,4,10,14]and multiparticle[6,14,17]models can also be constructed.Decoherence is the crucial problem for quantum computers[8],par-ticularly in the solid state[23].QLGA provide an extremely convenient arena in which to model this problem[24].Finally,the question of for which quantum computational tasks QLGA are best suited deserves serious investigation.

(?gure available from author)

Figure15.Evolution of the same wave packet with the same rule parameters as in Figure14, but now in a square well of depthφ=π/4.There is both re?ection and transmission as the wave packet scatters o?the right wall of the square well.

(?gure available from author)

Figure16.Evolution of the same wave packet with the same rule parameters as in Figures14 and15,but now in a square well of depthφ=π/3.This well is deep enough that the wave packet is almost entirely re?ected by the right wall of the square well.

Acknowledgements

It is a pleasure to thank Peter Doyle,Mike Freedman,Peter Monta and Richard Stong for discussions on various aspects of this project.I also bene?ted from conversations with Herbert Bernstein,Thomas Beth,P¨a ivi T¨o rm¨a and Harald Weinfurter at the4th Workshop on Quantum Computation sponsored by the ISI Foundation.

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B.Hasslacher,“Parallel billiards and monster systems”,in N.Metropolis and G.-

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Rota,eds.,A New Era in Computation(Cambridge:MIT Press1993)53–65;

R.Mainieri,“Design constraints for nanometer scale quantum computers”,preprint (1993)LA-UR93-4333,cond-mat/9410109;

M.Biafore,“Cellular automata for nanometer-scale computation”,Physica D70 (1994)415–433.

[8]D.P.DiVincenzo,“Quantum computation”,Science270(1995)255–261;

I.L.Chuang,https://www.doczj.com/doc/df11384199.html,?amme,P.W.Shor and W.H.Zurek,“Quantum computers,

factoring,and decoherence”,Science270(1995)1633–1635;

A.Barenco and A.Ekert,“Quantum computation”,Acta Physica Slovaca45(1995)

1–12;

and references therein.

[9]R.P.Feynman,“Simulating physics with computers”,Int.J.Theor.Phys.21(1982)

467–488.

[10]D.A.Meyer,“On the absence of homogeneous scalar unitary cellular automata”,

UCSD preprint(1995),quant-ph/9604011,to appear in Phys.Lett.A.

[11]C.D¨u rr,H.L?e Thanh and M.Santha,“A decision procedure for well-formed linear

quantum cellular automata”,in C.Puecha and R.Reischuk,eds.,STACS96:Pro-ceedings of the13th Annual Symposium on Theoretical Aspects of Computer Science, Grenoble,France,22–24February1996,Lecture notes in computer science1046(New York:Springer-Verlag1996)281–292;

C.D¨u rr and M.Santha,“A decision procedure for unitary linear quantum cellular

automata”,preprint(1996),quant-ph/9604007;

D.A.Meyer,“Unitarity in one dimensional nonlinear quantum cellular automata”,

UCSD preprint(1996),quant-ph/9605023;

W.van Dam,“A universal quantum cellular automaton”,preprint(1996).

[12]R.P.Feynman,“Quantum mechanical computers”,Found.Phys.16(1986)507–531.

[13]P.Benio?,“Quantum ballistic evolution in quantum mechanics:application to quan-

tum computers”,preprint(1996),quant-ph/9605022.

[14]B.M.Boghosian and W.Taylor,IV,“A quantum lattice-gas model for the many-

particle Schr¨o dinger equation in d dimensions”,preprint(1996)BU-CCS-960401, PUPT-1615,quant-ph/9604035.

[15]D.A.Meyer,“Quantum lattice gases and their invariants”,UCSD preprint.

[16]P.A.M.Dirac,The Principles of Quantum Mechanics,fourth edition(Oxford:Oxford

University Press1958).

[17]H.Weyl,The Theory of Groups and Quantum Mechanics,translated from the second

(revised)German edition by H.P.Robertson(New York:Dover1950).

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1548;

Z.Cheng,J.L.Lebowitz and E.R.Speer,“Microscopic shock structure in model particle systems:The Boghosian-Levermore cellular automaton revisited”,Commun.

Pure Appl.Math.XLIV(1991)971–979;

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Phys.68(1992)575–590.

[19]W.L.Briggs and V.E.Henson,The DFT:An owner’s manual for the discrete Fourier

transform(Philadelphia:SIAM1995).

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systems”,J.Stat.Phys.53(1988)323–344.

[21]J.C.Slater,Quantum Theory of Matter(New York:McGraw-Hill1951)Chapter10.

[22]D.A.Meyer,“Quantum mechanics of lattice gas automata.II.Boundary conditions

and other inhomogeneities”,UCSD/IPS preprint.

[23]T.Ando,A.B.Fowler and F.Stem,“Electronic properties of two-dimensional sys-

tems”,Rev.Mod.Phys.54(1982)437–672;

D.K.Ferry and H.L.Grubin,“Modeling of quantum transport in semiconductor

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and references therein.

[24]D.A.Meyer,in preparation.

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目录

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摘要
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关键词:FPGA 自动增益控制 等精度测量法
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目录
摘 要....................................................................................................................1 目录........................................................................................................................ 2 1. 系统方案...................................................................................................3
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1.2. 方案描述............................................................................................4 2. 电路设计...................................................................................................4
2.1. 宽带通道放大器分析........................................................................4 2.2. 正弦波整形电路................................................................................5 3. 软件设计...................................................................................................6 4. 测试方案与测试结果...............................................................................6 4.1. 测试仪器............................................................................................6 4.2. 测试方案及数据................................................................................7
4.2.1. 频率测试.....................................................................................7 4.2.2. 时间间隔测量.............................................................................7 4.2.3. 占空比测量.................................................................................8 4.3. 测试结论............................................................................................9 参考文献................................................................................................................ 9
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