Hi There,(adsbygoogle = window.adsbygoogle || []).push({});

Im having a little trouble with scalings required for a fft performed in Matlab. So here is the story:

I construct a position space gaussian, plot this guy out to 6 stdDev. My frontfactor is such that my probability density is normalized.

I know what the actual fourier transform should be in momentum space, thus when I use fft in matlab I should get something that lies ontop of this solution.

My problem is that I can't, also I cannot really see where things are going wrong. So here is some code to peruse if you're curious about helping,

Thanks in advance :D

%A gaussian wavepacket

asimParam %physical constants [SI units]

%experimental parameters

k0=0; %planewave with this wavenumber is modulated by a Gaussian

w=100e-6; %Gaussian width parameter

%simulation parameters

stdDevNum=6; %number of standard deviations before truncation

disp(['Proportion of total Gaussian within (' num2str(stdDevNum) ') std dev. :'...

num2str(erf(stdDevNum/sqrt(2)))]);

%calculate the maximum momentum to use before truncating the Gaussian, this

%is done in terms of the maximum number of std. dev. to consider. This can

%be done because an explicit form of the std dev. is known (See Sakurai)

stdDevX=w/sqrt(2);

xMax=stdDevNum*stdDevX;

grX=1024; %make sure this is base 2pwr as this optimises fft

x=linspace(-xMax,xMax,grX)';

dx=x(2,1)-x(1,1);

%calculate the x-space wavefunction |psi(x,t=0)>

xPsi=1/(pi^(1/4)*sqrt(w))*exp(i*k0*x-x.^2/(2*w^2));

rhoPsiX=conj(xPsi).*xPsi; %rho as a func of x

rhoPsiXint=sum(dx*rhoPsiX); %sum of density (step size uniform);

%Momentum space wavefunction: |phi(p,t=0)>

%First use the explicit form given in Sakurai, then compare the fft

%function in Matlab to ensure things are working correctly.

pPhiAn=@(pIn)sqrt(w/(hb*sqrt(pi)))*exp(-(pIn-hb*k0).^2*w.^2/(2*hb^2));

stdDevP=hb/(sqrt(2)*w);

N=grX; %same number of points as for x-space

dk=1/(N*dx); %such that fourier transform has correct units

dp=hb*dk; %now rescale to momentum

%pMax=stdDevP*stdDevNum;

pMax=dp*N/2;

pIn=linspace(-(pMax+hb*k0), (pMax+hb*k0), N)';

pPhiS=pPhiAn(pIn);

rhoPhiPs=conj(pPhiS).*pPhiS;

rhoPhiPsInt=sum(dp*rhoPhiPs); %sum of density (step size uniform);

%Now using Matlab's fft

pPhi=fftshift(fft(xPsi));

rhoPhiP=conj(pPhi).*pPhi;

%test if it works back to x-space

xPsiF=ifft(fft(xPsi));

rhoPsiXF=conj(xPsiF).*xPsiF;

%The rest of this m-file is comprised of plotting

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# [Matlab] FFT Gaussian x-space -> p-space scalings.

Can you offer guidance or do you also need help?

Draft saved
Draft deleted

**Physics Forums | Science Articles, Homework Help, Discussion**