Based on: J. R. Johansson (robert@riken.jp), http://jrjohansson.github.io, and Eunjong Kim.
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from sympy import *
init_printing()
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from sympsi import *
from sympsi.boson import *
from sympsi.pauli import *
The Jaynes-Cummings model is one of the most elementary quantum mechanical models light-matter interaction. It describes a single two-level atom that interacts with a single harmonic-oscillator mode of a electromagnetic cavity.
The Hamiltonian for a two-level system in its eigenbasis can be written as
$$ H = \frac{1}{2}\omega_q \sigma_z $$and the Hamiltonian of a quantum harmonic oscillator (cavity or nanomechanical resonator, NR) is
$$ H = \hbar\omega_r (a^\dagger a + 1/2) $$$$ H = \hbar\omega_{NR} (b^\dagger b + 1/2) $$The atom interacts with the electromagnetic field produced by the cavity (NR) mode $a + a^\dagger$ ($b + b^\dagger$) through its dipole moment. The dipole-transition operators is $\sigma_x$ (which cause a transition from the two dipole states of the atom). The combined atom-cavity Hamiltonian can therefore be written in the form
$$ H = \hbar \omega_r (a^\dagger a + 1/2) + \hbar \omega_{NR} (b^\dagger b + 1/2) + \frac{1}{2}\hbar\Omega\sigma_z + \hbar \sigma_x \left( g(a + a^\dagger) + \lambda(b + b^\dagger)\right) $$Although the Jaynes-Cumming Hamiltonian allow us to evolve the given initial state according to the Schrödinger Equation, in an experiment we would like to predicte the response of the coupled cavity-NR-qubit system under the influence of driving fields for the cavity and qubit, and also account for the effects of dissipation and dephasing (not treated here)
The external coherent-state input may be incorporated in the Jaynes-Cummings Hamiltonian by addition of terms involving the amplitude of the driving field $\vec{E_d} \left(\vec{E_s}\right)$ and it's frequency $\omega_d\left(\omega_s\right)$
$$ H_{cavity} = E_d \left(e^{i\omega_dt}a +e^{-i\omega_dt}a^\dagger\right) $$$$ H_{qubit} = E_s \left(e^{i\omega_st}\sigma_- +e^{-i\omega_st}\sigma_+\right) $$$$ H_{NR} = E_p \left(e^{i\omega_st}b +e^{-i\omega_st}b^\dagger\right) $$To obtain the Jaynes-Cumming Hamiltonian
$$ ? H = \hbar\omega_r (a^\dagger a + 1/2) %-\frac{1}{2}\Delta\sigma_x + \frac{1}{2}\hbar\Omega\sigma_z + \hbar \sigma_x \left( g(a + a^\dagger) + \lambda(b + b^\dagger)\right) $$we also need to perform a rotating-wave approximation which simplifies the interaction part of the Hamiltonian. In the following we will begin with looking at how these two Hamiltonians are related.
To represent the atom-cavity Hamiltonian in SymPy we creates an instances of the operator classes BosonOp and SigmaX, SigmaY, and SigmaZ, and use these to construct the Hamiltonian (we work in units where $\hbar = 1$).
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# CPW, qubit and NR energies
omega_r, omega_q, omega_nr = symbols("omega_r, omega_q, omega_{NR}")
# Coupling CPW-qubit, NR_qubit
g, L, chi, eps = symbols("g, lambda, chi, epsilon")
# Detuning
# Drives and detunnings
Delta_d, Delta_s, Delta_p = symbols(" Delta_d, Delta_s, Delta_p ")
A, B, C = symbols("A,B,C") # Electric field amplitude
omega_d, omega_s, omega_p = symbols("omega_d, omega_s, omega_p") # drive frequencies
# Detunning CPW-qubit, NR-qubit
Delta_CPW, Delta_NR = symbols("Delta_{CPW},Delta_{NR}")
# auxilary variables
x, y, t, Hsym = symbols("x, y, t, H ")
Delta_SP, Delta_SD = symbols("Delta_{SP},Delta_{SD}")
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# omega_r, omega_q, g, Delta_d, Delta_s, t, x, chi, Hsym = symbols("omega_r, omega_q, g, Delta_d, Delta_s, t, x, chi, H")
# A, B, C = symbols("A,B,C") # Electric field amplitude
# omega_d, omega_s = symbols("omega_d, omega_s") #
# omega_nr, L = symbols("omega_{NR},lambda")
# Delta,Delta_t = symbols("Delta,Delta_t")
# y, omega_t = symbols("y, omega_t")
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sx, sy, sz, sm, sp = SigmaX(), SigmaY(), SigmaZ(), SigmaMinus(), SigmaPlus()
a = BosonOp("a")
b = BosonOp("b")
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H = omega_r * Dagger(a) * a + omega_q/2 * sz + omega_nr * Dagger(b) * b
H_int = sx * (g * (a + Dagger(a)))
H_int_2 = sx *( L * (b + Dagger(b)))
H_drive_r = A * (exp(I*omega_d*t)*a + exp(-I*omega_d*t)*Dagger(a))
H_drive_q = B * (exp(I*omega_s*t)*sm + exp(-I*omega_s*t)*sp)
H_drive_NR = C * (exp(I*omega_p*t)*b + exp(-I*omega_p*t)*Dagger(b))
H_total = H + H_drive_r + H_drive_q + H_drive_NR #+ H_int + H_int_2 +
Eq(Hsym,H_total)
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First move to the interaction picture:
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U = exp(I * omega_r * t * Dagger(a) * a)
U
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H2 = hamiltonian_transformation(U, H_total.expand(),independent = True)
H2
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U = exp(I * omega_q * t * sp * sm)
U
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H3 = hamiltonian_transformation(U, H2.expand(), independent = True)
H3 = H3.subs(sx, sm + sp).expand()
H3 = powsimp(H3)
H3
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We introduce the detuning parameter $\Delta = \omega_q - \omega_r$ and substitute into this expression
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# trick to simplify exponents
def simplify_exp(e):
if isinstance(e, exp):
return exp(simplify(e.exp.expand()))
if isinstance(e, (Add, Mul)):
return type(e)(*(simplify_exp(arg) for arg in e.args))
return e
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H4 = simplify_exp(H3).subs(-omega_r + omega_q, Delta_CPW)
H4
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Now, in the rotating-wave approximation we can drop the fast oscillating terms containing the factors $ e^{\pm i( (\omega_q + \omega_r)t}$
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H5 = drop_terms_containing(H4, [exp( I * (omega_q + omega_r) * t),
exp(-I * (omega_q + omega_r) * t)])
H5 = drop_c_number_terms(H5.expand())
Eq(Hsym, H5)
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This is the interaction term of in the Jaynes-Cumming model in the interaction picture. If we transform back to the Schrödinger picture we have:
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U = exp(-I * omega_r * t * Dagger(a) * a)
H6 = hamiltonian_transformation(U, H5.expand(), independent =True)
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U = exp(-I * omega_q * t * sp * sm)
H7 = hamiltonian_transformation(U, H6.expand(),independent = True)
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H8 = simplify_exp(H7).subs(Delta_CPW, omega_q - omega_r)
H8 = simplify_exp(powsimp(H8)).expand()
H8 = drop_c_number_terms(H8)
H = collect(H8, [g])
Eq(Hsym, H)
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H = collect(H,[L*sm])
H = collect(H,[L*sp])
H = collect(H,[L*(Dagger(b)+b), A, B ,C])
H = H.subs(sm+sp,sx)
H
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U = exp(I * Dagger(a) * a * omega_d * t)
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H1 = hamiltonian_transformation(U, H_total, independent=True)
H1
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U = exp(I * Dagger(b) * b * omega_p * t)
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H2 = hamiltonian_transformation(U, H1, independent=True)
H2
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U = exp(I * Dagger(sm) * sm * omega_s * t)
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H3 = hamiltonian_transformation(U, H2, independent=True)
H3
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# H4 = simplify_exp(H3).subs(omega_d - omega_s, -Delta_SD)
H4 = H3.collect([A,B,C,Dagger(a)*a,Dagger(b)*b, g, L])
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H4
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H5 = H4.expand().subs(-sp*sm , -sz/2 ).collect([A,B,C,Dagger(a)*a,Dagger(b)*b, g, L, sz])
H5
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H5 = H5.subs(-omega_d + omega_r, -Delta_d).subs(-omega_p + omega_nr, - Delta_p).subs(omega_q/2 - omega_s/2, Delta_s); H5
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H_total_01 = H5
Eq(Hsym,H_total_01)
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H_total_01 = H_total_01 + H_int + H_int_2;
Eq(Hsym,H_total_01)
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U = exp(I * omega_r * t * Dagger(a) * a)
U
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H2 = hamiltonian_transformation(U, H_total_01.expand(),independent = True)
H2
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U = exp(I * omega_q * t * sp * sm)
U
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H3 = hamiltonian_transformation(U, H2.expand(), independent = True)
H3 = H3.subs(sx, sm + sp).expand()
H3 = powsimp(H3)
H3
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H4 = simplify_exp(H3).subs(-omega_r + omega_q, Delta_CPW)
H4
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H5 = drop_terms_containing(H4, [exp( I * (omega_q + omega_r) * t),
exp(-I * (omega_q + omega_r) * t)])
H5 = drop_c_number_terms(H5.expand())
Eq(Hsym, H5)
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U = exp(-I * omega_r * t * Dagger(a) * a)
H6 = hamiltonian_transformation(U, H5.expand(), independent =True)
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U = exp(-I * omega_q * t * sp * sm)
H7 = hamiltonian_transformation(U, H6.expand(),independent = True)
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H8 = simplify_exp(H7).subs(Delta_CPW, omega_q - omega_r)
H8 = simplify_exp(powsimp(H8)).expand()
H8 = drop_c_number_terms(H8)
H = collect(H8, [g])
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H = collect(H,[L*sm])
H = collect(H,[L*sp])
H = collect(H,[L*(Dagger(b)+b), A, B ,C])
H = H.subs(sm+sp,sx)
H
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U = exp((x * ( -Dagger(a) * sm + a * sp )).expand())
U
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H1 = hamiltonian_transformation(U, H, expansion_search=False, N=4).expand()
H1 = qsimplify(H1)
# H1
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H2 = drop_terms_containing(H1.expand(), [x**3, x**4,x**5,x**6])
# H2
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H2
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H3 = H2.subs(x, g/Delta_CPW)
# H3
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H4 = drop_c_number_terms(H3)
# H4
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H5 = collect(H4, [Dagger(a) * a, sz])
# H5
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U = exp(I * omega_r * t * Dagger(a) * a)
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H6 = hamiltonian_transformation(U, H5.expand(),independent = True);
# H6
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U = exp(I * omega_q * t * Dagger(sm) * sm)
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H7 = hamiltonian_transformation(U, H6.expand(),independent=True);
# H7
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H8 = drop_terms_containing(H7, [exp(I * omega_r * t), exp(-I * omega_r * t),
exp(I * omega_q * t), exp(-I * omega_q * t)])
# H8
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H9 = qsimplify(H8)
H9 = collect(H9, [Dagger(a) * a, sz])
# H9
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U = exp(-I * omega_r * t * Dagger(a) * a)
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H10 = hamiltonian_transformation(U, H9.expand(),independent = True);
# H10
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U = exp(-I * omega_q * t * Dagger(sm) * sm)
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H11 = hamiltonian_transformation(U, H10.expand(),independent=True);
# H11
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# H11 = drop_c_number_terms(H11)
H12 = qsimplify(H11)
# H12 = collect(H12, [Dagger(a) * a, sz])
H12 = H12.subs(omega_r, omega_q-Delta_CPW).expand().collect([A,B,C, L,g]).subs(omega_q-Delta_CPW,omega_r)
Eq(Hsym, H12)
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H12.collect(C)
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