Heat flow from environment approach

Gð Þ ¼y X

n, m∣hm∣U nij j²p_ne^iyð^εmf�εinÞ ¼X

n, mDnjU^†e^iyεmfjmE

mjUe^�iyεinp_njn

D E

¼X

n, m�njU^†e^iyHfjm�

mjUe^�iyHiρjn

� �

¼Tr U� ^†ð Þeτ ^iyHfUð Þeτ ^�iyHiρ�

: (58)

Hence, it may be concluded that

G ¼Tr U� ^†ð Þeτ ^iyHfUð Þeτ ^�iyHiρ�

: (59)

This turns out to be somewhat easier to work with thanPð Þ, and (59) plays aW similar role as Z in equilibrium statistical mechanics. From G yð Þ, the statistical moments of W can be found by expanding

Gð Þ ¼y �e^iyW�

¼1þiy Wh i �y² 2 �W²�

�y³ 6�W³�

þ⋯: (60)

A formula for the quantum mechanical formula for the moments can be found as well. The average work is Wh i ¼�H_f�

�h i, where for any operator A, we haveH_i h iA _t¼Tr U� ^†ð ÞAU tt ð Þρ�

as the expectation value of A at time t. This follows from the fact that the state of the system at t isρð Þ ¼t U tð ÞρU tð Þ^†. From the definition ofG, it ought to be the case that G yð ¼iβÞ ¼�e^�βW�

. However,ρin (38) and (59) yields Gð Þ ¼iβ 1

Z_iTr U� ^†e^�βHfU�

¼ 1

Z_iTr e� ^βHf�

¼Zf

Z_i : (61)

Using Z¼e^�βF, (61) yields (50)

Gð Þ ¼iy �e^�βW�

¼e^�βΔF: (62)

Nothing has been assumed about the speed of this process. Thus inequality (50) must hold for a process arbitrarily far from equilibrium.

Gð Þ ¼y X

n, m∣hm∣U nij j²p_ne^iyð^εmf�εinÞ ¼X

n, mDnjU^†e^iyεmfjmE

mjUe^�iyεinp_njn

D E

¼X

n, m�njU^†e^iyHfjm�

mjUe^�iyHiρjn

� �

¼Tr U� ^†ð Þeτ ^iyHfUð Þeτ ^�iyHiρ�

: (58)

Hence, it may be concluded that

G ¼Tr U� ^†ð Þeτ ^iyHfUð Þeτ ^�iyHiρ�

: (59)

This turns out to be somewhat easier to work with thanPð Þ, and (59) plays aW similar role as Z in equilibrium statistical mechanics. From G yð Þ, the statistical moments of W can be found by expanding

Gð Þ ¼y �e^iyW�

¼1þiy Wh i �y² 2�W²�

�y³ 6�W³�

þ⋯: (60) A formula for the quantum mechanical formula for the moments can be found as well. The average work is Wh i ¼�H_f�

�h i, where for any operator A, we haveH_i h iA _t¼Tr U� ^†ð ÞAU tt ð Þρ�

as the expectation value of A at time t. This follows from the fact that the state of the system at t isρð Þ ¼t U tð ÞρU tð Þ^†. From the definition ofG, it ought to be the case that G yð ¼iβÞ ¼�e^�βW�

. However,ρin (38) and (59) yields Gð Þ ¼iβ 1

Z_iTr U� ^†e^�βHfU�

¼ 1

Z_iTr e�^βHf�

¼Zf

Z_i: (61)

Using Z¼e^�βF, (61) yields (50)

Gð Þ ¼iy �e^�βW�

¼e^�βΔF: (62)

Nothing has been assumed about the speed of this process. Thus inequality (50) must hold for a process arbitrarily far from equilibrium.

4. Heat flow from environment approach

There is another somewhat different way in which the Jarzynski inequality can be generalized to quantum dynamics. In a classical system, the energy of the system can be continuously measured as well as the flow of heat and work. Continuous measurement is not possible in quantum mechanics without disrupting the dynam- ics of the system [20].

A more satisfactory approach is to realize that although work cannot be contin- uously measured, the heat flow from the environment can be measured. To this end, the system of interest is divided into a system of interest and a thermal bath.

The ambient environment is large, and it rapidly decoheres and remains at thermal equilibrium, uncorrelated and unentangled with the system. Consequently, we can measure the change in energy of the bathð�QÞwithout disturbing the dynamics of the system. The open-system Jarzynski identity is expressed as

e^�βW

� �

¼�e^�βEfe^βQe^βEi�

¼e^�βΔF: (63) For a system that has equilibrated with Hamiltonian H interacting with a ther- mal bath at temperature T, the equilibrium density matrix isρ¼e^βH=Z¼e^�βF�βH, Quantum Mechanics

whereβ¼1=K_BT. The dynamics of an open quantum system is described by a quantum operator~ρ¼Sρ, a linear trace-preserving, complete positive map of operators. Any such complete positive superoperator has an operator-sum representation

Sρ¼X

α

AαρA^†_α: (64)

Conversely, any operator-sum represents a complete positive superoperator.

The set of operators Af gα is often called Krauss operators. The superoperator is trace-preserving and conserves probability ifP

αA^†_αAα¼I. In the simplest case, the dynamics of an isolated quantum system is described by a single unitary operator U^† ¼U^�1.

The interest here is in the dynamics of a quantum system governed by a time- dependent Hamiltonian weakly coupled to an extended, thermal environment. Let the total Hamiltonian be

H¼H^Sð Þt ⊗I^BþI^S⊗H^BþεH^int, (65) where I^Sand I^Bare system and bath identity operators, H^Sð Þt the system Hamiltonian, H^Bthe bath Hamiltonian, and H^intthe bath-system interaction withε a small parameter. Assume initially the system and environment are uncorrelated such that the initial combined state isρ^S⊗ ρ^B_eq, whereρ^B_eqis the thermal density equilibrium matrix of the bath.

By following the unitary dynamics of the combined total system for a finite time and measuring the final state of the environment, a quantum operator description of the system dynamics can also be obtained:

S s, tð Þρ^S¼ TrBU�ρ^S⊗ ρ^B_eq�

U^†¼X

i, f

hbf∣Uðρ^S⊗ðX

i

e^�βεBi

Z^B ∣bii�bijÞÞU^†jbf�

¼ 1 Z_B

X

i, f

e^�βεBi�b_fjUjbi�

ρ^S�bijU^†jb_f� :

(66)

Here U is the unitary evolution operator of the total system U¼ exp i

ℏ ð_t

sHð Þdττ

� �

, (67)

and TrBis the partial trace over the bath degrees of freedom,� �ε^B_i

are the energy eigenvalues,fjbigis the orthonormal energy eigenvectors of the bath, and Z^Bis the bath partition function. Assume the bath energy states are nondegenerate. Then (66) implies the Krauss operators for this dynamics are

A_i,f ¼ 1 ffiffiffiffiffiffi Z_B

p e^�βεBi=2�b_fjUjbi�

: (68)

Suppose the environment is large, with a characteristic relaxation time short compared with the bath-system interactions, and the system-bath couplingεis small. The environment remains near thermal equilibrium, unentangled and uncorrelated with the system. The system dynamics of each consecutive time Entropy in Quantum Mechanics and Applications to Nonequilibrium Thermodynamics DOI: http://dx.doi.org/10.5772/intechopen.91831

interval can be described by a superoperator derived as in (66) which can then be chained together to form a quantum Markov chain:

ρð Þ ¼t S tð �1, tÞ⋯S sð þ1, sþ2ÞS s, sð þ1Þρ: (69) The Hermitian operator of a von Neumann-type measurement can be broken up into a set of eigenvaluesλσand orthonormal projection operatorsπσsuch that H¼ P

σλσπσ. In a more general sense, the measured operator of a positive operator- valued measurement need not be projectors or orthonormal. The probability of observing the a-th outcome is

p_a ¼Tr A� aρA^†_a�

: (70)

The state of the system after this interaction is

~ρ_a¼ AaρA^†_a

Tr A� aρA^†_a�: (71)

The result of the measurement can be represented by using a Hermitian map superoperatorA:

A ¼X

α

aαAαρA^†_α: (72)

An operator-value sum maps Hermitian operators into Hermitian operators:

½AH�^†¼ aαAαHA^†α

� �†

¼X

α aα� �A^† †

H^†A^†α¼ AH: (73) In the other direction, any Hermitian map has an operator-value-mean repre- sentation. Hermitian maps provide a particularly concise and convenient repre- sentation of sequential measurements and correlation functions. For example, suppose Hermitian mapArepresents a measurement at time 0,Cis a different measurement at time t, and the quantum operation Strepresents the system evolution between the measurements. The expectation value of a single measure- ment is

h i ¼a TrðAρÞ ¼X

α

aαTrAαρA^†_α¼X

α

p_αaα: (74)

The correlation function b th ð Þa 0ð Þican be expressed as b tð Þa 0ð Þ

h i ¼ Tr BSð tAρð Þ0 Þ ¼X

α,β

aαbβTr Bα�St�Aαρð ÞA0 ^†_α��

B^†_β: (75)

It may be shown that just as every Hermitian operator represents some mea- surement on the Hilbert space of pure states, every Hermitian map can be associ- ated with some measurement on the Liouville space of mixed states.

A Hermitian map representation of heat flow can now be constructed under assumptions that the bath and system Hamiltonian are constant during the mea- surement and the bath-system coupling is very small. A measurement on the total system is constructed, and thus the bath degrees of freedom are projected out. This leaves a Hermitian map superoperator that acts on the system density matrix alone.

Let us describe the measurement process and mathematical formulation together.

Quantum Mechanics

Begin with a composite system which consists of the bath, initially in thermal equilibrium weakly coupled to the system:

ρ^S⊗ ρ^B_eq: (76)

Measure the initial energy eigenstate of the bath so based on (76):

I^S⊗ ∣biihbi∣

� �

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihbj∣

� �

: (77)

Now allow the system to evolve together with the bath for some time:

U I�^S⊗ ∣biihbi∣�

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihbj∣

� �

U^†: (78)

Finally, measure the final energy eigenstate of the bath:

I^S⊗ ∣biihb_f∣

� �

U I�^S⊗ ∣biihb_i∣�

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihb_j∣

� �

U^†�I^S⊗ ∣bfihb_f∣�

: (79) Taking the trace over the bath degrees of freedom produces the final normalized system density matrix where trace over S gives the probability of observing the given initial and final bath eigenstates. Multiply by the Boltzmann weighted heat, and sum over the initial and final bath states to obtain the desired average Boltzmann weighted heat flow:

e^βQ

� �

¼X

i, f

e^{�β εBf�εBi}

� �

TrSTrBðI^S⊗ ∣b_fihb_f∣ÞUðI^S⊗ ∣biihbi∣Þ ρ^S⊗ ρ^B_eq

� �

ðI^S⊗ ∣bjihb_j∣ÞU^†ðI^S⊗ ∣bjihb_j∣Þ:

(80)

Replace the heat bath Hamiltonian by I^S⊗H^B ¼H�H^Sð Þt ⊗I^B�εH^int. The total Hamiltonian commutes with the unitary dynamics and cancels. The interac- tion Hamiltonian can be omitted in the small coupling limit giving

e^βQ

� �

¼ TrSTrB�e^βHS=2⊗I^S�

U e�^�β=2HS⊗I^B�

ρ^S⊗ ρ^B_eq

� �

e^�βHS=2⊗I^B

� �

U^†�e^βHS=2⊗I^B� (81) Collecting the terms acting on the bath and system separately and replacing the Krauss operators describing the reduced dynamics of the system, the result is

e^βQ

� �

¼ TrSe^βHS=2 TrB�Ue^�βHS=2ρ^Se^�βHS=2�

⊗ ρ^B_eqU^†

� �

e^βHS=2

¼TrS

X

α

e^βHS=2Aαe^�βHS=2ρ^Se^βHS=2A^†_αe^�βHS=2: (82) To summarize, it has been found that the average Boltzmann weighted heat flow is represented by

e^βQ

� �

¼Tr�R^�1SRρ^S�

: (83)

where S represents the reduced dynamics of the system. The Hermitian map superoperatorRtis given by

Entropy in Quantum Mechanics and Applications to Nonequilibrium Thermodynamics DOI: http://dx.doi.org/10.5772/intechopen.91831

interval can be described by a superoperator derived as in (66) which can then be chained together to form a quantum Markov chain:

ρð Þ ¼t S tð �1, tÞ⋯S sð þ1, sþ2ÞS s, sð þ1Þρ: (69) The Hermitian operator of a von Neumann-type measurement can be broken up into a set of eigenvaluesλσ and orthonormal projection operatorsπσsuch that H¼ P

σλσπσ. In a more general sense, the measured operator of a positive operator- valued measurement need not be projectors or orthonormal. The probability of observing the a-th outcome is

p_a¼ Tr A� aρA^†_a�

: (70)

The state of the system after this interaction is

~ρ_a ¼ AaρA^†_a

Tr A� aρA^†_a�: (71)

The result of the measurement can be represented by using a Hermitian map superoperatorA:

A ¼X

α

aαAαρA^†_α: (72)

An operator-value sum maps Hermitian operators into Hermitian operators:

½AH�^† ¼ aαAαHA^†α

� �†

¼X

α aα� �A^† †

H^†A^†α¼ AH: (73) In the other direction, any Hermitian map has an operator-value-mean repre- sentation. Hermitian maps provide a particularly concise and convenient repre- sentation of sequential measurements and correlation functions. For example, suppose Hermitian mapArepresents a measurement at time 0,Cis a different measurement at time t, and the quantum operation Strepresents the system evolution between the measurements. The expectation value of a single measure- ment is

h i ¼a TrðAρÞ ¼X

α

aαTrAαρA^†_α¼X

α

p_αaα: (74)

The correlation function b th ð Þa 0ð Þican be expressed as b tð Þa 0ð Þ

h i ¼ Tr BSð tAρð Þ0Þ ¼X

α,β

aαbβTr Bα�St�Aαρð ÞA0 ^†_α��

B^†_β: (75)

It may be shown that just as every Hermitian operator represents some mea- surement on the Hilbert space of pure states, every Hermitian map can be associ- ated with some measurement on the Liouville space of mixed states.

A Hermitian map representation of heat flow can now be constructed under assumptions that the bath and system Hamiltonian are constant during the mea- surement and the bath-system coupling is very small. A measurement on the total system is constructed, and thus the bath degrees of freedom are projected out. This leaves a Hermitian map superoperator that acts on the system density matrix alone.

Let us describe the measurement process and mathematical formulation together.

Quantum Mechanics

Begin with a composite system which consists of the bath, initially in thermal equilibrium weakly coupled to the system:

ρ^S⊗ ρ^B_eq: (76)

Measure the initial energy eigenstate of the bath so based on (76):

I^S⊗ ∣biihbi∣

� �

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihbj∣

� �

: (77)

Now allow the system to evolve together with the bath for some time:

U I�^S⊗ ∣biihbi∣�

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihbj∣

� �

U^†: (78)

Finally, measure the final energy eigenstate of the bath:

I^S⊗ ∣biihb_f∣

� �

U I�^S⊗ ∣biihb_i∣�

ρ^S⊗ ρ^B_eq

� �

I^S⊗ ∣bjihb_j∣

� �

U^†�I^S⊗ ∣bfihb_f∣�

: (79) Taking the trace over the bath degrees of freedom produces the final normalized system density matrix where trace over S gives the probability of observing the given initial and final bath eigenstates. Multiply by the Boltzmann weighted heat, and sum over the initial and final bath states to obtain the desired average Boltzmann weighted heat flow:

e^βQ

� �

¼X

i, f

e^{�β εBf�εBi}

� �

TrSTrBðI^S⊗ ∣b_fihb_f∣ÞUðI^S⊗ ∣biihbi∣Þ ρ^S⊗ ρ^B_eq

� �

ðI^S⊗ ∣bjihb_j∣ÞU^†ðI^S⊗ ∣bjihb_j∣Þ:

(80)

Replace the heat bath Hamiltonian by I^S⊗H^B¼H�H^Sð Þt ⊗I^B�εH^int. The total Hamiltonian commutes with the unitary dynamics and cancels. The interac- tion Hamiltonian can be omitted in the small coupling limit giving

e^βQ

� �

¼ TrSTrB�e^βHS=2⊗I^S�

U e�^�β=2HS⊗I^B�

ρ^S⊗ ρ^B_eq

� �

e^�βHS=2⊗I^B

� �

U^†�e^βHS=2⊗I^B� (81) Collecting the terms acting on the bath and system separately and replacing the Krauss operators describing the reduced dynamics of the system, the result is

e^βQ

� �

¼TrSe^βHS=2 TrB�Ue^�βHS=2ρ^Se^�βHS=2�

⊗ ρ^B_eqU^†

� �

e^βHS=2

¼TrS

X

α

e^βHS=2Aαe^�βHS=2ρ^Se^βHS=2A^†_αe^�βHS=2: (82) To summarize, it has been found that the average Boltzmann weighted heat flow is represented by

e^βQ

� �

¼ Tr�R^�1SRρ^S�

: (83)

where S represents the reduced dynamics of the system. The Hermitian map superoperatorRtis given by

Entropy in Quantum Mechanics and Applications to Nonequilibrium Thermodynamics DOI: http://dx.doi.org/10.5772/intechopen.91831

Rtρ¼e^�βHt=2ρe^βHt=2: (84) The paired Hermitian map superoperators act at the start and end of a time interval. They give a measure of the change in the energy of the system over that interval. This procedure does not disturb the system beyond that already incurred by coupling the system to the environment. The Jarzynski inequality now follows by applying this Hermitian map and quantum formalism. Discretize the experi- mental time into a series of discrete intervals labeled by an integer t.

The system Hamiltonian is fixed within each interval. It changes only in discrete jumps at the boundaries. The heat flow can be measured by wrapping the

superoperator time evolution of each time interval Stalong with the corresponding Hermitian map measurementsR^�1_t SRt. In a similar fashion, the measurement of the Boltzmann weighted energy change of the system can be measured with e� ^�βΔE�

¼ TrRτSR^�1_τ . The average Boltzmann weighted work of a driven, dissipative quan- tum system can be expressed as

e^�βW

� �

¼ Tr Rτ

Y

t �R^�1_t StRt� R^�1τ ρ^eq₀

!

, (85)

In (85),ρ^t_eqis the system equilibrium density matrix when the system Hamilto- nian is H^S_t.

This product actually telescopes due to the structure of the energy change Hermitian map (84) and the equilibrium density matrix (65). This leaves only the free energy difference between the initial and final equilibrium ensembles, as can be seen by writing out the first few terms

e^�βW

� �

¼Tr R^τ�R^�1τ SτR^τ�

⋯�R^�1₂ S₂R2�

R^�1₁ S₁R1

� �

R^�1₀ ρ⁰_eq

h i

¼Tr τ�R^�1τ SτR^τ�

⋯�R^�1₂ S₂R2�

R^�1₁ S₁R1

� � I

Z 0ð Þ

� �

¼Tr½Rτ�R^�1τ SτRτ�

⋯�R^�1₂ S₂R2�

R^�1₁ S₁ρ¹_eq Z 1ð Þ Z 0ð Þ

� �

⋯¼Zð Þτ

Z 0ð Þ¼e^�βΔF¼e^�βΔF:

(86)

In the limit in which the time intervals are reduced to zero, the inequality can be expressed in the continuous Lindblad form:

e^�βW

� �

¼TrRð Þt exp ð_t

0Rð Þξ ^�1Sð ÞRξ ð Þξ dξ

� �

Rð Þ0^�1ρ^eq₀ ¼e^�βΔF: (87)