Hall Conductivity, Transition-State Theory and Their Effect on Entanglement and the Energy Density ()
1. Introduction
A great deal of interest has been recently given to the formulation and possible experimental consequences of the extension of the standard quantum physical formalism to accommodate the non-commutativity of phase space operators [1]-[5]. The idea was inspired by quantum field theory and string theory [6]-[8]. The crucial difference with the standard quantum theory is to replace the usual product with the Moyal product [9] [10]. Therefore, they allow us to better understand various phenomena. So far, many examples have been studied intensively, such as the spectrum of the hydrogen atom [11] [12], the harmonic oscillator [13] [14], the Aharonov-Bohm effect under the action of a magnetic field [15]-[17], the Landau problem [18] [19], etc. We specify the particular system of two isotropic harmonic oscillators in a magnetic field, in the framework of non-commutative quantum mechanics, accordingly, because of the magnetic field application considering the Hall effect. This can be stated as follows: a semiconductor material through which an electric current flows perpendicular to the movement of charge carriers, a voltage is produced from the latter, so called Hall voltage, which has been attributed to the Hall effect. The Hall conductivity connected to the Hall effect has been taken into consideration in this work. Also, in the presence of the magnetic field
, we discuss the localized energy density, which is the amount of energy stored in a point of the material conductor. These two concepts are discussed in detail in this paper to clarify some properties of an entangled system. Entanglement concepts in non-commutative quantum mechanics are studied by many references, see for example [20]-[22] and we applied the transition-state theory to examine their effect on entanglement and the energy density.
2. Theoretical Framework (Definitions, Hamiltonian
Diagonalization)
A harmonic oscillator can behave as an electron under a magnetic field of induction
, in this framework, consider two isotropic harmonic oscillators with unit masses and are exposed to a magnetic field [23]. The Hamiltonian is written:
(2.1)
where
is the common angular frequency of the two harmonic oscillators,
is the electric charge. (2.1) can be reformulated as
(2.2)
where
and
. To diagonalize (2.2), we define the unitary operator
(2.3)
where
acts on Hamiltonian (2.2) as
to get the form
(2.4)
where
(2.5)
The solution (2.4) gives
(2.6)
3. Methods
The non-commutativity of positions
and momentums
operators in two-dimensional space is imposed by the relations [24]
,
and
, where
,
and
,
are the non-commutativity variables. As usual, we assume after, in the numerical section that
and
. Quantization deformation is the suitable method to study eignensolution of Hamiltonian (2.4) in non-commutative phase space [25].
is the Moyal product introducing the non-commutativity, applied when we treat the classical quantities
to replacing the ordinary product, it is given as
(3.1)
where
is defined as a matrix of dimension 2 and we have
Hamiltonian (2.4) can be reformulated as the sum of two Hamiltonians
and
such as
(3.2)
and
(3.3)
In expressions (3.2) and (3.3):
(3.4)
where
(3.5)
The Moyal product acts between
and
commutation relation as
(3.6)
and verify the ordinary relation
(3.7)
The eigenvalue solution of Hamiltonian (2.4) is given by solving the eigenequation
(3.8)
where
is the Wigner function of the
quantum states and
is the corresponding eigenenergy. (3.8) corresponds to the standard two-dimensional Schrödinger equation, the solution is given by the following Wigner functions
(3.9)
and
(3.10)
and
in expressions (3.9) and (3.10) have the forms:
and
(3.11)
We write the Wigner function and the eigenenergy of expression (3.8) as
(3.12)
and
(3.13)
Consequently, we get respectively
(3.14)
(3.15)
One can easily show that
(3.16)
Particularly the vacuum state, (3.14) becomes
(3.17)
To calculate various entropies in non-commutative phase space, we need first to calculate the reduced Wigner functions of each of two harmonic oscillators. Using (3.17), one has
(3.18)
and
(3.19)
In expressions (3.18) and (3.19),
and
(3.20)
Since the system is entangled then expression (3.18) or (3.19) is sufficient to calculate entanglement entropies. The purity function is a derivation of the linear entropy and it also provides another form to examine entanglement, its expression is
(3.21)
Following problem in ref. [26] and by applying the Moyal product
on expression (3.18), we obtain
(3.22)
where
.
By substituting (3.22) in (3.21), we have
(3.23)
Consequently, we can show that
(3.24)
For all integer
, the Rényi entropy writes
(3.25)
In order
,
(3.26)
Insert (3.26) in (3.25), we have
(3.27)
The particular case where
, expression (3.27) reduces to the von Neumann entropy as
(3.28)
From (3.28), we can obtain
as
(3.29)
Such expressions (3.24), (3.27) and (3.29) are interesting because they show all the ingredients to investigate the system. In the ordinary case when
, purity function, Rényi and the von Neumann entropies vanish. Recently, the quantization deformation method is extended to three dimensional to study entanglement of three isotropic harmonic oscillators by ref. [27].
4. Hall Conductivity and Energy Density
A natural generalization of two harmonic oscillators under a magnetic field in the framework of non-commutative quantum mechanics is devoted to deriving some properties of these two concepts: Hall conductivity and energy density, from the aspect of quantum information (entangled system). To start, we calculate the current density. Suppose that from (2.4),
(4.1)
and using expression (3.18), we have the current density as
(4.2)
By applying the Moyal product (3.1), we can write
(4.3)
Substituting (3.22) in (4.3), after some calculation we find expression (4.2) as:
(4.4)
The average of the current density operator
is defined as
(4.5)
where
is the probability distribution in the phase space. It is defined with the integral of the Wigner function on the momentum space as
(4.6)
We develop this expression using (3.22), we have
(4.7)
Using expression (4.7), we can then express (4.5) which interests us as
(4.8)
where
(4.9)
Hall conductivity is defined as the ratio of the current density
and the electric field
:
(4.10)
From (4.1), we have
(4.11)
consequently
(4.12)
So, it is easy to verify that
(4.13)
The average of (4.13)
(4.14)
Insert (4.8), (4.14) in (4.10), we obtain
(4.15)
Equation above summarizes some important specificity of an entangled system that can be quantified using Hall effect. Similarly, Hall conductivity is calculated in three dimensions using the Kubo formula from the bulk-edge [28]. Note that we have studied from two harmonic oscillators, entanglement entropies, Hall conductivity. It would be interesting to study their energy density. Their calculation in this context represents an important vision. Energy density reads
(4.16)
Go back expression (4.3), after integration, (4.16) becomes
(4.17)
where
(4.18)
The most famous examples that reflect the usefulness of both concepts are provided by the quantum Hall effect [29] [30], semiconductor quantum devices [31] [32], etc.
5. Application of Transition-State Theory
We will here briefly introduce some aspects of transition-state theory, focusing on the aspects that will be useful to us later. For more details, see references [33]-[35], from which this presentation is strongly inspired. We have performed a transition to the first excited state. Harmonic oscillator one moves into the first excited state, second harmonic oscillator in the ground state, so Wigner function (3.14) becomes
(5.1)
Consequently, of expression (5.1), the reduced Wigner function of the variables
, is written
(5.2)
In expression (5.2),
and
(5.3)
where
and
(5.4)
Using ref. [36] and applying the Moyal product (3.1), we have
(5.5)
At the barrier, the oscillator frequency is perturbed, defining thus the potential using (4.11) as
(5.6)
where
, (5.6) becomes
(5.7)
Insert (5.7) in (5.5), we have
(5.8)
At the barrier, we read the Wigner function as
(5.9)
To compute analytically integral of expression (5.9), we use ref. [37] and we ended up with
(5.10)
is the Step function, it is defined as
(5.11)
Now, we have just evaluated entanglement and the energy density via the transition-state theory, starting from expression
(5.12)
This allows us to write
(5.13)
In expression (5.13),
(5.14)
Expression (5.13) is a direct consequence of
(5.15)
In expression (5.15),
is the purity function of expression (3.24). Although with the transition performed to the first exited state and at the height of the barrier, the star product of (4.1) and (5.10) is written
(5.16)
Therefore, we generalize the energy density in the framework of transition-state theory as
(5.17)
More explicitly, we have
(5.18)
At the barrier, the energy density is written
(5.19)
which gives according to (5.18)
(5.20)
where
(5.21)
6. Some Particular Cases
Let us interpret the results obtained in (3.24), (3.27), (3.29), (4.15), (4.17), (5.15) and (5.20) in terms of the non-commutativity variables
,
and the magnetic field
.
6.1. The First Case
If we assume from expression (3.4) that
(6.1)
(3.11) become
(6.2)
As a consequence, (3.20) read
(6.3)
and
(6.4)
where
and
are fields and are functions of position and momentum, we consider from Figure 1, the evolution of the von Neumann entropy and the Hall conductivity with respect
and
. These two concepts of quantum mechanics have a similar behavior and they increase with
and
. This explains that non-commutativity introduces an intrinsic correlation into the system, affecting both the global quantum state (entanglement) and the electrical response (Hall conductivity). This behavior shows a duality between the quantities defined in the momentum space and those defined in the position space. The fields
and
that appear naturally in the expression of von Neumann entropy and Hall conductivity have a singularity at the origin of the coordinate position and momentum. Figure 2 shows that the transition in the quantum state performs clearly increases entanglement.
6.2. The Second Case
The second case is considered as follows:
which gives from (3.4),
. We define
in expression (3.4) such as
and
Figure 1. von Neumann entropy
(3.29) and Hall conductivity
(4.15) for
and
.
Figure 2. Purity functions
(3.24) and
(5.15) for
and
.
. This formalism leads us to generalize the evolution of the Rényi entropy and the energy density by considering a field that depends on the position non-commutativity parameter
and the magnetic field
. (3.11) and (3.20) become
(6.5)
(6.6)
and
(6.7)
Let us now evaluate the Rényi entropy on another basis according to
and
parameters.
represents the lowest state,
represents a more excited state.
We set
and
.
Observe Figure 3 as the magnetic field increases, Rényi entropy values of the highest excited states appear together and merge into one curve to a single value close to zero. The transition performs clearly increases entanglement, acts as an information-relevant perturbation. Let’s see how the energy density of an entangled system spreads according to the magnetic field
and the non-commutativity parameter
.
Figure 3. Rényi entropy (3.27) for different values of
;
(blue solid line),
(brown solid line),
(red solid line),
(black solid line).
We set
.
The coupling
of the position operator expression to the non-commutative property implies the non-localization of the two harmonic oscillators to the atom. Figure 4 and Figure 5 show that, an increase of
increases the energy density, so it increases the energy stored in the masses of the system. Consequently, it accelerates the non-localization to the atom. By comparing the two (Figure 4 and Figure 5), we prove that at the height of the barrier, a harmonic oscillator in a magnetic field
, which is a priori a function of the position
, has involuntarily and considerably increased its stored energy, so it increases its instabilization inside the atom. The effect of the
-field then only manifests in the presence of a position-dependent potential
and
.
Figure 4. Energy density
of (4.17).
Figure 5. Energy density
of (5.20).
7. Summary
In this work, we have examined quantum entanglement, as well as we have found the Hall conductivity and the energy density of two isotropic harmonic oscillators under a magnetic field in non-commutative phase space. The transition-state theory is applied to calculate the purity function and the energy density. Entanglement and Hall conductivity show a similar pattern because of the non-commutative parameters. A strong magnetic field disturbs the exact information of the highest excited states, and it accelerates the non-localization of the system in the atom by increasing its energy density. Transition-state theory makes the system more entangled and more instable.
Acknowledgements
We thank Prof. Hichem Eleuch for useful discussions.
Conflicts of Interest
The author declares no conflicts of interest.