1. Introduction
Permutation symmetric multiqubit states form an important class among quantum states due to their experimental significance and mathematical elegance [1] - [7] . The well-known Greenberger-Horne-Zeilinger (GHZ) [8] , W, and Dicke states [9] , belong to this class. Mathematical simplicity in addressing pure symmetric N-qubit states is owing to the fact that they are confined to the N + 1 dimensional subspace of the 2N dimensional Hilbert space. The N + 1 dimensional subspace is the maximal multiplicity space of the collective angular momentum space of N-qubits with Dicke states [9] , the common eigenstates of the squared collective angular momentum operator J2 and its z-component Jz forming its basis. In 1932 Majorana [10] proposed an elegant geometrical visualization for pure symmetric N-qubit states as a constellation of N-points on the Bloch sphere S2. The representation of pure symmetric multiqubit states in terms of constitutent N-qubits (spinors) is called Majorana representation [10] . Majorana geometric representation has found several significant applications in quantum information processing [11] [12] [13] [14] .
Quantum steering ellipsoid [15] offers a novel geometric picturization of two-qubit states and is useful in understanding quantum correlations such as non-locality, entanglement [16] [17] [18] [19] and quantum discord [16] [17] . The set of all Bloch vectors to which one of the qubits of a two-qubit system can be “steered” when all possible measurements are carried out on the other qubit correspond to quantum steering ellipsoid [15] . It has been identified that the volume of the steering ellipsoids [15] corresponding to the two-qubit subsystems of an N-qubit state,
, effectively captures monogamy properties of the state [18] [20] . Milne et al. [18] proposed a monogamy relation, in terms of the volumes of the quantum steering ellipsoids of two-qubit subsystems of a 3-qubit pure state which is stricter than the Coffman-Kundu-Wootters (CKW) monogamy relation [21] . A volume monogamy relation satisfied by pure as well as mixed N-qubit states has been obtained in [20] and is helpful in quantifying the shareability properties of the N-qubit state.
The steering ellipsoid of a two-qubit state that has attained a canonical form under suitable local operations on both the qubits is the so-called canonical steering ellipsoid [22] [23] [24] and provides another geometric representation of a two-qubit state. The canonical steering ellipsoid of any two-qubit state is shown to have only two distinct forms [24] and provide a much simpler geometric picture representing two-qubit states.
The volume monogamy relations for permutation symmetric 3-qubit pure states with two and three distinct spinors are established in [25] using the features of their respective steering ellipsoids. The canonical steering ellipsoids of the entire class of permutation symmetric N-qubit states with two distinct spinors are obtained in [26] and the nature of the volume monogamy relation with increasing N is analyzed [26] . In addition, the obesity of the steering ellipsoids is made use of to obtain expressions for concurrence of the two-qubit subsystems of the N-qubit states under consideration [25] [26] . In this paper, we construct the canonical steering ellipsoids of the N-qubit GHZ and
states and analyze the volume monogamy relations satisfied by them.
A flow chart of the paper is given here: In Sec. 2, following a brief overview on Majorana representation [10] [11] [12] [13] of pure permutation symmetric multiqubit states, we obtain the nature of N distinct spinors characterizing GHZ,
states and their generalized counterparts. Using these, we show that 3-qubit GHZ and
states are interconvertible under local operations on each qubit. In Sec. 3 we analyze the pairwise entanglement features as well as monogamous nature of N-qubit GHZ and
states. Following a primer on canonical forms of two-qubit subsystems of pure N-qubit state in Sec. 4, we construct the canonical steering ellipsoids of GHZ and
states in Sec. 5. The nature of the volume monogamy relation satisfied by
states is obtained and a comparison with that of GHZ and W-class of states is carried out in Sec. 6. Concluding remarks are given in Sec. 7.
2. Majorana Representation of Pure Symmetric Multiqubit States
A system of N-qubits obeying exchange symmetry gets restricted to a (N + 1) dimensional Hilbert space spanned by the basis vectors
where,
(1)
are the N + 1 Dicke states—expressed in the standard qubit basis
.
An arbitrary pure symmetric state,
(2)
is specified by the (N + 1) complex coefficients
. Eliminating an overall phase and normalizing the state (i.e.,
) implies that N complex parameters are required to completely characterize a pure symmetric state of N qubits.
Alternately, Majorana [10] expressed the pure state
as a superposition of symmetrized states of N spin-1/2 particles:
(3)
where
(4)
denote spinors constituting the pure symmetric state
. Here
denotes the set of all
permutations and 𝒩 corresponds to an overall normalization factor. The N complex parameters
, where
correspond to orientations of the spinor
(see Equation (4)), offer an alternate parametrization for the pure symmetric N qubit state
. The two representations given in Equations (2) and (3) of
together lead to the so-called Majorana polynomial equation: [13]
(5)
• The solutions
of the Majorana polynomial Equation (5) determine the orientations
of the spinors constituting the state
, in terms of the collective parameters
(see Equation (2)).
• When the Majorana Polynomial
is of degree
, it is necessary to recast the polynomial
in terms of
so that the
solutions
(6)
of Equation (6) determine the orientations of the remaining
spinors constituting the state
. In other words, given the parameters
, the N roots
of the Majorana polynomials (5), (6) determine the orientations
of the spinors constituting the pure symmetric N-qubit state
.
• When all the spinors in (Equation (3)) are distinct, the family of states is denoted by
indicating that each of the N-spinors in the N-qubit pure symmetric state appear only once in the symmetrized combination Equation (3). The family of states
denotes the family of states with two distinct spinors one of them repeating k times and the other
times in Equation (3),
.
• Dicke states are prominent members of the family
. In the following, we show that N-qubit GHZ and
states belong to the family
, with N-distinct spinors.
2.1. Majorana Spinors of N-Qubit GHZ and
States
Consider the N-qubit GHZ state
(7)
expressed in terms of the angular momentum states
,
,
to j as
(8)
Comparing Equation (8) with Equation (3), it can be seen that there are only two non-zero coefficients
. The Majorana polynomial Equation (5) for
turns out to be
(9)
• Thus the Nth roots of unity determine the N-distinct spinors of
when N is odd and Nth roots of −1 when N is even.
• When
,
,
,
where
are the cube roots of unity and the Majorana spinors
(10)
constitute
.
• One may verify explicitly that symmetrization of the spinors (10) leads to the GHZ state:
(11)
• In a similar manner, the fourth roots of −1 lead to the following spinors corresponding to
:
(12)
The symmetrization of the four spinors (12) as in (3) results in
(13)
the 4-qubit GHZ state expressed in the qubit basis.
It is evident from the discussion above, that the N-qubit GHZ state is a pure symmetric state characterized by N-distinct spinors. In the following, we show that the superposition of N-qubit W state
and its obverse state
is a pure symmetric state with N distinct spinors
,
.
We first express the N-qubit W state
, its obverse state
in the angular momentum and qubit basis respectively:
The equal superposition of
,
, which we refer to as
state, is given by
(14)
(15)
• Comparing Equation (14) with Equation (2), we have
as the only non-zero coefficients and hence the corresponding Majorana polynomial Equation (5) turns out to be
(16)
• The solutions of Equation (16) determine the
spinors corresponding to
and the solution
of the Majorana polynomial Equation (6) determines its Nth spinor. Recalling that
,
we proceed to determine the nature of Majorana spinors (see Equation (4)) constituting
:
(17)
Let us choose
to obtain
(18)
• The other
spinors of the
state are then given by
(19)
(20)
• Note that the Nth spinor corresponds to the solution
of Equation (6) and we get
,
arbitrary. Choosing
we find that
(21)
• In adition to the two spinors
,
(which are irrespective of any N) constituting the state
, rest of the distinct spinors are obtained to be the (N − 2)th roots of unity when N is odd and (N − 2)th roots of −1 when N is even [27] .
• For
, the spinor corresponding to the solution
of Equation (19) is characterized by the parameters
and
. In other words, the three distinct spinors
,
constituting
are given by
(22)
• When
, the Majorana spinors corresponding to
are given by
(23)
• One may verify explicitly that
(24)
Similarly one may verify explicitly that
is constructed by symmetrizing the 4 spinors given in Equation (23).
• From the above discussion it is clear that both
and
belong to the family
of pure symmetric N-qubit states characterized by N-distinct spinors.
2.2. Majorana Spinors of Generalised N-Qubit GHZ and
States
Generalized N-qubit GHZ and
states are respectively given by
(25)
and
(26)
where
.
• From Equations (2), (25), (26), we have
,
for
;
and
for
.
• For the 3-qubit state
, the Majorana polynomial Equation (5) takes the form
leading to the roots
(27)
are the cube roots of unity.
• The three Majorana spinors corresponding to
(see Equation (25)) are therefore given by
(28)
• With
and
for
, the independent roots of Majorana polynomial equation (see Equation (5))
and (see Equation (6))
,
are
(29)
• Thus the Majorana spinors of
turn out be
(30)
• In general, the N spinors corresponding to
are given by
(31)
• Note that
when N is odd and
when N is even.
• Similarly the N spinors corresponding to
are,
(32)
Here too,
when
is odd and
when
is even.
2.3. Interconvertibility of 3-Qubit GHZ and
States under Local Operations
If the states (11), (24) are related to each other by identical local operations of the form
, i.e.,
(33)
the corresponding Majorana spinors (10) and (22) must be related to each other through
,
; N is the normalization constant. We consider an arbitrary 2 × 2 invertible matrix
,
and explicitly solve the following equations relating Majorana spinors (10) and (22):
(34)
It can be seen that the SL (2, C) matrix
(35)
and the proportionality constants
(36)
lead to the transformation (33). In other words, the 3-qubit
state (24) gets transformed to the 3-qubit GHZ state (11) under identical local operations (35) on all the three qubits.
3. Pairwise Entanglement Features of N-Qubit
States
We adopt concurrence [28] [29] to quantify the pairwise entanglement in
states. The concurrence
of any two-qubit state
is defined as [28] [29]
(37)
where
,
are the eigenvalues of the 4 × 4 matrix
arranged in the decreasing order
.
In order to evaluate the pairwise entanglement features of N-qubit
states, we need to evaluate the structure of its two-qubit subsystems. The state
being a symmetric state, its two qubit reduced density matrices are all identical and from its structure in the qubit basis (see Equation (15)) one can readily evaluate the two-qubit reduced density matrices
. We denote
associated with the reduced two-qubit system
of the N-qubit state
.
• For the state
(see Equation (24)), the two qubit reduced density matrix obtained by tracing out any one qubit is given by
(38)
On expressing
in 4 × 4 matrix form (in the basis
, we have
(39)
• For the 4 qubit
state
the two-qubit reduced density matrix
, obtained by tracing over any two-qubits of
, is given by
(40)
Thus, the 4 × 4 matrix representation of
in the standard two-qubit basis is
(41)
• For any
, tracing out (N − 2) qubits of
(see Equation (15)) leads to the two qubit reduced density matrix
:
(42)
• On explicit evaluation of the eigenvalues of
, we find that
,
,
for
. Thus, the concurrence of the state
is
. For
, the matrix
has only two non-zero eigenvalues
, implying that the concurrence of the two-qubit reduced system
of
state is zero.
• For any
, the eigenvalues of
are
(43)
• For
, the values of
,
are found to be
(44)
• It can be seen from Equation (44) that
turns out to be negative and hence concurrence (see Equation (37)) of the
state vanishes when
.
• From Equation (43), we have
when
. The highest eigenvalue
being doubly repeated, concurrence of the state
vanishes for
. In other words, except for the reduced two-qubit systems
of the the 3-qubit state
, the concurrence for
drawn from
vanishes when
. Thus, there is no pairwise entanglement in
state except when
.
It is well-known that there is a trade-off between entanglement shared by multiple parties. We explore these restrictions on shareability or monogamy of entanglement in N-qubit
states. The Coffman-Kundu-Wootters inequality for monogamy of concurrence in a pure three-qubit state is given by [21]
(45)
where
denote concurrences of two-qubit reduced systems 1,2 and 1,3 respectively;
is the concurrence of the bipartition 1-23. The 3-tangle of a three-qubit state
, defined by [21]
(46)
is a measure of residual entanglement [30] , which is not accounted by the entanglement between two-qubit subsystems of a three-qubit pure state. The 3-tangle
for three-qubit GHZ states and
for W states.
A generalization of the 3-tangle to any N-qubit pure states, called N-tangle, has also been proposed [31] [32] . The N-tangle
for a pure symmetric N-qubit state
is given by
(47)
• One can readily see, by tracing out a qubit from the two-qubit density matrices (39), (41)), that the single qubit density matrices
,
of
,
are obtained as,
(48)
(49)
• Also, by tracing out a single qubit from the two-qubit states
(see Equation (42)), it can be readily seen that the single qubit density matrix
of the state
turns out to be
for any
, where
is the 2 × 2 identity matrix.
• For the 3-qubit
state, we have concurrence
and
(see Equation (48)). The 3-tangle
.
• The concurrence C between any pair of qubits in
being zero and
,
, we obtain the N-tangle (see Equation (47))
for
.
From the above results, we conclude that two-qubit entanglement in N-qubit
states with
vanishes—but their residual entanglement quantified by N-tangle
is maximum. The three-qubit
state possesses both two-qubit entanglement (quantified by concurrence
) and residual 3-way entanglement (characterized by
).
It is important to notice that N-qubit GHZ state also has vanishing concurrence and maximum concurrence tangle [21]
for any
. The identical pairwise entanglement and monogamy features of GHZ and
states for any
is worth noticing. We examine this aspect with the help of canonical steering ellipsoids in Sec. 5.
4. Canonical Steering Ellipsoids as Geometric Representation of Two-Qubit States
It has been shown [24] [25] [26] that the SLOCC transformation on a two-qubit state is equivalent to the Lorentz congruent transformation (up to a scalar factor) on the real representative of the state. This helps in obtaining the Lorentz canonical form of the real representative of the state and thereby its geometric representation in terms of canonical steering ellipsoiods [24] [25] [26] . While the canonical steering ellipsoids corresponding to two SLOCC inequivalent families of 3-qubit states are obtained in [25] , the canonical steering ellipsoids associated with SLOCC inequivalent families of N-qubit Dicke class of states are analysed in [26] . In the following, after a brief description of the concepts used in [24] [25] [26] , we proceed to work on the geometrical representation of N-qubit GHZ and
states.
4.1. Real Representation of Two-Qubit States and Their Lorentz Canonical Forms
Consider a two-qubit density matrix
expressed in the Hilbert-Schmidt basis
:
(50)
The coefficients of expansion
(51)
form a real 4 × 4 matrix
representing the density matrix
. Here,
,
are the Pauli spin matrices and
is the 2 × 2 identity matrix.
• When the qubits are subjected to local operations on their respective parts, the two-qubit density matrix
transforms to
as
(52)
Here,
denote 2 × 2 complex matrices with unit determinant and represent local operations on the qubits A, B. One can choose suitable local operations A and B such that the two-qubit density matrix
attains its canonical form
.
• When the two-qubit state
undergoes the transfomation (52), its real representative
transforms as [24] [25]
(53)
Here
are 4 × 4 proper orthochronous Lorentz transformation matrices [33] corresponding respectively to
and the superscript “T” denotes transpose operation.
• The Lorentz canonical form [24]
can be obtained by constructing the 4 × 4 real symmetric matrix
, where
denotes the Lorentz metric [33] .
• Using the defining property [33]
of Lorentz transformation L, it can be seen that the matrix
undergoes a similarity transformation
.
• It has been shown [24] [25] [26] that the canonical form of
can either be a diagonal matrix or a non-diagonal matrix with only one off-diagonal element [22] [24] [25] [26] depending on the eigenvalues and eigenvectors of
.
• When the eigenvector
associated with the highest eigenvalue
of
obeys the Lorentz invariant condition
,
assumes the diagonal canonical form
given by
(54)
where
are the non-negative eigenvalues of
.
• Suppose that the non-negative eigenvalues
and
(
) of
are doubly degenerate and an eigenvector
of
, belonging to the highest eigenvalue
, satisfies the Lorentz invariant condition
. In such cases, the Lorentz canonical form of
turns out to be a non-diagonal matrix (with only one non-diagonal element):
(55)
The parameters
,
in Equation (55) are given by [24] [25] [26]
(56)
where
is the 00th element of the canonical form
:
(57)
4.2. Steering Ellipsoids Corresponding to Lorentz Canonical Form of Two-Qubit States
In the two-qubit state
, local projective valued measurements (PVM)
(58)
on Bob’s qubit leads to collapsed states of Alice’s qubit characterized by Bloch vectors
through the transformation [24]
(59)
Here
,
represents points on the Bloch sphere representing all possible PVMs at Bob’s end. The steered Bloch vectors
of Alice’s qubit constitute an ellipsoidal surface
, enclosed within the Bloch sphere.
• For the diagonal canonical form
(see Equation (54)) of the two-qubit state, it follows from Equation (59) that
(60)
are steered Bloch points
of Alice’s qubit. They obey the equation
(61)
of an ellipsoid with semiaxes
,
,
) and center
inside the Bloch sphere
. We refer to this as the canonical steering ellipsoid representing the set of all two-qubit density matrices which are SLOCC equivalent to the canonical form
(see Equation (52)) corresponding to
.
• For the non-diagonal canonical form
(see Equation (55)), we get the coordinates of steered Alice’s Bloch vector
, on using Equation (59);
(62)
and they satisfy the equation
(63)
Note that Equation (63) represents the canonical steering spheroid (traced by Alice’s Bloch vector
) inside the Bloch sphere with its center at
and lengths of the semiaxes given by
,
. In other words, a shifted spheroid inscribed within the Bloch sphere, represents two-qubit states that possess a non-diagonal Lorentz canonical form
(see Equation (55)).
In the following, we obtain the Lorentz canonical forms of the real representation
associated with the two-qubit subsystems
of N-qubit
states for all N and arrive at their canonical steering ellipsoids.
5. Canonical Steering Ellipsoids Corresponding to
In Sec. 2, we have seen that the N-qubit
(
) states do not have pairwise entanglement (zero concurrence) and exhibit maximum restriction in the shareability of entanglement among its subsystems (concurrence tangle
). In contrast to this, the 3-qubit
state have non-zero concurrence (
) and the concurrence tangle is not maximum (
). In this section, we explore how the geometric picture of the two-qubit subsystems of the states
reflect this aspect.
5.1. Canonical Steering Ellipsoids Corresponding to
The real matrix representation
of the two-qubit subsystem
(see Equation (39)) of
and the real symmetric matrix
are respectively given by
(64)
The normalized eigenvectors
of
belonging to its eigenvalues
,
can be determined by solving the eigenvalue equation
,
. On explicit determination, we get
(65)
It can be readily verified that
where
,
,
when
,
i.e.,
and
when
. Thus, the set
forms an orthonormal tetrad of Minkowski four-vectors [33] . The 4 × 4 real matrix
, constructed using the eigenvectors of
is a Lorentz matrix [33] satisfying the relation
and
. Explicitly, we have
(66)
From the relations
,
, one can readily see that
is a diagonal matrix with elements
. With
,
, we have
,
.
From the discussion in Sec. 4.2 leading to Equation (61), it can be seen that the canonical steering ellipsoid corresponding to
is an oblate spheroid with radius 1/2 centered at the origin and touching the Bloch sphere (see Figure 1).
5.2. Geometrical Representation of 4-Qubit
States
The two-qubit subsystem
of the 4-qubit
state given in Equation (41) leads to the diagonal structure of its real representative
;
(67)
Thus
and it is already in its canonical form. The eigenvector belonging to the largest eigenvalue
is
of
and satisfies the relation
. With the other two eigenvalues of
being
, the parameters
,
are
,
i.e., the tip of Bloch vectors of one qubit, steered by another qubit of the 4-qubit
state trace a straight line joining the north and south poles of the Bloch sphere.
It is of interest to note here that the N-qubit GHZ states have the same geometrical representation as that of 4 qubit
states. More specifically, the two-qubit subsystem density matrix of
is given by
and its real representative is found to be
(68)
It readily follows that
. The eigenstructures of
corresponding to N-qubit GHZ states and 4-qubit
state are identical. The two-qubit subsystem density matrix of N-qubit GHZ state is represented by a straight line joining north and south poles of the Bloch sphere.
5.3. Canonical Steering Ellipsoids Corresponding to N-Qubit
States; N > 4
In order to obtain the geometrical representation of N-qubit
states for
, we first evaluate the real representative
of two-qubit subsystem
obtained in Equation (42): We find that
Eigenvalues of
are given by
and the eigenvector
corresponding to the highest eigenvalue
satisfies the relation
. Thus, the semi-axes of the steering ellipsoid (see Sec. 4.2) corresponding to two-qubit subsystem density matrix
of
are given by
(69)
Figure 1. Oblate spheroid representing the state
: Length of the semi-axes:
,
.
Thus, the N-qubit
state is represented geometrically by a spheroid (
) centered at the origin of the Bloch sphere. For
, we obtain an oblate spheroid (see Figure 2).
When
, it is readily seen from Equation (69) that we have a sphere of radius 1/3 (see Figure 3).
For
, the canonical steering ellipsoids of
are prolate spheroids with their radius 2/N decreasing with N (see Figure 4 and Figure 5).
One can obtain similar canonical steering ellipsoids for generalized counterparts of GHZ and
states by constructing the real representatives of their two-qubit subsystems. We notice that the nature of the canonical ellipsoids remain the same as that for
and
but the volume of the ellipsoid increases when
,
. This indicates the increase in shareability of entanglement among its subsystems. Quite in accordance with increase in pairwise entanglement (concurrence) and decrease in monogamous nature (decrease in concurrence tangle) of these states (see Sec. 3), their shareability of entanglement decreases when
,
as reflected through the volume of the corresponding canonical steering ellipsoids.
6. Volume Monogamy Features of
and
The restriction on shareability of quantum correlations in a multipartite state get captured in various monogamy relations, which find interesting applications in ensuring security in quantum key distribution [34] [35] . A geometrically intuitive monogamy relation in terms of the volumes of the steering ellipsoids representing the two-qubit subsystems of multiqubit pure states has been proposed and extensively studied in Refs. [16] [17] [18] [19] . The volume monogamy relation is stronger than the well-known Coffman-Kundu-Wootters monogamy relation given by Equation (45) and is applicable to N-qubit pure states.
The normalized volume
of the quantum steering ellipsoid corresponding to pure symmetric N-qubit state
is given by [26]
Figure 2. Canonical steering ellipsoid representing
with semiaxes
,
.
Figure 3. Sphere of radius 1/3, the canonical steering ellipsoid of
.
Figure 4. Prolate spheroid with semiaxes
,
representing
.
Figure 5. Prolate spheroid with semiaxes
,
representing
.
(70)
Here
is the real representative of the two-qubit density matrix
of
and
(
) is the Bloch-vector of the single qubit subsystem. It has been shown in [26] that the volume monogamy relation in the state
is given by
(71)
We proceed to study the volume monogamy relation governing N-qubit
state. The LHS of the monogamy inequality (71) is a measure of the degree of restriction on shareability of quantum correlations in any arbitrary N-qubit pure symmetric state. The lowest possible value 0 indicates maximum restriction on shareability of entanglement. Equality sign in (71) indicates least restriction and largest allowed shareability of entanglement.
For
, the real representative
of the two-qubit subsystem
of
is given in Equation (64). It is seen that the Bloch vector components
,
(see Equation (64)) leading to
. We find that
. Thus we obtain (see Equation (70))
. Thus, volume monogamy relation is readily satisfied:
For
, we obtain
as
(see Equation (67)). The volume monogamy relation is thus maximally satisfied. It may be noted that the real matrix corresponding to two-qubit subsystem of N-qubit GHZ state is given by
and hence,
irrespective of the value N. Thus, the LHS of the volume monogamy inequality (71) takes its minimum value 0 for GHZ state, highlighting the strongest restrictions on sharability of entanglement.
Let us turn our attention to volume monogamy property of
,
. The real representative
of
,
is a diagonal matrix
with the magnitude of single qubit Block vector
. Substituting
(see Equation (70)) we obtain the volume monogamy relation (71) for
,
:
(72)
We have plotted the LHS of volume monogamy relation (72) in Figure 6. It is evident from Figure 6 that the sharability of entanglement gets restricted as number of qubits N increase and is quantified by reducing volume of the canonical steering ellipsoids On explicit evaluation of the volumes of steering ellipsoids (as a function of the parameter
) of generalized
and GHZ states, we find analogous features.
It is worth recalling here that the volume monogamy inequality for W state belonging to the family
—which are constituted by two distinct spinors—is shown [26] to be
. A comparison of the LHS of monogamy relation of
and the W-class of states is depicted in Figure 7. It is seen that W states exhibit stricter volume monogamy for pairwise entanglement than
states beyond
when the cross-over happens (see Figure 7).
Figure 6. LHS. of the volume monogamy relation (72) obeyed by
state as a function of the number of qubits N.
Figure 7. Comparison of the LHS of the volume monogamy relation governing
and the W-class of N-qubit states.
7. Summary
In this work, we have compared the features of the N-qubit GHZ and equal superposition of N-qubit W,
states, which are constituted by N-distinct Majorana spinors. Such a comparison is carried out for the first time, to the best of our knowledge. We have evaluated the explicit structure of Majorana spinors of both the families. Using the structure of Majorana spinors for 3-qubit GHZ and
states, we have shown that they are interconvertible under identical local operations on their qubits. Geometric visualization of N-qubit
states in terms of canonical steering ellipsoids inscribed within the Bloch sphere is highlighted. Furthermore, we have investigated the volume monogamy inequality governing shareability of entanglement in N-qubit
states.
Acknowledgements
ARU and Sudha are supported by the Department of Science and Technology (DST), India through Project No. DST/ICPS/QUST/2018/107. ASH is supported by the Foundation for Polish Science (IRAP Project, ICTQT, contract no. MAB/2018/5, co-financed by EU within Smart Growth Operational Programme). HSK is supported by the Institute of Information & Communications Technology Planning & 14 Evaluation (IITP) Grant funded by the Korean government (MSIT) (No. 2022-0-00463, Development of a quantum repeater in optical fiber networks for quantum internet).