1. Introduction
Probability, as formally viewed in stochastic processes (see, e.g., [1]), is a fundamental ingredient for understanding countless natural phenomena. For instance, it is ubiquitous in the general framework of classical statistical mechanics. Probability is also a keystone in quantum mechanics, where the concept of probability amplitudes relates to the distribution of outcomes upon measurements. However, there are fundamental distinctions between the concept of randomness [2] in classical and quantum physics since probability can have a contrasting character in these two realms [3]. These differences are particularly noticeable in the correlation functions of physical quantities, i.e., observables [2,4].
Although many restrictions apply [5], correlations can be used as measures of the degree of determinism/randomness in a system. Moreover, certain situations might be relatively easy to pinpoint. Indeed, on the one hand, if there is a well-behaved mapping between a and b, with a and b possible values for bona fide observables A and B describing a problem (we are obviously not considering logical/philosophical digressions, like “the sun raises every morning” and “humans are mortal”, such that A and B are true, but with no causal association between A and B), e.g., and r in a classical Keplerian orbit, the correlation is “perfect”, and there should be a fully deterministic relation between them. On the other hand, if a specific value for A determines a range of allowable values for B as well as their frequencies of occurrence (with the same being true for A regarding B), this would indicate a stochastic connection between A and B. So, at least in principle, one could infer a joint probability for A and B, allowing one to define a proper correlation function for these observables, which we represent by .
But as already mentioned, will display different properties if resulting from either classical or quantum processes. We should mention that there are some general ways to identify between classical and quantum correlations, e.g., through the idea of distance measures as relative entropy [6]. Other procedures may be problem-oriented, for instance, those employed in the study of Gaussian states [7] or of system–reservoir interactions [8]. Quantum correlations are often more general than classical correlations [4,9,10], and in some cases stronger [11,12,13], creating scalings (due to entanglements) in many-body systems, which are classically absent [14,15]. Furthermore, there are distinct types of quantum correlations [9], ranging from the most basic—associated with the quantum construction itself, which we call “quantum”—to those exhibiting an increasing order of restrictiveness, typically entanglement, steering, and nonlocality; see Figure 1. Although the aim here is not to provide a general account of quantum correlations, it is useful for our later discussions to briefly mention a few characteristics of some of them. Also, since we shall specifically address EPR states (see Section 2), we restrict our analysis to pure states.
The first paradigmatic quantum correlation is entanglement, which includes all possible forms of interrelations in multipartite states. Focusing on the bipartite case, consider a composite state, , where and are the Hilbert spaces of systems I and II; if this state cannot be written as the direct product , for and , then is entangled. From this broad definition, one can have distinct degrees of entanglement for . Nonetheless, the entanglement of is maximum if its von Neumann (or if one prefers, Shannon) entropy is also maximum. EPR states exactly meet this condition [16]. A second, somewhat stronger, coupling between systems I and II is that in which the state of one (e.g., I) can be driven or steered through measurements on the other (e.g., II); nevertheless, the contrary is not true. In such an inseparability context, the composite system exhibits a steering correlation (reviews in [9,17,18]). All steered systems are entangled, but not all entangled systems are steered. And third, suppose we measure the observable A for system II and C for system I (with I and II spatially apart), obtaining, respectively, a and c. In quantum mechanics, one finds that usually the joint probability . The reason is that the quantum world is non-local—a notion heavily criticized by the famous EPR (Einstein–Podolsky–Rosen) paper [19] in the early days of the theory. An alternative interpretation would be to assume that quantum mechanics is incomplete, i.e., there exist local hidden variables inaccessible through its framework. In Bell’s groundbreaking contribution [20], it has been demonstrated that and some inequalities for the associated correlations should be observed. Nonetheless, the local hidden variables have been ruled out through experiments [21,22,23,24,25,26,27,28,29,30] showing violations of Bell’s inequalities. Thus, in opposition to EPR’s concept of local realism, nonlocality is inherent to quantum mechanics and indeed represents its most restrictive correlation [9,31]. We finally mention that any entangled pure state is non-local in some appropriately chosen set of observable bases, but the situation is far more involved for mixed states [9]; see also later. This previous (rather heuristic) hierarchical classification for quantum correlations is pictorially represented in Figure 1.
As a significant example of Bell’s inequality for two-level systems (qubits), we refer to the CHSH inequality [32]. For an ensemble of composites formed by systems I and II, suppose that for all copies, parts I and II are spatially separated and then placed in regions 1 and 2, respectively. This should be done without altering the original properties that we intend to measure. So, detectors 1 and 2 in these regions can infer certain observables for I ( and , whose results are denoted as and ) and II ( and , of results and ); Figure 2. From appropriate averages, we can calculate the CHSH correlation function ; for an explicit expression, see Section 5, and for an overview of the whole procedure, refer to [16]. For arbitrary X and Y observables, local hidden variables would necessarily lead to . But quantum mechanics does allow situations where , corresponding to Bell’s inequality violation.
The above considerations might give the incorrect impression that identifying and classifying the quantum correlations of any state is straightforward. Actually, from a technical point of view this is often not true [9,33,34,35]. Moreover, determining all states that comply with certain specified correlation traits can be even harder [36]. One potential issue is that the interpretation of quantum correlations may contradict our intuitive understanding of classical correlations [2,4].
In this contribution, we address two concrete features for the observables of a bipartite state. Suppose we have a collection of observables for I and for II, with . For any j, we assume that the allowed outcomes are always drawn from the same set of numerical values, respectively, and . Notice that this is not a too-restrictive assumption. For instance, if the observables are the spin-1/2 components , , , then the possible numerical values of measurements are always the same, i.e., . Also, let be a quantum correlation function for these possible observables, taken in pairs . With the exception of potentially overly specific functional forms for , typically, we can anticipate that larger (smaller) values for are associated with a stronger (weaker) interdependence between . On the other hand, the range of variation for should be connected to the particular relations within and within . Thus, we consider two, in principle, independent conditions: (a) Regardless of j, only pairs in the form can describe the composite system state. So, by determining the value of (), we ascertain the value of (). Furthermore, they are equally probable, i.e., we have a uniform distribution for these values, so that in an ensemble description, any pair, n, would contribute with the same probability, . Hence, for , and fixed N, this premise maximizes the normalized entropy (note that is well-defined even in the hypothetical limit of ). This represents the maximum entanglement of the composite. (b) There is no reinforcement dependence among s or s. On the contrary, knowing the value of () precludes knowing with certainty the value of (), . This naturally emerges from the non-commutation, and , of the corresponding self-adjoint operators for the observables and .
Observe that (a) represents a substantial link between systems I and II. Indeed, knowing an observable in one system entails complete knowledge of the corresponding observable (because of the pairing ) in the other. Also, we recall that for arbitrary entangled states, it is always possible to find suitable sets of observable bases, such that Bell’s inequalities are violated [9,37,38]. Hence, solely assuming (a), one might be led to conclude that and constitute these bases. Consequently, for , taking as and supposing all possible combinations between and (e.g., representing the different choices of , , , and in Figure 2), we would expect the non-observance of the Bell’s inequality, at least in some instances. On the contrary, considered alone, the condition (b) seems to act in a different direction, tending to decrease the quantum correlations. In this way, two pertinent queries arise: (i) Possible contexts where (a) and (b) occur together and (ii) given so, which values can assume.
The arguments in [19] criticizing the nonlocality of quantum mechanics were fully based on particular pure bipartite states, the so-called EPR states — for a very complete and solid refutation of the EPR reasoning, demonstrating its inadequacy, see, e.g., [39] and the refs. therein. Interestingly, these s verify the assumption (a) (as well as a second extra feature; see the next section). The conclusions in [19], and indirectly the structures of these s, motivated the investigations in [20] (also refer to [40,41]). But as mentioned earlier, local realism has been overturned by testing Bell’s inequalities for arbitrary s.
Here, we prove very generally—and without the need for sophisticated mathematical techniques—that s also comply with the assumption (b), thus, providing a concrete example of (i). In addition, as for (ii), setting (qubits), we calculate considering only observables associated with EPR states. We determine that ; thus, not exceeding the nonlocality threshold of 2. It is worth recalling that maximally entangled pure states do not need to map into maximum violation of the CHSH inequality [42,43] (a fact motivating the proposal of distinct Bell-like inequalities capable of detecting maximal entanglement; see [44,45]). Actually, some maximally entangled pure states can adhere to the CHSH inequality under special local measurements. So, another result in the present contribution is that this is also always the case across all EPR bases.
This paper is organized as follows: We review important aspects of EPR states in Section 2. We give a simple demonstration of condition (b) for EPR states in Section 3. We exemplify our results for the three-level systems in Section 4. We show that EPR states do not violate Bell’s inequalities for CHSH correlations in Section 5. Our final remarks and conclusions are drawn in Section 6. For the sake of completeness, known but here very systematized facts about the concept of observables and features of observables bases and bases transformations are presented in the appendices.
2. Some Key Aspects in Forming and Measuring EPR States
In this Section we briefly review the basic characteristics of EPR states [46], essentially those considered in the original work by Einstein–Podolsky–Rosen [19]. A much more general and rigorous definition of finite dimensional systems can be found in [47,48] (for infinite-dimensional spaces, see [49,50]).
The main relevant steps in the preparation and posterior measurements of are schematically depicted in Figure 3. Initially (), one has two non-interacting systems (systems I and II), with their composite state simply being the direct product of the individual states of I and II. Then, during the time interval , systems I and II interact in such a way that certain physical observables of I (say C, associated with the Hermitian operator having eigenvalues , for ) and some of II (say A, associated with of eigenvalues ) become entangled, resulting in the state . Next, during a time interval , I and II are sufficiently brought apart so to cease any eventual influence of one another. Since the exact value of is not relevant to the present discussion, for simplicity we just set . Here, by influence we mean the type of interaction described by potentials in the Schrödinger equation, coupling the degrees of freedom of systems I and II. Hence, for , one has that . Moreover, one must guarantee that regardless of the separation procedure and the subsequent dynamics (for ), the entanglement attained between systems I and II (during ) is maintained. This should hold until any eventual measurement at any . In this way, for , the components of corresponding to the eigenvectors of I and of II would not be altered, or
(1)
In Equation (1), the labels (for I) and (for II) denote how other observables, distinctly from C of I and A of II, will evolve in time, given that the systems I and II no longer interact.If, by means of measurements, we aim to assess only the correlated quantities C and A, the information given by and is not fundamental. Thus, one can drop these explicit dependencies in Equation (1), just written it as for . It is a common practice in the analysis of EPR states [46,47]: (1) To suppose maximally entangled states, i.e., all in being equally probable, namely, for any n to assume that . (2) To disregard eventual relative phases by trivially reincorporating them into the state’s definition, or . Thus, the existence (or not) of phases multiplying the eigenvectors is irrelevant to all our general results.
A second fundamental feature of an EPR state is that (here, for , i.e., prior to any measurement) can be written in the following distinct ways:
(2)
where the set () is composed by the eigenvectors of (), or(3)
From Equation (2), we have that from a measurement performed only on I at —determining the observable C (D)—we should obtain complete knowledge about the value of A (B) for II; Figure 3. In fact, if afterward () we test system II for A (B), we will find the same previously inferred value. Paramount to our analysis is the fact that Equation (2) and, thus, the EPR state, complies with the condition (a) discussed in the introduction.One last feature in defining an EPR state is to ascribe to the observables A and B—associated with system II, cf. Equation (2)—operators that do not commute, i.e., . In this respect, we mention that given an EPR state, one interesting and sometimes challenging issue is to determine all associated pairs of non-commuting observables [48]. We further remark that is a key assumption in [19], attempting to show that quantum mechanics is incomplete (of course misguidedly; for a summary of many valid objections to the EPR arguments see [39]).
Below we derive an extra general property for , arising from Equation (2) and . We show that for the observables C and D: . Considering the pairs of observables A, B for II and C, D for I, this is precisely the condition (b) in Section 1. Therefore, it is established that EPR states simultaneously obey (a) and (b).
3. A Necessary Condition for EPR States: The Observables Are Pair-Wisely Associated with Non-Commuting Operators
Hereafter, we suppose the Hilbert space for the composed systems (refer to Equation (1)) . Note that and distinguish I from II. Further, we assume that all specific aspects that we wish to determine for our individual systems, either I or II, are described by the proper separable Hilbert space (of dimensions N and spanned by a countable, i.e., discrete, basis—see Equation (4)). We should observe that although EPR states are usually addressed for finite Ns, which is also the case in this contribution, quantum correlations, including CHSH, and s for countable infinite-dimensional () systems are also analyzed in the literature, see, e.g., Refs. [51,52], as well as the general considerations in [47].
Moreover, the reasons for choosing countable Hilbert spaces are twofold. First, EPR-like states have been vital to test certain fundamental predictions of quantum mechanics, even motivating the development of the Bell inequalities [20,40]. Albeit some theoretical proposals [53] and devised experimental arrangements [54,55,56] are based on continuous variables (for a review, see [57]), historical breakthroughs [21,22,23,24,25,26] and recent loophole-free measurements [27,28,29,30,58] consider discrete observables. Second, the spectrum theorem for self-adjoint operators—relevant for determining the suitable basis for —holds true very generally [59]. However, establishing solidly grounded properties of transformations (similar to those presented in Section 3.1) between continuous bases may require additional technicalities; this goes far beyond the scope of this contribution. For instance, for continuous bases associated with operators such as position and momentum, one should work with generalized eigenvectors in a rigged Hilbert space [60].
As quantum observables, we suppose linear self-adjoint—or Hermitian, the usual jargon in physics—operators, whose domains are the whole (eventually, one could also consider linear subsets of , which are dense in if the self-adjoint operators are unbounded [61]; see Appendix A). For our goals, it is not necessary to explicitly address formal constructions of the join probability measures associated with any assessment of observables, e.g., as rigorously conducted in [47]. We adopt fairly well-established definitions (refer to [47]) that are consistent with actual procedures in concrete measurement realizations [21,22,23,24,25,26].
For and , the sets of eigenvectors of and forming the orthonormal basis (ONB) of , the basis change reads as follows (which in fact is valid for N either finite or infinite):
(4)
To derive the results of the next Sec., we will rely on some known properties of the unitary matrix , dependent on the commutation relation of and , as discussed in Appendix A. Considerations about appropriate self-adjoint operators and , representing observables for EPR states, are reviewed in Appendix A.1.3.1. Correlations in the Observables of EPR States
From the above and the remarks in Section 2, for systems I and II described by EPR states, as seen in Equation (2), we can focus on the “reduced” Hilbert space . Also, for the self-adjoint operators , , , and (see Equation (3)), we assume that the features described in Appendix A.2 and and ( and ) act just on I (II). Thus, the set of eigenvectors and , respectively, of and , are suitable ONBs for . The same is true for and ) (of and ) with respect to . We also recall that .
By inserting the second relation in Equation (4) into the second equality in Equation (2), we obtain the following:
(5)
Comparing Equation (5) with Equation (2), (recalling that is an ONB), we must have the following:(6)
Hence, from Equation (6), we see that up to the complex conjugation of the elements of (an operation that certainly does not change the matrix structural relations), the basis transformation is fully akin to . In other words, Equation (6) has exactly the same functional form of the mapping between and .But the matrix —hence also —represents a change of basis associated with non-commuting operators, and . Thence, according to Appendix A.2, cannot be transformed into an identity, or more generally, in a permutation , matrix. In Appendix A.3, we illustrate such universal facts considering the bases transformations of a spin- system in arbitrary directions. So, , cannot commute; otherwise, it would be possible to find a basis where they are simultaneously diagonal, leading to (or ), so a contradiction.
To summarize, if a state can be expanded as in Equation (2)—with the different bases related to the Hermitian operators , , , —and , we conclude that . This is implied in our aforementioned condition (b). As far as we know, this property of the EPR state has gone somewhat unexplored in the literature, even though it is implicit in particular contexts, like in the Bohm construction of EPR states [62], in Bohr’s response [63] to the EPR paper [19], and in certain constructive empiricism critics to EPR reasoning [64].
4. An Explicit Example: Three-Level System
To illustrate the previous general result, we discuss qutrits, i.e., . In this case, I and II can be described as three components of angular moment systems. Then, the equivalent to the Pauli matrices are (for the usual direction association: 1 for x, 2 for y, 3 for z)
(7)
Note that for the permutation symbol of . The eigenvalue equation associated with (with ) reads , where . One has(8)
Just as a mathematical digression, we observe all the above eigenvectors can be condensed into a single formula as follows:(9)
From the above expressions, we can readily construct the basis transformation matrices , such that . For example, is given by the following:(10)
Furthermore, in a compact form we can write the following:(11)
and(12)
Also, the matrices and follow from (obviously, ).In Table 1, we show that regardless of w, v, u, and m, we will always have the following:
(13)
where ; and , but with the rather important restriction, as follows:(14)
Next, by constructing EPR states for three-level systems, we show that as it should be, obligatorily the observables need to be associated with non-commuting operators. We start with (for totally arbitrary), as follows:(15)
For any , and using the properties of the transformation matrices , for Equation (15) we have the following:(16)
For , the operators and for system II do not commute. We recall that this is one of the requirements of EPR states. Moreover, in such a case, due to Equation (13) and Table 1, we have that —see the last equality in Equation (16)—is necessarily one of the eigenvectors of the operator (eventually multiplied by a phase ). Also, according to the condition in Equation (14), does not commute with . So, this is in agreement with condition (b) of the EPR states proved in the previous section. Lastly, for f a one-to-one index function from to (easily inferred from Table 1) and integer numbers, we find very generally that ( and )(17)
In this way, is an EPR state , displaying all the necessary properties regarding the non-commutation of the associated observables, i.e., and .It should not be difficult to show that, using the above results, the same holds true if we construct EPR states from the eigenstates of the operators .
Finally, the findings in Section 3 follow irrespective of whether the states considered are degenerate or not. Hence, it is instructive to provide an example of the former situation. We note that, with the help of [65], it is straightforward to engender Hermitian matrices displaying degenerated eigenvalues. Thus, suppose the eigenvalues (or , ) of the eigenvectors and associated observable operator:
(18)
We set(19)
Repeating the transformations implemented in Equation (16), i.e., changing the basis from to and considering the explicit form of , we obtain (for , with )(20)
Here, with (so , ) and(21)
Then, from the matrices for and , it reads that and given the exact form of Equations (19) and (20), as well as , we conclude that is an EPR state, thus satisfying (b).5. Observables Exclusively Involving Qubits in EPR States Do Not Violate Bell Inequalities
As already mentioned, in principle, it is possible to find appropriate bases for an entangled state, such that violates Bell’s inequality [9,37,38]. But referring to Figure 2, this implies properly choosing , , and , to be measured, respectively, in detectors 1 and 2. A traditional example is that of entangled photons [16], where one can have the CHSH correlation by adjusting the relative angles of the polarizers employed as detectors.
However, EPR states display a rather special link between their observables, adhering to conditions (a) and (b). Indeed, when considering Equation (2), on the one hand, if we know an observable value for system I (II), this fully determines the value of the associated observable for system II (I). On the other hand, observables in each system are pair-wisely incompatible, namely, we cannot simultaneously determine A and B for II and C and D for I. Conceivably, this must strongly affect the state’s correlation features. Hence, a pertinent question regards the possible range for if computed only from EPR states. This seems to be a simple and direct way to characterize correlations for such a particular class of states.
Assuming the EPR-like state in Equation (2), let with the possible eigenvalues of ) being (so , ). For simplicity, we disregard the eventual relative phases between the states, thus, we have the following:
(22)
Nonetheless, we emphasize that writing and does not change the following results.The transformation from the basis to is given by an arbitrary unitary matrix , whereas from to is given by the complex conjugate of (cf., Equations (5) and (6)), or . Using an appropriate parameterization for U(2) matrices (see, e.g., [66]), we generally have
(23)
with complex numbers such that and . Moreover, we suppose , so that (see Appendix A.3). Thus,(24)
Therefore, for the observables , the CHSH correlation function [32] reads as follows:(25)
with (see Figure 2)(26)
To obtain (as well as ) and (as well as ), we consider Equation (26) with given, respectively, by the first and second expansions in Equation (22). So, straightforwardly, we find the following:(27)
For and we obtain the following:(28)
Now, using Equation (24) in Equation (28)(29)
Finally (for , , )(30)
Analyzing all the combinations (i.e., , or , ) for , , , one realizes that the only possibilities for are(31)
Hence, if we calculate for any set of observables for which the state has the EPR structure, we always find that . However, the violation of Bell’s inequalities in the CHSH construction corresponds to (in fact, [67]).The above finding is a bit curious. EPR states have been discussed in [19] as an attempt to show that quantum mechanics is incomplete, opening up the possibility of alternatives like local hidden variables. The violation of the Bell inequalities [20,40] for a quantum system overturns such claims [41]. Moreover, many breakthrough experiments have clearly demonstrated the existence of quantum correlations violating Bell’s inequalities [21,22,23,24,25,26,27,28,29,30]. However, when probing only the EPR observables (when ) in a CHSH-like experiment—without considering any additional information about the composite—one most likely would not be able to fully discard local hidden variables.
Also, the present is complementary to Hardy’s results [68,69], refuting local hidden variables without invoking inequalities. Indeed, he demonstrated Bell’s nonlocality through clever interference-like experimental setups for particle–antiparticle pairs. More recently, this idea has been extended to many particle systems [70].
6. Final Remarks and Conclusions
Quantum correlations are notoriously more diverse and conceptually more complex than their classical counterparts. For instance, certain works (see, e.g., [71]) argue that tests on violations of Bell-type inequalities should be interpreted as statistical inference of the local incompatibility of observables. So, they might deceive our intuitive predictions about trending behaviors. This is a direct consequence of the complex way in which observables are interrelated in certain particular states. Here, we have investigated one of these special states: bipartite EPR states.
EPR states are known to display maximum entanglement, in the sense that the determination of an observable for I (II) fully specifies the observable for II (I). Moreover, all pairs of observable values are equally probable. These characteristics constitute condition (a)—introduction section. In this contribution, through a rather simple procedure, we further established that, generally, if can be expanded in terms of eigenvectors, either for the observables C (of I) and A (of II) or D (of I) and B (of II), then C is incompatible with D, and A is incompatible with B. In other words, for the associated Hermitian operators, it follows that and . This corresponds to condition (b). To illustrate this general result, we analyzed the EPR states in three-level systems ().
From (a), there is a great interdependence between systems I and II: , , etc. But from (a) and (b), all possible pairs n (, , etc) of measurement outcomes are equiprobable and the observables within each system (C with D, A with B, etc.) are incompatible. This conceivably should lead to high fluctuations in the observable values, tending to decrease correlations. Combined, these opposite traits can make it difficult to foresee ranges for quantum s obtained solely from the observables forming the s. In the particular situation of qubits (), by calculating only for EPR states, we found that , thus not violating Bell’s inequality.
Finally, a natural issue concerns the behavior of quantum correlations for EPR states in higher-dimensional bipartite systems, i.e., for . For such an analysis, it would be necessary to properly extend Bell’s inequalities, particularly for the CHSH. Some interesting proposals have already been addressed in the literature [72,73,74]. Nevertheless, irrespective of the exact analytic form of a generalized , the scheme should involve, as in Section 5, unitary matrices of order . Since a U(N) has free parameters, exact analytic calculations may be laborious, although not unfeasible (at least in some particular instances). It is our plan to discuss these and other aspects related to in a future contribution.
Conceptualization, M.G.E.d.L.; methodology, D.F.O.; software, D.F.O. and L.R.N.O.; validation, D.F.O. and L.R.N.O.; formal analysis, D.F.O. and L.R.N.O.; investigation, D.F.O., L.R.N.O. and M.G.E.d.L.; writing—original draft preparation, M.G.E.d.L.; writing—review and editing, D.F.O., L.R.N.O. and M.G.E.d.L.; supervision, M.G.E.d.L.; project administration, M.G.E.d.L.; funding acquisition, M.G.E.d.L. All authors have read and agreed to the published version of the manuscript.
Data is contained within the article.
We would like to thank the following for the fruitful and elucidating discussions: A.M. Ozorio de Almeida, R.M. Angelo, M.W. Beims, N.P. Neto, A.D. Ribeiro, B.A. Mello and L.S. Schulman.
The authors declare no conflicts of interest.
The following abbreviations are used in this manuscript:
EPR | Einstein-Podolsky-Rosen |
CHSH | Clauser–Horne–Shimony–Holt |
ONB | Orthonormal basis |
Footnotes
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Figure 1. Schematics of the hierarchy of quantum correlations (see, e.g., [9]). However, there may be overlaps, which are not represented here. The basic one, simply ‘quantum’, relates to the very construction of quantum mechanics. It arises from superposition allied to interference: the probability density function [Forumla omitted. See PDF.] is obtained from [Forumla omitted. See PDF.] with [Forumla omitted. See PDF.], so that the “parts” [Forumla omitted. See PDF.]s, whatever they represent, generate correlations. Beyond the basics, entanglement is typically considered the most fundamental form of quantum correlation, with nonlocality being the most restrictive. Steering falls in between, serving as a middle ground.
Figure 2. Illustration of the setup to compute the CHSH correlation function [Forumla omitted. See PDF.]. By separating systems I and II, one can perform measurements of distinct observables, e.g., [Forumla omitted. See PDF.] and [Forumla omitted. See PDF.] for I at detector 1 and [Forumla omitted. See PDF.] and [Forumla omitted. See PDF.] for II at detector 2. From numerous realizations of the experiment, leading to distinct outcomes, [Forumla omitted. See PDF.], [Forumla omitted. See PDF.], [Forumla omitted. See PDF.], [Forumla omitted. See PDF.] (assuming only the values −1 or +1, in two-level systems), one obtains the correlation function [Forumla omitted. See PDF.] (the actual expression is given in Section 5).
Figure 3. Schematics of typical processes in forming and measuring an EPR state [Forumla omitted. See PDF.] (constituted by the entanglement of two systems, I and II): the preparation of the entangled state; the spatial separation of its two parts, I and II; and the possible measurements of an observable for system I, allowing the inference (with 100% certainty) of an observable for system II.
List of all the states in the form
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
Appendix A. Transformations between Eigenbasis Associated with Commuting and Non-Commuting Operators
Below we review some important characteristics of observables and transformations between the countable basis of a separable Hilbert space
We start by addressing the dimensions of the unitary
The answer is positively provided: (i) We deal with suitable ONBs for
Appendix A.1. The Observables Assumed for EPR States
For an EPR state,
In standard quantum physics, the pure point and absolutely continuous parts of the spectrum are associated, respectively, with bound and scattering states [
In our present context, according to Equations (
The multiplicity (in quantum mechanics, degeneracy) of an eigenvalue, a, is the dimension of its eigenspace. In other words, the degeneracy of any a is the number of distinct eigenvectors solving the corresponding eigenvalue equation. We have that
Lastly, for a self-adjoint
-
N is finite [
78 ]; -
N is infinite and
is compact; -
N is infinite and
belongs to certain classes of unbounded operators, see [ 61 ].
Few remarks are in order here. We recall that an operator
As illustrations, for (ii), we mention a not-too-singular attractive potential (like the hydrogen atom [
Appendix A.2. The Structure of the Transformation Matrix Γ
For both
We now consider two possible situations for the operators
and commute: (1) The operators have common eigenvectors. (2) So, based on which one is diagonal, say , is block-diagonal with the distinct blocks having dimensions (all finite, see Appendix A.1 ) equal to the multiplicity of the differenteigenvalues. (3) But from the previous remarks about spectra decomposition, these blocks can always be put in a diagonal form. Therefore, there is at least one basis that simultaneously diagonalizes and . As a consequence of (3), in Equation ( 4 ) can be reduced to an identity matrixof size N. In fact, more generally is a permutation matrix, once in Equation ( 4 ), we can havefor some subsets of indices . Nonetheless, given that for any permutation matrix, , a trivial relabeling of one of the basis indices, say , turns into . and do not commute: (4) There are no common eigenvectors and, thus, no basis can simultaneously diagonalize and . (5) In this way, the matrix may (due to occasional symmetries associated with and ) or may not display a block diagonal format. However, in either situation, neither itself nor its eventual diagonal blocks can be transformed into identity matrices. If they were, it would imply the existence of mutual eigenvectors, violating condition (4).
An illustration of the above is given next, for spin-
Appendix A.3. The Basis Transformation Matrix for Spin-1/2 Systems in Arbitrary Directions
For
Thus, a basis transformation from
From the direct analysis of Equation (
Therefore, for
References
1. Brémaud, P. Probability Theory and Stochastic Processes; 1st ed. Springer: Cham, Switzerland, 2020.
2. Knill, E.; Zhang, Y.; Bierhorst, P. Generation of quantum randomness by probability estimation with classical side information. Phys. Rev. Res.; 2020; 2, 033465. [DOI: https://dx.doi.org/10.1103/PhysRevResearch.2.033465]
3. Beltrametti, E.G. Classical versus quantum probabilities. Chance in Physics: Foundations and Perspectives; Bricmont, J.; Ghirardi, G.; Dürr, D.; Petruccione, F.; Galavotti, M.C.; Zanghi, N. Springer: Berlin, Germany, 2001; pp. 225-232.
4. Gallego, R.; Masanes, L.; De La Torre, G.; Dhara, C.; Aolita, L.; Acín, A. Full randomness from arbitrarily deterministic events. Nat. Commun.; 2013; 4, 2654. [DOI: https://dx.doi.org/10.1038/ncomms3654] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24173040]
5. Pearl, J. Causality: Models, Reasoning and Inference; 2nd ed. Cambridge University Press: Cambridge, UK, 2009.
6. Modi, K.; Paterek, T.; Son, W.; Vedral, V.; Williamson, M. Unified view of quantum and classical correlations. Phys. Rev. Lett.; 2010; 104, 080501. [DOI: https://dx.doi.org/10.1103/PhysRevLett.104.080501] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/20366919]
7. Adesso, G.; Datta, A. Quantum versus classical correlations in Gaussian states. Phys. Rev. Lett.; 2010; 105, 030501. [DOI: https://dx.doi.org/10.1103/PhysRevLett.105.030501]
8. Maziero, J.; Werlang, T.; Fanchini, F.F.; Céleri, L.C.; Serra, R.M. System-reservoir dynamics of quantum and classical correlations. Phys. Rev. A; 2010; 81, 022116. [DOI: https://dx.doi.org/10.1103/PhysRevA.81.022116]
9. Adesso, G.; Bromley, T.R.; Cianciaruso, M. Measures and applications of quantum correlations. J. Phys. A; 2016; 49, 473001. [DOI: https://dx.doi.org/10.1088/1751-8113/49/47/473001]
10. Guo, Z.; Cao, H.; Chen, Z. Distinguishing classical correlations from quantum correlations. J. Phys. A; 2012; 45, 145301. [DOI: https://dx.doi.org/10.1088/1751-8113/45/14/145301]
11. Xu, J.; Xu, X.; Li, C.; Zhang, C.; Zou, X.; Guo, G. Experimental investigation of classical and quantum correlations under decoherence. Nat. Commun.; 2010; 1, 7. [DOI: https://dx.doi.org/10.1038/ncomms1005]
12. Branciard, C.; Gisin, N.; Pironio, S. Characterizing the nonlocal correlations created via entanglement swapping. Phys. Rev. Lett.; 2010; 104, 170401. [DOI: https://dx.doi.org/10.1103/PhysRevLett.104.170401]
13. Fritz, T. Beyond Bell’s theorem: Correlation scenarios. Phys. Rev. Lett.; 2012; 14, 103001. [DOI: https://dx.doi.org/10.1088/1367-2630/14/10/103001]
14. de Chiara, G.; Sanpera, A. Genuine quantum correlations in quantum many-body systems: A review of recent progress. Rep. Prog. Phys.; 2018; 81, 074002. [DOI: https://dx.doi.org/10.1088/1361-6633/aabf61] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/29671752]
15. Frérot, I.; Fadel, M.; Lewenstein, M. Probing quantum correlations in many-body systems: A review of scalable methods. Rep. Prog. Phys.; 2023; 86, 114001. [DOI: https://dx.doi.org/10.1088/1361-6633/acf8d7] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/37699388]
16. Audretsch, J. Entangled Systems: New Directions in Quantum Physics; 1st ed. Wiley-VCH: Weinheim, Germany, 2007.
17. Cavalcanti, D.; Skrzypczyk, P. Quantum steering: A review with focus on semidefinite programming. Rep. Prog. Phys.; 2016; 80, 024001. [DOI: https://dx.doi.org/10.1088/1361-6633/80/2/024001] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28008876]
18. Uola, R.; Costa, A.C.S.; Nguyen, H.C.; Gühne, O. Quantum steering. Rev. Mod. Phys.; 2020; 92, 015001. [DOI: https://dx.doi.org/10.1103/RevModPhys.92.015001]
19. Einstein, A.; Podolsky, B.; Rosen, N. Can quantum-mechanical description of physical reality be considered complete?. Phys. Rev.; 1935; 47, 777. [DOI: https://dx.doi.org/10.1103/PhysRev.47.777]
20. Bell, J.S. On the Einstein Podolsky Rosen paradox. Physics; 1964; 1, pp. 195-200. [DOI: https://dx.doi.org/10.1103/PhysicsPhysiqueFizika.1.195]
21. Aspect, A.; Grangier, P.; Roger, G. Experimental realization of Einstein-Podolsky-Rosen-Bohm gedankenexperiment: A new violation of Bell’s inequalities. Phys. Rev. Lett.; 1982; 49, pp. 91-94. [DOI: https://dx.doi.org/10.1103/PhysRevLett.49.91]
22. Aspect, A.; Grangier, P.; Roger, G. Experimental test of Bell’s inequalities using time-varying analyzers. Phys. Rev. Lett.; 1982; 49, pp. 1804-1807. [DOI: https://dx.doi.org/10.1103/PhysRevLett.49.1804]
23. Tapster, P.R.; Rarity, J.G.; Owens, P.C.M. Violation of Bell’s inequality over 4 km of optical fiber. Phys. Rev. Lett.; 1994; 73, 1923. [DOI: https://dx.doi.org/10.1103/PhysRevLett.73.1923]
24. Tittel, W.; Brendel, J.; Zbinden, H.; Gisin, N. Violation of Bell inequalities by photons more than 10 km apart. Phys. Rev. Lett.; 1998; 81, 3563. [DOI: https://dx.doi.org/10.1103/PhysRevLett.81.3563]
25. Weihs, G.; Jennewein, T.; Simon, C.; Weinfurter, H.; Zeilinger, A. Violation of Bell’s inequality under strict Einstein locality conditions. Phys. Rev. Lett.; 1998; 81, 5039. [DOI: https://dx.doi.org/10.1103/PhysRevLett.81.5039]
26. Aspect, A. Quantum mechanics: To be or not to be local. Nature; 2007; 446, pp. 866-867. [DOI: https://dx.doi.org/10.1038/446866a]
27. Hensen, B.; Bernien, H.; Dréau, A.E.; Reiserer, A.; Kalb, N.; Blok, M.S.; Ruitenberg, J.; Vermeulen, R.F.L.; Schouten, R.N.; Abellán, C. et al. Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers. Nature; 2015; 526, pp. 682-686. [DOI: https://dx.doi.org/10.1038/nature15759]
28. Shalm, L.K.; Meyer-Scott, E.; Christensen, B.G.; Bierhorst, P.; Wayne, M.A.; Stevens, M.J.; Gerrits, T.; Glancy, S.; Hamel, D.R.; Allman, M.S. et al. Strong loophole-free test of local realism. Phys. Rev. Lett.; 2015; 115, 250402. [DOI: https://dx.doi.org/10.1103/PhysRevLett.115.250402]
29. Hensen, B.; Kalb, N.; Blok, M.S.; Dréau, A.E.; Reiserer, A.; Vermeulen, R.F.L.; Schouten, R.N.; Markham, M.; Twitchen, D.J.; Goodenough, K. et al. Loophole-free Bell test using electron spins in diamond: Second experiment and additional analysis. Sci. Rep.; 2016; 6, 30289. [DOI: https://dx.doi.org/10.1038/srep30289]
30. Rosenfeld, W.; Burchardt, D.; Garthoff, R.; Redeker, K.; Ortegel, N.; Rau, M.; Weinfurter, H. Event-ready Bell test using entangled atoms simultaneously closing detection and locality loopholes. Phys. Rev. Lett.; 2017; 119, 010402. [DOI: https://dx.doi.org/10.1103/PhysRevLett.119.010402] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28731745]
31. Genovese, M.; Gramegna, M. Quantum correlations and quantum non-locality: A review and a few new ideas. Appl. Sci.; 2019; 9, 5406. [DOI: https://dx.doi.org/10.3390/app9245406]
32. Clauser, J.F.; Horne, M.A.; Shimony, A.; Holt, R.A. Proposed experiment to test local hidden-variable theories. Phys. Rev. Lett.; 1969; 23, pp. 880-884. [DOI: https://dx.doi.org/10.1103/PhysRevLett.23.880]
33. Brodutch, A.; Modi, K. Criteria for measures of quantum correlations. Quantum Inf. Comput.; 2012; 12, pp. 721-742. [DOI: https://dx.doi.org/10.26421/QIC12.9-10-1]
34. Slofstra, W. The set of quantum correlations is not closed. Forum Math. Pi; 2019; 7, e1. [DOI: https://dx.doi.org/10.1017/fmp.2018.3]
35. Fu, H.; Miller, C.A.; Slofstra, W. The membership problem for constant-sized quantum correlations is undecidable. arXiv; 2022; arXiv: 2101.11087
36. Guo, Y.; Wu, S. Quantum correlation exists in any non-product state. Sci. Rep.; 2014; 4, 7179. [DOI: https://dx.doi.org/10.1038/srep07179]
37. Gisin, N. Bell’s inequality holds for all non-product states. Phys. Lett. A; 1991; 154, pp. 201-202. [DOI: https://dx.doi.org/10.1016/0375-9601(91)90805-I]
38. Yu, S.; Chen, Q.; Zhang, C.; Lai, C.H.; Oh, C.H. All entangled pure states violate a single Bell’s inequality. Phys. Rev. Lett.; 2012; 109, 120402. [DOI: https://dx.doi.org/10.1103/PhysRevLett.109.120402] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23005926]
39. Fine, A.; Thomas, A.R. The Einstein-Podolsky-Rosen argument in quantum theory. The Stanford Encyclopedia of Philosophy, Summer 2020 Edition; Zalta, E.N. Available online: https://plato.stanford.edu/archives/sum2020/entries/qt-epr/ (accessed on 4 April 2024).
40. Bell, J.S. EPR correlations and EPW distributions. Ann. N. Y. Acad. Sci.; 1986; 480, 263. [DOI: https://dx.doi.org/10.1111/j.1749-6632.1986.tb12429.x]
41. Bell, J.S. Speakable and Unspeakable in Quantum Mechanics; Cambridge University Press: Cambridge, UK, 1987.
42. Munro, W.J.; Nemoto, K. Maximally entangled mixed states and the Bell inequality. Z. Naturforschung; 2001; 56a, pp. 152-154. [DOI: https://dx.doi.org/10.1515/zna-2001-0123]
43. Chu, S.-K.; Ma, C.T.; Miao, R.-X.; Wu, C.-H. Maximally entangled state and Bell’s inequality in qubits. Ann. Phys.; 2018; 395, pp. 183-195. [DOI: https://dx.doi.org/10.1016/j.aop.2018.05.016]
44. Salavrakos, A.; Augusiak, R.; Tura, J.; Wittek, P.; Acín, A.; Pironio, P. Bell inequalities tailored to maximally entangled states. Phys. Rev. Lett.; 2017; 119, 040402. [DOI: https://dx.doi.org/10.1103/PhysRevLett.119.040402]
45. Lipinska, V.; Curchod, F.J.; Máttar, A.; Acín, A. Towards an equivalence between maximal entanglement and maximal quantum nonlocality. New J. Phys.; 2018; 20, 063043. [DOI: https://dx.doi.org/10.1088/1367-2630/aaca22]
46. Reid, M.D.; Drummond, P.D.; Bowen, W.P.; Cavalcanti, E.G.; Lam, P.K.; Bachor, H.A.; Andersen, U.L.; Leuchs, G. The Einstein-Podolsky-Rosen paradox: From concepts to applications. Rev. Mod. Phys.; 2009; 87, 1727. [DOI: https://dx.doi.org/10.1103/RevModPhys.81.1727]
47. Arens, R.; Varadarajan, V.S. On the concept of Einstein-Podolsky-Rosen states and their structure. J. Math. Phys.; 2000; 41, 638. [DOI: https://dx.doi.org/10.1063/1.533156]
48. Werner, R.F. EPR states for von Neumann algebras. arXiv; 1999; arXiv: quant-ph/9910077
49. Keyl, M.; Schlingemann, D.; Werner, R.F. Infinitely entangled states. Quantum Inf. Comput.; 2003; 3, pp. 281-306. [DOI: https://dx.doi.org/10.26421/QIC3.4-1]
50. Huang, S. Generalized Einstein-Podolsky-Rosen states. J. Math. Phys.; 2007; 48, 112102. [DOI: https://dx.doi.org/10.1063/1.2809269]
51. Coladangelo, A.; Stark, J. An inherently infinite-dimensional quantum correlation. Nat. Commun.; 2020; 11, 3335. [DOI: https://dx.doi.org/10.1038/s41467-020-17077-9]
52. Musat, M.; Rordam, M. Non-closure of quantum correlation matrices and factorizable channels that require infinite dimensional ancilla. Commun. Math. Phys.; 2020; 375, pp. 1761-1776. [DOI: https://dx.doi.org/10.1007/s00220-019-03449-w]
53. Larsson, J.A. Bell inequalities for position measurements. Phys. Rev. A; 2004; 70, 022102. [DOI: https://dx.doi.org/10.1103/PhysRevA.70.022102]
54. Bohm, D.; Aharonov, Y. Discussion of experimental proof for the paradox of Einstein, Rosen, and Podolsky. Phys. Rev.; 1957; 108, 1070. [DOI: https://dx.doi.org/10.1103/PhysRev.108.1070]
55. Abouraddy, A.F.; Yarnall, T.; Saleh, B.E.A.; Teich, M.C. Violation of Bell’s inequality with continuous spatial variables. Phys. Rev. A; 2007; 75, 052114. [DOI: https://dx.doi.org/10.1103/PhysRevA.75.052114]
56. Buono, D.; Nocerino, G.; Solimeno, S.; Porzio, A. Different operational meanings of continuous variable Gaussian entanglement criteria and Bell inequalities. Laser Phys.; 2014; 24, 074008. [DOI: https://dx.doi.org/10.1088/1054-660X/24/7/074008]
57. Adesso, G.; Illuminati, F. Entanglement in continuous-variable systems: Recent advances and current perspectives. J. Phys. A; 2007; 40, pp. 7821-7880. [DOI: https://dx.doi.org/10.1088/1751-8113/40/28/S01]
58. Giustina, M.; Versteegh, M.A.M.; Wengerowsky, S.; Handsteiner, J.; Hochrainer, A.; Phelan, K.; Steinlechner, F.; Kofler, J.; Larsson, J.; Abellán, C. et al. Significant loophole-free test of Bell’s theorem with entangled photons. Phys. Rev. Lett.; 2015; 115, 250401. [DOI: https://dx.doi.org/10.1103/PhysRevLett.115.250401]
59. Leinfelder, H. A geometric proof of the spectral theorem for unbounded self-adjoint operators. Math. Annal.; 1979; 242, pp. 85-96. [DOI: https://dx.doi.org/10.1007/BF01420484]
60. Celeghini, E.; Gadella, M.; del Olmo, M.A. Applications of rigged Hilbert spaces in quantum mechanics and signal processing. J. Math. Phys.; 2016; 57, 072105. [DOI: https://dx.doi.org/10.1063/1.4958725]
61. Schmüdgen, K. Unbounded Self-Adjoint Operators on Hilbert Space; Springer: Dordrecht, The Netherlands, 2012.
62. Bohm, D. Quantum Theory; 2nd ed. Prentice-Hall: Englewood Cliffs, NJ, USA, 1951.
63. Bohr, N. Can quantum-mechanical description of physical reality be considered complete?. Phys. Rev.; 1935; 48, 696. [DOI: https://dx.doi.org/10.1103/PhysRev.48.696]
64. Van Fraassen, B.C. Quantum Mechanics: An Empiricist View; Clarendon Press: New York, NY, USA, 1991.
65. Caspers, W.J. Degeneracy of the eigenvalues of hermitian matrices. J. Phys. Conf. Ser.; 2008; 104, 012032. [DOI: https://dx.doi.org/10.1088/1742-6596/104/1/012032]
66. Schneider, H.; Barker, G.P. Matrices and Linear Algebra; 2nd ed. Dover: Mineola, NY, USA, 1973.
67. Cirel’son, B.S. Quantum generalizations of Bell’s inequality. Lett. Math. Phys.; 1980; 4, pp. 93-100. [DOI: https://dx.doi.org/10.1007/BF00417500]
68. Hardy, L. Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories. Phys. Rev. Lett.; 1992; 68, 2981. [DOI: https://dx.doi.org/10.1103/PhysRevLett.68.2981]
69. Hardy, L. Nonlocality for two particles without inequalities for almost all entangled states. Phys. Rev. Lett.; 1993; 71, 1665. [DOI: https://dx.doi.org/10.1103/PhysRevLett.71.1665]
70. Tran, D.M.; Nguyen, V.D.; Ho, L.B.; Nguyen, H.Q. Increased success probability in Hardy’s nonlocality: Theory and demonstration. Phys. Rev. A; 2023; 107, 042210. [DOI: https://dx.doi.org/10.1103/PhysRevA.107.042210]
71. Khrennikov, A. Get rid of nonlocality from quantum physics. Entropy; 2019; 21, 806. [DOI: https://dx.doi.org/10.3390/e21080806] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33267519]
72. Chen, J.; Kaszlikowski, D.; Kwek, L.C.; Oh, C.H. Wringing out new Bell inequalities for three-dimensional systems (qutrits). Mod. Phys. Lett. A; 2002; 17, pp. 2231-2237. [DOI: https://dx.doi.org/10.1142/S0217732302008885]
73. Collins, D.; Gisin, N.; Linden, N.; Massar, S.; Popescu, S. Bell inequalities for arbitrarily high-dimensional systems. Phys. Rev. Lett.; 2002; 88, 040404. [DOI: https://dx.doi.org/10.1103/PhysRevLett.88.040404] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/11801097]
74. Li, M.; Zhang, T.; Hua, B.; Fei, S.M.; Li-Jost, X. Quantum nonlocality of arbitrary dimensional bipartite states. Sci. Rep.; 2015; 5, 13358. [DOI: https://dx.doi.org/10.1038/srep13358] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26303075]
75. Jorgensen, P.E.T.; Kornelson, K.A.; Shuman, K.L. Iterated function systems, moments, and transformations of infinite matrices. Memoirs of the American Mathematical Society; AMS: Providence, RI, USA, 2011.
76. Goertzen, C.M. Operations on Infinite × Infinite Matrices and Their Use in Dynamics and Spectral Theory. Ph.D. Thesis; University of Iowa: Iowa City, IA, USA, 2013.
77. Kim, Y.S.; Noz, M.E. Theory and Applications of the Poincaré Group; 3rd ed. D. Reidel Publishing Company: Dordrecht, The Netherlands, 1986.
78. Akhiezer, N.I.; Glazman, I.M. Theory of Linear Operators in Hilbert Space; Dover: Mineola, NY, USA, 1993.
79. Halmos, P.R. A Hilbert Space Problem Book; Springer: New York, NY, USA, 1982.
80. Jorgensen, P.E.T.; Pearse, E.P.J. Spectral reciprocity and matrix representations of unbounded operators. J. Funct. Anal.; 2011; 261, pp. 749-776. [DOI: https://dx.doi.org/10.1016/j.jfa.2011.01.016]
81. Dutkay, D.E.; Jorgensen, P.E.T. Fourier duality for fractal measures with affine scales. Math. Comput.; 2012; 81, pp. 2253-2273. [DOI: https://dx.doi.org/10.1090/S0025-5718-2012-02580-4]
82. Michael, M.; Simon, B. Methods of Modern Mathematical Physics, 1: Functional Analysis; 2nd ed. Academic Press: San Diego, CA, USA, 1980.
83. Hassani, S. Mathematical Physics: A Modern Introduction to Its Foundations; 2nd ed. Springer: Basel, Switzerland, 2013.
84. Last, Y. Quantum dynamics and decompositions of singular continuous spectra. J. Funct. Anal.; 1996; 142, pp. 406-445. [DOI: https://dx.doi.org/10.1006/jfan.1996.0155]
85. Thirring, W. Quantum Mathematical Physics; 2nd ed. Springer: Berlin, Germany, 2002.
86. Teschl, G. Mathematical Methods in Quantum Mechanics: With Applications to Schrödinger Operators; 2nd ed. AMS: Providence, RI, USA, 2014.
87. Cycon, H.L.; Froese, R.G.; Kirsch, W.; Simon, B. Schrödinger Operators with Application to Quantum Mechanics and Global Geometry; 1st ed. Springer: Berlin, Germany, 1987.
88. Last, Y. Exotic spectra: A review of Barry Simon’s central contributions. Spectral Theory and Mathematical Physics: A Festschrift in Honor of Barry Simon’s 60th Birthday; Gesztesy, F.; Deift, P.; Galvez, C.; Perry, P.; Schlag, W. AMS: Providence, RI, USA, 2007; pp. 697-714.
89. Tchebotareva, O. An example of embedded singular continuous spectrum for one-dimensional Schrödinger operators. Lett. Math. Phys.; 2005; 72, pp. 225-231. [DOI: https://dx.doi.org/10.1007/s11005-005-7650-z]
90. Pearson, D. Singular continuous measure in scattering theory. Commun. Math. Phys.; 1978; 60, pp. 13-36. [DOI: https://dx.doi.org/10.1007/BF01609472]
91. Kiselev, A.; Last, Y.; Simon, B. Modified Prfer and EFGP transforms and the spectral analysis of one-dimensional Schrödinger operators. Commun. Math. Phys.; 1998; 194, pp. 1-45. [DOI: https://dx.doi.org/10.1007/s002200050346]
92. Remling, C. Embedded singular continuous spectrum for one-dimensional Schrödinger operators. Trans. Am. Math. Soc.; 1999; 351, pp. 2479-2497. [DOI: https://dx.doi.org/10.1090/S0002-9947-99-02495-2]
93. Davis, E.B. Linear Operators and Their Spectra; 1st ed. Cambridge University Press: Cambridge, UK, 2007.
94. Buck, R.C. Multiplication operators. Pac. J. Math.; 1961; 11, pp. 95-103. [DOI: https://dx.doi.org/10.2140/pjm.1961.11.95]
95. Cancès, E. Introduction to first-principle simulation of molecular systems. Computational Mathematics, Numerical Analysis and Applications; Mateos, M.; Alonso, P. Springer: Berlin, Germany, 2017.
96. Amrein, W.O. Non-Relativistic Quantum Dynamics; 1st ed. D. Reidel Publishing Company: Dordrecht, The Netherlands, 1981.
97. Carreau, M.; Farhi, E.; Gutmann, S. Functional integral for a free particle in a box. Phys. Rev. D; 1990; 42, 1194. [DOI: https://dx.doi.org/10.1103/PhysRevD.42.1194] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/10012954]
98. da Luz, M.G.E.; Cheng, B.K. Quantum-mechanical results for a free particle inside a box with general boundary conditions. Phys. Rev. A; 1995; 51, 1811. [DOI: https://dx.doi.org/10.1103/PhysRevA.51.1811]
99. Kuhn, J.; Zanetti, F.M.; Azevedo, A.L.; Schmidt, A.G.M.; Cheng, B.K.; da Luz, M.G.E. Time-dependent point interactions and infinite walls: Some results for wavepacket scattering. J. Opt. B; 2005; 7, pp. S77-S85. [DOI: https://dx.doi.org/10.1088/1464-4266/7/3/011]
100. Beltrametti, E.G.; Cassinelli, G. The logic of quantum mechanics. Encyclopedia of Mathematics and Its Applications; Addison-Wesley: Reading, MA, USA, 1981.
101. Shankar, R. Principles of Quantum Mechanics; 2nd ed. Plenum Press: New York, NY, USA, 1994.
102. Chaichian, M.; Hagedorn, R. Symmetries in Quantum Mechanics; Institute of Physics Publishing: Bristol, UK, 1998.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
© 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.
Abstract
The identification and physical interpretation of arbitrary quantum correlations are not always effortless. Two features that can significantly influence the dispersion of the joint observable outcomes in a quantum bipartite system composed of systems I and II are: (a) All possible pairs of observables describing the composite are equally probable upon measurement, and (b) The absence of concurrence (positive reinforcement) between any of the observables within a particular system; implying that their associated operators do not commute. The so-called EPR states are known to observe (a). Here, we demonstrate in very general (but straightforward) terms that they also satisfy condition (b), a relevant technical fact often overlooked. As an illustration, we work out in detail the three-level systems, i.e., qutrits. Furthermore, given the special characteristics of EPR states (such as maximal entanglement, among others), one might intuitively expect the CHSH correlation, computed exclusively for the observables of qubit EPR states, to yield values greater than two, thereby violating Bell’s inequality. We show such a prediction does not hold true. In fact, the combined properties of (a) and (b) lead to a more limited range of values for the CHSH measure, not surpassing the nonlocality threshold of two. The present constitutes an instructive example of the subtleties of quantum correlations.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer