Possible considerations to teleport fermionic particles via studying on teleportation of twoparticle state with a four-particle pure entangled state

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1 ossible considerations to teleport fermionic particles via studying on teleportation of twoparticle state with a four-particle pure entangled state Shilan Savan Semnan university, faculty of science, hysics department. Mehrdad Ghominejad Semnan university, faculty of science, hysics department Abstract: In this research we have firstly studied some evolutionary theorems in new conceptual quantum information matters such as ER gedanken eperiment, Bell states and CHSH inequality. Then we have introduced those kinds of eperiments which are dealing with spin-correlation of photons and protons by which the mentioned inequalities are violated. roton as a famous fermionic-hadronic particle in low and high energy QCD interactions has been the center of our attentions to focus. This study has been followed by an idea we have, on possible considerations on proton teleportation by discussing out probilistic teleportation of an unknown two-particle state with a four-particle pure entangled state and positive operator valued measure (OVM). Introduction: The parado of Einstein, odolsky and Rosen [] was advanced as an argument that quantum mechanics could not be a complete theory but should be supplemented by additional variles, by which the causality and locality theories are restored. It is well-known that Einstein, odolsky and Rosen (ER) [] asserted quantum mechanics is incomplete in terms of local realism, which has been a basic view on nature of classical theories. According to the local realistic theories, objects should have definite properties whether they are measured or not (reality), and there is no action-at a-distance in nature (locality). Some attempts were made to eplain quantum mechanical phenomena from a view of the local realistic theories. Bell, however, showed quantum mechanical correlations between entangled systems, can be stronger than those by the local realistic theories []. Since Bell s proof was given by an inequality which could be tested eperimentally, many eperiments have been devoted to test the inequality by measuring polarization correlations between entangled two photons and have yielded results which are consistent with quantum mechanics. A brief history of these eperiments is summarized in Refs. [3] and []. The mentioned movement to pursue and check eperimentally the bell inequality in QCD interactions started with the key paper of Clauser, Horne, Shimony and Holt (CHSH) [5] who generalized Bells theorem such that it applies to realizle eperimental tests with pair detection of all local hidden varile theories. A proof of the CHSH inequality by Bell can also be found in [6] here, we rather give the final result. For any local hidden varile theory the epectation values E(i; j) of spin measurements on two particles have to fulfill the following inequality: i, and j, S cl (,,, ) E (, ) E(, ) E(, ) E(, ) S (,,, ) > Orientations of angels QM S cl for maimal violation of CHSH inequality shsavan@yahoo.com mghominejad@semnan.ac.ir

2 Quantum mechanically thinking, the mentioned inequality can not be totally true though. To check this non reliility for photons and most importantly for QCD like scatterings such as roton-roton (called ) scattering in low and rarely for high energies to actually observe such a violation of Bells inequality in a loratory some eperiments was done. It was first shown for a set of local measurements on the two subsystems of an ER singlet state by Fredman and Clauser and later most prominently by A. Aspect et al. [7] with photons and by M. Lamehi-Rachti and W. Mittig [] with protons in entangled systems. In this eperiment (figure below), we see a spin-correlation measurement of two protons produced by the H ( d, He) n reaction. Here the decay of particle in the spin-singlet state[ S ] into two spin-half particles, for two protons;. Making use of this reaction, our eperiment has a distinguished feature that the directions of the spin-measuring aes are not pre-fied by the eperimental setup, which enled us to solve the problem in this eperiment. They used strong interaction to test the Bell s inequality. Since the strong interaction is a short range interaction, entangled particles are produced with etremely short coherence length. It is of considerle interest to investigate whether an entanglement between two particles is robustly maintained even if the two particles are spatially separated from each other by a distance etremely beyond their coherence length. Measured spin-correlations between two protons in the spin-singlet ( S ) state which was produced by the proton-proton S-wave elastic scattering. This is because proton-proton scattering at large angle and low energy, say a few MeV, goes mainly in S wave But the antisymmetry of the final wave function then requires the antisymmetries singlet spin state. In this state, when one spin is found up the other is found down. This follows formally from the quantum epectation value, sin glet σ () σ () sin glet. In these eperiments, protons of MeV l energy are scattered at a l angle of 5, and spin correlation of scattered and recoil protons are measured. They obtained results which showed violation of Bell s inequality, we consider there is a crucial weak point in this eperiment. In their setup, one of the spin-measuring aes of the polarimeters was fied along the normal of the scattering plane of the pp-scattering. He [ S ] [ S ] : { } entangled state (ER pair) { C ( ) cos } { ( ) θ} QM θ θ C BI θ. π This eperiment anyhow, shows agrees with quantum mechanics and disagrees (in the sense of a certain etrapolation) with the locality inequality, and is the first serious test of Bell inequality by spin fermion z z

3 system with mass. Violating the Bell inequality for the sake of ER states, opens up the world of teleportation for us, that has recently been the source of many researches in this domain [9]. Quantum Teleportation Quantum teleportation is a technique for moving quantum states around, even in the sence of a quantum communications channel linking the sender of quantum state to the recipient. Now it seems necessary to eplain some crucial concepts, namely, entanglement, qubit and Bell state, which have key roles to perceive teleportation. What is the entanglement? In order to understand the concept of quantum communication the phenomena called entanglement, described for the first time at 935 by Einstein, odolsky and Rosen. They eplained the theoretical possibility, from the point of view of quantum physics, to create a system composed by two particles with a strong interaction between them and which is known as ER system in honour of its discoverers. The interaction which was described in these particles is so deep to the limit that if one of them was modified, the other one was instantly affected on a determined and unique way. Until this point, nothing escapes to the classical physics being this kind of interactions present, for eample, at a planet satellite system. But the ER interactions go further, and have three main characteristics which make them unique and etremely interesting: Any modification applied to one of the particles, from now on particle, would imply a variation on its couple (particle ). The alteration on particle would depend on the modification done on particle. Different modifications would imply different variations. The alteration of particle, due to the modification of particle does not depend on the distance between Them. article is instantly altered once particle is modified. What is qubit? Qubit is a fundamental element for quantum computation and quantum information. In the early days of quantum mechanics the qubit structure was not at all obvious, and people struggle with phenomena that we may now understand in terms of qubits, that is, in the terms of two level quantum systems. A decisive early eperiment indicating the qubit structure we conceived by Stern in 9 and performed with Gerlach in 9 in Frankfurt. Qubit can be in a state other than or. It is also possible to form where the numbers and are comple numbers. In general a qubits state is a unit vector in a two-dimensional comple vector θ iϕ θ space. Because we write, cos e sin where the angels θ and ϕ define a point on the unit three-dimensional sphere. This sphere is often called the Bloch sphere, which provides a useful means of visualizing the state of single qubit and often serves as an ecellent testbed for ideas out quantum computation and quantum information. Bloch sphere representation of a qubit 3

4 What is Bell state? Suppose we have two qubit system whose four computational basis states are denoted by,,,. Obviously an infinite number of different entangled pairs can be defined by only varying probility amplitudes of Φ a b. Moreover computational basis states and also form a suitle subset of all possible two-qubit basis vectors. There are four distinguished entangled pairs called Bell states or ER pairs Bell state generator quantum circuit Bell states have an often eploited important property, namely they form an orthonormal vector set which is equivalent to the fact as we will see later that they can be distinguished unambiguously. Now here s how quantum teleportation works. Alice and Bob met long ago but now live far apart. While together they generated an ER pair, each taking one qubit of the ER pair when they separated. Many years later, Bob is in hiding, and Alices mission, should she choose to accept it, is to deliver a qubit to Bob. She does not know the state of the qubit, and moreover can only send classical information to Bob. In outline, the steps of the solution are as follows: Alice interacts the qubit with half of ER pair (singlet state), and then measures the two qubits in her possession, obtaining one of four possible classical results,,, and. She sends this information to Bob. Depending on Alices classical message, Bob performs one of four operations on his half of the ER pair. Amazingly, by doing this he recovers the original state!. The state to be teleported is, where and are unknown amplitudes. ER pair of Alice and Bob as seen below is the package B,C. Qubit A which belongs to Alice, now should be teleported to Bob. Using a special quantum circuit figured in [9, pp. 7] the state input into the quantum circuit is [ ( ) ( )]. The first two qubits (on the left) belong to Alice, and the third qubit to Bob. Alice sends her qubits through a CNOT gate, and then sends the first qubit through a Hadamard gate, obtaining,

5 [ ( ) ( )]. [ ( )( ) ( )( )]. The latter state may be re-written in the following way, simply by regrouping terms: [ ( ) ( ) ( ) ( )]. Obviously Alice obtains one of the four possible two-bit results among,, or as measurement outcome. Each of them is in close connection with the state of Bob s qubit hence Alice sends these two classical bits to Bob. After a short hesitation Bob compares to the potential states of his half Bell pair. It is easy to realize the Ι Z X ZX Therefore Bob has only to apply the inverse of the appropriate transform(s) in compliance with the received classical bits. So as it can be seen, quantum teleportation is a process of transmission of an unknown quantum state via a previously shared ER pair with the help of only two classical bits transmitted through a classical channel [8]. It was regarded as one of the most striking progress of quantum information theory [9]. It may have a number of useful applications in quantum computer [, ], quantum dense coding [], quantum cryptography [3] and quantum secure direct communication. The first scheme for teleporting an unknown quantum state from a sender to a receiver was proposed by Bennett et al. 5

6 About three years ago, YAN Feng-Li, DING He-Wei designed a scheme for probilistic teleporting an unknown two particle state with a four-particle pure entangled state and a projective measurement on an auiliary particle. It is shown that by performing two Bell state measurements, a proper OVM and a unitary transformation, the unknown two-particle state can be teleported from the sender Alice to the receiver Bob with certain probility. Suppose Alice has two particles and in an unknown state, ( a b c ) a b c d d Where a, b, c and d are arbitrary comple numbers. Alice and Bob share quantum entanglement in the form of the following four-particle pure entangled state, which is used as the quantum channel below, ( 3 ) 3 The particles 3 and, and particle pair (, ) are in Alices possession, and particles 5 and 6 are in Bobs possession. The overall state of si particles is w 3 If the outcomes of the Alices two Bell state measurements are are in the state, Φ and Φ, then the particles 5 and 6 3 ( a b c d ) a b c d Now, we will not write out the states of the particles 5 and 6 corresponding to the other outcomes of Alices two Bell state measurements [6]. Then Alice informs Bob her two Bell state measurements on particles, 3 and, Without loss of generality, we give the case for, all other cases can be deduced similarly. In order to realize the teleportation, Bob introduces two auiliary qubits a and b in the state. Thus the state of particles 5, 6, a, and b becomes, ( a b c d ) a b c d Then Bob performs two controlled-not operations(c NOT gate) with particles 5 and 6 as the control qubits and the auiliary particles a and b as the target qubits respectively. After completing this operation the particles 5, 6, a, and b are in the following state, ( a b c d ) a b c d 6

7 After some rearrangement one obtains, [( a b c d ) ( a b c d ) ( a b c d ) ( ) ( a b c d ) ( ) a b c d ) ( ) ] ( Now it should be noted that, there is a mathematical tool known as the OVM formalism which is especially well adapted to the analysis of the measurements. [7] Now Bob makes an optimal OVM [5,8] on the ancillary [7, pp.8] particles a and b to conclusively distinguish the ove states. We choose the optimal OVM in this subspace as follows: Ρ i i i, i,,3,; 5 Ι i i Ι 3 i F ( ) F ( ) 3 F ( ) F ( ) Where, F the matri I in matri ove, is an identity operator, is a coefficient relating to,,,, and makes, 5 to be a positive operator. For eactly determining, we would like to write the five operators 5,, 3, and 5 in the matri form, 7

8 F F 3 F F ) ( A D C B A F 5 ) ( B ) ( C ) ( D Obviously, we should carefully choose such that all the diagonal elements of be nonnegative. If the 5 result of Bobs OVM is, then Bob can safely conclude that the state of the particles 5 and 6 is then, ) ( d c b a If Bobs OVM outcome is, Bob can obtain by applying the unitary transformation Ι z σ on the particles 5 and 6. Two other possible results can be achieved similarly. Therefore, in these four cases stated ove, the teleportation is realized successfully. However if Bobs OVM outcome is, Bob can infer nothing out the identity of the state of the particles 5 and 6. In this case the teleportation fails. As a matter of fact, the key point is that Bob never makes a mistake identifying the state of the particles 5 and 6. This infallibility comes at the price that sometime Bob obtains no information out the identity of the state of the particles 5 and 6. By the similar method we can make the teleportation successful in the other outcomes of Alices two Bell state measurements. 5 8

9 Evidently, when the Bell states Φ Φ 3, are acquired in Alices two Bell state measurements, the F probility of successful teleportation is Φ Φ Φ ( Ι 3 w 5 Ι ) Synthesizing all Alices Bell state measurement cases (siteen kinds in all), the probility of successful tele- 6F portation in this scheme is. Hence the smallest corresponds to the highest probility of successful teleportation. Because is a positive operator, i.e., A,B,C, and D are nonnegative, it is easy to obtain that the smallest must be 5 μ F. Here is the smallest one in the set of F μ {,,, }. Hence if is chosen, the probility of successful teleportation in this scheme is μ. μ Conclusion: It was shown that the possibility of teleportation of protons as fermions ( in low energy scales) can be focused using some availle eperimental techniques. However in the last, we have used a two-particle state in one hand, with a four-particle pure entangled state in other hand to teleport a fermionic characteristic (spin), but it should be noted that we have no idea on how to prepare a successful setup to eamine our suggestion. It seems that the mathematics behind it works well. But it is honestly a long way between a mythic and gedanken Idea and a viewle-eperimental idea to have reasonle data in loratory. 9

10 References: - A. Einstein, B. odolski and N. Rosen, hys. Rev. 7, 777 (935). - J. S. Bell, hysics letter (96) J. F. Clauser and A. Shimony, Rep. rog. hys. (978) A. Aspect, Nature 398 (999) John F. Clauser, Michael A. Horne, Abner Shimony and Richard A. Holt, hys. Rev. Lett. 3 (969) J. F. Clauser, A. Shimony. eperimental tests and implications. Rep. rog. hys.,, 88 (978). 7- A. Aspect, J. Dalibard, G. Roger. Eperimental Test of Bells Inequalities Using Time-Varying Analyzers. hys. Rev. Lett., 9, (5) 8 (98). 8- C.H.Bennett et al 993 hys. Rev. Lett M.A.Nielsen and I.L.Chuang Quantum Computation and Quantum Information (Cambridge: Cambridge University ress. - Cirac J I and Zoller 995 hys. Rev. Lett Barenco A et al 995 hys. Rev. Lett C.H.Bennett and S.J.Wiesner 99 hys. Rev. Lett Gisin N, Ribordy G, Tittel W and Zbinden H Rev. Mod. hys M. Lamehi-Rachti, W. Mittig. Quantum mechanics and hidden variles: A test of Bells inequality by measuring the spin correlation in low-energy proton-proton scattering. hys. Rev. D,, () 53 (976). 5- Bandyopadhyay S hys. Rev. A arxiv: quant-ph/3969 v 3 Sep Asher eres, Quantum theory: concepts and methods. Kluwer academic publishers. 8- Sandor Imre, Ferenc Balazs, Quantum computing and communications, John Wiley & sons, Ltd.