Quantum Teleportation is a means to transfer the state of a quantum system over large lengths by using entanglement (Zeilinger, 2007). Since ancient times, it is a desire to be able to travel from one place to another instantaneously. The basic idea of teleportation is that the sender examines the object to be teleported and sends the read information to the receiver, which then uses this information to restructure the object (Bouwmeester et al., 2000). However, success can not be achieved in this manner according to quantum mechanics, since both portray the concept in a different way. Determining the quantum state by measurement is impossible if only one object is within reach. Quantum state represents all probabilistic information about an object and thus making use of quantum entanglement, quantum teleportation can be made possible (Zeilinger, 2007). Experimentally, the teleported thing is the information represented by an object and not the substance, of which it is made. Entanglement can be used for teleportation where sender (Alice) is in control of the teleportee photon in a quantum state, not aware of it, and the receiver (Bob) shares an auxiliary pair of entangled photons, as shown in figure1.
The sender has an original particle and both share an auxiliary entangled pair. Performing the Bell measurement, the sender transmits the random outcome to the receiver who, by performing unitary transformation, can change the ancillary photon into a copy of the original (Zeilinger, 2007). Quantum teleportation is considered to be the basic building block of future quantum computer networks and it would permit the transmission of the quantum output to another quantum computer as a quantum input (Zeilinger, 2007). According to Zhang et al., (2000), quantum teleportation implies the transfer of a state over an absolute spatial distance by manipulating the prearranged entanglement of quantum systems in relation to the transmission of minimum information. Later researchers developed protocols for teleportation with three stages; preparation of Einstein-Podolsky-Rosen (EPR) entangled particle source where sender and receiver share the particle from the source, secondly, performing the Bell-operator measurement by sender on his EPR particle and the target particle with unknown quantum state and finally transmitting the result of measurement to the receiver. After this applying the unitary operation on receiver’s EPR particle and the unknown state of particle is removed at sender site and its replica appears on receiver’s site. The state of teleportation is not found between the two sites during transfer (Zhang et al., 2000). Noh and Carmichael (2006) considered an optical field for teleportation protocol with a finite bandwidth as input. Figure2. illustrates the teleportation protocol based on the concept of sender and receiver (Zeilinger, 2007).
Theoretical and Experimental Realizations
Many attractive theoretical developments were brought in from the time of publication of protocols especially the one presented by Zhang et al., (2000). According to the research conducted by (Noh and Carmichael, 2006), Vaidman (1994) presented the use of non-local measurements for teleportation of unknown quantum states with continuous variables, also provided a method for two way teleportation. Further to this research, Braunstein & Kimble (1998) analysed to integrate finite degree of correlation between relevant particles and also to include inefficiencies in the process of measurement. Zubairy et al., (2003) dealt with the teleportation of a field state from a high Q cavity to another one. Introducing the interspecies teleportation system Maierle et al., (1998) proposed that the information in a superposition of molecular chiral amplitudes needs to be teleported to a photon. Then presenting the QT as an essential ingredient for quantum computing and implementation of it with a simple circuit, Brassard et al., (2004) pursued QT as a primitive operation in quantum computing (Zhang et al., 2000).
Moving towards the experimental realizations, the first laboratory implementation of quantum teleportation was conducted in 1997 that involved the successful transmission of polarization state from photon to photon (Zhang et al., 2000). To generate the polarization entangled EPR source, a type-II degenerate pulse parametric down-conversion process was applied with an easy experimental design. But here only one EPR-Bell state can be identified. Later on it was shown by Pan et al., (1998) that free propagating particles with no physical interaction could be easily entangled. During the experiment, Bell-state measurements were applied to single photon from each of the two pairs of entangled photons and as an outcome the other two photons extended to an entangled state. Also, it was shown that it is not necessary that entangled particles originate from a common source. Afterwards, another experiment related to quantum teleportation by Boschi et al (1998) involved implementation of quantum optical, where polarization degree of freedom of one photon in the pair was used for determining the unknown state. Here the objective to be achieved was the two degrees of freedom of a photon could be k-vector entangled, but the scheme cannot be used for teleporting an external unknown quantum state. Experimental evidence is there for the conservation of energy and time photon entanglement through telecommunication fiber network over distances more than 10km. Also the distribution of cryptographic quantum keys over open space optical paths of around 1km in the similar style. The actual huge economic and national security significances of a successful realization of a loop holes-free quantum teleportation system have directed to a competition between laboratories hat have the experimental abilities to properly handle this challenge. Figure 3 shows an experimental setup for measurement of entanglement and interference. The quantum teleportation can not be achieved by the use of methods implemented in experiments that are presented to date. The reason behind this is the impossibility to perform Bell operator measurements without using the interaction between quantum states of the particles (Zhang et al., 2000).
There are varieties of applications of quantum teleportation and a few are discussed is this section. The Application of quantum teleportation is well considered to be a mean of communication between quantum computers in the near future. According to Bouwmeester et al., (2000), the long distance quantum teleportation has been possible as shown in figure 4. Here the fiber channel is placed in a sewage pipe tunnel below the River Danube in Vienna although the microwave channel passes above it (Bouwmeester et al., 2000). Wang and Kais (2006) discussed the application of quantum teleportation in one-dimensional quantum dot system as shown in figure 5.
The application of continuous variable approach on creation and detection of multipartite quantum entanglement to a quantum teleportation network is considered by Furusawa et al. (2005) (Xiao-Ming et al., 2007). In this piece of work, the tripartite quantum protocols were created with quantum optically created tripartite entanglement and thus calling it as a quantum teleportation network as shown in figure 6.
Application of quantum mutual entropy to characterize the quantum teleportation process with the help of nonlinear channel involves the basic concept of quantum teleportation channel based on the original teleportation system which was used along with deriving the mutual entropy for the information transmission in the quantum teleportation channel. As a result the information could be transmitted through the linear quantum teleportation channel but generally the associated channel become nonlinear and the information conservation becomes difficult to be studied (Inoue et al., 1998).
Another application involves the abstraction of quantum teleportation and use of local cloning for splitting of entanglement having multiple senders and receivers (Ghiu, 2007). Moving to another application of quantum teleportation Sherson et al., (2006) discussed the quantum teleportation of light and matter. This was performed as a first successful teleportation between objects of different natures. Previous experiments were performed with two different light beams, then it became possible to transfer attributes of a stored photon to another object of same kind and recently scientists have shown the transfer of quantum states of a light pulse to a macroscopic object, which is a group of 1012 atoms. This provides a way for practical application in realization of quantum computers or transmission of coded data i.e., quantum cryptography.