Quantum Communication And Application Of Probabilistic Cloning In Quantum Computation

1. Probabilistic cloning and quantum computationWe discuss the usefulness of quantum cloning and present examples of quantum computation tasks for which quantum cloning offers an advantage which cannot be matched by any approach that does not resort to it. In these quantum computations, we need to distribute quantum information contained in states about which we have some partial information. To perform quantum computations, we use state-dependent probabilistic quantum cloning procedure to distribute quantum information in the middle of a quantum computation. Furthermore we give achievable efficiencies and construct efficient quantum logic network for probabilistic cloning the quantum states used in implemented quantum computation tasks for which cloning provides some enhancement in performance.2. Quantum secure direct communicationTwo theoretical schemes for controlled and secure direct communication are proposed. The communications are based on shared triplet of entangled particles and controlled quantum tele-portation. The distributed entangled particles shared by the sender Alice, the remote receiver Bob and the supervisor Charlie function as a quantum information channel for faithful transmission. After insuring the security of the quantum channel, Alice encodes her secret message directly on a sequence of particle states and transmits them to Bob supervised by Charlie using controlled quantum teleportation. Bob can read out the encoded messages directly by the measurement on his qubits. In addition, two schemes for quantum secure conditional direct communication are also proposed, where a set of EPR pairs of maximally entangled particles in Bell states, initially made by the supervisor Charlie, but shared by the sender Alice and the receiver Bob, functions as quantum information channels for faithful transmission. After insuring the security of the quantum channel and obtaining the permission of Charlie (i.e. Charlie is trustworthy and cooperative, which means the 'conditional' in the two schemes), Alice and Bob begin their private communication under the control of Charlie. In the first scheme, Alice transmits secret message to Bob in a deterministic manner with the help of Charlie by means of Alice's local unitary transformations, both Alice and Bob's local measurements, and both of Alice and Charlie's public classical communication. In the second scheme, the secure communication between Alice and Bob can be achieved via public classical communication of Charlie and Alice, and the local measurements of both Alice and Bob. The common feature of theseprotocols is that the communications between two communication parties Alice and Bob depend on the agreement of the third side Charlie. Moreover, transmitting one bit secret message, the sender Alice only needs to apply a local operation on her one qubit and send one bit classical information.We present two quantum deterministic secure direct communication schemes achieved by swapping quantum entanglement. Here a set of ordered Einstein-Podolsky-Rosen (EPR) pairs and a set of ordered maximally entangled three-particle states (GHZ states) are used as a quantum information channel for sending secret messages directly, respectively. In the first scheme, after insuring the safety of the quantum channel, the sender Alice encodes the secret messages directly by applying a series local operations on her particles sequence according to their stipulation. Using three EPR pairs, three bits of secret classical information can be faithfully transmitted from Alice to remote Bob without revealing any information to a potential eavesdropper. By both Alice and Bob's GHZ state measurement results, Bob is able to read out the encoded secret messages directly. In the second scheme, after insuring the safety of the quantum channel, Alice and Bob apply a series local operations on their respective particles according to the tripartite stipulation and the secret message they both want to send to Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is able to i...