We propose a protocol for direct quantum communication between two parties (Alice and Bob) which is achieved by controlling the phase of the signal photon. Without imperfection and noise, Alice's two detectors always click with unit probability according to Bob's different decisions. The probability of detecting a photon in the transmission channel can be controlled and can be made arbitrarily small by increasing the resources.
We propose a protocol for direct quantum communication between two parties (Alice and Bob) which is achieved by controlling the phase of the signal photon. Without imperfection and noise, Alice's two detectors always click with unit probability according to Bob's different decisions. The probability of detecting a photon in the transmission channel can be controlled and can be made arbitrarily small by increasing the resources.
We present a controlled quantum teleportation protocol. In the protocol, quantum information of an unknown state of a 2-level particle is faithfully transmitted from a sender Alice to a remote receiver Bob via an initially shared triplet of entangled particles under the control of the supervisor Charlie. The distributed entangled particles shared by Alice, Bob and Charlie function as a quantum information channel for faithful transmission. We also propose a controlled and secure direct communication scheme by means of this teleportation. After ensuring the security of the quantum channel, Alice encodes the secret message directly on a sequence of particle states and transmits them to Bob supervised by Charlie using this controlled quantum teleportation. Bob can read out the encoded message directly by the measurement on his qubit. In this scheme, the controlled quantum teleportation transmits Alice's message without revealing any information to a potential eavesdropper. Because there is not a transmission of the qubit carrying the secret message between Alice and Bob in the public channel, it is completely secure for controlled and direct secret communication if perfect quantum channel is used. The special feature of this scheme is that the communication between two sides depends on the agreement of a third side to co-operate.
We propose a quasi-counterfactual quantum swap gate for exchanging Alice's unknown photon state and Bob's unknown atomic state under the condition that only Alice's photon may appear in the transmission channel between Alice and Bob, while the probability of the existence of photon in the transmission channel is controllable and can tend to zero. Unlike standard counterfactual quantum communication protocols, quantum states exchange in present scenario is achieved by multiple phase operations, rather than multiple measurements. The total effect of those operations can be considered as a unitary time evolution operator. Therefore, the communication fidelity and efficiency of our protocol are always one if system imperfection and channel noise are not considered. Compared to standard counterfactual communication protocols, our protocol is easy to implement. We also show that it can be easily converted to a standard counterfactual one.
We show that in any quantum direct communication protocol that is based on $\ensuremath{\Psi}$ and $\ensuremath{\Phi}$ Bell states, an eavesdropper can always tell $\ensuremath{\Psi}$ from $\ensuremath{\Phi}$ states without altering the transmission in any way in the message mode. This renders all protocols that make use of only one $\ensuremath{\Psi}$ state and one $\ensuremath{\Phi}$ state completely insecure in the message mode. All four-Bell-state protocols require a revision and this might be of importance for new implementations of entanglement-based cryptographic protocols. The detection rate of an eavesdropper is 25 per control transmission, i.e., a half of the rate in the two-state (ping-pong) protocol. An eavesdropper can detect control probes with certainty in the standard control transmission without a photon in the Alice-to-Bob's travel mode and with near certainty in a transmission with a fake photon in the travel mode. Resending of measured control photons via the travel mode would make an eavesdropper completely invisible.
A remote control scheme on setting up quantum communication channel between three legitimate participants by using only linear optical elements and photon detectors was presented. In the theoretical scheme, the two partially entangled coherent states are utilized as the quantum channel lining three remote users (Alice, Bob and Clara), Wherein, Alice is the controller, Bob and Clara were the users. First, Alice establishes the quantum communication channels and Bob and Clara, respectively (equivalent the telecommunications to establish contact with each user). When there is a direct confidential communications needs between Bob and Clara, they send a request to the Alice through a classical channel. Alice (equivalent the operator) then makes a measurement of the number of photons on own particles, respectively. The secure quantum channel between Bob and Clara was successfully established. It was proved that, although the system is designed for two uses systems, but for multiusers systems, it is still reasonable.
A theoretical scheme for alternative and secure direct communication is proposed. The communication is based on controlled quantum teleportation via state. After insuring the security of the quantum channels (a set of qubits in the states), Alice encodes the secret messages directly on a sequence of particle states and transmits them to Bob supported by Charlie, or to Charlie supported by Bob, using controlled quantum teleportation. Bob or Charlie can read out the encoded messages directly by the measurement on his qubits. In this scheme, the controlled quantum teleportation transmits Alice's message without revealing any information to a potential eavesdropper. Because there is no a transmission of the qubits carrying the secret messages between legitimate parties in the public channel, it is completely secure for direct secret communication if perfect quantum channel is used. We also discuss that when one of the three legitimate parties does not cooperate with others because of some reasons, the remaining two persons can still communicate with each other by entanglement purification. The main feature of this scheme is that the communicate between two sides is alternative but depends on the agreement of the third, and more over, the communication between two sides does not suspend with loss of one of three qubits ( i.e., one of the three legitimate parties does not cooperate with others).
Two schemes for quantum secure conditional direct communication are 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 these protocols 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 also show that the two schemes are completely secure if quantum channels are perfect.