Secure direct communication using step-split Einstein-Podolsky-Rosen pair

We presen a secure direct communication protocol by using step-split Einstein-Podolsky-Rosen (EPR) pair. In this communication protocol, Alice first sends one qubit of an EPR pair to Bob. Bob sends a receipt signal to Alice through public channel when he receives Alice's first qubit. Alice performs her encoding operations on the second qubit and sends this qubit to Bob. Bob performs a Bell-basis measurement to draw Alice's information. The security of this protocol is based on `High fidelity implies low entropy'. If Eve want to eavesdrop Alice's information, she has to attack both qubits of the EPR pair, which results in that any effective eavesdropping attack can be detected. Bob's receipt signal can protect this protocol against the eavesdropping hiding in the quantum channel losses. And this protocol is strictly secure to perform a quantum key distribution by using Calderbank-Shor-Steane codes.Comment: PACS:03.67.Hk,03.65.U

Measurement, Trace, Information Erasure and Entropy

We show that both information erasure process and trace process can be realized by projective measurement. And a partial trace operation consists to a projective measurement on a subsystem. We show that a nonunitary operation will destroy the wave-behavior of a particle. We give a quantum manifestation of Maxwell's demon and give a quantum manifestation of the second law of therodynamics. We show that, considering the law of memontum-energy conversation, the evolution of a closed system should be unitary and the von Neumann entropy of the closed quantum system should be least.Comment: 8pages. Reported on the Conference of Chinese Physics Society (2003 autumn):Quantum information conncil hous

Eavesdropping on the Bostroem-Filbinger Communication Protocol in Noisy Quantum Channel

We show an eavesdropping scheme on Bostr\UNICODE{0xf6}m-Felbinger communication protocol (called ping-pong protocol) [Phys. Rev. Lett. 89, 187902 (2002)] in an ideal quantum channel. A measurement attack can be perfectly used to eavesdrop Alice's information instead of a most general quantum operation attack. In a noisy quantum channel, the direct communication is forbidden. We present a quantum key distribution protocol based on the ping-pong protocol, which can be used in a low noisy quantum channel. And we give a weak upper bound on the bit-error ratio that the detection probability $d$ should be lower than 0.11, which is a requirement criterion when we utilize the ping-pong protocol in a real communication.Comment: 4 pages; PSCA, 03.67.Hk, 03.65.U

Deterministic Secure Direct Communication Using Mixed state

We show an improved ping-pong protocol which is based on the protocol showed by Kim Bostrom and Timo Felbinger [Phys. Rev. Lett. 89, 187902 (2002); quant-ph/0209040]. We show that our protocol is asymptotically secure key distribution and quasisecure direct communication using a single photon resource. And this protocol can be can be carried out with great efficiency and speed using today's technology

Deterministic Secure Direct Communication Using Ping-pong protocol without public channel

Based on an EPR pair of qubits and allowing asymptotically secure key distribution, a secure communication protocol is presented. Bob sends either of the EPR pair qubits to Alice. Alice receives the travel qubit. Then she can encode classical information by local unitary operations on this travel qubit. Alice send the qubit back to Bob. Bob can get Alice's information by measurement on the two photons in Bell operator basis. If Eve in line, she has no access to Bob's home qubit. All her operations are restricted to the travel qubit. In order to find out which opeartion Alice performs, Eve's operation must include measurements. The EPR pair qubits are destroyed. Bob's measurement on the two photons in Bell operator basis can help him to judge whether Eve exist in line or not. In this protocal, a public channel is not necessary.Comment: 3 figure

Optimal Experimental Scheme for Bennett-Brassard 1984 Quantum Key Distribution Protocol with Weak Coherent Sources, Noisy and Lossy Channel

It is the first scheme which allows the detection apparatus to achieve both the photon number of arriving signals and quantum bit error rate of the multiphoton pulses precisely. We show that the upper bound of the fraction of the tagged multiphoton pulses counts is $\mu$, which is independent of the channel loss and the intensity of the decoy source. Such upper bound is $inherent$ and cannot be reduced any longer as long as the weak coherent scouces and high lossy channel are used. We show that our scheme can be implemented even if the channel loss is very high. A stronger intensity of the pulse source is allowable to improve the rate of quantum key distribution. Both the signal pulses and decoy pulses can be used to generate the raw key after verified the security of the communication. We analyze that our scheme is optimal under today's technology. PACS: 03.67.DdComment: RevTex, 2figures, submitted for publication on 30 March 200

An one-time-pad key communication protocol with entanglement

We present an one-time-pad key communication protocol that allows secure direct communication with entanglement. Alice can send message to Bob in a deterministic manner by using local measurements and public communication. The theoretical efficiency of this protocol is double compared with BB84 protocol. We show this protocol is unconditional secure under arbitrary quantum attack. And we discuss that this protocol can be perfectly implemented with current technologies.Comment: 4 pages;PACS: 03.67.Hk, 03.65.U

Hawking radiation as tunneling from the Kerr and Kerr-Newman black holes

Recent work, which treats the Hawking radiation as a semi-classical tunneling process at the horizon of the Schwarzschild and Reissner-Nordstrom spacetimes, indicates that the exact radiant spectrum is no longer pure thermal after considering the black hole background as dynamical and the conservation of energy. In this paper, we extend the method to investigate Hawking radiation as massless particles tunneling across the event horizon of the Kerr black hole and that of charged particles from the Kerr-Newman black hole by taking into account the energy conservation, the angular momentum conservation, and the electric charge conservation. Our results show that when self-gravitation is considered, the tunneling rate is related to the change of Bekenstein-Hawking entropy and the derived emission spectrum deviates from the pure thermal spectrum, but is consistent with an underlying unitary theory.Comment: 10 pages, no figure, Revtex4, typos removed, final version to appear in PR

Hawking Radiation of Dirac Particles in a Variable-mass Kerr Black Hole

Hawking effect of Dirac particles in a variable-mass Kerr space-time is investigated by using method of the generalized tortoise coordinate transformation. The location and the temperature of event horizon of the non-stationary Kerr black hole are derived. It is shown that the temperature and the shape of event horizon depend not only on the time but also on the polar angle. However, our results demonstrate that the Fermi-Dirac spectrum displays a new spin-rotation effect which is absent from that of Bose-Einstein distribution.Comment: 6 pages, revtex (12pt), no figure. Chin. Phys. Lett. 18 (2001) 485 (in press

Hawking Radiation of Weyl Neutrinos in a Rectilinearly Non-uniformly Accelerating Kinnersley Black Hole

Quantum thermal effect of Weyl neutrinos in a rectilinearly non-uniformly accelerating Kinnersley black hole is investigated by using the generalized tortoise coordinate transformation. The equation that determines the location, the Hawking temperature of the event horizon and the thermal radiation spectrum of neutrinos are derived. Our results show that the location and the temperature of the event horizon depend not only on the time but also on the angle.Comment: 9 pages, no figure, Latex 2.09, accepted for Chinese Physics Vol. 11, No. 7 (2002