Though it was proven that secure quantum sealing of a single classical bit is impossible in principle, here we propose an unconditionally secure quantum sealing protocol which seals a classical bit string. Any reader can obtain each bit of the sealed string with an arbitrarily small error rate, while reading the string is detectable. The protocol is simple and easy to implement. The possibility of using this protocol to seal a single bit in practice is also briefly discussed.

The model of the quantum protocols sealing a classical bit is studied. It is shown that there exist upper bounds on its security. For any protocol where the bit can be read correctly with the probability α, and reading the bit can be detected with the probability β, the upper bounds are β 1/2 and α + β 9/8.

Using the recursive Green's function technique, we study the coherent electron conductance of a quantum point contact in the presence of a scanning probe microscope tip. Images of the coherent fringe inside a quantum point contact for different widths are obtained. It is found that the conductance of a specific channel is reduced while other channels are not affected as long as the tip is located at the positions correspending to that channel. Moreover, the coherent fringe is smoothed out by increasing the temperature or the voltage across the device. Our results are consistent with the experiments reported by Topinka et al. (Science 289, 2323(Science 289, (2000).PACS numbers: 73.23.-b, 73.43.Cd, 73.20.At (Phys. Rev. B 65, 205321 (2002)) Quantum point contacts (QPCs) formed in twodimensional electron gases (2DEGs) have attracted significant attention for the past two decades. 1,2 Since the discovery of the conductance quantization in these structures, 1 QPCs have been widely used in a variety of investigations, including transport through quantum dots, the quantum Hall effect, magnetic focusing, and the Aharonov-Bohm effect. 2

An experimental scheme is proposed to test Bell's inequality by using superconducting nanocircuits. In this scheme, quantum entanglement of a pair of charge qubits separated by a sufficiently long distance may be created by cavity quantum electrodynamic techniques; the population of qubits is experimentally measurable by dc currents through the probe junctions, and one measured outcome may be recorded for every experiment. Therefore, both locality and detection-efficiency loopholes should be closed in the same experiment. We also propose a useful method to measure the amount of entanglement based on the concurrence between Josephson qubits. The measurable variables for Bell's inequality as well as the entanglement are expressed in terms of a useful phase-space Q function.

In a recent quantum oblivious transfer protocol proposed by Nagy et al., it was proven that attacks based on individual measurements and 2-qubit entanglement can all be defeated. Later we found that 5-body entanglement-based attacks can break the protocol. Here we further tighten the security bound, by showing that the protocol is insecure against 4-body entanglement-based attacks, while being immune to 3-body entanglement-based attacks. Also, increasing the number of qubits in the protocol is useless for improving its security.