Efficient Quantum Key Distribution

We devise a simple modification that essentially doubles the efficiency of a well-known quantum key distribution scheme proposed by Bennett and Brassard (BB84). Our scheme assigns significantly different probabilities for the different polarization bases during both transmission and reception to reduce the fraction of discarded data. The actual probabilities used in the scheme are announced in public. As the number of transmitted signals increases, the efficiency of our scheme can be made to approach 100%. The security of our scheme (against single-photon eavesdropping strategies) is guaranteed by a refined analysis of accepted data which is employed to detect eavesdropping: Instead of lumping all the accepted data together to estimate a single error rate, we separate the accepted data into various subsets according to the basis employed and estimate an error rate for each subset individually. Our scheme is the first quantum key distribution with an efficiency greater than 50%. We remark that our idea is rather general and can be used to improve the efficiency of a number of other existing schemes

Bell inequality, Bell states and maximally entangled states for n qubits

First, we present a Bell type inequality for n qubits, assuming that m out of the n qubits are independent. Quantum mechanics violates this inequality by a ratio that increases exponentially with m. Hence an experiment on n qubits violating of this inequality sets a lower bound on the number m of entangled qubits. Next, we propose a definition of maximally entangled states of n qubits. For this purpose we study 5 different criteria. Four of these criteria are found compatible. For any number n of qubits, they determine an orthogonal basis consisting of maximally entangled states generalizing the Bell states.Comment: 8 pages, no figur

Quantum Bit Commitment with a Composite Evidence

Entanglement-based attacks, which are subtle and powerful, are usually believed to render quantum bit commitment insecure. We point out that the no-go argument leading to this view implicitly assumes the evidence-of-commitment to be a monolithic quantum system. We argue that more general evidence structures, allowing for a composite, hybrid (classical-quantum) evidence, conduce to improved security. In particular, we present and prove the security of the following protocol: Bob sends Alice an anonymous state. She inscribes her commitment $b$ by measuring part of it in the + (for $b = 0$) or $\times$ (for $b=1$) basis. She then communicates to him the (classical) measurement outcome $R_x$ and the part-measured anonymous state interpolated into other, randomly prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde

Does Clauser-Horne-Shimony-Holt Correlation or Freedman-Clauser Correlation lead to the largest violation of Bell's Inequality?

An inequality is deduced from Einstein's locality and a supplementary assumption. This inequality defines an experiment which can actually be performed with present technology to test local realism. Quantum mechanics violate this inequality a factor of 1.5. In contrast, quantum mechanics violates previous inequalities (for example, Clauser-Horne-Shimony-Holt inequality of 1969, Freedman-Clauser inequality of 1972, Clauser-Horne inequality of 1974) by a factor of $\sqrt 2$. Thus the magnitude of violation of the inequality derived in this paper is approximately $20.7$ larger than the magnitude of violation of previous inequalities. This result can be particularly important for the experimental test of locality.Comment: 15 pages, LaTeX file, no figure

Optimal eavesdropping in quantum cryptography with six states

A generalization of the quantum cryptographic protocol by Bennett and Brassard is discussed, using three conjugate bases, i.e. six states. By calculating the optimal mutual information between sender and eavesdropper it is shown that this scheme is safer against eavesdropping on single qubits than the one based on two conjugate bases. We also address the question for a connection between the maximal classical correlation in a generalized Bell inequality and the intersection of mutual informations between sender/receiver and sender/eavesdropper.Comment: 4 pages, 1 figur

Intravenous Paracetamol Versus Patient-Controlled Analgesia With Morphine for the Pain Management Following Diagnostic Knee Arthroscopy in Trauma Patients: A Randomized Clinical Trial

Background: Most patients undergoing outpatient surgeries have the unpleasant experience of high level pain after surgery. Compared with open surgeries, arthroscopic procedures are less painful; however, inadequate pain management could be associated with significant concerns. Opioids alone or in combination with local anesthetics are frequently used for diminishing postoperative pain using intravenous or epidural infusion pumps. Despite morphine various disadvantages, it is commonly used for controlling pain after surgery. Objectives: The aim of this study was to compare intravenous paracetamol and patient-controlled analgesia (PCA) with morphine for the pain management following diagnostic knee arthroscopy in trauma patients. Patients and Methods: Sixty trauma patients who were scheduled to undergo knee arthroscopy were randomly divided into two groups. Patients immediately received intravenous infusion of 1 g paracetamol within 15 minutes after surgery and every 6 hours to 24 hours in the paracetamol group. The patient-controlled analgesia group received morphine through PCA infusion pump at 2 mL/h base rate and 1mL bolus every 15 minutes. Pain level, nausea and vomiting, and sedation were measured and recorded during entering the recovery, 15 and 30 minutes after entering the recovery, 2, 6, and 24 hours after starting morphine pump infusion in the morphine and paracetamol in the paracetamol groups. Results: There was no significant difference regarding the pain level at different times after entering the recovery between the two groups. No one from the paracetamol group developed drug complications. However, 22.3% in the PCA morphine suffered from postoperative nausea; there was a statistically significant difference regarding the sedation level, nausea, and vomiting at various times between the two groups. Conclusions: Intravenous administration of paracetamol immediately after knee arthroscopy improved postoperative pain, decreased analgesic administration, maintained stable hemodynamic parameters, had no complications related to opiates, no nausea and vomiting, and increased patient satisfaction and comfort in comparison to PCA with morphine

Deterministic secure direct communication using GHZ states and swapping quantum entanglement

We present a deterministic secure direct communication scheme via entanglement swapping, where a set of ordered maximally entangled three-particle states (GHZ states), initially shared by three spatially separated parties, Alice, Bob and Charlie, functions as a quantum information channel. After ensuring 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 infer the secret messages directly. The secret messages are faithfully transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ states without revealing any information to a potential eavesdropper. Since there is not a transmission of the qubits carrying the secret message between any two of them in the public channel, it is completely secure for direct secret communication if perfect quantum channel is used.Comment: 9 pages, no figur

Do all pure entangled states violate Bell's inequalities for correlation functions?

Any pure entangled state of two particles violates a Bell inequality for two-particle correlation functions (Gisin's theorem). We show that there exist pure entangled N>2 qubit states that do not violate any Bell inequality for N particle correlation functions for experiments involving two dichotomic observables per local measuring station. We also find that Mermin-Ardehali-Belinskii-Klyshko inequalities may not always be optimal for refutation of local realistic description.Comment: 4 pages, journal versio