Comment on "connection between entanglement and the speed of quantum evolution" and on "entanglement and the lower bounds on the speed of quantum evolution"

Batle and Borrás studied the connection between entanglement and speed of quantum evolution for certain low-dimensional bipartite quantum states. However, their studies did not cover all possible cases. And the relation between entanglement and the maximum possible quantum evolution speed for these uncovered cases can be very different from the ones that they have studied. © 2010 The American Physical Society.published_or_final_versio

Quantum-classical complexity-security tradeoff in secure multiparty computations

I construct a secure multiparty scheme to compute a classical function by a succinct use of a specially designed fault-tolerant random polynomial quantum error correction code. This scheme is secure provided that (asymptotically) strictly more than five-sixths of the players are honest. Moreover, the security of this scheme follows directly from the theory of quantum error correcting code, and hence is valid without any computational assumption. I also discuss the quantum-classical complexity-security tradeoff in secure multiparty computation schemes and argue why a full-blown quantum code is necessary in my scheme.published_or_final_versio

Practical scheme to share a secret key through a quantum channel with a 27.6% bit error rate

A secret key shared through quantum key distribution between two cooperative players is secure against any eavesdropping attack allowed by the laws of physics. Yet, such a key can be established only when the quantum channel error rate due to eavesdropping or imperfect apparatus is low. Here, a practical quantum key distribution scheme by making use of an adaptive privacy amplification procedure with two-way classical communication is reported. Then, it is proven that the scheme generates a secret key whenever the bit error rate of the quantum channel is less than 0.5-0.1sqrt[5]≈27.6%, thereby making it the most error resistant scheme known to date.published_or_final_versio

Unconditionally secure key distribution in higher dimensions by depolarization

This paper presents a prepare-and-measure scheme using N-dimensional quantum particles as information carriers where N is a prime power. One of the key ingredients used to resist eavesdropping in this scheme is to depolarize all Pauli errors introduced to the quantum information carriers. Using the Shor-Preskill-type argument, we prove that this scheme is unconditionally secure against all attacks allowed by the laws of quantum physics. For N = 2n > 2, each information carrier can be replaced by n entangled qubits. In this case, there is a family of eavesdropping attacks on which no unentangled-qubit-based prepare-and-measure (PM) quantum key distribution scheme known to date can generate a provably secure key. In contrast, under the same family of attacks, our entangled-qubit-based scheme remains secure whenever 2n ≥ 4. This demonstrates the advantage of using entangled particles as information carriers and of using depolarization of Pauli errors to combat eavesdropping attacks more drastic than those that can be handled by unentangled-qubit-based prepare-and-measure schemes. © 2005 IEEE.published_or_final_versio

Measuring time-energy resources for quantum processes

Parallel Session BThe speed of any quantum process is limited by quantum mechanics via time-energy uncertainty relations and they imply that time and energy are tradeoff with each other. As such, we propose to measure the time-energy as a single unit for quantum channels. We consider a time-energy measure for quantum channels and compute lower and upper bounds of it using the channel Kraus operators. For a special class of channels (which includes the depolarizing channel), we obtain the exact value of the time-energy measure. Our result can be used to compare the time-energy resources of similar quantum processes. In particular, we show that erasing quantum information requires √(n + 1)/n) times more time-energy resource than erasing classical information, where n is the system dimension. This work is published in [1].postprin

Measuring time-energy resources for quantum processes

Parallel Session BThe speed of any quantum process is limited by quantum mechanics via time-energy uncertainty relations and they imply that time and energy are tradeoff with each other. As such, we propose to measure the time-energy as a single unit for quantum channels. We consider a time-energy measure for quantum channels and compute lower and upper bounds of it using the channel Kraus operators. For a special class of channels (which includes the depolarizing channel), we obtain the exact value of the time-energy measure. Our result can be used to compare the time-energy resources of similar quantum processes. In particular, we show that erasing quantum information requires √(n + 1)/n) times more time-energy resource than erasing classical information, where n is the system dimension. This work is published in [1].postprin

Relation between physical time-energy cost of a quantum process and its information fidelity

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Practical entanglement distillation scheme using recurrence method and quantum low density parity check codes

Many entanglement distillation schemes use either universal random hashing or breeding as their final step to obtain almost perfect shared EPR pairs spite of a high yield, the hardness of decoding a random linear code makes the use of random hashing and breeding infeasible in practice this pilot study, we analyze the performance of the recurrence method, a well-known entanglement distillation scheme, with its final random hashing or breeding procedure being replaced by various efficiently decodable quantum codes. Among all the replacements investigated, the one using a certain adaptive quantum low density parity check (QLDPC) code is found to give the highest yield for Werner states over a wide range of noise level- the yield for using this QLDPC code is higher than the first runner up by more than 25% over a wide parameter range this respect, the effectiveness of using QLDPC codes in practical entanglement distillation is illustrated. © The Author(s) 2010.published_or_final_versionSpringer Open Choice, 21 Feb 201

A model of gas buildup and release in crater lakes

The sudden release of carbon dioxide gas, which may accumulate gradually within the stratified water bodies of a crater lake, caused two fatal disasters in Cameroon during the past decade. We model the various processes that may have led to the release by considering (1) gas injection, (2) an internal triggering mechanism, (3) propagation of a disturbance after triggering, and (4) the forces that end the outburst. We suggest that the 1986 Lake Nyos outburst was triggered at the lake bottom. The final explosive stage of the release ended quickly when a sufficiently large volume of gas bubbles prevented circulation of water eddies within the lake. A cellular automaton model is used to estimate the amount of carbon dioxide gas released and the characteristic time interval between successive outbursts. If both the gas accumulation rate and the diffusion rate of carbon dioxide through water are constant, then rapid gas release, will occur at fairly regular intervals. In which case, the amount of gas released from Lake Nyos is 0.17±0.05 km3 at standard temperature and pressure each 37±10 years. It is possible, however, that an external event could trigger a sudden release or that the diffusion rate of carbon dioxide may change as more gas accumulates, which could shorten the release time.published_or_final_versio

Practical issues in quantum-key-distribution postprocessing

Quantum key distribution (QKD) is a secure key generation method between two distant parties by wisely exploiting properties of quantum mechanics. In QKD, experimental measurement outcomes on quantum states are transformed by the two parties to a secret key. This transformation is composed of many logical steps (as guided by security proofs), which together will ultimately determine the length of the final secret key and its security. We detail the procedure for performing such classical postprocessing taking into account practical concerns (including the finite-size effect and authentication and encryption for classical communications). This procedure is directly applicable to realistic QKD experiments and thus serves as a recipe that specifies what postprocessing operations are needed and what the security level is for certain lengths of the keys. Our result is applicable to the BB84 protocol with a single or entangled photon source. © 2010 The American Physical Society.published_or_final_versio