# 几篇文章

Improving noise threshold for optical quantum computing with the EPR photon source

Z.-H. Wei, Y.-J. Han, C. H. OH, L.-M. Duan

We show that the noise threshold for optical quantum computing can be significantly improved by using the EPR-type of photon source. In this implementation, the detector efficiency $\eta_{d}$ is required to be larger than 50%, and the source efficiency $\eta_{s}$ can be an arbitrarily small positive number. This threshold compares favorably with the implementation using the single-photon source, where one requires the combined efficiency $\eta_{d}\eta_{s}>2/3$. We discuss several physical setups for realization of the required EPR photon source, including the photon emitter from a single-atom cavity.

Z. -H. Wei, Y. -J. Han, C. H. OH, & L. -M. Duan (2009). Improving noise threshold for optical quantum computing with the EPR photon source arXiv arXiv: 0912.1493v1
Zhang, R., Garner, S., & Hau, L. (2009). Creation of Long-Term Coherent Optical Memory via Controlled Nonlinear Interactions in Bose-Einstein Condensates Physical Review Letters, 103 (23) DOI: 10.1103/PhysRevLett.103.233602

# 通过博士答辩

12月5日，我完成了博士论文答辩。值得指出的是，与我一起答辩的，还有我们组的一位留学生，这也应该是交大物理学科第一位留学博士生Patrick，他来自马达加斯加。答辩一切顺利。我念博士期间已经发表和接收了6篇SCI论文，其中有四篇第一作者的论文，均发表在Phys. Rev. A上。论文发表以来已经被正面引用四十余次。非常感谢导师李福利教授多年来对我的帮助、支持和指导，也感谢留学基金委对我留学的资助。

# Slow measurement in quantum computation

In “Effective Fault-Tolerant Quantum Computation with Slow Measurements“, David P. DiVincenzo and Panos Aliferis discussed a very important problem in fault-tolerant quantum computation, does the speed of measurement influences the quantum accuracy threshold greatly? As all investigation on the threshold was based on fast measurement, which is no longer than a few gate operation times, they considered the slow measurement case, that is a measurement takes 1000 gate operations. They found that the slow measurement has a very minimal effect on the quantum accuracy thresold. Their results may be a good news for solid-state quantum computing, where the measurement canot be as short as gate operation times.

# Adiabatic Quantum algorithms and QC in DFS via measurement

Here I list two papers recently posted on the arxiv which I thought interesting or important.

Adiabatic quantum algorithms and quantum phase transitions in quant-ph/0608017, Ralf Schützhold, Gernot Schaller from Dresden University of Technology reveal the similarity between adiabatic quantum algorithms and quantum phase transitions. They found that the Grover algorithm was corresponding to the first order of quantum phase transitions. They found that the second or higher order transitions were much better than first order one. With this insight, they proposed a novel adiabatic quantum algorithm for the solution of 3-satisfiability (3-SAT) problems, which is much faster than the Grover algorithm, possibly even with an exponential speed-up.

Universal quantum computation in deocoherence-free subspace with neutral atoms in quant-ph/0607175, P. Xue and Y.F. Xiao from IQOQI and USTC proposed a scheme that realizing universal quantum computation between via measurement in deterministic way. They show how to implement cavity-assisted interaction between neutral atoms and coherent optical pulses. The quantum gates act on decoherence-free subspace. Therefore the dominant source of decoherence — dephasing is highly depressed. The homodyne detection of the coherent state directly measure the relative phase of the signal state, so the photon losses only decrease the fidelity but not lead to a failure of the measurement.

# Only years away for Quantum Computer?

David Deutsch posted an entry on his blog recently, in which he thought there were only years away for universal quantum computer.

For a long time my standard answer to the question ‘how long will it be before the first universal quantum computer is built?’ was ’several decades at least’. In fact, I have been saying this for almost exactly two decades … and now I am pleased to report that recent theoretical advances have caused me to conclude that we are within sight of that goal. It may well be achieved within the next decade.

The main discovery that has made the difference is cluster quantum computation, which is a marvellous new way of structuring quantum computations which makes them far easier to implement physically.

I am interested with cluster quantum computation these days. I am trying to generate cluster states by atoms trapped in cavities connected by fibres. Though I know something about cluster states, I know little about cluster quantum computation. After reading Deutsch’s words, I am curious about it. I downloaded the pdf file of the article and read it. I found it illuminating and worth to be read again. Maybe I can do something in this area.

I don’t agree with Deutsch’s prediction that quantum computer is only years away. But I am sure that thanks for the developing of several technologies and thories, we are closed to the birth of quantum computer. I hope there are still many important works to be done in quantum computer. If all things have been done, what can I do? Now the key point for me is finding those important questions in quantum computer&information and solving some of them.