**演讲人：**
Dr Syed Assad Australian National University

**时间：** 2013-11-19 16:00-2013-11-19 17:00

**地点：**FIT 1-222

**内容：**

A fundamental feature of the quantum world that we live in is that (non-orthogonal) quantum states cannot be cloned. This no cloning theorem lies at the heart of quantum mechanics and is a direct consequence of the Heisenberg uncertainty relation. While this feature is vital in some quantum communication protocols such as secure quantum key distributions and quantum random number generation, it is an annoyance in other protocols. For example, it was long (and correctly) believed that the no cloning theorem forbids the implementation of an ideal quantum repeater and quantum noiseless linear amplifier. This a major obstacle in the extending the range of quantum networks (for example for quantum key distribution) since the quantum signal would degrade after say 100 kilometres and cannot be recovered.

To overcome this obstacle, a recent novel protocol was discovered where it was shown that a quantum state can unexpectedly be noiselessly amplified. The price we pay for this amplification is that the process cannot be deterministic. This protocol has a finite probability of success so that on average, the Heisenberg uncertainty relation is still satisfied. However the successfully amplified quantum states can be heralded and this is helpful in extending the range of loss-sensitive protocols.

While probabilistic noiseless amplifier is useful, it is hard to implement experimentally. All implementations to date can only achieve low amplifications and low probability of success. In this talk, I will present an experiment of performing a probabilistic noiseless amplification based on a post-selection of the measurement outcomes. We will see that this post-selection process is much easier to implement and yet yields identical results as an actual physical probabilistic noiseless amplifier. It also has a higher probability of success which is close to the ultimate limit allowed by quantum mechanics.

We apply this procedure to two protocols: entanglement distillation and quantum key distribution and show that we can still recover quantum signals after propagation loss in a distance corresponding to 100 kilometers of fibre.

In the second part of the talk, I will present an experiment to reconstruct the Wigner function of a Schroedinger cat states basedon homodyne tomography [1].

[1] Reconstruction of photon number conditioned states using phase randomized homodyne measurements. J. Phys. B

**个人简介: **

Dr Syed Assad obtained his PhD jointly from the National University of Singapore and the Australian National University under the supervision of Prof Berthod-Georg Englert and Prof Lam Ping Koy. He graduated in 2011 and is currently doing a post-doc at the Australian National University.

His current research interests are mainly in experiments for continuous variable quantum information using optics such as: the generation of non-classical states, implementation of a quantum key distribution, realisation of a fast and robust realtime quantum random number generator and an implementation of a measurement based noiseless linear amplifier. He also works on quantum secure communication protocols using both discrete and continuous variable quantum states.