QUOXIC

The next meeting will be on Thursday 24th April at University College London in History Building at 25 Gordon Square room 107 - not to be confused with 25 Gordon Street (student union).

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This time we are having a special theme on entanglement, especially multiparty entanglement.

Schedule

• 13:30 Dan Browne (UCL) -- Introduction to Multiparty Entanglement and Graph States.
• 14:10 Janet Anders (UCL) -- Measurement-based classical computation.
• 14:50 Catherine Jarvis (Bristol) -- Collapse and Revival of Entanglement between Qubits interacting via a Quantum Bus.
• COFFEE BREAK
• 16:00 Sougato Bose (UCL) -- Sharing Entanglement using spin chains.
• 16:30 Dimitris Angelakis (Crete) -- Towards quantum simulation and quantum computation in strongly coupled atom-cavity systems.
• 17:10 Terry Rudolph (Imperial) -- TBA.

Abstracts

In measurement-based computation the computation proceeds by adaptive single-qubit measurements on a multi-qubit entangled state, such as the cluster state. The adaptive measurements require a classical control computer which processes the previous measurement outcomes to determine the correct bases for the following measurement. Hence feeding the control computer with an appropriate resource state raises its computational power considerably, in the case of the cluster state it reaches quantum universality. The resources states fuelling such computational jump can in this sense be considered to possess a "computational power" themselves which is used by the control computer.

While impressive work has been done to classify entangled states that boost the power of any classical control computer to quantum universality, little is known about the reverse question. Given the simplest possible control computer, the CNOT computer, what are resources that can boost its computational power at all? Asking this way around leads naturally to the notion of measurement-based "classical computation" and I will shed light on this problem and finally derive minimal resource states needed for universal classical computation.

Reference: J. Anders and D.E. Browne - Measurement-based classical computation - on the arXiv next week or soon afterwards...

We have studied the dynamics described but the Jaynes-Cummings Model which captures the salient features of two level systems interacting with a single mode of an electromagnetic cavity. The two level systems will be referred to as qubits and the cavity mode will be treated fully quantum mechanically, hence may be regarded as a 'quantum bus'. Firstly we note, for the one qubit case, that in between the well known phenomena of collapse and revival of Rabi oscillations there is a time 1/2 t_r when the qubit system is in the state \psi_{attractor}, independent of their initial state. This fact was discovered by Gea-Banacloche. Here we show that such attractor states also occur in the multi qubit sector of the model. Moreover, we find that when the qubits are in such a state they are not entangled, even if they were initially. We then argue that as time goes on past the attractor time, in general, the entanglement between the qubit revives. For the two qubit case such collapse and revival of the entanglement is illustrated by explicit calculation of the concurrence as a function of time. Further calculations for the three qubit case also supports our conjecture that this phenomena is a generic feature of qubits interacting with a 'quantum bus'

One of the most important problems in the area spanned by Physics, Computing, Engineering and Nanotechnology is to efficiently simulate the dynamics of quantum systems. A variety of brilliant ideas involving optical lattices have been developed as possible quantum simulators but lack of individual addressing due to the small atomic separation remains an issue.

In this talk, I will review the results of recent studies of a new system of individually addresable coupled cavities interacting strongly with various dopants in the form of atoms or quantum dots. Based on our 2004 preliminary results that coupled cavities can be used for inducing photonic phase shifts[1], we have recently shown that this system could exhibit a Mott phase for the combined atomic-photonic excitations (polaritons) in each site. This behaviour originates from the effective photon repulsion due to the photon blockade effect. A phase transition to a photonic superfluid could be observed by manipulation of the atom-photon interaction[2]. In addition the system dynamics are characterized by a generalized Hubbard model and the simulation of various spin models (Heisenberg, Ising etc) could be possible[3,4]. Finally applications in quantum communication[5], in creating cluster states[6], steady state polaritonic entanglement[7] and heralded generation of polarized entangled photon pairs[8] will also be discussed.

[1] Angelakis, M. Santos, V. Yanopappas, A.K. Ekert, "A proposal for the implementation of quantum gates with photonic-crystal coupled cavity waveguides", Phys. Lett. A. Vol.362, 377 (2007) (ArXiv:quant-ph/0410189)
[2] Dimitris G. Angelakis, Marcelo F. Santos, Sougato Bose, "Photon blockade induced Mott transitions and XY spin models in coupled cavity arrays", Phys. Rev. A (Rap. Com.) vol. 76, 031805 (2007) (ArXiv:quant-ph/0606159).
[3] Alastair Kay and Dimitris G. Angelakis, "Reproducing spin lattice models in strongly coupled atom-cavity systems." Submitted (ArXiv:0802.0488)
[4] Jaeyoon Cho, Dimitris G. Angelakis, Sougato Bose, "Simulation of high-spin Heisenberg chains in coupled cavities." Submitted. (ArXiv:0802.3365)
[5] Sougato Bose, Dimitris G. Angelakis, Daniel Burgarth. "Transfer of a Polaritonic Qubit through a Coupled Cavity Array", Journ. Of Mod. Opt. vol. 54, 2307 (2007) (ArXiv:0704.0984)
[6] Dimitris G. Angelakis, Alastair Kay, "Weaving light-matter qubits into a one way quantum computer", New J. Phys. Vol. 10, 023012 (2008). (ArXiv:quant-ph/0702133).
[7] Dimitris G. Angelakis, Stefano Mancini, Sougato Bose, "Steady state entanglement between distant hybrid light-matter qubits under classical driving", submitted. (ArXiv:0711.1830).
[8] Jaeyoon Cho, Dimitris G. Angelakis, Sougato Bose, "Heralded generation of two-photon polarization entanglement with coupled cavities". Submitted. (ArXiv:0712.2413)