QUOXIC

The next meeting will be on Tuesday 2nd December at Oxford University Computing Lab, Wolfson Building, Parks Road, Oxford.

Local information about the computing lab can be found here. The talks will take place in Lecture Theatre A.

Schedule

• 13:30 : Animesh Datta (Imperial)
  "Discord in Quantum Computation and Communication"
• 14:00 : Radu Ionicioiu (HP Labs)
  "Generalized parity module: an entanglement resource"
• 14:30 : Andrzej Dragan (Imperial / Warsaw)
  "Emergence of quantum indeterminacy from specal relativity"
• 15:00 COFFEE BREAK
• 15:30 : Abolfazl Bayat (UCL)
  "Long Range Entanglement by Bond Quenching in Spin Chain Kondo Model"
• 16:00 : Dimitris Tsomokos (Hertfordshire)
  "Chirality and entanglement in a spin liquid state"
• 16:30 : Daniel Burgarth (Oxford) (TBC)
  "Restricted quantum control of spin chains"

Abstracts

We will study two simple examples of quantum advantage where entanglement does not seem to play a role. One is the exponential speedup in the power of one qubit (DQC1) model of quantum computation, while the second in the scenario of entanglement transfer between distributed parties, via a third party. We will show how quantum discord might be used to explain these advantages

Measurements play an crucial role in both linear optics QC and cluster state QC. Parity measurements can be used as building blocks for preparing arbitrary stabilizer states, and, together with 1-qubit gates are universal for quantum computing. Here we generalize parity gates by using a higher dimensional (qudit) ancilla. This enables us to go beyond the stabilizer/graph state formalism and prepare other types of multi-particle entangled states. The generalized parity module introduced here can prepare in one-shot, heralded by the outcome of the ancilla, a large class of entangled states, including GHZ_n, W_n, Dicke states D_{n,k}, and, more generally, certain sums of Dicke states, like G_n states used in secret sharing. For W_n states it provides an exponential gain compared to linear optics based methods.

Reference: PRA 78, 052326 (2008); arXiv:0806.0982.

It is shown that the single-particle quantum theory with fundemental indeterminacy based on complex probability amplitudes undergoing linear superposition principle is an inevitable consequence of special relativity with the principle of relativity extedned to superluminal observers.

We show that a single local quench at one end of a Kondo spin chain can lead, through dynamical evolution, to the emergence of a high quality long distance entanglement between the spins located at the end points of the chain. We compare the quality of entanglement achieved in both dimer and Kondo regimes of the chain. We find that different quenches can lead to high quality entanglement in both phases though the dynamics of entanglement generation is slower and somewhat more dispersive in the dimer phase. The proposed mechanism is highly robust against temperature fluctuations and fast since the time evolution is determined by a non-perturbative quenching of the ending bonds. We find similar results in the analysis of the spin-1/2 XXZ chain and discuss potential experimental settings where signatures of the high quality long distance entanglement discussed in this paper may be observed.

The chirality of tripartite states is investigated and used as a witness of entanglement. Using the chirality, valuable insights are gained into the many-body entanglement properties of a two-dimensional spin system in a quantum phase transition from a ferromagnetic phase to a spin liquid state. Finally, evidence is provided that this state is topologically ordered.

We discuss the indirect control of a chain of coupled two-level systems by acting on its end(s) only. We show how it is possible to obtain knowledge on all coupling strengths despite the restricted access, and how this knowledge might be used to achieve simple control tasks such as coherent transfer, state preparation, and even arbitrary operations.