ArXiv identifier: 

0810.4340

Speakers: 

S. Virmani

Authors: 

M. B. Plenio and S. Virmani

We consider the possibility of adding noise to a quantum circuit to make it efficiently simulatable classically. In previous works this approach has been used to derive upper bounds to fault tolerance thresholds - usually by identifying a privileged resource, such as an entangling gate or a non-Clifford operation, and then deriving the noise levels required to make it `unprivileged'. In this work we consider extensions of this approach where noise is added to Clifford gates too, and then `commuted' around until it concentrates on attacking the non-Clifford resource.

ArXiv identifier: 

0901.4470

Speakers: 

Susana F. Huelga

Authors: 

Neil P. Oxtoby, Ángel Rivas, Susana F. Huelga, and Rosario Fazio

We consider non-interacting multi-qubit systems as controllable probes of an environment of defects/impurities modelled as a composite spin-boson environment. The spin-boson environment consists of a small number of quantum-coherent two-level fluctuators (TLFs) damped by independent bosonic baths. A master equation of the Lindblad form is derived for the probe-plus-TLF system.

ArXiv identifier: 

0812.4305

Speakers: 

R. F. Werner

Authors: 

V. B. Scholz and R. F. Werner

The situation of two independent observers conducting measurements on a joint quantum system is usually modelled using a Hilbert space of tensor product form, each factor associated to one observer. Correspondingly, the operators describing the observables are then acting non-trivially only on one of the tensor factors. However, the same situation can also be modelled by just using one joint Hilbert space, and requiring that all operators associated to different observers commute, i.e. are jointly measurable without causing disturbance.

Researchers at the National Institute of Standards and Technology (NIST) have proved, for the first time, that the lifetime of quantum-computing bits can be extended. In their experiment, they showed that by applying specially timed magnetic pulses to qubits, made of beryllium ions, they could prolong the life of the quantum bits from about one millisecond to hundreds of milliseconds. The work is described in this week's Nature.

The realization of a universal quantum computer that can carry out arbitrary computations remains a long term goal. But the technologies developed so far enable us to perform so called quantum simulations. Here assemblies of directly controllable quantum particles form models for complex systems which are difficult to manipulate. A new, promising technique was now developed in the group of Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching.

FACULTY OF MATHEMATICS AND PHYSICAL SCIENCES
SCHOOL OF PHYSICS AND ASTRONOMY
CHAIR IN QUANTUM INFORMATION SCIENCE

The intrinsic rotation of electrons - the "spin" - remains unused by modern electronics. If use as an information carrier were possible, the processing power of electronic components would suddenly increase to a multiple of the present capacity. In cooperation with colleagues from Dortmund, St. Petersburg and Washington, Bochum physicists have now succeeded in aligning electron spin, bringing it to a controlled "waver" and reading it out. The electron spin can also be realigned as required at any time using optical pulses.

ArXiv identifier: 

0901.4454

Speakers: 

Martin Plenio

Authors: 

Filippo Caruso, Alex W. Chin, Animesh Datta, Susana F. Huelga, Martin B. Plenio

Transport of excitations through networked systems plays an important role in many areas of physics, chemistry, and biology. The uncontrollable interaction of the transmission network with a noisy environment is usually assumed to deteriorate its transport capacity, especially so when the system is fundamentally quantum mechanical. Here we identify key mechanisms through which dephasing noise, contrary to expectation, may actually aid transport through a dissipative network.

ArXiv identifier: 

0903.0612

Speakers: 

Daniel Burgarth

Authors: 

Daniel Burgarth, Koji Maruyama

Identifying the nature of interactions in a quantum system is essential in understanding any physical phenomena. Acquiring information on the Hamiltonian can be a tough challenge in many-body systems because it generally requires access to all parts of the system. We show that if the coupling topology is known, the Hamiltonian identification is indeed possible indirectly even though only a small gateway to the system is used. Surprisingly, even a degenerate Hamiltonian can be estimated by applying an extra field to the gateway.

ArXiv identifier: 

0807.2444

Speakers: 

Alvaro Feito Boirac

Authors: 

J.S. Lundeen, A. Feito, H. Coldenstrodt-Ronge, K.L. Pregnell, Ch. Silberhorn, T.C. Ralph, J. Eisert, M.B. Plenio, I.A. Walmsley

Measurement connects the world of quantum phenomena to the world of classical events. It plays both a passive role, observing quantum systems, and an active one, preparing quantum states and controlling them. Surprisingly - in the light of the central status of measurement in quantum mechanics - there is no general recipe for designing a detector that measures a given observable. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (i.e.

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