Quantum Computation

Quantum speeding-up of computation demonstrated in a superconducting two-qubit processor

Date: 
2012-04-05
Author(s): 

A. Dewes, R. Lauro, F.R. Ong, V. Schmitt, P. Milman, P. Bertet, D. Vion, and D. Esteve

Reference: 

Phys. Rev. B 85, 140503(R) (2012)

We operate a superconducting quantum processor consisting of two tunable transmon qubits coupled by a swapping interaction, and equipped with nondestructive single-shot readout of the two qubits. With this processor, we run the Grover search algorithm among four objects and find that the correct answer is retrieved after a single run with a success probability between 0.52 and 0.67, which is significantly larger than the 0.25 achieved with a classical algorithm. This constitutes a proof of concept for the quantum speed-up of electrical quantum processors.

Universal quantum computation with ordered spin-chain networks

Date: 
2011-09-21
Author(s): 

Y. Tserkovnyak and D. Loss

Reference: 

Phys. Rev. A 84, 032333

It is shown that anisotropic spin chains with gapped bulk excitations and magnetically ordered ground states offer a promising platform for quantum computation, which bridges the conventional single-spin-based qubit concept with recently developed topological Majorana-based proposals. We show how to realize the single-qubit Hadamard, phase, and π/8 gates as well as the two-qubit controlled-not (cnot) gate, which together form a fault-tolerant universal set of quantum gates.

Controlling the Interaction of Electron and Nuclear Spins in a Tunnel-Coupled Quantum Dot

Date: 
2011-01-25
Author(s): 

C. Kloeffel, P.A. Dalgarno, B. Urbaszek, B.D. Gerardot, D. Brunner, P.M. Petroff, D. Loss, and R.J. Warburton

Reference: 

Phys. Rev. Lett. 106, 046802

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability. For a σ+ pump, the emission switches from σ+ to σ- at a particular detuning of the laser. Simultaneously, the nuclear spin polarization switches from positive to negative.

Absence of spontaneous magnetic order of lattice spins coupled to itinerant interacting electrons in one and two dimensions

Date: 
2011-09-01
Author(s): 

D. Loss, F.L. Pedrocchi, and A.J. Leggett

Reference: 

Phys. Rev. Lett. 107, 107201

We extend the Mermin-Wagner theorem to a system of lattice spins which are spin coupled to itinerant and interacting charge carriers. We use the Bogoliubov inequality to rigorously prove that neither (anti-) ferromagnetic nor helical long-range order is possible in one and two dimensions at any finite temperature. Our proof applies to a wide class of models including any form of electron-electron and single-electron interactions that are independent of spin.

Quantum Information Classification Scheme updated

Summary: 

Version 1.2 of the QICS is available.

The on-line version of the document can be browsed here.
A pdf of the full document will be generated following this link.
Old versions of the QICS are archived here.

Quantum Simulation of Interacting Fermions Lattice Models in Trapped Ions

Date: 
2012-03-07
Author(s): 

J. Casanova, A. Mezzacapo, L. Lamata, and E. Solano

Reference: 

Submitted to Physical Review Letters (2011)

We propose a method of simulating many-body interacting fermion lattice models in trapped ions, including highly nonlinear interactions in arbitrary spatial dimensions and for arbitrarily distant couplings. We encode the products of fermionic operators in nonlocal spin operators of an ion string, which are efficiently implementable, while a Trotter expansion of the total evolution operator can be applied by using only polynomial resources.

Workshop on the Quantum Physics of Phase

Date: 
2012-04-23 - 2012-04-26
Place: 
Stockholm

The workshop aims to gather world leading experts on the physics and applications of quantum phenomena in microwave circuits at low temperatures. This will include the following topics:

- Quantum computation schemes and devices
- Circuit MASERS and parametric amplifiers
- Quantum limited measurement and squeezing
- Quantum phase slips and geometric phases
- Single charge pumping and quantum metrology
- Superconducting and electromechanical technologies
Sponsored by:

QIPC cluster review meeting

Date: 
2012-04-18 - 2012-04-20
Place: 
NH Hotel Bingen, Museumstrasse 3, D-55411 Bingen (Mainz) Germany

This is the traditional QIPC cluster reviews. The program is as follows:

Call For Conference Proposals published

Summary: 

A call for proposals for the next QUIE2T sponsored QIPC conference has been published.

A call for proposals for the next QUIE2T sponsored QIPC conference has been published.

Holonomic quantum computing in ground states of spin chains with symmetry-protected topological order

Date: 
2011-03-30
Author(s): 

M. Renes, Akimasa Miyake, Gavin K. Brennen, and Stephen D. Bartlett

Reference: 

arxiv:1103.5076

While solid-state devices offer naturally reliable hardware for modern classical computers, thus far quantum information processors resemble vacuum tube computers in being neither reliable nor scalable. Strongly correlated many body states stabilized in topologically ordered matter offer the possibility of naturally fault tolerant computing, but are both challenging to engineer and coherently control and cannot be easily adapted to different physical platforms.

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