By Pengjun Wang, Shanxi University
In this talk, I will introduce the experimental realization of two-dimensional spin–orbit coupling in ultracold Fermi atomic gases. We apply three lasers to couple three atomic hyperfine spin state in ultracold 40K Fermi gases. Through spin-injection radio frequency spectroscopy, we probe the spin-resolved energy dispersions of the dressed atoms, and observe a highly controllable single Dirac point created by the 2D SOC. The Dirac point created in this scheme is robust, in the sense that it moves in momentum space without opening a gap when experimental parameters change. We expect our work to open a door to explore the physics of topological matters in using cold gases.
By Peng Xue, Southeast University
By Jianwei Wang, University of Bristol
By Jonathan Dowling, Louisiana State University
Aaronson and Arkhipov recently used computational complexity theory to argue that classical computers very likely cannot efficiently simulate linear, multimode, quantum-optical interferometers with arbitrary Fock-state inputs [Aaronson and Arkhipov, Theory Comput. 9, 143 (2013)]. Here we present an elementary argument that utilizes only techniques from quantum optics. We explicitly construct the Hilbert space for such an interferometer and show that its dimension scales exponentially with all the physical resources. We also show in a simple example just how the Schr\"odinger and Heisenberg pictures of quantum theory, while mathematically equivalent, are not in general computationally equivalent. Finally, we conclude our argument by comparing the symmetry requirements of multiparticle bosonic to fermionic interferometers and, using simple physical reasoning, connect the nonsimulatability of the bosonic device to the complexity of computing the permanent of a large matrix. We apply the results to quantum metrology and also show that other quantum optical states other than Fock states lead to similar conclusions.
By Catherine Laflamme, University of Innsbruck
By Jean-Francois Roch, Université Paris-Sud and ENS Cachan
The ability to quantitatively map magnetic field distributions is of crucial importance for fundamental studies ranging from materials science to biology, and also for the development of new devices in spintronics. Recently it has clearly demonstrated that scanning NV magnetometry is an efficient technique which combines high sensitivity and nanoscale resolution (for a review, see ). This technique relies on the optical detection of the electron spin resonance associated with a single NV center in diamond attached to a AFM tip. The magnitude of the stray magnetic field above a magnetic sample can then be determined from the Zeeman shifts of the energy levels associated to this artificial atom in the solid state.
Extending this technique to cryogenic environment will open the way to investigate many magnetic phenomena occuring in complex condensed matter systems, such as superconductivity or the magnetic properties of strongly correlated systems. I will present our recent realization of a scanning magnetometer based on NV centers in a nanodiamond, in a low-temperature setup which combines atomic force microscopy and optical confocal microscopy. This scanning NV magnetometer has been applied to the imaging of magnetic domain walls in GaMnAsP, a semiconductor with a Curie temperature around 100 K which displays dilute ferromagnetism  so that spin-polarized electrical currents can reverse the magnetization direction of the magnetic domains through torque .
1. L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V. Jacques, “Magnetometry with nitrogen-vacancy defects in diamond”, Rep. Prog. Phys. 77, 056503 (2014).
2. A. Lemaître, A. Miard, L. Travers, O. Mauguin, L. Largeau, C. Gourdon, V. Jeudy, M. Tran, and J.-M. George, “Strain control of the magnetic anisotropy in (Ga,Mn)(As,P) ferromagnetic semiconductor layers”, Appl. Phys. Lett. 93, 021123 (2008).
3. M. Yamanouchi, D. Chiba, F. Matsukara, and H. Ohno, “Current-induced domain-wall switching in a ferromagnetic semiconductor structure”, Nature 428, 539 (2004).
By Jörg Wrachtrup, University of Stuttgart
The precision of any measurement is limited by quantum mechanics. Yet, in practice, hardly any measurement reaches its quantum limits. This is because dephasing typically influences the measurement device, thus rendering sensitivity below its physical limits. A new class of quantum sensors based on spin defects in materials like diamond, however, reach quantum-limited precision even under ambient conditions. Such sensors, e.g. allow for very precise detection of quantities like magnetic and electric fields, temperature, and pressure. By using multi-spin entanglement, quantum algorithms or quantum memories, Heisenberg scaling of sensitivity is achieved. While the physics of engineering optimum quantum states is subject to intense research in laboratories around the world, diamond quantum sensors start to venture into applications. First proof of principle work has demonstrated their use in material science, biology, medical imaging, and even industry.
By Luiz Davidovich, Universidade Federal do Rio de Janeiro
Quantum Metrology concerns the estimation of parameters, like a phase shift in an interferometer, the magnitude of a weak force or a small displacement, or yet the time duration of a dynamical process, taking into account the quantum character of the systems and processes involved. Quantum mechanics brings in some new features to the process of parameter estimation. The precision of the estimation becomes now intimately related to the possibility of discriminating two different quantum states of the probe corresponding to two different values of the parameter to be estimated. Also, possible measurements must abide by the rules of quantum mechanics. At the same time, quantum properties, like squeezing and entanglement, may help to increase the precision. This talk will review recent results[1, 2] concerning the application of quantum metrology to open systems, with applications to optical interferometry[1, 2, 3] and the quantum speed limit. A quantum-metrology analysis of the problem of weak-value amplification5 will also be discussed.
1. B. M. Escher, R. L. de Matos Filho, and L. Davidovich, General framework for estimating the ultimate precision limit in noisy quantum-enhanced metrology, Nature Phys. 7, 406 (2011).
2. B. M. Escher, R. L. de Matos Filho, and L. Davidovich, Quantum metrology for noisy systems, Braz. J. Phys. 41, 229 (2011).
3. B. M. Escher, L. Davidovich, N. Zagury, and R. L. de Matos Filho, Quantum metrological limits via a variational approach, Phys. Rev. Lett. 109, 190404 (2012).
4. M. M. Taddei, B. M. Escher, L. Davidovich, and R. L. de Matos Filho, Quantum speed limit for physical processes, Phys. Rev. Lett. 110, 050402 (2013).
5. G. Bié Alves, B. M. Escher, R. L. de Matos Filho, N. Zagury and L. Davidovich, Weakvalue amplification as an optimal metrological protocol, Phys. Rev. A 91, 062107 (2014).
By Zhihui Yan, Shanxi University