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### Harold U. Baranger, Professor of Physics

The broad focus of Prof. Baranger's group is **quantum open systems at the nanoscale**, particularly the generation of **correlation** between particles in such systems. Fundamental interest in nanophysics-- the physics of small, nanometer scale, bits of solid-- stems from the ability to control and probe systems on length scales larger than atoms but small enough that the averaging inherent in bulk properties has not yet occurred. Using this ability, entirely unanticipated phenomena can be uncovered on the one hand, and the microscopic basis of bulk phenomena can be probed on the other. Additional interest comes from the many links between nanophysics and nanotechnology. Within this thematic area, our work ranges from projects trying to nail down realistic behavior in well-characterized systems, to more speculative projects reaching beyond regimes investigated experimentally to date.

Correlations between particles are a central issue in many areas of condensed matter physics, from emergent many-body phenomena in complex materials, to strong matter-light interactions in quantum information contexts, to transport properties of single molecules. Such correlations, for either electrons or bosons (photons, plasmons, phonons,…), underlie key phenomena in nanostructures. Using the exquisite control of nanostructures now possible, experimentalists will be able to engineer correlations in nanosystems in the near future. Of particular interest are cases in which one can tune the competition between different types of correlation, or in which correlation can be tunably enhanced or suppressed by other effects (such as confinement or interference), potentially causing a quantum phase transition-- a sudden, qualitative change in the correlations in the system.

My recent work has addressed correlations in both electronic systems (quantum wires and dots) and photonic systems (photon waveguides). We have focused on 3 different systems: (1) qubits coupled to a photonic waveguide, (2) quantum dots in a dissipative environment, and (3) low-density electron gas in a quantum wire. The methods used are both analytical and numerical, and are closely linked to experiments.

Office Location: | 291 Physics Bldg, Durham, NC 27708 |

Office Phone: | (919) 660-2598 |

Email Address: | |

Web Pages: | http://www.phy.duke.edu/research/cm/bghttps://phy.duke.edu/research/research-areas/condensed-matter-and-materials-physics/baranger-group |

**Teaching (Fall 2018):**

- PHYSICS 607.01,
*INTRO TO CONDEN MAT PHY*Synopsis- Bio Sci 130, MW 04:40 PM-05:55 PM

**Education:**Ph.D. Cornell University 1986 M.S. Cornell University 1983 A.B. Harvard University 1980

**Specialties:**-
Theoretical condensed matter physics

Nanophysics

**Research Interests:***Theoretical Condensed Matter Physics, Quantum Materials Physics, Nanoscience*The broad focus of Prof. Baranger's group is

**the interplay of electron-electron interactions and quantum interference at the nanoscale**. Fundamental interest in nanophysics - the physics of small, nanometer scale, bits of solid - stems from the ability to control and probe systems on length scales larger than atoms but small enough that the averaging inherent in bulk properties has not yet occurred. Using this ability, entirely unanticipated phenomena can be uncovered on the one hand, and the microscopic basis of bulk phenomena can be probed on the other. Additional interest comes from the many links between nanophysics and nanotechnology. Within this thematic area, our work ranges from projects trying to nail down realistic behavior in well-characterized systems, to more speculative projects reaching beyond regimes investigated experimentally to date. Currently, 5 topics are being actively pursued:**1. Kondo Effect in Nanoscale Systems**

The Kondo effect is a classic of many-body physics involving the correlation of an electron in an isolated level with a bulk Fermi sea. In contrast, we consider a finite size Fermi sea and so treat the non-zero level-spacing in the lead. The relevant experimental situation is two quantum dots connected by tunneling, a very small one to supply the electron in an isolated level and a large one to act as a nanoscale Fermi sea. [with Prof. S. Chandrasekharan]**2. Molecular Electronics**

We have established a state-of-the-art program to calculate the electric current through single molecules. This involved substantial program development in previous years; we are now concentrating on studying various systems. For instance, we carried out an extensive study of molecules containing*cobaltocene*, a sandwich molecule consisting of a Co atom between two 5-member carbon rings. Cobaltocene has spin 1/2, and manipulation of this spin strongly affects the electrical conduction. Thus we have introduced the first examples of true*molecular spintronics*- a spin filter, spin valve, and spin switch. [with Prof. W. Yang]**3. Quantum Computing: Decoherence in Quantum Error Correction**

We focus on the effects of*decoherence*- processes which break the quantum mechanical coherence at the basis of this type of computation. A key question is how decoherence scales as the computer becomes larger, that is, as the number of qubits increases so that the states of the computer become increasingly more complicated entangled states. Initial pessimistic estimates were circumvented by*quantum error correction*, a clever encoding of a single logical qubit using several physical qubits. We are studying how decoherence due to correlated noise scales in a computer using error correction.**4. Toward Strong Interactions in Circular Quantum Dots**

The “electron gas” model of electrons in solids – in which the conduction electrons interact via Coulomb forces but the ionic potential is neglected – has been a key paradigm of solid state physics. Quantum mechanically, the physical properties change dramatically depending on the balance between the strength of the Coulomb interaction and the kinetic energy. The limiting cases are well understood: for very weak interactions the particles are delocalized while for very strong interactions they localize in a Wigner crystal. The physics at intermediate densities is surprisingly rich and remains at the forefront of research. We are studying the*intermediate density electron gas*confined to various nanostructures by using quantum Monte Carlo techniques.**5. Quantum Phase Transitions**

We are studying models of strongly interacting systems in which there is a quantum (zero temperature) phase transition as a function of disorder strength. The models are chosen so that there is a cooperative many-body ground state (superconductivity or ferromagnetism), and the disorder introduces inhomogeneity through quantum interference. As the inhomogeneity in the system grows, the cooperative state is eventually killed at a quantum phase transition. Through careful study using recently developed algorithms, we identify a bosonic superconductor-insulator transition which has new critical exponents which sharply disagree with previous theoretical prejudices. [with Prof. S. Chandrasekharan]

**Keywords:**Coherent states • condensed matter • Electric Conductivity • Electrodes • electron correlations • electron transport • Many-body problem • materials physics • mesoscopic • Models, Chemical • molecular electronics • Molecular Structure • Nanophotonics • nanoscience • Nanoscience • Nanostructures • novel materials • Quantum chaos • Quantum communication • Quantum computers • Quantum Dots • Quantum entanglement • Quantum Hall effect • quantum interference • Quantum interference • Quantum optics • spintronics • Superconducting quantum interference devices • Transport theory

**Current Ph.D. Students**(Former Students)- Gu Zhang
- Yao-Lung Fang

**Postdocs Mentored**- Jose Hoyos Neto (11/07 - 8)
- Devrim Guclu (8/06 - 8)
- Eduardo Novais (10/04 - 5)
- Jaebeom Yoo (10/03 - 08)
- Ji-Woo Lee (09/03 - 07)
- Hong Jiang (08/03 - 08)
- Amit Ghosal (03/03 - 08)
- Martina Hentschel (08/02 - 04)
- San-Huang Ke (04/02 - 06)
- Gonzalo Usaj (09/01 - 04)
- Evgenii Narimanov (09/98 - 09)

**Recent Publications**(More Publications)- Zhang, XHH; Baranger, HU,
*Quantum interference and complex photon statistics in waveguide QED*, Physical Review A, vol. 97 no. 2 (February, 2018) [doi] [abs] - Zhang, XHH; Baranger, HU,
*Photon Statistics in a Waveguide: Beyond Bunching and Antibunching*, arXiv, vol. 1710 (October, 2017), arXiv - Fang, Y-LL; Ciccarello, F; Baranger, HU,
*Non-Markovian Dynamics of a Qubit Due to Single-Photon Scattering in a Waveguide*, arXiv, vol. 1707 (July, 2017), arXiv - Fang, Y-LL; Baranger, HU,
*Multiple emitters in a waveguide: Nonreciprocity and correlated photons at perfect elastic transmission*, Physical Review A, vol. 96 no. 1 (July, 2017) [doi] - Zhang, G; Novais, E; Baranger, HU,
*Rescuing a Quantum Phase Transition with Quantum Noise*, Physical Review Letters, vol. 118 no. 5 (February, 2017), pp. 050402 [doi] [abs]

- Zhang, XHH; Baranger, HU,

**Selected Invited Lectures***Waveguide QED: Quantum Transport of Strongly-Correlated Photons*, September 16, 2013, Colloquium at the Center for Nonlinear Studies, Los Alamos National Lab, NM*Quantum Mechanics and Dissipation: Connecting the Nano- and Macro- Worlds*, July 02, 2013, 2nd French-American Workshop, Grenoble France*Waveguide QED: Quantum Transport of Strongly-Correlated Photons*, April 25, 2013, Linnaeus Colloquium at Chalmers University of Technology, Goteborg, Sweden*Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment*, April 23, 2013, Seminar at the Niels Bohr Institute, Copenhagen, Denmark*Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment*, December 14, 2012, Workshop on Novel Quantum Phenomena in Mesoscopic Systems, National Center for Theoretical Sciences, Hsinchu Taiwan*Quantum Phase Transition and Emergent Symmetry in Quadruple Quantum Dot System*, June 21, 2011, Quantum Nanoelectronics Seminar, Institut Neel, Grenoble France*Interaction-Induced Localization in Quantum Dots and Wires: Quantum Monte Carlo Studies*, September 22, 2010, Conference on "New Frontiers in the Physics of Quantum Dots", Chernogolovka, Russia.*Tunable Electron-Electron Correlations in Quantum Wires and Dots*, May 05, 2010, Condensed Matter Seminar at USPI Sao Carlos, Sao Carlos, Brazil*Transport Through Single-Molecule Junctions: Interference, Thermopower, and the Role of Self-Interaction Effects*, May 04, 2010, Colloquium at the UFABC, Sao Paolo, Brazil*Transport Through Single-Molecule Junctions: Interference, Thermopower, and the Role of Self-Interaction Effects*, March 16, 2010, APS March Meeting (invited talk), Portland, Oregon.*Does Quantum Error Correction Keep Your Quantum Information Safe?*, February 12, 2010, Theoretical Physics Seminar, Grenoble, France*Toward Correlation Engineering in Electronic Nanostructures*, April 13, 2009, Colloquium at NC State University, Raleigh NC*Kondo Physics in Quantum Dots (a series of 3 lectures)*, October 13, 2008, School on Mesoscopic Physics, Cargese France