Lecture Videos

Quantum Computation and Quantum Information Lecture 4: Unitary Transformations and the Elitzur--Vaidman Bomb Carnegie Mellon Course 15-859BB, Fall 2018 https://www.cs.cmu.edu/~odonnell/quan... Course discussion board at https://www.diderot.one Email cmuquantum2018@gmail.com for access Weekly work: http://www.cs.cmu.edu/~odonnell/quant... Taught by Ryan O'Donnell Filmed by Dave A. for Panopto (http://www.panopto.com/) Thumbnail image by Eels and Ticha Sethapakdi

Quantum Computation and Quantum Information Lecture 3: Understanding and Measuring One Qubit Carnegie Mellon Course 15-859BB, Fall 2018 https://www.cs.cmu.edu/~odonnell/quan... Course discussion board at https://www.diderot.one Email cmuquantum2018@gmail.com for access Weekly work: http://www.cs.cmu.edu/~odonnell/quant... Taught by Ryan O'Donnell Filmed by Dave A. for Panopto (http://www.panopto.com/) Thumbnail image by Eels and Ticha Sethapakdi

Quantum Computation and Quantum Information Lecture 2: Rotate, Compute, Rotate Carnegie Mellon Course 15-859BB, Fall 2018 https://www.cs.cmu.edu/~odonnell/quan... Course discussion board at https://www.diderot.one Email cmuquantum2018@gmail.com for access Weekly work: http://www.cs.cmu.edu/~odonnell/quant... Taught by Ryan O'Donnell Filmed by Dave A. for Panopto (http://www.panopto.com/) Thumbnail image by Eels and Ticha Sethapakdi

Quantum Computation and Quantum Information Lecture 1: 10^500 Parallel Universes Carnegie Mellon Course 15-859BB, Fall 2018 https://www.cs.cmu.edu/~odonnell/quan... Course discussion board at https://www.diderot.one Email cmuquantum2018@gmail.com for access Weekly work: http://www.cs.cmu.edu/~odonnell/quant... Taught by Ryan O'Donnell Filmed by Dave A. for Panopto (http://www.panopto.com/) Thumbnail image by Eels and Ticha Sethapakdi

The laws of thermodynamics are fundamental laws of nature that classify energy changes for macroscopic systems as work performed by external driving and heat exchanged with the environment. In the past decades, these principles have been successfully extended to the level of classical trajectories of microscopic systems to account for thermal fluctuations. In particular, experimentally tested generalizations of the second law, known as fluctuation theorems, quantify the occurrence of negative entropy production. The extension of thermodynamics to include quantum fluctuations faces unique challenges such as the proper identification of heat and work and clarification of the role of quantum coherence. I will present experiments that allow us to track heat and work along single quantum trajectories of a superconducting qubit evolving under continuous unitary evolution and measurement. We are able to verify the first law of thermodynamics in that the measured heat and work sum to the total energy change of the quantum system.  We then verify the second law of thermodynamics in the form of the Jarzynski equality by employing a novel quantum feedback loop that cancels the heat exchanged at each point in time with additional work.  Our results successfully generalize stochastic thermodynamics to the quantum regime, paving the way for future experimental and theoretical investigations of quantum information and thermodynamics.

Geometrical effects influencing the measured spin coherence and quantum phase coherence in mesoscopic structures were characterized by low-temperature spin-dependent quantum transport experiments.  The findings are of possible relevance for the design of devices for quantum technologies, and have foundational aspects as well.  The materials studied have strong spin-orbit interaction and are heterostructures of InSb, InAs, or InGaAs, and the semimetal Bi with its surface states. The materials were patterned into mesoscopic stadia, narrow channels or quantum interferometers, of typical size ~ 1 micron, comparable to the spin and quantum phase coherence lengths.  Aharonov-Bohm experiments, antilocalization, and universal conductance fluctuations were used to quantify the spin- and quantum phase coherence lengths.  Using geometrical constraints on the accumulation of quantum geometric phases, the work shows a correspondence, in a diffusive transport regime, between mesoscopic dephasing effects due to time-reversal symmetry breaking by magnetic fields, and spin decoherence due to spin-orbit interaction (Aharonov-Bohm / Aharonov-Casher correspondence).  The work also reveals device-geometrical influences on quantum phase coherence from coupling to the classical environment and geometrical effects of electron-electron interactions.

Christopher White, Caltech IQIM

I will describe a method "DMT" for approximating density operators of 1D systems as low bond dimension matrix product operators that, when combined with a standard framework for time evolution (TEBD), makes possible simulation of the dynamics of strongly thermalizing systems to arbitrary times. The method performs well for both near-equilibrium initial states (Gibbs states with spatially varying temperatures) and far-from-equilibrium initial states, including quenches across phase transitions and pure states. I will also discuss ongoing work applying the method to the diffusive-subdiffusive transition in the ergodic phase of the random-field Heisenberg model.

Strontium titanate is a bulk insulator that becomes superconducting at remarkably low carrier densities. Even more enigmatic properties become apparent at the strontium titanate/lanthanum aluminate (STO/LAO) interface and it is important to disentangle the effects of reduced dimensionality from the poorly-understood pairing mechanism. Recent experiments measuring the surface photoemission spectrum[1] and bulk tunneling spectrum[1] have found a cross-over, as a function of carrier density, from a polaronic regime with substantial spectral weight associated with strongly coupled phonons, to a more conventional weakly coupled Fermi liquid. Interestingly, it is only the polaronic state that becomes superconducting at low temperatures, although the properties of the superconducting phase itself appear entirely conventional. We interpret these results in a simple analytical model that extends an Engelsberg-Schrieffer theory of electrons coupled to a single longitudinal optic phonon mode to include the response of the electron liquid, and in particular phonon-plasmon hybridization. We perform a Migdal-Eliashberg calculation within our model to obtain this material's unusual superconducting phase diagram.

[1] Z. Wang et al, Nat. Mater. (2016)
[2] A.G. Swartz et al, arXiv:1608.05621

In this talk, I am going to present some of the work that I have done during my PhD. In the first part I will mostly focus on building a low-temperature Andreev reflection spectroscopy probe (which is basically a simpler version of an STM). In the second part, I will briefly talk about the observation of a new phase of matter, tip-induced superconductivity (TISC), that emerges only under mesoscopic metallic point contacts on topologically non-trivial semimetals like a 3-D Dirac semimetal Cd3As2, and a Weyl semimetal, TaAs and comment on the possible mechanism that might lead to the emergence of such a surprising phase of matter. All these experiments were done using our home-built low-temperature probe.
If time permits, I will also talk about some experiments that we did using various scanning probe based microscopic techniques, e.g., piezo-response force microscopy (PFM) and ferroelectric lithography. I will show how certain “artifacts” can limit the application of PFM in the investigation of ferroelectric materials, and how, under certain circumstances, such “artifacts” can actually turn out to be useful.

References:
[1] L. Aggarwal, A. Gaurav, G. S. Thakur, Z. Haque, A. K. Ganguli & G. Sheet, Unconventional Superconductivity at Mesoscopic Point-contacts on the 3-Dimensional Dirac Semi-metal Cd3As2.” Nature Materials 15, 32 (2016).
[2] L. Aggarwal, S. Gayen, S. Das, R. Kumar, V. Sϋß, C. Shekhar, C. Felser & G. Sheet, “Mesoscopic superconductivity and high spin-polarization coexisting at metallic point contacts on Weyl semimetal TaAs.” Nature Communications, 8, 13974 (2017).
[3] J. S. Sekhon, L. Aggarwal & G. Sheet, “Voltage induced local hysteretic phase switching in silicon.” Applied Physics Letters 104, 162908 (2014). 

The two-dimensional diffusive metal stabilized at the interface of SrTiO3 and the Mott Insulator perovskite LaTiO3[1-2] has challenged many notions related to the formation and electronic behavior of the two-dimensional electron gas (2DEG) at the well studies LaAlO3-SrTiO3 interface. Here we discuss specifically the stability of the superconducting phase[3] at LaTiO3 – SrTiO3 interface, the nature of the superconductor – normal metal quantum phase transition (T=0 limit) driven by magnetic field, significance of the field vis-à-vis the Chandrasekhar - Clogston limit for depairing, and how the transition is initiated when the extent of Coulomb interaction amongst charge carriers is modulated by electrostatic gating[4]. The nature of the superconducting condensate is highlighted in the light of the Ti - t2g orbital driven bands and their filling in the presence of a strong Rashba spin – orbit interaction (SOI). Towards the end of the talk, we will discuss the prominent effects of Rashba SOI on normal state quantum transport and how it renormalizes a Kondo-like electronic behavior in range of temperature Tc< T < 5K[5-7]. The prominence of the Ti 3d0 and Ti 3d1 correlated electron physics in these systems will be demonstrated further from our recent studies of 2DEG in ion irradiated SrTiO3 crystals[8,9].
1. Advanced Materials 22, 4448(2010). 2. Phys. Rev. B 86, 075127(2012).
3. Nature Communications, 1, 89(2010). 4. Nature Materials 12, 542(2013).
5. Phys. Rev. B (Rapid Communication) 90, 081107(2014). 6. Phys. Rev. B 90, 075133(2014). 7. Phys. Rev. B 94, 115165 (2016).
8. Phys. Rev. B 91, 205117(2015). 9. Phys. Rev. B 92, 235115 (2015).

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