Seminar

Toward single atom qubits on a surface: Pump-probe spectroscopy and electrically-driven spin resonance

William Paul, IBM Research

Single Fe atoms placed on a thin MgO film have exceptional magnetic properties: Their spin relaxation lifetime can extend to many milliseconds, and their quantum state can be coherently manipulated by RF electric fields. In this talk, we will discuss a scanning tunneling microscopy (STM) investigation of the dynamics of spin-relaxation and the electric-field-driven spin resonance of individual Fe atoms on a MgO/Ag(001) surface.

Monday, January 4, 2016 2:30 pm - 2:30 pm EST (GMT -05:00)

Colloquium: Shalev Ben-David

Separations in query complexity using cheat sheets

Shalev Ben-David, Massachusetts Institute of Technology (MIT)

We show a power 2.5 separation between bounded-error randomized and quantum query complexity for a total Boolean function, refuting the widely believed conjecture that the best such separation could only be quadratic (from Grover's algorithm). We also present a total function with a power 4 separation between quantum query complexity and approximate polynomial degree, showing severe limitations on the power of the polynomial method.

Nitrogen-vacancy (NV) centers in diamond nanophotonic structures for quantum networking

Edward Chen, Massachusetts Institute of Technology

The exceptional optical and spin properties of the negatively charged nitrogen-vacancy (NV) center in diamond have led to a wide range of hallmark demonstrations ranging from super-resolution imaging to quantum entanglement, teleportation, and sensing. The solid-state environment of the NV allows us to engineer nano-structures that can enhance the properties of the NV and improve the readout and initialization fidelities of the spin.

Quantum Max-flow/Min-cut

Xingshan Cui, University of California, Santa Barbara

The classical max-ﬂow min-cut theorem describes transport through certain idealized classical networks. We consider the quantum analog for tensor networks. By associating a tensor to each node in an integral ﬂow network, we can also interpret it as a tensor network, and more speciﬁcally, as a linear map.

Sample-optimal tomography of quantum states

Nengkun Yu, IQC

It is a fundamental problem to decide how many copies of an unknown mixed quantum state are necessary and sufficient to determine the state. Previously, it was known only that estimating states to error ϵ in trace distance required O(dr2/ϵ2) copies for a d-dimensional density matrix of rank r. Here, we give a theoretical measurement scheme (POVM) that requires O((dr/δ)ln(d/δ)) copies of ρ to error δ in infidelity, and a matching lower bound up to logarithmic factors.

Monday, November 30, 2015 2:30 pm - 2:30 pm EST (GMT -05:00)

Colloquium: Karsten Flensberg

Towards demonstration of Majorana-based topological qubits

Karsten Flensberg, Niels Bohr Institute

The first part of the talk presents recent progress in the search for condensed matter systems hosting Majorana bound state in semiconductor-superconductor nanowire-based heterostructures. In the second part a proposal for the next steps towards manipulation of quantum information stored in topological qubits is presented.

Friday, November 6, 2015 2:30 pm - 2:30 pm EST (GMT -05:00)

Analysis seminar: Fred Shultz

Applications of order isomorphisms of C*-algebras

Fred Shultz, Wellesley College

We will review known results about order isomorphisms of C*-algebras,
and will describe some applications to complete positivity of maps and
a generalization of the Choi matrix. (This is joint work with Vern Paulsen.)
Then we will describe some applications to quantum information theory.

How hard is deciding trivial versus non-trivial in the dihedral coset problem

Nai-Hui Chia, Pennsylvania State University

The dihedral coset problem (DCP) is an important open problem in quantum algorithms and has been studied since the early days of quantum computing. This problem attracts attention even from experts in cryptography due to its application to the lattice-based cryptosystems. It has been shown by Oded Regev in 2005 that the DCP has deep connections to the unique shortest vector problem and the random subset sum problem.

Monday, November 2, 2015 2:30 pm - 2:30 pm EST (GMT -05:00)

Colloquium: Mukund Vengalattore

Measurement-induced localization of an ultracold lattice gas

Mukund Vengalattore, Cornell University

The act of observation has profound consequences on a quantum system. I will describe our experimental demonstration of a Heisenberg microscope based on nondestructive imaging of a lattice gas. We show that the act of imaging these atoms induces their localization - a manifestation of the quantum Zeno effect.

Monday, November 9, 2015 2:30 pm - 2:30 pm EST (GMT -05:00)

Colloquium: Bertrand Reulet

What are the properties of the electromagnetic field radiated by a quantum conductor, or how to generate entangled radiation with a normal metal ?

Bertrand Reulet, University of Sherbrooke

A classical current in a conductor radiates a classical electromagnetic field. We explore some properties of the field radiated by a conductor when electron transport must be described by quantum mechanics, i.e. when the electron current becomes quantum itself.