Events

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.

Security of differential quadrature phase shift quantum key distribution

Shun Kawakami, University of Tokyo

One of the simplest methods for implementing quantum key distribution over fiber-optic communication is the Bennett-Brassard 1984 protocol with phase encoding (PE-BB84 protocol), in which the sender uses phase modulation over double pulses from a laser and the receiver uses a passive delayed interferometer.

Estimating outcome probabilities of quantum circuits using quasiprobabilities

Hakop Pashayan, The University of Sydney

We present a method for estimating the probabilities of outcomes of a quantum circuit using Monte Carlo sampling techniques applied to a quasiprobability representation.

Nanocontacts atom-by-atom with a friction emulator

Dorian Gangloff, Massachusetts Institute of Technology

Friction is the basic, ubiquitous mechanical interaction between two surfaces that results in resistance to motion and energy dissipation. To test long-standing atomistic models of friction processes at the nanoscale, we implemented a synthetic nanofriction interface using laser cooled ions subject to the periodic potential of an optical standing wave.

Catalysis of Stark-tuned Interactions between Ultracold Rydberg Atoms

Aye Lu Win, Old Dominion University

The strong long-range interaction between ultracold Rydberg atoms gives rise to a number of interesting phenomena that have been studied in recent years including resonant energy transfer collisions, many-body quantum simulations, quantum information processing, and ultracold plasmas. The dipole-dipole interaction between a pair of Rydberg atoms can result in a state-changing interaction if the energy defect for the process is small.

Progress toward a spin squeezed optical atomic clock beyond the standard quantum limit

Boris Braverman, Massachusetts Institute of Technology

State of the art optical lattice atomic clocks have reached a relative
inaccuracy level of order $10^{-18}$, making them the most stable time
references in existence.

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.

Measuring Entanglement Entropy in a Many-body System

K. Rajibul Islam, MIT-Harvard Center for Ultracold Atoms

Entanglement, perhaps the most counter-intuitive feature of quantum mechanics, describes non-local correlations between quantum objects. In recent years, entanglement has emerged as a central concept in our understanding of quantum many-body physics. It allows us to characterize phases of quantum matter that cannot be distinguished by broken symmetries, such as topological states.

Quantum Randomness Expansion - New Results

Carl Miller, University of Michigan

Is it possible to create a source of provable random numbers? An affirmative answer to this question would be highly useful in information security, where random numbers are needed to provide the keys for encryption algorithms. Bell inequality violation experiments offer hope for this problem, since the outputs of a Bell violation must be non-classical and therefore not fully predictable to an adversary. The challenge is to prove something stronger: that the outputs can be processed (extracted) to obtain uniformly random data. This leads to some complex and beautiful mathematics.

Quantum optics of strongly correlated many-body systems

Igor Mekhov, University of Oxford

We show that quantum backaction of weak measurement constitutes a novel source of competitions in many-body systems, thus leading to new phenomena. We consider a system of ultracold atoms in optical lattices trapped inside a high-Q cavity, which requires a fully quantum description of both light and matter waves. The QND measurements lead to the generation of genuinely multipartite entangled modes of the matter fields, which have analogies in quantum optics (e.g. two-mode squeezing), but are non-Gaussian.