Events

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.

Sean Walker of the Department of Chemistry will be defending his thesis:

Molecular nanomagnets for novel spintronics devices

Sean is supervised by Professor Jonathan Baugh.

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.

Jamie Sikora, Centre for Quantum Technologies, National University of Singapore

In this talk, I will discuss correlations that can be generated by performing local measurements on bipartite quantum systems. I'll present an algebraic characterization of the set of quantum correlations which allows us to identify an easy-to-compute lower bound on the smallest Hilbert space dimension needed to generate a quantum correlation. I will then discuss some examples showing the tightness of our lower bound.

Poompong Chaiwongkhot of the Department of Physics and Astronomy will be defending his thesis:

Detection Efficiency Mismatch and Finite-Key-Size Attacks on Practical Quantum Cryptography Systems

Poompong is supervised by Research Assistant Professor Vadim Makarov.

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.

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.