QuICS Special Seminar

Lindbladians, one of the simplest extensions of Hamiltonian-based quantum mechanics, are used to describe “drainage” (i.e., decay) and decoherence of a quantum system induced by the system's environment. While traditionally viewed as detrimental to fragile quantum properties, a tunable environment offers the ability to drive the system toward exotic phases of matter, which may be difficult to stabilize in nature, or toward protected subspaces, which can be used to store and process quantum information.

This research presents the design of a classical networking protocol that supports distributed quantum state tomography, which provides necessary information for quantum error correction to work properly.
 
Also, the main audience would be familiar with classical communication, but not with quantum physics, because the conference focuses on classical networking as a whole. Therefore the paper provides some of the backgrounds on quantum communication as well.
 

The yield of physical qubits fabricated in the laboratory is much lower than that of classical transistors in production semiconductor fabrication. Actual implementations of quantum computers will be susceptible to loss in the form of physically faulty qubits. Though these physical faults must negatively affect the computation, we can deal with them by adapting error correction schemes.

Surface codes exploit topological protection to increase error resilience in quantum computing devices and can in principle be implemented in existing hardware. They are one of the most promising candidates for active error correction, not least due to a polynomial time decoding algorithm which admits one of the highest predicted error thresholds.

Tensor networks provide a natural framework for exploring holographic dualities because their entanglement entropies automatically obey an area law. We study the holographic properties of networks of random tensors. We review several interesting structural features of the AdS/CFT correspondence and derive them in our model.  Entropies of random tensor networks satisfy the Ryu-Takayanagi formula for all boundary regions, including corrections due to bulk entanglement.

The QRAM model of quantum computing describes how a (hypothetical) quantum computer and a classical computer work together to produce sophisticated quantum algorithms. The classical computer handles the bulk of the computation and sends circuits to the quantum computer for execution. In this talk I will introduce the Qwire circuit language, which encodes circuits in a classical programming language of our choice and facilitates communication with an attached quantum computer.

Many tasks in quantum information rely on accurate knowledge of a system's Hamiltonian, including calibrating control, characterizing devices, and verifying quantum simulators. In this talk, we pose the problem of learning Hamiltonians as an instance of parameter estimation. We then solve this problem with Bayesian inference, and describe how rejection and particle filtering provide efficient numerical algorithms for learning Hamiltonians.

We show that the multiqubit (including qubit) Clifford group in any even prime power dimension is not only a unitary 2-design, but also a unitary 3-design. Moreover, it is a minimal unitary 3-design except for dimension 4. As an immediate consequence, any orbit of pure states of the multiqubit Clifford group forms a complex projective 3-design; in particular, the set of stabilizer states forms a 3-design. By contrast, the Clifford group in any odd prime power dimension is only a unitary 2-design.

Counterfactual communication is communication without particles in the transmission channel. It is argued that an interaction-free measurement of the presence of opaque objects can be named `counterfactual', while proposed ``counterfactual'' measurements of the absence of such objects are not counterfactual. The quantum key distribution protocols which rely only on measurements of the presence of the object are counterfactual, but quantum direct communication protocols are not.

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.