Seminar

Quantum memory will play an important role in quantum networks, notably as components in quantum repeaters. One promising technique for realizing broadband quantum memory, the atomic frequency comb (AFC) protocol, calls for a material with large inhomogeneous broadening and small homogeneous broadening: spectral-hole burning techniques can be used to prepare the absorption spectrum in a periodic pattern of narrow peaks (an AFC). A single photon, absorbed as a collective excitation, will be re-emitted after a time interval fixed by the AFC tooth spacing.

For quantum computers to achieve their promise, regardless of the qubit technology, significant improvements to both performance and scale are required.  Quantum-dot-based qubits in silicon have recently enjoyed dramatic advances in fabrication and control techniques.  The “exchange-only” modality is of particular interest, as it avoids control elements that are difficult to scale such as microwave fields, photonics, or ferromagnetic gradients.  In this control scheme, the entirety of quantum computation may be performed using only asynchronous, baseband voltage p

The Solovay-Kitaev algorithm is a fundamental result in quantum computation. It gives an algorithm for efficiently compiling arbitrary unitaries using universal gate sets: any unitary can be approximated by short gates sequences, whose length scales merely poly-logarithmically with accuracy. As a consequence, the choice of gate set is typically unimportant in quantum computing. However, the Solovay-Kitaev algorithm requires the gate set to be inverse-closed.

Recent work has predicted that quenched near-integrable systems can exhibit dynamics associated with thermal, quantum, or purely non-equilibrium phase transitions, depending on the initial state [1]. Using a trapped-ion quantum simulator with intrinsic long-range interactions, we investigate collective non-equilibrium properties of critical fluctuations after quantum quenches.

Nonequilibrium fluctuation theorems provide a correspondence between properties of quantum systems in thermal equilibrium and work distributions arising in nonequilibrium processes. Building upon these theorems, we present a quantum algorithm to prepare a purification of the thermal state of a quantum system of interest.

Quantum computers hold the promise of solving computational problems which are intractable using conventional methods. For fault-tolerant operation quantum computers must correct errors occurring due to unavoidable decoherence and limited control accuracy. Here, we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors. Using 17 physical qubits in a superconducting circuit we encode quantum information in a distance-three logical qubit building up on recent distance-two error detection experiments [1].

Quantum technology has the potential to revolutionize how we acquire and process experimental data to learn about the physical world. An experimental setup that transduces data from a physical system to a stable quantum memory, and processes that data using a quantum computer, could have significant advantages over conventional experiments in which the physical system is measured and the outcomes are processed using a classical computer.

In this talk I want to introduce shadow sequence estimation. This is a protocol for learning noise in (random) quantum circuits in a flexible and scalable manner. It arises essentially as a combination of randomised shadow estimation (in the Huang-Kueng-Preskill sense) and randomised benchmarking, a time-honoured gate-fidelity estimation protocol.

Quantum complexity is a measure of the minimal number of elementary operations required to approximately prepare a given state or unitary channel. Recently this concept has found applications beyond quantum computing---in the classification of topological phases of matter and in the description of chaotic many-body systems. Furthermore, within the context of the AdS/CFT correspondence, it has been postulated that the complexity of a specific time-evolved many-body quantum state is sensitive to the long-time properties of AdS-black hole interiors.

We argue that all locality-preserving mappings between fermionic observables and Pauli matrices on a two-dimensional lattice can be generated from the exact bosonization (arXiv:1711.00515), whose gauge constraints project onto the subspace of the toric code with emergent fermions. Starting from the exact bosonization and applying Clifford finite-depth generalized local unitary (gLU) transformation, we can achieve all possible fermion-to-qubit mappings (up to the re-pairing of Majorana fermions).