Speaker |
Title |
Abstract |

M. Aspelmeyer | Quantum Optical Control of Levitated Solids: a novel probe for the gravity-quantum interface |
The increasing level of control over motional quantum states of massive, solid-state mechanical devices opens the door to an hitherto unexplored parameter regime of macroscopic quantum physics. I will report on our recent progress towards controlling levitated solids in the quantum regime. I will discuss the prospects of using these systems for fundamental tests of physics, including the interface between quantum and gravitational physics. |

D. Bouwmeester | Investigating the reduction of the quantum wavefunction |
An overview will be given of experimental efforts towards detecting the reduction of the quantum wave function. In particular experiments with optomechanical systems will be presented because such systems are well suited to create quantum superpositions involving large masses. The process of Stimulated Raman adiabatic passage (STIRAP) in optomechanics will be demonstrated, enabling the creation and investigation of entanglement of multiple optomechanical systems. |

Y. Chen | Toward Testing Quantum mechanics and Gravity in the Lab |
As experimental techniques allow the preparation, manipulation and measurement of macroscopic objects in the quantum regime, new opportunities arise for testing quantum mechanics, quantum measurement theory, and possibly the quantum nature of gravity. These tests may simply confirm the "conventional predictions", in which case we will be demonstrating the power of quantum mechanics in new, unprecedented regimes. These tests could also deviate from conventional predictions, and reveal "new physics". I will review the status of some efforts that motivate what types of deviations that the experiments can search for. |

J. Emerson | Assessing and Suppressing Decoherence to Scale up Quantum Computing Performance |
Quantum computers have disruptive capabilities, but are limited by an unprecedented sensitivity to uniquely quantum error modes that lead to inaccurate and incorrect quantum computing solutions. In this talk I will describe the break-through tools we have recently developed to capture the exponential complexity of full-system errors affecting today's noisy intermediate-scale quantum processors. I will show experimental results, on today's leading ion trap and superconducting qubit processors, that illustrate how these tools allow us to both improve performance and also predict how accurately quantum hardware, with and without hardware-specific compiling tricks, can find solutions to quantum chemistry, materials science and other problems of interest. Given sufficient interest, I can also provide either an introductory or detailed review of the pros and cons of standard quantum computing benchmarking methods. |

R. Harris | Outrunning the bear: Quantum annealing in the prescence of an environment |
What happens when one violates the speed limits imposed on users of D-Wave's technology? Can one outrun the decoherence mechanisms that bring such physical systems into thermal equilibrium with their environment? I will present recent experimental results pertaining to these questions. |

M. Kasevich | Tests of quantum mechanics and gravity with atom interferometry |
This talk will describe tests of quantum superpostion and entanglement at macroscopic scales (time, distance, particle number) using atomic systems. |

D. Lidar | Quantum algorithmic breakeven: on scaling up with noisy qubits |
As quantum computing proceeds from perfecting physical qubits towards testing logical qubits and small scale algorithms, an urgent question being confronted is how to decide that critical milestones and thresholds have been reached. Typical criteria are gates exceeding the accuracy threshold for fault tolerance, logical qubits with higher coherence than the constituent physical qubits, and logical gates with higher fidelity than the constituent physical gates. In this talk I will argue in favor of a different criterion I call "quantum algorithmic breakeven", which focuses on demonstrating an algorithmic scaling improvement in an error-corrected setting over the uncorrected setting. I will present evidence that current experiments with commercial quantum annealers have already crossed this threshold. I will also discuss our latest evidence for a "limited quantum speedup" with such devices. The lessons we have learned from experimenting with commercial devices with many noisy qubits will hopefully inform other approaches to quantum computing. |

J. Moore | How is quantum many-body physics modified in a world of bounded entanglement? |
Entanglement is one of the key properties of quantum mechanics and often described as a "resource" that enables quantum computing and creates difficulties for quantum computing. The focus on this talk is on two examples, one static and one dynamic, of how some aspects of physics are modified in matrix simulations that can only retain a finite amount of entanglement but are otherwise highly accurate. The first example is from a few years ago: at quantum critical points, one obtains "finite-entanglement scaling", the creation of a finite correlation length analogous to finite-size scaling. The second example is from recent work with Maxime Dupont on the emergence of Kardar-Parisi-Zhang scaling in the physics of a variety of isotropic spin chains. |

R. Penrose | How Gravity May Change Quantum Mechanics |
It used to be said that there would be a "quantum limit" constraining the ultimate capabilities of electronic computers. Yet quantum computers allows these very quantum effects to be harnessed, thereby extending the potential physical capabilities of computers.
However, I shall argue that current quantum mechanics must itself have limits, where gravitational effects restrict the lifetimes of massive quantum superpositions, leading to objective wave-function collapse ("OR" process). Yet, a successful OR theory could provide a route to devices that might even supersede the capabilities of quantum computers. This talk will argue not only for gravitational OR to be a genuine physical effect, but will also present suggestions (from twistor theory) for possible directions that such an OR theory might take. |

R. Raussendorf | A computationally universal phase of quantum matter |
We provide the first example of a symmetry protected quantum phase that has universal computational power. Throughout this phase, which lives in spatial dimension two, the ground state is a universal resource for measurement based quantum computation. Joint work with Cihan Okay, Dong-Sheng Wang, David T. Stephen, Hendrik Poulsen Nautrup; J-ref: Phys. Rev. Lett. 122, 090501 |

J. Raymond | Studying phase transitions in lattices using a superconducting quantum processor |
D-Wave systems quantum annealing processors implement the transverse-field Ising model in superconducting flux qubits. Although the initial intention was to solve classical computation problems such as sampling and optimization, recent publications show that with the newly introduced knobs, one can probe quantum systems with a significant transverse field. In this lecture, I will start with a brief introduction of quantum annealing, transverse-field Ising model and the type of classical computation problems that D-Wave quantum annealers can tackle. Rest of the lecture, I will talk about two recent publications: investigation of spin-glass / ferromagnetic / paramagnetic phase transitions in a 3D cubic lattice (Science 165, pp162-165, 2018) and investigation of a Kosterlitz-Thouless phase transition in a fully-frustrated 2D lattice (Nature 560, pp456-460, 2018). |

T.F. Rosenbaum | Quantum Annealing and Computation (scheduled public lecture) |
Why climb mountains when you can tunnel through them? Harnessing quantum tunneling holds great promise to speed up solutions to optimization problems, ranging from design of circuit boards to protein folding. When computers optimize, they are doing the analog of the physical process of annealing. I will discuss experiments on disordered magnets that quantitatively compare quantum and classical annealing, and demonstrate quantum speedup for reasons that can be understood at a microscopic level. This type of computation follows from Richard Feynman's concept of a quantum computer, and underlies the power of D-Wave machines. Finally, I will discuss the possibility of programmability for spins in disordered magnetic systems, showing recent results from our Lab on quantum spin liquids. |

J. Salfi | Ultra-long coherence times of spin-orbit qubits |
Electron spin qubits have exceptionally long coherence times suitable for quantum technologies. Spin-orbit coupling promises to greatly improve spin qubit scalability and functionality, allowing qubit coupling via photons, phonons, or mutual capacitances, and to enable new engineered hybrid quantum systems. Despite much recent interest, studies of spin-orbit qubits to date yielded disappointingly short coherence times, from 100 nanoseconds to 1 microsecond. Here we demonstrate ultra-long ~10 millisecond spin coherence times for holes bound to acceptor atoms in silicon where spin-orbit coupling yields quantized total angular momentum J=3/2. Our results rival the best electron spin qubits and are 10^4 to 10^5 times long than previous spin-orbit qubits. This is highly non-trivial because our J=3/2 system has spin 1/2 coupled to orbital angular momentum, unlike S=1/2 donors in Si or S=1 NV centres in diamond with negligible spin-orbit coupling and similar coherence times. The key ingredient is the use of intentional crystal strain to control the longitudinal electric dipole that causes decoherence from electrical fluctuations. These results open a new pathway to develop controllable quantum systems. |

D. Silevitch | Direct Measurement of the Soft Mode Driving a Quantum Phase Transition |
The quantum Ising model in transverse field describes a set of mutually interacting quantum spins in a potential that orients each spin along an easy axis, with quantum fluctuations controlled by an external magnetic field applied perpendicular to the easy axis. The competition between these three interactions provides extremely rich behavior, of long-standing interest in statistical mechanics and condensed matter physics, quantum field theory, quantum optics, quantum computing, neural networks, chemistry, and economics. Key features of this behavior have been demonstrated in the LiHoxY1−xF4 family of compounds, including quantum phase transitions, random-field effects, quantum spin glasses and liquids, and quantum annealing. However, the collective modes have not been observed—they are at the wrong energy scale to be seen using probes such as neutron scattering or nuclear magnetic resonance. Here we use low-energy, resonant microwave spectroscopy to reveal the collective mode spectrum of the pure ferromagnet, LiHoF4. Instead of the single excitation expected for a simple quantum Ising system, we find a remarkable array of modes arising from coupling of the Ising electronic spins to a 'bath' of spin-7/2 Ho nuclear spins, the lowest of which softens at the quantum critical point. This suggests that similar modes may exist in other quantum Ising systems, including adiabatic quantum computers. |

S. Simmons | The T-centre spin-photon interface |
Silicon simultaneously hosts the most well-developed integrated electronic and integrated photonic circuit platforms. Furthermore, silicon is also host to some of the highest-performance spin qubits (whose coherence lifetimes extend into the hours [1] in isotopically-purified silicon-28) and photon qubits (whose two-qubit gate fidelities approach 99% [2]). Correspondingly, a spin with an efficient optical interface in silicon could enable a wide range of integrated quantum technologies such as optically-linked qubits and quantum computing modules, photon memories, quantum transducers, repeaters, and more. Early research into silicon spin-photon interfaces revealed interesting long-lived deep-donor candidates which interface with mid-infrared photons [3]. In this talk I will introduce our research into the recently rediscovered 'T-centre', a so-called 'radiation damage centre' in silicon, which possesses up to four long-lived spin qubits and an excitonic spin-dependent optical transition in the telecommunications 'O' band. [1] Saeedi, K., Simmons, S., Salvail, J. Z., Dluhy, P., Riemann, H., Abrosimov, N. V., et al. (2013). Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28. Science, 342(6160), 830–833. http://doi.org/10.1126/science.1239584 [2] Qiang, X., Zhou, X., Wang, J., Wilkes, C. M., Loke, T., O’Gara, S., et al. (2018). Large-scale silicon quantum photonics implementing arbitrary two-qubit processing. Nature Photonics, 12(9), 534–539. http://doi.org/10.1038/s41566-018-0236-y [3] Morse, K. J., Abraham, R. J. S., DeAbreu, A., Bowness, C., Richards, T. S., Riemann, H., et al. (2017). A photonic platform for donor spin qubits in silicon. Science Advances, 3(7), e1700930. http://doi.org/10.1126/sciadv.1700930 |

P.C.E. Stamp | Multipartite Entanglement and its Dynamics: Theory and Experiment |
A long-standing problem in Quantum Information theory is how to define multipartite entanglement. Here I give a solution to this problem in terms of a set of 'entanglement correlators', and exhibit a hierarchy of coupled equations of motion for these. I then discuss how this can beapplied to multi-qubit systems. This is done first for a quantum Ising model; and then detailed results are given for the real life rare earth magnet LiHoF_4, where the dynamics can be understood in terms of a set of interacting "electronuclear" degrees of freedom. |