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QFarm

QFarm Quantum Seminar Series

Q-Farm Quantum Seminar Series presents "Learning Adaptive Quantum State Tomography with Neural Networks and Differentiable Programming"

Topic: 
Learning Adaptive Quantum State Tomography with Neural Networks and Differentiable Programming
Abstract / Description: 

Quantum State Tomography is the task of determining an unknown quantum state by making measurements on identical copies of the state. Current algorithms are costly both on the experimental front -- requiring vast numbers of measurements -- as well as in terms of the computational time to analyze those measurements. In this talk, we address the problem of analysis speed and flexibility, introducing Neural Adaptive Quantum State Tomography (NA-QST), a machine learning based algorithm for quantum state tomography that adapts measurements and provides orders of magnitude faster processing while retaining state-of-the-art reconstruction accuracy. Our algorithm is inspired by particle swarm optimization and Bayesian particle-filter based adaptive methods, which we extend and enhance using neural networks. The resampling step, in which a bank of candidate solutions -- particles -- is refined, is in our case learned directly from data, removing the computational bottleneck of standard methods. We successfully replace the Bayesian calculation that requires computational time of O(poly(n)) with a learned heuristic whose time complexity empirically scales as O(log(n)) with the number of copies measured n, while retaining very comparable reconstruction accuracy. This corresponds to a factor of a million speedup for 10^7 copies measured. We demonstrate that our algorithm learns to work with basis, symmetric informationally complete (SIC), as well as other types of POVMs. We discuss the value of measurement adaptivity for each POVM type, demonstrating that its effect is significant only for basis POVMs. Our algorithm can be retrained within hours on a single laptop for a two-qubit situation, which suggests a feasible time-cost when extended to larger systems. It can also adapt to a subset of possible states, a choice of the type of measurement, and other experimental details.

Paper link: https://arxiv.org/abs/1812.06693

Date and Time: 
Wednesday, February 19, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Quantum technology with fiber Fabry-Perot cavities"

Topic: 
Quantum technology with fiber Fabry-Perot cavities
Abstract / Description: 

Atom-Photon interaction is everywhere in quantum information, and it is never good enough. Optical cavities with small mode cross-section and lowest loss therefore play a key role, enhancing the interaction in quantum interfaces, but also providing effective coupling that creates entanglement between particles. Fiber Fabry-Perot cavities (FFPs) with laser- machined, ultralow roughness micromirrors are one cavity type that is being used quite successfully with an increasing range of emitters, ranging from ultracold atoms to diamond NV centers and carbon nanotubes. I will give an overview of this cavity technology and its latest developments, and describe two experiments where FFP cavities are used to generate many-particle entanglement. In the first, the cavity measurement effectively blocks part of the qubits' Hilbert space, and entanglement emerges when the coherently driven qubits approach this blocked bart. The second experiment is a trapped-atom clock on an atom chip, where the FFP cavity has allowed us to generate long-lived spin squeezed states in a metrological environment.

Date and Time: 
Wednesday, February 5, 2020 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Surprises from Time Crystals"

Topic: 
Surprises from Time Crystals
Abstract / Description: 

Time crystals are new states of matter that only exist in an out-of-equilibrium setting. I will review the state of this rapidly evolving field, focusing in particular on some of the remarkable properties of this phase, and the surprises coming out of its study. I will provide a detailed overview of existing experiments, with a view towards identifying the ingredients needed for an unambiguous observation of this phase in the future.

Date and Time: 
Wednesday, January 29, 2020 - 12:00pm
Venue: 
Hansen Physics & Astrophysics Building, 102/103

Q-Farm & Geballe Laboratory for Advanced Materials (GLAM) special seminar: "Generative Modelling of Quantum Systems via Quantum Neural Networks"

Topic: 
Generative Modelling of Quantum Systems via Quantum Neural Networks
Abstract / Description: 

In this talk I will first provide some context on the state of the field of quantum machine learning and recent successes of quantum deep learning. Quantum deep learning consists of learning representations of data using composite networks of parameterized quantum transformations, also known as parameterized quantum circuits or quantum neural networks. Following this introduction, I will introduce a new class of generative quantum-neural-network-based models called Quantum Hamiltonian-Based Models (QHBMs). These models provide a novel paradigmatic approach for quantum-probabilistic hybrid variational learning, where one efficiently decomposes the tasks of learning classical and quantum correlations in a way which maximizes the utility of both classical and quantum processors, thereby combining the capabilities of classical probabilistic deep learning and quantum deep learning. Following this will be the introduction of the Variational Quantum Thermalizer (VQT) algorithm for generating the thermal state of a given Hamiltonian and target temperature, and an explanation of how QHBM's are naturally suited for this task. The VQT can be seen as a generalization of the Variational Quantum Eigensolver (VQE) to thermal states. As another application of QHBM's, we will define the tasks of Quantum Modular Hamiltonian learning, where one learns to generatively model mixed quantum states as the thermal state of a learned Hamiltonian. I will provide numerical results demonstrating the efficacy of these techniques in illustrative examples. Namely, using QHBMs for modelling Heisenberg spin systems, to learn entanglement Hamiltonians and compression codes in simulated free Bosonic systems, and to create thermal states for quantum simulation of Fermionic systems.

Date and Time: 
Thursday, November 21, 2019 - 1:30pm
Venue: 
McCullough Building Room 130

Q-Farm Quantum Seminar Series presents Two Speakers

Topic: 
Universal Programmable photonic architecture for quantum information processing; Piezo-optomechanical transduction for microwave to optical quantum frequency conversion
Abstract / Description: 

Universal Programmable photonic architecture for quantum information processing
Ben Bartlett, Stanford University

Photonics provides a range of unique advantages as a platform for quantum information processing, but presents intrinsic challenges which make compact deterministic devices difficult to implement. In this talk, we describe an architecture for a photonic integrated circuit which can be dynamically programmed to implement any quantum operation, in principle deterministically and with perfect fidelity. Our architecture consists of a lattice of beamsplitters and phase shifters, which perform rotations on path-encoded photonic qubits, and embedded quantum emitters, which use a two-photon scattering process to implement two-qubit controlled gates deterministically. We discuss how to program the device and we show how machine learning techniques can be used to automatically implement highly compact approximations to desired quantum circuits.


 

Piezo-optomechanical transduction for microwave to optical quantum frequency conversion
Wentao Jiang, Stanford University

Efficient modulation of the optical properties of a material or a device has long been pursued for both classical and quantum applications. Mechanical deformation strongly perturbs the refractive index compared to other methods and has the potential to be orders of magnitude more energy efficient. In this talk, we compare different approaches for microwave to optical quantum frequency conversion and show how existing optomechanical converters with gigahertz-frequency mechanical mode suffer from some combination of low optical quality factor, low electrical-to-mechanical transduction efficiency, and low optomechanical interaction rate. We consider all these design challenges and demonstrate a piezo-optomechanical converter that combines an efficient piezoelectric transducer with an optimized optomechanical crystal, and improves the conversion efficiency by nearly three orders of magnitude.

Date and Time: 
Wednesday, November 20, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Entanglement phase transition in random unitary circuits with measurements"

Topic: 
Entanglement phase transition in random unitary circuits with measurements
Abstract / Description: 

Parametric resonance of linear harmonic oscillators is a well known phenomenon. Experimental advances in the recent past now make it possible to study parametric phenomena in a wide array of classical and quantum systems. Spanning single to many-body systems, we will show how parametric resonance can be harnessed for novel force sensing applications as well as the realisation of new phases of matter including time crystals.

Date and Time: 
Wednesday, November 6, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Entanglement phase transition in random unitary circuits with measurements"

Topic: 
Entanglement phase transition in random unitary circuits with measurements
Abstract / Description: 

Open quantum systems can display rich dynamics of quantum information: the information may be scrambled within the system, leaked to an environment, or shared between the system and the environment. In this talk, we discuss the dynamics of quantum information in a generic open system, modeled by local random unitary circuits that are interspersed by projective measurements. The interplay between unitary information scrambling and measurements leads to a sharp phase transition: at sufficiently high rates of measurements, any coherent information in the system is completely lost, while at sufficiently low rates, an extensive amount of information is robustly protected and survives --- this is a consequence of the natural quantum error correction enabled by scrambling dynamics. We will elaborate on how these two phases can be characterized from a variety of complementary perspectives based on the average entanglement entropy within the system, the Fisher information in measurement outcomes, and the quantum channel capacity of the open dynamics. Furthermore, we will present an original method to analyze the dynamics of quantum information in the open system by mapping random unitary circuits with measurements into well-known classical statistical mechanics models. This mapping reveals that the phase transition in a certain limit belongs to the universality class of the bond percolation in the 2D square lattice. We will discuss the implications of our results on characterizing near-term quantum devices.

Date and Time: 
Wednesday, November 6, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Universality in the dynamics of quantum information"

Topic: 
Universality in the dynamics of quantum information
Abstract / Description: 

The far-from-equilibrium dynamics of closed quantum systems has become a central topic in condensed matter physics, due to incredible experimental advances in cold atomic and other systems. Concepts of quantum information have taken center stage in this context, with entanglement, in particular, playing a central role in the emergence of thermodynamics. Predicting the behavior of these quantities, however, is notoriously hard as most existing analytical and numerical tools, designed for systems in equilibrium, do not carry over to the dynamical setting. In my talk I will discuss how studying a set solvable minimal models has allowed us to partially conquer this problem and uncover universal features of quantum dynamics. In particular, I will describe some simple hydrodynamic models of how quantum information spreads in time, and discuss a recent prediction for entanglement growth that is particularly relevant for cold atom experiments.

Date and Time: 
Wednesday, October 23, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series

Topic: 
TBA
Abstract / Description: 

Anna will give an overview of the process we use to model and design large-scale superconducting quantum circuits at Rigetti. I will introduce the different stages of the iterative design flow focusing on our modeling approach which combines classical lumped-element circuit models with circuit quantization to calculate the lossy eigenmodes of the system. Using this circuit theoretic technique, we studied multiplexed qubit readout schemes for scalable circuit architectures, for example coupling multiple resonators to a single Purcell filter to protect the qubit from radiative decay channels. I will present our findings on loss and mode hybridization as a function of the Purcell filter geometry and coupling capacitances.

Date and Time: 
Wednesday, September 25, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

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