QFarm

QFarm Quantum Seminar Series

Q-FARM presents "Emergent quantum randomness and its application for quantum device benchmarking"

Topic: 
Emergent quantum randomness and its application for quantum device benchmarking
Abstract / Description: 

In this talk, we describe a novel, universal phenomenon that occurs in strongly interacting many-body quantum dynamics beyond the conventional thermalization. The observed universality leads to the development of a novel benchmarking method applicable for a wide variety of near-term quantum devices. More specifically, we point out that a single many-body wavefunction can encode an ensemble of a large number of pure states defined on a subsystem. For a wide class of many-body wavefunctions, we show that the ensembles encoded in them display universal statistical properties by using a notion in quantum information theory, called quantum state k-designs. The special case (k=1) reduces to the conventional quantum thermalization. The universality is corroborated by two theorems, solvable models, extensive numerical simulations of Hamiltonian dynamics, and recent experimental observations based on a Rydberg quantum simulator. Our results offer a new approach for studying quantum chaos and provide a practical method for sampling pseudorandom quantum states. As an example of practical utility, I will explain how our results allow us to develop a novel sample-efficient benchmarking protocol, which has been already demonstrated in an experiment.

Ref:
arXiv:2103.03535,
arXiv:2103.03536

Date and Time: 
Wednesday, May 26, 2021 - 12:00pm to Thursday, May 27, 2021 - 11:55am

Q-FARM Colloquium - two talks -

Topic: 
"Memory and optimization with multimode cavity QED" and "Transverse-Field Ising Dynamics by Rydberg Dressing in a cold atomic gas"
Abstract / Description: 

Memory and optimization with multimode cavity QED

Quantum systems are driving a revolution in computing and information theory. Driven-dissipative quantum systems, which are both pumped by an external source and are open to environmental interactions, have not been explored as a computational resource as fully as their closed counterparts. In this talk, I will describe how a driven-dissipative system is realized by coupling ultracold atoms to a multimode optical cavity and how it can perform various computational tasks. Through a combination of unitary and dissipative dynamics, the system can learn and recognize arbitrary sets of patterns, an ability known as associative memory, and also functions as a heuristic solver for (NP hard) Ising optimization problems. These functionalities can be understood in terms of semiclassical theories that describe the transition to a superradiant ordered state, which encodes a learned pattern or a solution to an Ising problem. While a fully quantum description remains intractable, experimental progress will be discussed that demonstrates the required ingredients for near-term realization.


 

Transverse-Field Ising Dynamics by Rydberg Dressing in a cold atomic gas

With recent progress on building large and controllable quantum systems, we are on the cusp of harnessing these systems for quantum computation and metrology. Optically controlled interactions are a necessary tool to implement computation and metrology schemes in a system of cold atoms. In this talk, we will present a realization of long-range optically-controllable Ising interactions in a cold gas of cesium atoms by Rydberg dressing. By adding microwave coupling between the clock states we emulate the transverse-field Ising model and detect dynamical signatures of the paramagnetic-ferromagnetic phase transition. We will discuss current progress towards producing spin squeezing by using local and dynamical control of interactions. Finally, we will describe prospects of encoding a quantum algorithm in a hardware-efficient way in this system.

Date and Time: 
Wednesday, May 12, 2021 - 12:00pm to Thursday, May 13, 2021 - 11:55am

Q-FARM presents "Quantum probes of two-dimensional materials"

Topic: 
Quantum probes of two-dimensional materials
Abstract / Description: 

Spin qubits based on diamond NV centers can detect tiny magnetic fields; thin two-dimensional materials produce tiny magnetic fields. Do they make a good match? I will discuss two works that explored how NV magnetometry can uniquely probe the spins and currents in crystals that are one-atom thick.

Outline:
How we discovered, via local magnetic noise measurements, that graphene electrons at high bias undergo cherenkov radiation of phonons
How we performed the first NMR measurement on a single-atom thick crystal.
Some thoughts and outlook on the benefits and challenges of using spin qubits to measure condensed matter systems.
Preview of UC Irvine work on nanomanipulation of 2d material heterostructures

References:
"Electron-phonon instability in graphene revealed by global and local noise probes"
T. I. Andersen*, B. L. Dwyer*, J. D. Sanchez-Yamagishi*, J. F. Rodriguez-Nieva, K. Agarwal, K. Watanabe, T. Taniguchi, E. A. Demler, P. Kim, H. Park, M. D. Lukin
Science 364 ,6436 (2019) *equal contribution

"Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit"
I. Lovchinsky, J. D. Sanchez-Yamagishi, E. K. Urbach, S. Choi, S. Fang, T. Andersen, K. Watanabe, T. Taniguchi, A. Bylinskii, E. Kaxiras, P. Kim, H. Park, and M. D. Lukin
Science 355, 6324 (2017)

Date and Time: 
Wednesday, May 5, 2021 - 12:00pm to Thursday, May 6, 2021 - 11:55am

QFARM Quantum Seminar Series, two talks

Topic: 
1-A photonic quantum computer design with only one controllable qubit; 2-Towards MEMS-driven photonic computing
Abstract / Description: 

Talk Title #1: A photonic quantum computer design with only one controllable qubit

Ben Bartlett (Stanford University, Prof. Shanhui Fan's group)

We describe a design for a photonic quantum computer which requires minimal quantum resources: a single coherently-controlled atom. Quantum operations applied to the atomic qubit can be teleported onto the photonic qubits via projective measurement, and arbitrary quantum circuits can be compiled into a sequence of these teleported operators. The proposed device has a machine size which is independent of quantum circuit depth, does not require single-photon detectors, operates deterministically, and is robust to experimental imperfections.

Talk title #2: Towards MEMS-driven photonic computing

Sunil Pai (Stanford University, advised by Prof. Olav Solgaard and co-advised by Profs. David Miller and Shanhui Fan)

ABSTRACT: Programmable nanophotonic networks of Mach-Zehnder interferometers are energy-efficient circuits for matrix-vector multiplication that benefit a wide variety of applications such as artificial intelligence, quantum computing and cryptography. In this talk, we discuss the theory and algorithms to set up field-programmable photonic networks using MEMS (microelectromechanical systems)-actuated phase shifts, which unlike currently more commonplace thermal phase shifters, cost no energy to maintain a constant phase shift and therefore significantly improve overall energy efficiency. We discuss the corresponding implications on gradient-based optimization (on-chip machine learning), scalability and dispersion of such networks for commercial applications such as training and inference in photonic neural networks, optical cryptocurrency, and low-loss quantum computers. We will then attempt a live demonstration of programming a small thermally-driven 6x6 triangular photonic network and observe the practical considerations such as thermal drift, dispersion, and phase shift calibration in these devices.

Date and Time: 
Wednesday, April 28, 2021 - 12:00pm to Thursday, April 29, 2021 - 11:55am

Q-FARM presents "Spin qubits in silicon carbide for quantum technologies"

Topic: 
Spin qubits in silicon carbide for quantum technologies
Abstract / Description: 

Defect spin qubits in silicon carbide (SiC) with associated nuclear spin quantum memories can leverage near-telecom emission and wafer-scale semiconductor device engineering for creating quantum technologies. Here, I highlight recent advances with the neutral divacancy defect (VV0) in SiC within the context of long-distance quantum communication and repeater schemes. Broadly, I will illustrate how quantum states can be controlled, tuned, and enhanced through their integration into SiC mechanical, photonic, and electrical devices. I will first describe the isolation of single VV0 defects in functional SiC optoelectronic devices, which allows for deterministic charge state control and terahertz tuning, but also surprisingly eliminates spectral diffusion in the optical structure of these defects. I will then discuss the entanglement and control of nuclear spin registers, and show how isotopic engineering can enhance both nuclear quantum memories and electron spin coherence times, while also demonstrating high fidelity control (99.98%), initialization, and readout. Briefly, I will further highlight recent results that universally protect spin coherence from electrical, magnetic, and thermal noise, resulting in T2*>20 ms in a naturally abundant crystal. This suite of results establishes SiC as a promising platform for scalable quantum science with optically-active spins.

Date and Time: 
Thursday, April 29, 2021 - 12:30pm

Q-FARM presents "Continuous variables quantum complex networks"

Topic: 
Continuous variables quantum complex networks
Abstract / Description: 

Experimental procedures based on optical frequency combs and parametric processes produce quantum states of light involving large numbers of spectro-temporal modes that can be mapped and analyzed in terms of quantum complex networks. The protocols enable the implementation of reconfigurable entanglement structures that can go beyond the regular geometry of cluster states and implement graphs with more complex topology. Quantum complex networks, mimicking real-world structures, can then be explored to study tailored quantum communication and information protocols. When non-Gaussian statistics is induced in such quantum systems, they are hard to benchmark theoretically and hard to experimentally reconstruct. I will show theoretical benchmarks based on complex network theory and machine learning technique for experimental detection of Wigner negativity.

Date and Time: 
Wednesday, April 21, 2021 - 12:00pm to Thursday, April 22, 2021 - 11:55am

QFARM: "Ultra-low-power second-order nonlinear optics on a chip" and "Quantum Dynamics of Ultrafast Nonlinear Photonics"

Topic: 
"Ultra-low-power second-order nonlinear optics on a chip" and "Quantum Dynamics of Ultrafast Nonlinear Photonics"
Abstract / Description: 

Thin-film lithium niobate is a promising platform for integrated photonics because it can tightly confine light in small waveguides which allows for large interactions between light, microwaves, and mechanics. Recent advances in nanofabrication have made such high performance photonic circuits possible. In this talk, I will present a recent demonstration of efficient frequency doubling and parametric oscillation in a thin-film lithium niobate resonator. The operating regimes of this system are controlled using the relative detuning of the intracavity resonances, and the emission frequency of parametric oscillation is tuned over one THz simply by adjusting the pump laser over a few hundreds of MHz. We also observe highly-enhanced effective third-order nonlinearities caused by cascaded second-order processes resulting in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms.


Broadband optical pulses propagating in highly nonlinear nanophotonic waveguides can significantly leverage optical nonlinearity by tight temporal and spatial field confinements, promising a route towards all-optical quantum engineering and information with single-photon nonlinearities. Modeling and engineering quantum optical devices operating in this strongly interacting regime, however, pose significant theoretical challenges; a large number of frequency modes undergo non-Gaussian dynamics, whose description naïvely requires exponentially large Hilbert space. Using an example of broadband parametric downconversion, we show that such systems can exhibit rich quantum dynamics that are qualitatively different from semi-classical predictions. In the talk, we introduce a prescription based on matrix product state (MPS) representations to realize an efficient full-quantum simulation of generic pulse propagation. Specifically, we unravel the dynamics of an optical soliton, highlighting features that cannot be captured by a conventional theoretical framework.

Date and Time: 
Wednesday, April 14, 2021 - 12:00pm to Thursday, April 15, 2021 - 11:55am

Q-FARM presents "Quantum sensing with unlimited optical bandwidth"

Topic: 
Quantum sensing with unlimited optical bandwidth
Abstract / Description: 

Squeezed light is a major resource for quantum sensing, which has been already implemented in high-end interferometric sensing, such as gravitational wave detection. However, standard squeezed interferometry methods suffer from two severe limitations. First, the detection bandwidth of squeezing-enhanced interferometry is limited by the narrowband response (MHz to GHz) of photodetectors, which critically prevents efficient utilization of the optical bandwidth (tens of THz and more) offered by standard sources of squeezed light. Second, current methods require near ideal photo-detectors with unity efficiency, prohibiting real-life applications, where ideal detection is not available. To overcome these limitations and to benefit from the orders-of-magnitude enhancement in the sensing throughput offered by the optical bandwidth, a paradigm shift is required in terms of broadband quantum sources, detection schemes, and interferometric design.

I will present a set of new methods for sub-shot-noise sensing, based on nonlinear interferometry, which overcome these limitations. By placing the phase object in question between two parametric amplifiers in series, the first amplifier generates broadband squeezed light to interrogate the object and the second amplifier acts as an ideal broadband quantum detector to measure the object's response. This technique is robust to detection inefficiency and provides an unprecedentedly broad optical bandwidth for quantum measurement, exceeding the possibilities of photodetectors by several orders of magnitude.

I will discuss in detail two specific examples of ultra broadband parametric-homodyne measurement [1] and of squeezing-enhanced Raman spectroscopy [2].

References:
[1] Y. Shaked, Y. Michael, R. Vered, L. Bello, M. Rosenbluh and A. Pe'er, "Lifting the Bandwidth Limit of Optical Homodyne Measurement", Nature Communications 9, 609 (2018).
[2] Y. Michael, L. Bello, M. Rosenbluh, and A. Pe'er, "Squeezing-enhanced Raman Spectroscopy", npj Quantum Information 5, 81 (2019).

Date and Time: 
Wednesday, March 24, 2021 - 10:00am to Thursday, March 25, 2021 - 9:55am
Venue: 
Zoom ID: 987 676 025; +password

Q-FARM presents "Unconventional computing with liquid light"

Topic: 
Unconventional computing with liquid light
Abstract / Description: 

The recent advances in the development of physical platforms for solving combinatorial optimisation problems reveal the future of high-performance computing for quantum and classical devices. Unconventional computing architectures were proposed for numerous systems including superconducting qubits, CMOS hardware, optical parametric oscillators, memristors, lasers, photonic simulators, trapped ions, polariton and photon condensates. A promising approach to achieve computational supremacy over the classical von Neumann architecture explores classical and quantum hardware as Ising and XY machines. Gain-dissipative platforms such as the networks of optical parametric oscillators, coupled lasers and non-equilibrium Bose-Einstein condensates such as exciton-polariton or photon condensates use an approach to finding the global minimum of spin Hamiltonians which is different from quantum annealers or quantum computers. In my talk, I will discuss the principles of the operation of the devices based on such systems with a focus on polariton graph platform that we recently realised in experiments.

Date and Time: 
Wednesday, March 10, 2021 - 12:00pm to Thursday, March 11, 2021 - 11:55am

Q-FARM presents "Coupling diamond defects to high-finesse optical microcavities"

Topic: 
Coupling diamond defects to high-finesse optical microcavities
Abstract / Description: 

Defect centers in diamond can offer atomic-like optical transitions and long-lived spin degrees of freedom. Integrating them into high quality optical resonators opens a route toward realizing a cavity quantum electrodynamics system combining atomic-like coherence with a robust solid-state platform. While approaches based on diamond nanophotonics have been pursued for more than a decade, Fabry-Perot microcavities present a complementary approach that has recently received significant attention. This talk will consider the potential benefits and challenges to open micro-cavities, and examine progress toward coupling them to diamond defect centers

Date and Time: 
Wednesday, February 24, 2021 - 12:00pm to Thursday, February 25, 2021 - 11:55am

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