Monday, September 30, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)
Monday, September 30, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)IQC Colloquium featuring Artur Izmaylov
Quantum computing for quantum chemistry
Quantum computing for quantum chemistry
Understanding the dynamics of quantum many-body systems is one of the fundamental objectives of physics. The existence of an efficient quantum algorithm for simulating these dynamics with reasonable resource requirements suggests that this problem might be among the first practically relevant tasks quantum computers can tackle. Although an efficient classical algorithm for simulating such dynamics is not generally expected, the classical hardness of many-body dynamics has been rigorously proven only for certain commuting Hamiltonians. In this talk, I will show that computing the output distribution of quantum many-body dynamics is classically difficult, classified as #P-hard, also for a large class of non-commuting many-body spin Hamiltonians. Our proof leverages the robust polynomial estimation technique and the #P-hardness of computing the permanent of a matrix. By combining this with the anticoncentration conjecture of the output distribution, I will argue that sampling from the output distribution generated by the dynamics of a large class of spin Hamiltonians is classically infeasible. Our findings can significantly reduce the number of qubits required to demonstrate quantum advantage using analog quantum simulators.
Semiconductor spin qubits are well recognized as a promising platform for scalable fault-tolerant quantum computers (FTQCs) because of relatively long spin coherence time in solid state devices and high-electrical tuneability of the quantum states [1]. In addition, semiconductors have a great potential for applications in quantum communications because of their abilities in optical devices. Therefore, especially in quantum repeater applications, the semiconductor spin qubits provide a route to efficiently connect qubit modules or quantum computers via optical fibers and construct global quantum networks, contributing to realize secure quantum communications and distributed quantum computing [2]. In this talk, we present the physical process enabling the quantum state conversion from single photon polarization states to single electron spin states in gate-defined quantum dots (QDs) and its experimental demonstration [3]. As recent significant achievements, we discuss that the enhancement of the conversion efficiency from a single photon to a single spin in a quantum dot using photonic nanostructures [4]. Finally, we present a perspective of high conversion efficiency quantum repeater operating directly at a telecom wavelength based on semiconductor spin qubits.
[1] G. Burkard et al., Rev. Mod. Phys. 95, 025003 (2023). [2] A. Oiwa et al., J. Phys. Soc. Jpn. 86, 011008 (2017); L. Gaudreau et al., Semicond. Sci. Technol. 32, 093001 (2017). [3] T. Fujita et al., Nature commun. 10, 2991 (2019); K. Kuroyama et al., Phys. Rev. B 10, 2991 (2019). [4] R. Fukai et al., Appl. Phys. Express 14, 125001 (2021); S. Ji et al., Jpn. J. Appl. Phys. 62, SC1018 (2023).
Scaling up the number of qubits available in experimental systems is one of the most significant challenges in quantum computation. A promising path forward is to modularize the quantum processors and then connect many processors using quantum channels, realized using photons and optical fibers. For Rydberg atom arrays, one of the leading platforms for quantum information processing, this could be done by developing an interface for photons, such as an optical cavity. In addition, an optical cavity can be used for fast mid-circuit readout for error detection. In this talk, I will discuss recent progress with two types of cavities and their feasibility as a photonic link. First, we show coherent control of Rydberg qubits and two-atom entanglement as close as 130um away from a nanophotonic cavity. Second, we show fast high-fidelity qubit state readout at a fiber Fabry Perot cavity. In addition, a fiber cavity also allows for cavity-mediated atom-atom gates, which could enable novel quantum networking capabilities.
Finding the minimum of the energy of a many-body system is a fundamental problem in many fields. Although we hope a quantum computer can help us solve this problem faster than classical computers, we have a very limited understanding of where a quantum advantage may be found. In this talk, I will present some recent theoretical advances that shed light on quantum advantages in this domain. First, I describe rigorous analyses of the Quantum Approximate Optimization Algorithm applied to minimizing energies of classical spin glasses. For certain families of spin glasses, we find the QAOA has a quantum advantage over the best known classical algorithms. Second, we study the problem of finding a local minimum of the energy of quantum systems. While local minima are much easier to find than ground states, we show that finding a local minimum under thermal perturbations is computationally hard for classical computers, but easy for quantum computers. These results highlight exciting new directions in leveraging physics-inspired algorithms to achieve quantum advantages in broadly useful problems.
Quantum many-body scars (QMBS) consist of a few low-entropy eigenstates in an otherwise chaotic many-body spectrum and can weakly break ergodicity resulting in robust oscillatory dynamics. The notion of QMBS follows the original single-particle scars introduced within the context of quantum billiards, where scarring manifests in the form of a quantum eigenstate concentrating around an underlying classical unstable periodic orbit. A direct connection between these notions remains an outstanding question. Here, I will first show that a spinor condensate, owing to its collective interactions, is amenable to the diagnostics of scars. We characterize this system's rich dynamics, spectrum, and phase space, consisting of both regular and chaotic states. The former are low in entropy, violate the Eigenstate Thermalization Hypothesis, and can be traced back to integrable effective Hamiltonians, whereas most of the latter are scarred by the underlying classical unstable periodic orbits, while satisfying Eigenstate Thermalization Hypothesis. I will exhibit evidence on how the existing QMBS in the literature are akin to the regular states, rather than the quantum scars. Then I will move on to introduce a spatially many-body model with a mean-field limit by decreasing the range of the interactions. Remarkably, we find that unstable periodic orbits affect the early-time many-body dynamics giving rise to a new type of QMBS. I will classify the QMBS in two main classes, discuss their distinct properties, and show how both QMBS states show up in our model in different parameter regimes. This talk aims (i) to clarify the connection of QMBS to quantum scars and regular eigenstates, and (ii) illustrate the fundamental principle of classical-quantum correspondence in a many-body system, and its current limitations.
Can we compute on small quantum processors? In this talk, I explore the extent to which noise presents a barrier to this goal by quickly drowning out the information in a quantum computation. Noise is a tough adversary: we show that a large class of error mitigation algorithms -- proposals to "undo" the effects of quantum noise through mostly classical post-processing – can never scale up. Switching gears, we next explore the effects of non-unital noise, a physically natural (yet analytically difficult) class of noise that includes amplitude-damping and photon loss. We show that it creates effectively shallow circuits, in the process displaying the strongest known bound on average convergence of quantum states under such noise. Concluding with the computational complexity of learning the outputs of small quantum processors, I will set out a program for wrapping these lower bounds into new directions to look for near-term quantum computational advantage.
The first part of this presentation will provide a brief overview of optical technologies that enabled high-capacity fiber-optic communication systems, from single-mode fibers to fibers supporting multiple spatial modes. A perspective on the evolution of high-capacity systems will be discussed. The second part of the talk will focus on power-e ciency optical detection systems. More specifically, we will describe an experimental demonstration of a system operating at 12.5 bits/photon with optical clock transmission and recovery on free-running transmitters and receivers.
About René-Jean Essiambre Dr. Essiambre worked in the areas of fiber lasers, nonlinear fiber optics, advanced modulation formats, space-division multiplexing, information theory, and high-photon-e ciency systems. He participated in the design of commercial fiber-optic communication systems where several of his inventions were implemented. He has given over 150 invited talks and helped prepare and delivered the 2018 Physics Nobel Prize Lecture on behalf of Arthur Ashkin. He served on or chaired many conference committees, including OFC, ECOC, CLEO, and IPC. He received the 2005 Engineering Excellence Award from OPTICA and is a fellow of the IEEE, OPTICA, IAS-TUM, and Bell Labs. He was President of the IEEE Photonics Society (2022-2023) and is currently the Past-President (2024-2025).
The search for new fundamental physics -- particles, fields, new objects in the sky, etc -- requires a relentless supply of more and more sensitive detection modalities. Experiments looking for new physics are starting to regularly encounter noise sources generated by the quantum mechanics of measurement itself. This noise now needs to be engineered away. The search for gravitational waves with LIGO, and their recent use of squeezed light, provides perhaps the most famous example. More broadly, searches for various dark matter candidates, precision nuclear physics, and even tests of the quantization of gravity are all now working within this quantum-limited regime of measurement. In this talk, I will give an overview of this set of ideas, focusing on activity going on now and what can plausibly be achieved within the next decade or so.
Quantum error-correcting codes play a pivotal role in enabling fault-tolerant quantum computation. These codes protect quantum information through intricately designed redundancies that encode the information in a global manner. Unlike classical objects, in a quantum error-correcting code, the knowledge of individual subregions, even when combined, reveals nothing about the overall state.
In this talk, we explore the quantification of how far quantum error-correcting code are from classical states. We examine this question from three different perspectives: circuit complexity (the mimimal number of circuit depth needed to prepare a quantum state), expansion number (the minimal number of terms needed to expand the wavefunction), and a quantity we termed product overlap, which characterizes the maximal overlap between a given state and any product state. We will demonstrate why any quantum error-correcting code states must exhibit exponentially small product overlap, and how it implies lower bounds for the circuit complexity and the expansion number.