Practicality of Quantum Random Access Memory

Quantum computers are expected to revolutionize the world of computing, but major challenges remain to be addressed before this potential can be realized. One such challenge is the so-called data-input bottleneck: Even though quantum computers can quickly solve certain problems by rapidly analyzing large data sets, it can be difficult to load this data into a quantum computer in the first place. In order to quickly load large data sets into quantum states, a highly-specialized device called a Quantum Random Access Memory (QRAM) is required. Building a large-scale QRAM is a daunting engineering challenge, however, and concerns about QRAM’s practicality cast doubt on many potential quantum computing applications. In this thesis, I consider the practical challenges associated with constructing a large-scale QRAM and describe how several of these challenges can be addressed. I first show that QRAM is surprisingly resilient to decoherence, such that data can be reliably loaded even in the presence of realistic noise. Then, I propose a hardware-efficient error suppression scheme that can further improve QRAM’s reliability without incurring substantial additional overhead, in contrast to conventional quantum error-correction approaches. Finally, I propose experimental implementations of QRAM for hybrid quantum acoustic systems. The proposed architectures are naturally hardware-efficient and scalable, thanks to the compactness and high coherence of acoustic modes. Taken together, the results in this thesis both pave the way for small-scale, near-term experimental demonstrations of QRAM and improve the reliability and scalability of QRAM in the long term

Metrology of Quantum Control and Measurement in Superconducting Qubits

Quantum computers have the potential to solve problems which are classically intractable. Superconducting qubits present a promising path to building such a computer. Recent experiments with these qubits have demonstrated the principles of quantum error correction, quantum simulation, quantum annealing, and more. Current research with superconducting qubits is focused on two primary goals: creating a fully fault tolerant logical qubit out of many physical qubits using surface code error correction, and demonstrating an exponential speedup over any classical computer for a well-defined computational problem. To achieve either of these goals requires high precision control of three components: single qubit gates, two qubit gates, and qubit measurement. In this thesis, we use randomized benchmarking to characterize single qubit gates with 99.95\% fidelity and two qubit gates wiht 99.5\% fidelity in superconducting transmon qubits. In addition, we use standard decoherence measurements as well as newly developed extensions of randomized benchmarking to determine the limiting sources of error. Finally, we explore the surprisingly complicated dynamics of measuring the transmon state through a coupled resonator, and show that fully understanding this process requires breaking a few "standard" assumptions

Quantum money and scalable 21-cm cosmology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 165-170).This thesis covers two unrelated topics. The first part of my thesis is about quantum money, a cryptographic protocol in which a mint can generate a quantum state that no one can copy. In public-key quantum money, anyone can verify that a given quantum state came from the mint, and in collision-free quantum money, even the mint cannot generate two valid quantum bills with the same serial number. I present quantum state restoration, a new quantum computing technique that can be used to counterfeit several designs for quantum money. I describe a few other approaches to quantum money, one of which is published, that do not work. I then present a technique that seems to be secure based on a new mathematical object called a component mixer, and I give evidence money using this technique is hard to counterfeit. I describe a way to implement a component mixer and the corresponding quantum money using techniques from knot theory. The second part of my thesis is about 21-cm cosmology and the Fast Fourier transform telescope. With the FFT telescope group at MIT, I worked on a design for a radio telescope that operates between 120 and 200 MHz and will scale to an extremely large number of antennas N. We use an aperture synthesis technique based on Fast Fourier transforms with computational costs proportional toN logN instead of N2. This eliminates the cost of computers as the main limit on the size of a radio interferometer. In this type of telescope, the cost of each antenna matters regardless of how large the telescope becomes, so we focus on reducing the cost of each antenna as much as possible. I discuss the FFT aperture synthesis technique and its equivalence to standard techniques on an evenly spaced grid. I describe analog designs that can reduce the cost per antenna. I give algorithms to analyze raw data from our telescope to help debug and calibrate its components, with particular emphasis on cross-talk between channels and I/Q imbalance. Finally, I present a scalable design for a computer network that can solve the corner-turning problem.by Andrew Lutomirski.Ph.D

LIPIcs, Volume 251, ITCS 2023, Complete Volume

LIPIcs, Volume 251, ITCS 2023, Complete Volum