JQI-QuICS-CMTC Seminar

Abstract: In this talk, I will summarise recent theoretical work on understanding nonlinear response functions in systems with interacting quasiparticles.

Abstract: Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a non-perturbative problem, where BCS theory does not apply. What sets the superconducting phase stiffness, and relatedly the transition temperature, in this limit is of both fundamental and practical interest. This question has become especially relevant with the discovery of superconductivity in moiré materials.

The field of quantum Hamiltonian complexity lies at the intersection of quantum many-body physics and computational complexity theory, with deep implications to both fields. The main object of study is the LocalHamiltonian problem, which is concerned with estimating the ground-state energy of a local Hamiltonian. A major challenge in the field is to understand the complexity of the LocalHamiltonian problem in more physically natural parameter regimes.

Quantum error-correction remains a critical component to realizing the full promise of quantum algorithms.  In this talk, I will discuss experimental progress towards creating and controlling logical qubits on a trapped ion quantum computer. Our code of choice is the Bacon-Shor [[9,1,3]] subsystem code, which consists of 9 data qubits, encoding 1 logical qubit, with stabilizer measurements mapped to 4 ancilla qubits capable of correcting any single qubit error.

We present an optimal protocol for encoding an unknown qubit state into a multiqubit Greenberger-Horne-Zeilinger-like state and, consequently, transferring quantum information in large systems exhibiting power-law (1/r^α) interactions. For all power-law exponents α between d and 2d+1, where d is the dimension of the system, the protocol yields a polynomial speedup for α>2d and a superpolynomial speedup for α≤2d, compared to the state of the art.

The quantum beats are a well-understood phenomenon that has long been used as a spectroscopic technique in various systems. Here we demonstrate two new aspects in understanding and using quantum beats - (i) coupling to the electromagnetic vacuum allows for beating without an initial superposition between the excited levels, and (ii) by detecting the transmission in the forward direction in a superradiant burst, quantum beats can be collectively enhanced, increasing the signal strength useful in systems with low signal-to-noise.

Competition between unitary dynamics that scrambles quantum information non-locally and local measurements that probe and collapse the quantum state can result in a measurement induced entanglement phase transition. Here we study this phenomenon in an all-to-all Brownian hybrid circuit model of qubits that is analytically tractable. A part of the system is initially entangled with a reference which remains mixed at low measurement rates but is purified at high measurement rates.

Analog quantum algorithms are formulated in terms of Hamiltonians rather than unitary gates and include quantum adiabatic computing, quantum annealing, and the quantum approximate optimization algorithm (QAOA).  These algorithms are promising candidates for near-term quantum applications, but they often require fine tuning via the annealing schedule or variational parameters.  In this work we connect all these algorithms to the optimal analog procedure.  Notably, we explore how the optimal procedure approaches a smooth adiabatic procedure but with a superposed oscillato

Please note that this is a special industry speaker seminar.

We finally made it to what seemed like sci-fi wishful thinking. Quantum computers are real and available on the cloud, and their power is growing at a greater-than-Moore’s-Law pace. What does this mean for those entering the job market soon? What will we be using these qubit-loaded behemoths for? Join us for some informal Q&A about this post-quantum era we find ourselves within.