Seminar

Fermionic Linear Optics (FLO) is a restricted model of quantum computation which in its original form is known to be efficiently classically simulable. We show that, when initialized with suitable input states, FLO circuits can be used to demonstrate quantum computational advantage with strong hardness guarantees. Based on this, we propose a quantum advantage scheme which is a fermionic analogue of Boson Sampling: Fermion Sampling with magic input states.

Hyperbolic lattices are tessellations of the hyperbolic plane using, for instance, heptagons or octagons. They are relevant for quantum error correcting codes and experimental simulations of curved space quantum physics in circuit quantum electrodynamics. Underneath their perplexing beauty lies a hidden and, perhaps, unexpected periodicity that allows us to identify the unit cell and Bravais lattice for a given hyperbolic lattice.

Quantum computing devices can solve problems that are infeasible for classical computers.

Marcus is recognised as a principal contributor to the emergence of diamond-based quantum technologies, including quantum microscopy, quantum computing and quantum communications. These technologies represent new paradigms of microscopy, computing and communications that have the potential to revolutionise many disciplines of science and technology. During this seminar Marcus will share more about how the industry can expand the vision for quantum computing.

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.

The presence of noise is currently one of the main obstacles to achieving large-scale quantum computation. Strategies to characterise and understand noise processes in quantum hardware are a critical part of mitigating it, especially as the overhead of full error correction and fault-tolerance is beyond the reach of current hardware. Non-Markovian effects are a particularly unfavorable type of noise, being both harder to analyse using standard techniques and more difficult to control using error correction.

Quantum metrology, which studies parameter estimation in quantum systems, has many important applications in science and technology, ranging from frequency spectroscopy to gravitational wave detection. Quantum mechanics imposes a fundamental limit on the estimation precision, called the Heisenberg limit, which is achievable in noiseless quantum systems, but is in general not for noisy systems. This talk is a summary of some recent works by the speaker and collaborators on quantum metrology enhanced by quantum error correction.

In many natural situations where the input consists of n quantum systems, each associated with a state space C^d, we are interested in problems that are symmetric under the permutation of the n systems as well as the application of the same unitary U to all n systems. Under these circumstances, the optimal algorithm often involves a basis transformation, known as (quantum) Schur transform, which simultaneously block-diagonalizes the said actions of the permutation and the unitary groups.

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.

One of the most fundamental and counterintuitive features of quantum mechanics is entanglement, which is central to many demonstrations of the quantum advantage. Studying quantum correlations generated by local measurements on an entangled physical system is one of the direct ways to gain insights into entanglement. The focus of this dissertation is to get better understanding of the hardness of determining if a given correlation is quantum, which is also known
as the membership problem of quantum correlations.