Recent highlights

Interface between hybrid quantum systems

In order to utilize the advantages of different quantum platforms, we need an interface to send information from one system to another. Nevertheless, practical quantum systems usually suffer from limitations that restrict the properties of the implementable interface. In this line of research, we aim to tackle such restrictions and imperfections. We completely characterized the linear interface between two bosonic systems, this allows us to understand how "bad" is an imperfect interface. Despite the simplicity and ubiquity of linear bosonic interface, we discovered a new class, swapped-QND gate, that has attractive features in many applications. We also developed strategies to engineer arbitrary interface by applying a bad interface for multiple times. We also studied a specific implementation of interface with cavity optomechanics, and devised simple methods to remove the noise due to "leaky" cavities.

Relevant papers:

Pak-Tik Fong, Sheung Chi Poon, Hoi-Kwan Lau (in preparation)
Hoi-Kwan Lau and Aashish A. Clerk, Physical Review Letters 124, 103602 (2020)
Hoi-Kwan Lau and Aashish A. Clerk, npj Quantum Information 5, 31 (2019)


Manipulation and application of spin ensemble

Each spin is a qubit, but a collection of them behaves like bosons, at least in the low excitation regime. In this line of works, we explore novel strategies (e.g. engineered dissipation, coupling to parametric cavity) to control and prepare a spin ensemble to resource for quantum technologies, such as spin-squeezed state for quantum sensing. In the high excitation regime, we observed effects that are surprisingly similiar to (e.g. amplification) and different from (e.g. even-odd effect) bosonic systems. Furthermore, utilizing the collectively enhanced light-matter coupling in atomic spin systems, we devised new architectures of quantum platforms for cooling mechanical oscillators and storing entanglement for quantum communication.

Relevant papers:

Hoi-Kwan Lau, Hong Qiao, Aashish A. Clerk, Tian Zhong, arXiv:2208.00886
Martin Koppenhöfer, Peter Groszkowski, Hoi-Kwan Lau, Aashish A. Clerk, Physical Review X Quantum 3, 030330 (2022)
Peter Groszkowski, Martin Koppenhöfer, Hoi-Kwan Lau, A. A. Clerk, Physical Review X 12, 011015 (2022)
Peter Groszkowski, Hoi-Kwan Lau, C. Leroux, L. C. G. Govia, A. A. Clerk, Physical Review Letters 125, 203601 (2020)
Hoi-Kwan Lau, Alexander Eisfeld, and Jan-Michael Rost, Physical Review A 98, 043827 (2018)

Non-Hermitian quantum sensing

Recent literatures suggested that sensors' sensitivity can be surprisingly enhanced by adding suitable loss and amplification (i.e. system becomes non-Hermitian). This contradicts with conventional wisdom of quantum sensing, which loss and amplification usually introduce noise that is generally harmful. In this work, we conducted the first-of-its-kind full quantum analysis for non-Hermitian quantum sensors. As opposed to previous reports, we find that exceptional point is neither a necessary nor sufficient for improving sensitivity when fundamental quantum noise is properly considered. On the other hand, we find that non-reciprocity, another important non-Hermitian property, can provide sensitivity enhancement even in the full quantum regime. We have patented our new design of quantum sensors.

Relevant paper:

Hoi-Kwan Lau and Aashish A. Clerk, Nature Communications 9, 4320 (2018)

Bosonic quantum computing

Conventionally, building a bosonic quantum computer requires: 1) Cool the system to the ground state; 2) Specify two pure physical states as the qubit basis; 3) engineer encoding-specific interaction to implement logic gates.  In this line of works, we show that all of these procedures are only sufficient but not necessary. We found a unified operation, exponential-swap gate, that can implement logic gates for any bosonic code qubit. Such a "universal coupler" had been regarded as a cornerstone of bosonic quantum computation (Nature 566, 460 (2019)), and was experimentally realized (Nature 566, 509 (2019)).  Our formalism also permits a new class of encoding that can represent pure logical information by bosonic systems with arbitrary thermal excitation. Additionally, the exponential-swap gate can be used in quantum machine learning of continuous-variable quantum information.

Relevant papers:

Kevin Marshall, Daniel F. V. James, Alexandru Paler, and Hoi-Kwan Lau, Physical Review A 99, 032345 (2019)
Hoi-Kwan Lau, Raphael Pooser, George Siopsis, Christian Weedbrook, Physical Review Letters 118, 080501 (2017)
Hoi-Kwan Lau and Martin B. Plenio, Physical Review A 95, 022303 (2017)
Hoi-Kwan Lau and Martin B. Plenio, Physical Review Letters 117, 100501 (2016)