Publications

The tangled state of quantum hypothesis testing

Mario Berta, Fernando G. S. L. Brandão, Gilad Gour, Ludovico Lami, Martin B. Plenio, Bartosz Regula, and Marco Tomamichel, Nature Physics 20, 172–175 (2024).

The Generalised Quantum Stein's Lemma would allow us to calculate the ultimate efficiency of entanglement testing, a key task in entanglement theory that involves deciding whether an unknown state is entangled or not. Recently, we discovered a serious issue with the original proof of this cornerstone result, whose status is unclear at this point. However, we conjecture that the result in the original form is still valid. We discuss the implications of this for the reversibility of entanglement and general quantum resources.

Exact solution for the quantum and private capacities of bosonic dephasing channels

Ludovico Lami and Mark M. Wilde, Nature Photonics 17, 525–530 (2023).

When transmitting quantum information across an optical link, e.g. an optical fibre, a typical error that may occur is a random dephasing. Quantum states encode coherences in the phases of certain complex numbers, and dephasing consists in adding a bit of noise to those phases, hindering their transmission. What makes the resulting dephasing channel difficult to analyse is that it is an example of a non-Gaussian channel. Here  however we show how to solve this model completely: we obtain exact formulas for the quantum capacity, the entanglement distribution (a.k.a. "two-way assisted") quantum capacity, and the private capacity, which turn out to coincide.

Presented at QIP 2023.

No second law of entanglement manipulation

Ludovico Lami and Bartosz Regula, Nature Physics 19 184–189 (2023).

We prove that the theory of entanglement manipulation is asymptotically irreversible "from first principles", i.e. under all non-entangling operations. This entails that entanglement cannot be quantified in a unique way, i.e. there is no unique entanglement measure that determines all physical transformations, mimicking the role of the entropy in thermodynamics.

Presented as a plenary talk at QIP 2022.

Entanglement and superposition are equivalent concepts in any physical theory

Guillaume Aubrun, Ludovico Lami, Carlos Palazuelos, and Martin Plávala, Geom. Funct. Anal. 31(2):181-205, 2021.

Guillaume Aubrun, Ludovico Lami, Carlos Palazuelos, and Martin Plávala, Physical Review Letters 128, 160402 (2022). 

The first lecture of your standard quantum mechanics course presents the fundamental concept of superposition, demonstrated via the two-slit experiment. The second lecture (optimistically) introduces another fundamental notion, that of entanglement, which we now understand as lying at the heart of the advantages of quantum over classical computing. How are these two notions connected? Well, via the formalism of quantum theory, that is, via the math. But since they are ultimately operational concepts, it is natural to find an operational connection, one that does not rely on the mathematical formalism of quantum theory, and that is thus valid beyond the realm of quantum physics. Here we find such a connection, proving that, within the formalism of general probabilistic theories, two theories are entangleable if and only if they both exhibit local superpositions. To do so, we solve a 45-year old mathematical conjecture about tensor products of cones!

Presented at QIP 2021.