Wednesday 14 September 2022

Edificio 18

9:00 – 9:35


Quantum cryptography beyond quantum key distribution

Quantum key distribution is probably both the most well-known and the most investigated application in quantum communication. However, there’s more to communication than sending secret messages. This talk will give an overview of some other things that “quantum” enables us to do, such as quantum finger- printing and quantum coin flipping. We will also discuss multiparty computation, which “quantum” only enables us to do partly (still better than ”classical”, but not perfectly). This is where two or more parties do not trust each other, but want to compute something together, without revealing any more than necessary about their individual input data. Examples include determining the outcome of an election, or determining which bid is the highest, among a number of secret bids. If we limit the quantum memory adversaries can use, perfect quantum multiparty computation is impossible. Otherwise, cheating probabilities are still lower than for classical protocols.

9:35 – 10:00


A new ultracold ytterbium experiment with single-atom control

A fundamental goal in quantum science and technology is the control and preser- vation of coherence in large qubit ensembles. Neutral atoms stored in optical traps offer a platform with large potential for flexibility and scalability, as wit- nessed by recent experimental progress enabled by precise motional and con- figurational control obtained using tailored optical potentials. While most efforts in neutral atom quantum information experiments have been focused on alkali atoms, alkaline-earth-like atoms provide new capabilities for realizing and prob- ing pristine quantum systems, featuring extremely robust nuclear and electronic excited states. Here, I will report on the ongoing development of a new exper- imental apparatus in Trieste, aiming to achieve single-atom control in ultracold ytterbium ensembles. I will outline our main research directions, targeting im- portant questions at the interface between quantum many-body physics, open quantum systems and quantum information science.

10:00 – 10:25


Orientational melting in planar ion crystals.

The crystallization behavior of a finite number of long-range interacting particles is of fundamental interest for a wide variety of physical systems, from nanoparti- cle to atomic and molecular physics. Nevertheless, our understanding of meso- scopic systems is limited by the fact that there is no universal description of their collective properties which are highly particle number-dependent. I will report on our studies of melting of a planar trapped ion crystal which, notably, is also the first trapped ion experiment in Italy. The geometry of our trap electrodes makes it possible to continuously change the structure of the crystal from a 1D string to a 2D crystal by changing a DC voltage. When the confining potential is made isotropic, the ions undergo a structural transition from a Coulomb crys- tal to a rotor, in which the particles are no longer localized in space, but are rather delocalized along circular trajectories. Interestingly, for sufficiently large numbers of ions two or more concentric rings are populated, and the rings can exhibit independent dynamics, controllable by pinned impurities.

10:25 – 10:50


Towards hole-spin qubits in planar Si pMOSFETs

Hole spins in Si and Ge quantum dots represent a viable route towards the im- plementation of electrically controlled qubits. In particular, the qubit imple- mentation based on pMOSFETs offers great potentialities in terms of integra- tion with the control electronics and long-term scalability. Moreover, the future down scaling of these devices will possibly improve the performance of both the classical (control) and quantum components of such monolithically integrated circuits. Here we develop and use a multi-scale approach to simulate hole-spin qubits in a down scaled version of a commercial Si pMOSFET [1]. Our simulations show the formation of well-defined hole quantum dots within the Si channel, the possibility of a general electrical control (with Rabi frequencies of the order of 100 MHz), and the presence of sweet spots (where the qubit sensitivity to electri- cal noise is possibly suppressed). In all these respects, a crucial role is played by the channel geometry, and specifically by its aspect ratio, for which we identify an optimal range of parameter values. [1] L. Bellentani et al., Phys. Rev. Applied 16, 054034 (2021).

10:50 – 11:20

Coffee Break

11:20 – 11:45


Restoration of the non-Hermitian bulk-boundary correspondence via topological amplification

Non-Hermitian lattices display a unique kind of energy gap and extreme sen- sitivity to boundary conditions [1]. Within the current description of NH topo- logical phases, any NH Hamiltonian featuring a point gap in the spectrum is regarded as topologically nontrivial. This, in turn, leads to the breakdown of the bulk-boundary correspondence for NH systems, since under open boundary conditions the separation between edges and bulk is lost—the so-called NH skin effect. In this presentation, I show how to restore the bulk-boundary correspon- dence for the most paradigmatic class of NH lattice models, namely single-band models with no symmetry.


Imaging stars with quantum error correction

The development of high-resolution, large-baseline optical interferometers would revolutionize astronomical imaging. However, classical techniques are hindered by physical limitations including loss, noise, and the fact that the received light is generally quantum in nature. We show how to overcome these issues using quantum communication techniques. We present a general framework for using quantum error correction codes for protecting and imaging starlight received at distant telescope sites. In our scheme, the quantum state of light is coher- ently captured into a non-radiative atomic state via Stimulated Raman Adiabatic Passage, which is then imprinted into a quantum error correction code. The code protects the signal during subsequent potentially noisy operations nec- essary to extract the image parameters. We show that even a small quantum error correction code can offer significant protection against noise. For large codes, we find noise thresholds below which the information can be preserved. Our scheme represents an application for near-term quantum devices that can increase imaging resolution beyond what is feasible using classical techniques. arXiv:2204.06044

11:45 – 12:10


Topological matter from topological light: Hermitian and non-Hermitian scenarios.

Topology and quantum optics are two fields whose interplay can give rise to new physics [1]. Fractional decay in a topological continuum and topological dependent atom-atom interactions mediated by topological light are just few examples of a pletora of unconventional phenomena [2]. In parallel to this, non- Hermitian Hamiltonians, often used to describe nanophotonic platforms [3], have been shown to possess topological properties with no Hermitian counterpart [4].
In this talk I will present the relation between the Hermitian (or non-Hermitian) topology of a photonic lattice and the topological nature of the atom-atom Hamiltonian mediated by such lattice [5]. [1], [2] M. Bello, G. Platero, J. I. Cirac, A. González-Tudela. Sci. Adv. 2019, [3] F. Roc- cati, S. Lorenzo, G. Calajò, G. M. Palma, A. Carollo, F. Ciccarello. Optica 2022,[4]
E. J. Bergholtz, J. C. Budich, F. K. Kunst. Rev. Mod. Phys. 2021,[5] F. Roccati et al., in preparation


Distributed quantum sensing

We propose an estimation scheme based on the distribution of a single squeezed state among d interferometers to achieve highly sensitive estimation of multiple parameters. The scheme admits different implementations ranging from opti- cal to atom interferometry. The fundamental component of our scheme is the “quantum circuit” (QC), a linear network that optimally distributes the squeezing generated at one of its inputs among d simple (Mach-Zehnder or Ramsey) inter- ferometers, where d unknown parameters are then imprinted and the number of particles at the outputs finally measured. For any given linear combination of the parameters, we identify the optimal configuration of the QC that allows its estimation with maximal, sub-shot-noise sensitivity. Our “entangled” strategy, based on the mode-entanglement created by the QC, outperforms the rival and more common “separable” strategy, in which the same unknown parameters are estimated independently: the sensitivity gain being a factor d, at most. We show that these results are robust against the noise which may arise in the sensor net- work. Our new scheme paves the ways to a variety of applications in distributed quantum sensing.

12:10 – 12:35


Topologically frustrated systems

I summarize our main findings on topologically frustrated one-dimensional spin models. Topological frustration arises when we consider periodic boundary con- ditions in a system with short-range antiferromagnetic interactions consisting of an odd number of spins. Despite its simplicity, which paves the way for the pos- sibility of using fully analytical methods, this kind of frustration gives rise to an entirely new phenomenology. Among the different results obtained, I will focus on three of them, that provide a clear picture of the richness of such models. The three properties that I will discuss are the Loschmidt-echo and its peculiar chaotic properties, the extra contribution to Magic inherent in the geometry of topolog- ically frustrated states, and the peculiar spatial dependence of magnetization in such systems.

Di Candia

Remote quantum sensing: quantum illumination and quantum doppler radar/lidar

Remote quantum sensing has recently gained interest from the scientific com- munity due to its potential capability in achieving a quantum advantage in radar- like scenarios. In the quest of creating a concept working with state-of-the-art technology, we discuss the feasibility of two such protocols in the optical and microwave regime. We give the reasons that the quantum illumination proto- col fails in reaching a quantum advantage from a practical point of view. At the same time, we recognize that other protocols, such as velocity estimation with a quantum doppler radar/lidar, can achieve a substantial quantum advan- tage with respect to a coherent state used as a classical benchmark. We show that our quantum doppler radar/lidar is robust to the use of amplifiers, reaching large SNR values without spreading the signal on an unfeasibly large bandwidth, as needed in the quantum illumination protocol. Finally, we discuss the role of losses and not-optimal measurements setup in the performance of both protocols.

12:35 – 13:00


Coherence and Majorana qubits in Josephson circuits featuring pi-periodic elements

A Josephson junction based on a π-periodic energy-phase relation has emerged as a novel element that can provide augmented freedom in engineering of su- perconducting circuits. In ordinary Josephson circuits, the dependence of the energy spectrum on the offset charges on different islands is 2e periodic through the Aharonov-Casher effect and resembles a crystal band structure. The employ- ment of cos(2￿) Josephson junctions enables tailoring of the Josephson potential and designing spectra featuring multiplets of flat bands, providing us with noise- insensitive energy levels. Furthermore, the suppression of individual Cooper pair tunneling in π-periodic Josephson junctions results in parity-protected supercon- ducting qubits. We propose to couple such a qubit to a Majorana qubit based on Majorana zero energy modes. By properly driving the system we can ob- tain a SWAP gate between the superconducting qubit and the Majorana qubit and employ the latter as a topologically protected memory. The system enables fast gates and long-lived quantum memories, a key requirement for high fidelity quantum information processing in a noisy quantum computing environment.


Quantum noise limits and squeezed-light enhancement of optical magnetometry

Optically-pumped magnetometers (OPMs), in which an atomic ensemble is optically pumped and the spin precession is optically detected, are among the most sensitive devices to measure low-frequency magnetic fields. As in other atomic quantum sensors, the achievable sensitivity of OPMs is limited by three contributions of quantum noise: photon shot noise, atomic projection noise, and quantum backaction, the latter due to the effective field produced by ac-Stark shift.
Here we first describe a pulsed scalar magnetic gradiometer [1] that achieves state-of-the-art differential sensitivity of 14 fT/√Hz over a broad dynamic range, including Earth’s field magnitude. We discuss the theoretical Cramer-Rao lower bound, in the presence of non-white spin noise and atomic diffusion, and we compare it against the experimental standard deviation of the estimated frequency difference.
Secondly, we describe the quantum enhancement of an OPM by polarization squeezing of the probe beam [2]. We report an improvement in high-frequency sensitivity and measurement bandwidth with no loss of sensitivity in any region of the frequency spectrum, a direct demonstration of the evasion of measurement backaction.

[1] V. G. Lucivero et al. “Femtotesla nearly-quantum-noise-limited pulsed gradiometer at Earth-scale fields”, Phys. Rev. Applied 18, L021001 (2022)
[2] C. Troullinou et al. “Squeezed-light enhancement and backaction evasion in a high-sensitivity optically pumped magnetometer”, Phys. Rev. Lett. 127, 193601 (2021)

13:00 – 15:00


15:00 – 16:40

General Discussion

16:40 – 17:10

Coffee Break

Guided Tour at 17:10
Social Dinner at 20:15