The field of quantum computing has evolved into a large interdisciplinary community where significant resources are invested worldwide. As a result, accelerated progress is achieved and we are at the beginning of a new age of computation, developing programmable quantum systems towards universal quantum computers. Quantum computers promise to solve certain mathematical problems that are intractable to classical computers. Quantum computing systems are built from the bottom up reaching the limits of what can be classically simulated. The IBM Quantum Development Roadmap describes our vision of creating a quantum computing ecosystem delivering quantum applications through the cloud. This requires developing the entire quantum computing stack starting from the qubit and quantum processor technology, control electronics to software, algorithms and applications for quantum computing, implemented in the cloud and integrated with high performance computing. In this presentation the recent developments of our quantum computing systems and the scientific advances that enabled scaling superconducting quantum processors to 127 qubits are presented. Besides scale, also quality and speed will be discussed building the key metric for measuring the performance of quantum computation. Examples of applications where the computational power of quantum computing could make a difference are provided.

This conference presentation, “Long-lived spin squeezing in a trapped-atom clock on a chip” was recorded for the Quantum Technologies conference at SPIE Photonics Europe 2022.

First temperature measurement in neurons by ODMR techniques G.Petrini, G.Tomagra, E.Bernardi, E,. Moreva, I.P.Degiovanni, P.Traina, P.Olivero, V.Carabelli, M.Genovese In this talk I will present our results that demonstrate for the first time the possibility of making localized temperature measurement with precision under 0.1 K in neurons by exploiting ODMR techniques. After a general introduction to ODMR techniques based on NV colour centers in diamond, I will present this breakthrough result that will have huge impact to biology and medicine. In little more detail, we probe temperature variations at single-cell scale using color centers in nanodiamonds. We activate the firing of a neuronal network using a drug that stops the inhibitory mechanism. In these conditions, we detect a significant local temperature increase. We associate the observed temperature increase to the firing state of the network, since no significant increase is observed in cases when the drug is not injected. We discuss the possible physiological mechanisms that causes the heat production leading to the observed temperature increase. In perspective, this phenomenon could be extremely promising as a mean of indirect detection of the action potential in future experiments in which the nanodiamonds are functionalized in order to target specific ions channels.

Hong-Ou-Mandel interferometry takes advantage of the quantum nature of two-photon interference to increase the resolution of precision measurements of time-delays. Relying on few-photon probe states, this approach is applicable also in cases of extremely sensible samples and it achieves attosecond (nanometer path length) scale resolution, which is relevant to cell biology and two-dimensional materials. Here, we theoretically analyze how the precision of Hong-Ou-Mandel interferometers can be significantly improved by engineering the spectral distribution of two-photon probe states. In particular, we assess the metrological power of different classes of biphoton states with non-Gaussian time-frequency spectral distributions, considering the estimation of both time-and frequency-shifts. We find that grid states, characterized by a periodic structure of peaks in the chronocyclic Wigner function, can outperform standard biphoton states in sensing applications. The considered states can be feasibly produced with atomic photon sources, bulk non-linear crystals and integrated photonic waveguide devices.

Recent developments, such as the experimental realization of large-scale cluster states, have built a valid case for continuous-variable quantum optics as a promising platform for quantum information processing. The capability of creating non-Gaussian states is key to building a universal quantum computer and achieving a quantum computational advantage. On the other hand, quantum correlations are also at the core of current developments in quantum technologies. Yet, quantum correlations in non-Gaussian states are still poorly understood for continuous-variable systems. In this contribution we will focus on quantum steering, where Alice and Bob each share a part of bipartite quantum state and perform local measurements on their respective subsystem. Quantum steering from Alice to Bob occurs when Bob can exploit information from Alice’s measurements to infer the outcome of his observables’ measurement more precisely than allowed by classical correlations. The paradigmatic example for this phenomenon is found when Alice and Bob both measure field quadratures. In this case, Bob can construct conditional variances that violate Heisenberg’s inequality. This violation, known as Reid’s criterion, is a signature of quantum steering that relies purely on Gaussian features of the state. More generally, we speak of Gaussian steering when we can violate steering inequality using only information from the state’s covariance matrix. For non-Gaussian states, the covariance matrix only offers limited information about the state, and many properties remain under the radar. Therefore, certification protocols of quantum steering for non-Gaussian states are scarce and generally highly demanding from an experimental point of view. In this contribution, we use a recently established connection between quantum steering and the (quantum) Fisher information to develop a new protocol for detection of quantum steering in non-Gaussian. This protocol relies exclusively on homodyne measurements.

Spin noise spectroscopy (SNS) allows optically detect the fluctuations of a spin ensemble. Such fluctuations induce noise in the birefringence of the medium, which can be probed by recording polarization fluctuations of a laser beam after its propagation through the sample. As spin fluctuations are centered on the zero frequency, a transverse magnetic field is applied: spin precession then shifts the spin noise resonance at the Larmor frequency, so that it is not hidden by technical noises. The first SNS experiment was reported in the early 1980s by Alexandrov and Zapasskii, but it is only in 2004 that advances in narrow linewidth lasers and low noise electronics allow using this technique to probe various systems, such as thermal atomic vapors, semiconductors, or quantum wells. It was also proposed for magnetic field sensing, and possible measurements of correlations beyond the second-order raise a lot of interest, as they can give access to new quantum phenomena. We have performed spin noise spectroscopy in a metastable helium gas cell, and show that we can get two polarization dependent noise peaks when we record the linear instead of the circular birefringence fluctuations. Moreover, the relatively simple structure of helium allows us to show that it depends on the closest optical transition even when we are detuned by more than the Doppler broadening. We can also show that the behavior of the polarization dependence is strongly affected by time dependent B-field fluctuations. We performed simulations using a density matrix time evolution model, with added random fluctuations. It reproduces quite well the experimental results, including the transition dependence and the time dependent B-field noise effect, which have not been reported yet.

Integrated quantum photonics is a key tool towards large scale quantum technologies. In this work we present an AlGaAs-based photonic circuit for on-chip generation and manipulation of broadband polarization-entangled photon pairs. The quantum state is generated by Type-II spontaneous parametric down conversion in AlGaAs Bragg reflection waveguides at telecom wavelengths and room temperature. Polarization entangled photons are manipulated over a frequency band of around 50 nm by an integrated polarizing beam splitter. We demonstrate Hong-Ou-Mandel visibility of 80% for a quantum device where the photon pairs generation and manipulation are implemented in a single chip.

For scalable quantum communication and networks, a key step is to realize a quantum repeater node that can efficiently connect different segments of atom-photon entanglement using quantum memories. We report a compact and hardware-efficient realization of a quantum repeater node using a single atomic ensemble for multicell quantum memories. A millisecond lifetime is achieved for individual memory cells after suppressing the magnetic-field-induced inhomogeneous broadening and the atomic-motion-induced spin-wave dephasing. Based on these long-lived memory cells, we achieve heralded asynchronous entanglement generation in two quantum repeater segments one after another and then an on-demand entanglement connection of these two repeater segments. As another application of the multicell atomic quantum memory, we further demonstrate storage and on-demand retrieval of heralded atomic spin-wave qubits by implementing a random access quantum memory with individual addressing capacity. This work provides a promising constituent for efficient realization of quantum repeaters for large-scale quantum networks.

High-dimensional entangled states of light provide novel capabilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the frequency degree of freedom is attracting a growing interest due to its robustness to propagation in optical fibers and its capability to convey large scale of quantum information into a single spatial mode. This provides a strong incentive for the development of efficient and scalable methods for the generation and manipulation of frequency-encoded quantum states. Nonlinear parametric processes are powerful tools to generate such states, but up to now the manipulation of the generated frequency states has been carried out mostly by post-manipulation, which demands complex and bulk-like experimental setups. Direct production of on-demand frequency-states at the generation stage, and preferably using a chip-based source, is crucial in view of practical and scalable applications for quantum information technologies. Here we employ parametric down-conversion in an integrated AlGaAs chip to engineer the wavefunction and exchange statistics of frequency-entangled photon pairs directly at the generation stage, without post-manipulation [1]. Tuning the pump spatial intensity allows to produce frequency-anticorrelated, correlated and separable states, while tuning the spatial phase allows to tailor the symmetry of the spectral wavefunction. As revealed by Hong-Ou-Mandel interferometry, this allows to simulate either bosons, fermions, or anyons, i.e. particles displaying a fractional exchange statistics intermediate between bosons and fermions [2]. Finally, we show that the frequency entanglement can be combined with the polarization entanglement of our source to produce hybrid frequency-polarization entangled states [3], leading to quantum beating in two-photon interference. These results, obtained at room temperature and telecom wavelength, open promising perspectives for implementing quantum simulation tasks with tailored wavefunction and particle statistics in a chip-integrated platform. [1] S. Francesconi et al, Optica 7, 316 (2020). [2] S. Francesconi et al., ACS Photonics 8, 2764 (2021). [3] S. Francesconi et al., in preparation.

Resolving light sources below the diffraction limit is a fundamental task both for astronomy and microscopy. Several recent works, analysed this problem through the lens of quantum parameter estimation theory and proved that the separation between two point sources can be estimated at the quantum limit using intensity measurements after spatial-mode demultiplexing. However, most previous works have either consider low-intensity, or thermal sources. To broaden the applicability of this approach, it is important to extend these results to more general light sources. To this goal, we will present an analytical expression for the Quantum Fisher Information, determining the ultimate resolution limit, for the separation between two sources in an arbitrary Gaussian state. Applying this result to different quantum states, we can shine some light on some relevant questions. We can for example explore the role of partial coherence considering displaced and correlated thermal states, or investigate the importance of quantum correlations by considering squeezed light. In addition to the ultimate quantum limit, we will discuss a simple estimation technique, requiring access only to the mean value of a linear combination of demultiplexed intensity measurements, which is often sufficient to saturate these limits, and can easily be adapted to incorporate the most common noise sources. Finally, we will present our experimental setup that allows for the generation of the images of two sources with different photon statistics, as well as for spatial mode demultiplexing and we will discuss the first practical implementations if the above mentioned estimation techniques.

Quantum fluctuations play a central role in the properties of quantum matter. In non-interacting ensembles, they manifest as fluctuations of non-commuting observables, quantified by Heisenberg inequalities. In the presence of interactions, additional quantum fluctuations appear, from which many-body correlations and entanglement originate. In the context of many-body physics, the Bogoliubov theory provides us with an illuminating microscopic picture of how this occurs for weakly-interacting bosons, with the appearance of the quantum depletion formed by pairs of bosons with opposite momenta. This conceptually simple example yet lacks experimental confirmation. Exploiting the single particle resolution of our metastable Helium detector, we report the direct observation of pairs of atoms with opposite momenta in the depletion of an equilibrium interacting Bose gas. We show that the pair correlation signal rapidly drops as temperature rises, as expected for the quantum depletion. A quantitative study of the atom-atom correlations, both at opposite and close-by momenta, allows us to fully characterise the quantum correlations in the interacting Bose gas. Our results demonstrate how an equilibrium many-body quantum state acquires specific correlations - those of two-mode squeezed states here - as a result of the interplay between quantum fluctuations and interactions. In addition, the measured amplitudes of the correlation signals reveal sub-poissonian number differences between modes at opposite momenta, an important step towards characterising entanglement in equilibrium many-body quantum states. In the future, our approach can be used to characterize more strongly correlated systems for which a theoretical description is lacking, thus fully entering the realm of quantum simulation. - A. Tenart, G. Hercé, J.-P. Bureik, A. Dareau, D. Clément, arXiv:2105:05664 - T. D. Lee, K. Huang, and C. N. Yang, Phys. Rev. 106 (1957) - R. Lopes, C. Eigen, N. Navon, D. Clément, R. P. Smith, and Z. Hadzibabic, PRL 119 (2017) - H. Cayla, C. Carcy, Q. Bouton, R. Chang, G. Carleo, M. Mancini, and D. Clément, PRA 97 (2018) - A. Tenart, C. Carcy, H. Cayla, T. Bourdel, M. Mancini and D. Clément, PRR. 2 (2020) - H. Cayla, S. Butera, C. Carcy, A. Tenart, G. Hercé, M. Mancini, A. Aspect, I. Carusotto, and D. Clément, PRL 125 (2020)

This conference presentation, “Integrated photonic quantum information processing with silicon nitride photonic processors” was recorded for the Quantum Technologies conference at SPIE Photonics Europe 2022.

This conference presentation, “Broad diversity of near-infrared single-photon emitters in silicon ” was recorded for the Quantum Technologies conference at SPIE Photonics Europe 2022.

The negatively charged nitrogen-vacancy (NV) center in diamond is among the most promising solid-state systems implementations of a quantum bit. However, integration of the NV center into any efficient photonic environment requires microstructuring the diamond at below-micrometer scale. Preserving the low NV zero-phonon line inhomogeneous broadening during this process is a major challenge with standard NV creation methods. This issue severely limits practical applications of NV centers. Initial studies on pulsed-laser assisted creation of NVs yielded promising results by creating NVs with low inhomogeneous broadening at desired spatial locations in diamond [1]. Crucially, the lattice damage resulting from implantation of energetic ions was avoided. However, the method relied on a narrow window of parameters for successful writing. We widen this window by using a solid immersion lens (SIL), which facilitates laser writing over a broad range of pulse energies and allows for vacancy formation close to a diamond surface without inducing surface graphitization. We operate in the previously unexplored regime where lattice vacancies are created following tunneling breakdown rather than multiphoton ionization [2]. We present NV arrays that have been created between 1 and 40 µm from a diamond surface, all presenting optical linewidth distributions with means as low as 61.0±22.8 MHz [2], including spectral diffusion induced by off-resonant repump for charge stabilization. This emphasizes the exceptionally low charge-noise environment of laser-written NVs. Such high-quality NV centers are excellent candidates for practical applications employing two-photon quantum interference with separate NV centers. Finally, we propose a model for disentangling power broadening from inhomogeneous broadening in the NV zero-phonon line optical linewidth [2]. [1] Y.-C. Chen, P. S. Salter, S. Knauer, … and J. M. Smith, Nat. Photonics 11, 77 (2017). [2] V. Yurgens, J.A Zuber, S. Flågan, M. De Luca, B.J. Shields, I. Zardo, P. Maletinsky, R.J. Warburton, and T. Jakubczyk, ACS Photonics 8, 1726–1734 (2021)

Ultracold atoms and trapped ions are among the most formidable sources of coherent matter available in a laboratory. In a hybrid quantum system of atoms and ions, ultracold atoms and trapped ions are combined in a single experimental apparatus, thus realizing an innovative platform to experimentally investigate open problems of quantum physics from a new standpoint. An atom-ion hybrid system not only brings together the advantages of each single physical system, but moreover gives rise to atom-ion interactions, which are two orders of magnitude longer-ranged than atom-atom interactions. Despite a strong interest in atom-ion experiments in recent years, atom-ion systems have not yet been brought to an s-wave scattering regime in which atoms and ions experience a long-living coherent evolution. I will report on the advancements in the realization of an experimental apparatus in which a single trapped Barium ion will be immersed into an ultracold gas of fermionic Lithium. In particular, I will report on the a number of technical advancements that we have realized in order to achieve the highest possible level of control over the atom-ion quantum mixture. These include a new ion trap for creating confining potentials for the ions with a combination of optical and electric fields [1], and a high-finesse optical cavity for confining and rapidly cool neutral atoms via unconventional sideband cooling [2]. Notably, this apparatus was recently used to successfully trap Ba+ ions, thus realizing the first ion trapping experiment in Italy. [1] E. Perego, L. Duca, C. Sias Appl. Sci. 10, 2222 (2020) [2] F. Berto, E. Perego, L. Duca, C. Sias arXiv:2107.04110 Phys. Rev. Research (in press)

Quantum memories are a key element for the realisation of quantum repeaters, essential for long-distance quantum communication. Especially for satellite-based quantum networks, alkali metal vapours constitute an excellent storage platform, as neither cryogenics, nor strong magnetic fields are required. We have realised a technologically simple, in principle satellite-suited quantum memory in Caesium vapor, based on electromagnetically induced transparency (EIT) on the Cs D1 line. We focus on the simultaneous optimization of end-to-end efficiency and signal-to-noise level in the memory, which will make our system suitable for many different applications. We have achieved light storage at the single-photon level with end-to-end efficiencies up to 12%, which correspond to internal memory efficiencies of up to 30%. Simultaneously we achieve a minimal noise level corresponding to µ_1=0.029 signal photons. Furthermore, we have determined the limiting noise source at this level to be four-wave mixing noise in the Lambda-system and present solutions to minimise this read-out noise.

Interacting Fermi systems in two dimensions display intriguing phenomena such as high temperature superfluidity and pseudogap physics. These features emerge from the collective behavior of the constituent particles but are extremely hard to predict from microscopic theory. I will report on experiments that realize microscopic two-dimensional systems with ultracold fermionic lithium atoms. With the ability to deterministically prepare few-body ground states and to observe individual particles in momentum space, they enable a microscopic view of strongly interacting two-dimensional Fermi systems. Surprisingly, we find that characteristic features of many-body Fermi gases can already be found in systems of no more than a dozen particles: In spectroscopy, we observe collective excitations that are the few-body precursor of the Higgs amplitude mode of a superfluid. Moreover, in spin-resolved momentum space probes, we can directly image individual ‘Cooper pairs’ and show the presence of fermionic pairing even in a microscopic setting. These findings may broaden our understanding of finite-size Fermi systems, such as atomic nuclei and superconducting grains.