# Publications

## Double-sided coaxial circuit QED with out-of-plane wiring

APPLIED PHYSICS LETTERS **110** (2017) ARTN 222602

## Generation and detection of squeezed phonons in lattice dynamics by ultrafast optical excitations

NEW JOURNAL OF PHYSICS **19** (2017) ARTN 023032

## Probing the Fluctuations of Optical Properties in Time-Resolved Spectroscopy.

Physical review letters **119** (2017) 187403-

We show that, in optical pump-probe experiments on bulk samples, the statistical distribution of the intensity of ultrashort light pulses after interaction with a nonequilibrium complex material can be used to measure the time-dependent noise of the current in the system. We illustrate the general arguments for a photoexcited Peierls material. The transient noise spectroscopy allows us to measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well they thermalize during the coherent lattice vibrations. The proposed statistical measurement developed here provides a new general framework to retrieve dynamical information on the excited distributions in nonequilibrium experiments, which could be extended to other degrees of freedom of magnetic or vibrational origin.

## Optimal control of two qubits via a single cavity drive in circuit quantum electrodynamics

PHYSICAL REVIEW A **95** (2017) ARTN 042325

## Circuit quantum acoustodynamics with surface acoustic waves.

Nature communications **8** (2017) 975-

The experimental investigation of quantum devices incorporating mechanical resonators has opened up new frontiers in the study of quantum mechanics at a macroscopic level. It has recently been shown that surface acoustic waves (SAWs) can be piezoelectrically coupled to superconducting qubits, and confined in high-quality Fabry-Perot cavities in the quantum regime. Here we present measurements of a device in which a superconducting qubit is coupled to a SAW cavity, realising a surface acoustic version of cavity quantum electrodynamics. We use measurements of the AC Stark shift between the two systems to determine the coupling strength, which is in agreement with a theoretical model. This quantum acoustodynamics architecture may be used to develop new quantum acoustic devices in which quantum information is stored in trapped on-chip acoustic wavepackets, and manipulated in ways that are impossible with purely electromagnetic signals, due to the 105 times slower mechanical waves.In this work, Manenti et al. present measurements of a device in which a tuneable transmon qubit is piezoelectrically coupled to a surface acoustic wave cavity, realising circuit quantum acoustodynamic architecture. This may be used to develop new quantum acoustic devices.

## Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.

Physical review letters **119** (2017) 147701-

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

## Simultaneous Bistability of a Qubit and Resonator in Circuit Quantum Electrodynamics

Physical Review Letters **118** (2017)

## Phase separation in the nonequilibrium Verwey transition in magnetite

PHYSICAL REVIEW B **93** (2016) ARTN 054305

## Surface acoustic wave resonators in the quantum regime

PHYSICAL REVIEW B **93** (2016) ARTN 041411(R)

## Erratum: Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics (Nature Communications (2015) 6:10249 DOI: 10.1038/ncomms10249)

Nature Communications **7** (2016)

## Quantum interferences reconstruction with low homodyne detection efficiency

EPJ Quantum Technology **3** (2016)

© 2016 Esposito et al. Optical homodyne tomography consists in reconstructing the quantum state of an optical field from repeated measurements of its amplitude at different field phases (homodyne data). The experimental noise, which unavoidably affects the homodyne data, leads to a detection efficiency η < 1. The problem of reconstructing quantum states from noisy homodyne data sets prompted an intense scientific debate about the presence or absence of a lower homodyne efficiency bound (η > 0.5) below which quantum features, like quantum interferences, cannot be retrieved. Here, by numerical experiments, we demonstrate that quantum interferences can be effectively reconstructed also for low homodyne detection efficiency. In particular, we address the challenging case of a Schrödinger cat state and test the minimax and adaptive Wigner function reconstruction technique by processing homodyne data distributed according to the chosen state but with an efficiency η < 0.5. By numerically reproducing the Schrödinger’s cat interference pattern, we give evidence that quantum state reconstruction is actually possible in these conditions, and provide a guideline for handling optical tomography based on homodyne data collected by low efficiency detectors.

## Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics.

Nature communications **6** (2015) 10249-

Fluctuations of the atomic positions are at the core of a large class of unusual material properties ranging from quantum para-electricity to high temperature superconductivity. Their measurement in solids is the subject of an intense scientific debate focused on seeking a methodology capable of establishing a direct link between the variance of the atomic displacements and experimentally measurable observables. Here we address this issue by means of non-equilibrium optical experiments performed in shot-noise-limited regime. The variance of the time-dependent atomic positions and momenta is directly mapped into the quantum fluctuations of the photon number of the scattered probing light. A fully quantum description of the non-linear interaction between photonic and phononic fields is benchmarked by unveiling the squeezing of thermal phonons in α-quartz.

## Quantum optics for studying ultrafast processes in condensed matter

Conference on Lasers and Electro-Optics Europe - Technical Digest **2015-August** (2015)

© 2015 OSA. Quantum state reconstruction techniques are used to address non equilibrium matter states. Proof of principle experiments reveal that few photons coherent pulses can be squeezed by the interaction with impulsively excited coherent phonons in quartz.

## Quantum optics for studying ultrafast processes in condensed matter

CLEO: QELS - Fundamental Science, CLEO_QELS 2015 (2015) 1551p-

Quantum state reconstruction techniques are used to address non equilibrium matter states. Proof of principle experiments reveal that few photons coherent pulses can be squeezed by the interaction with impulsively excited coherent phonons in quartz. © 2014 Optical Society of America.

## Surface acoustic wave devices on bulk ZnO crystals at low temperature

APPLIED PHYSICS LETTERS **106** (2015) ARTN 063509

## Coherence and decay of higher energy levels of a superconducting transmon qubit.

Physical review letters **114** (2015) 010501-

We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive π pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20 μs for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.

## Witnessing the formation and relaxation of dressed quasi-particles in a strongly correlated electron system.

Nature communications **5** (2014) 5112-

The non-equilibrium approach to correlated electron systems is often based on the paradigm that different degrees of freedom interact on different timescales. In this context, photo-excitation is treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast dynamics of quasi-particles in an archetypal strongly correlated charge-transfer insulator (La2CuO(4+δ)), we show that the interaction between electrons and bosons manifests itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard-Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles, which are suddenly dressed by an ultrafast reaction of the bosonic field.

## Pulsed homodyne Gaussian quantum tomography with low detection efficiency

NEW JOURNAL OF PHYSICS **16** (2014)

## Speed limit of the insulator-metal transition in magnetite.

Nature materials **12** (2013) 882-886

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

## Applied physics. Storing quantum information in Schrödinger's cats.

Science **342** (2013) 568-569