Seminar

Quantum states of massive objects have fascinated since the inception of quantum mechanics [1]. Nowadays molecules of thousands of atoms and nanomechanical resonators weighing picograms can be brought into quantum states [2]. This capability enables tests of the validity of quantum mechanics and provides new avenues for quantum technologies. To explore even more massive quantum systems requires exceptional isolation of the system from the environment and precise control over its quantum state. I will present first steps in the development of a new experimental platform that may allow quantum control over the motion of objects with masses larger than 10^13 atomic mass units. This platform is based on magnetically levitating a superconducting microparticle in cryogenic vacuum [3].

[1] Erwin Schrödinger, Naturwissenschaften 23, 807–812 (1935)
[2] Markus Arndt and Klaus Hornberger, Nature Physics 10, 271 (2014)
[3] Martí Gutierrez Latorre, Gerard Higgins, Achintya Paradkar, Thilo Bauch, Witlef Wieczorek, Phys. Rev. Applied 19, 054047 (2023)

We have been working on a variety of tests regarding on the one hand fundamental questions in quantum physics such as hypothetical higher-order interference or features and characteristics of multi-particle interference and, on the other hand on quantum communication applications like quantum range-finding. For either type of use one needs readily available, reliable and high-brightness sources of single photons and (entangled) photon pairs. While parametric down-conversion sources are simple and cost-effective, I will show how multi-pair emission seriously limits the achievable performance. We have thus embarked on perfecting single semiconductor quantum dot sources, especially regarding coherent optical excitation for optimal output. Our results indicate that quantum dots are getting very close to being the optimal sources for many applications.

Quantum sensors offer very promising applications in a variety of tasks.  In particular, continuously monitored quantum systems are especially interesting due to the recent advances in experimental techniques, allowing data to be captured and processed in "real time".  This creates a challenge for theoretical analysis, as often analytic solutions to open system dynamics are not available and it is necessary to resort to slow, computationally demanding numerical simulations.  Here, I will present work on how we can circumvent such issues through algorithmic sampling approaches, in particular "Approximate Bayesian Computation" (ABC).  To demonstrate the performance of these methods, I will consider as a sensor a nonlinear optomechanical system, monitored through continuous photon counting, where numerical solutions are expensive to obtain. Instead, I will show that ABC is capable of performing the inference significantly faster, with only a small sacrifice in accuracy, thus paving the way towards real implementations of continuous quantum sensing in complex systems.

Light can be described using different tools, and in particular through the ones linked to its wave behaviour, such as its spatial and temporal coherence properties. The knowledge of these coherence properties provides many information : about the light source itself, such as its angular intensity profile if one measures the spatial coherence, but also on the underlying light matter interaction processes when, for example, one measures the temporal coherence of light emitted or scattered by a medium. Thanks to the work of Glauber, it is well known that to have a full description of the temporal coherence properties, one has to know the correlation functions at all orders. In this presentation, I will focus on the first order correlation function, corresponding to electric field correlations, and second order correlation function or intensity correlations. In addition, for chaotic light sources, a relation exists between field and intensity temporal correlations, this relation being known as the Siegert relation [1,2]. I will show how this relation can be verified and used in two different domains: for light scattered by atoms that can be considered as quantum emitters, and in astronomy.

I will first discuss the Siegert relation for the light scattered by cold atoms. To validate the Siegert relation, one needs to verify different assumptions. One of this assumption is that the emitted or scattered phases should be random and uncorrelated. While the process of phase randomization is obvious in stars with thermal radiation, it is a bit more complex for light scattered by quantum scatterers which depends on the scattering regime. We will show how we can probe the transition from the classical regime, where the loss of coherence is due the atomic thermal motion [3], to the quantum regime dominated by spontaneous emission [4, 5].

Finally, I will discuss the Siegert relation in astronomy, for light coming from stars. We can show that this relation can be used to extract astrophysical information, such as the coherence time due to emission lines. Combined to spatial interferometry, this can be used to determine the angular diameter of the source. We will show that it is especially interesting to perform such measurements on emission lines, allowing characterizing the extended atmosphere of the star, but also giving access to other fundamental parameters such as its distance [3], giving the opportunity to propose a new method for astrophysical distance calibration.

[1] D. Ferreira et al., "Connecting field and intensity correlations: The Siegert relation and how to test it", AJP 88, 831 (2020) [2] P. Lassègues et al., "Field and intensity correlations: the Siegert relation from stars to quantum emitters", EPJD 76, 246 (2022)

Maxwell's demon principle of extracting valuable resources through measuring fluctuations in the system already stimulated modern quantum physics. In contrast to classical physics, a free coupling to a probe and its free measurement fundamentally shape the system state. This becomes a new dimension of the Maxwell demon effect, as in addition to the gained information, the back action on the system can be exploited and essential for further applications. We investigate quantum bosonic Maxwell's demon nonlinearly coupled to a two-level system to shift its paradigm to quantum nonlinear physics. The deterministic multiple subtractions of single energy quantum by an energetically conservative Jaynes-Cummings interaction leads to an out-of- equilibrium state. Although still super-Poissonian, it can resonantly excite another two-level system better than any thermal state. Making use of an already available and energetically conserved nonlinear Jaynes-Cummings to absorb multiple bosons, nonlinear subtractions can exclusively suppressed the tails of bosonic noise to reach an out-of-equilibrium state whose statistics and mean-to-noise ratio are close to that of Possonian distribution. It is, to our knowledge, the first demonstration of using two- quanta process to desirably shape the statistics of a bosonic mode close to that of Poissonian without any involvement of classical drive and nonlinear bosonic saturation as in the case of an ideal laser.

The superposition principle is one of the most fundamental principles of quantum mechanics. According to the Schrödinger equation, a physical system can be in any linear combination of its possible states. Although the validity of this principle is routinely confirmed for microscopic systems, it remains unclear why macroscopic objects are not observed to be in superpositions of distinguishable states. Our experiments demonstrate the preparation of a mechanical resonator with an effective mass of 16 micrograms in nonclassical states of motion, such as Fock states and Schrödinger cat states. Furthermore, we investigate the decoherence dynamics of these states by observing the disappearance of Wigner negativities, which allows us to derive constraints on possible modifications to the Schrödinger equation. Our results have potential applications in continuous variable quantum information processing, quantum sensing, and in the fundamental investigation of quantum mechanics on massive systems.

References

Björn Schrinski, Yu Yang, Uwe von Lüpke, Marius Bild, Yiwen Chu, Klaus Hornberger, Stefan Nimmrichter, and Matteo Fadel, Phys. Rev. Lett. 130, 133604 (2023).

Marius Bild, Matteo Fadel, Yu Yang, Uwe von Lüpke, Phillip Martin, Alessandro Bruno, and Yiwen Chu, Science 380, 274 (2023).

One of the central tasks in AMO physics is to specify how the atomic nuclei in molecules move, for example, upon a vertical excitation by an external trigger. A standard approach to get the information about the dynamics of such motion is to do pump-probe experiments with light pulses which have duration shorter than its timescale (femtochemistry). In my talk, I will present a completely different approach to obtaining dynamical information – resonant electron scattering. The trick is to create a short-lived anion state with the electron autodetachment lifetime in the order of femto- to picoseconds. Such an anion state – also called a resonance – has a total energy in continuum and can decay via several competing processes, such as electron loss or dissociation. By experimentally monitoring efficiency and kinematics of these decay processes, we are able to ‘track’ the motion of nuclei on the resonant potential energy surface. In the first part of my talk I will present several recent examples of probing such electron-induced dynamics. In the second part, I will present practical applications of this approach, such as identification of suitable gases for the use in high-voltage insulation or the improvement of the precursor design for focused-beam nanofabrication.

Understanding and controlling microscopic quantum devices represents a major milestone. Their precision is related to the fluctuations of their measurable output, an aspect that becomes preponderant at the nanoscale. Achieving a regime where the machine operates at a given reliability/precision inevitably comes at a cost in terms of thermodynamic resources, such as dissipated heat or excess work, thus massively impacting the machines’ performances. Thermodynamic uncertainty relations (TURs) have represented a landmark first step in understanding this balance, as they express a trade-off between precision, defined as the noise-to-signal ratio of a generic current, and the amount of associated entropy production. These results have deep consequences for quantum thermal machines, imposing an upper bound for their efficiency in terms of the power yield and its fluctuations. Such engines can be divided into two classes: steady-state heat engines and periodically driven heat engines. In this talk I will present and discuss the derivation of genuinely quantum corrections to TURs in both cases, which were obtained by combining techniques from quantum information theory and thermodynamics of geometry. Finally, I will report on an experimental measurement of such quantum correction in a trapped-ion experiment.

Quantum key distribution (QKD) is a promising technology to secure communication in the post-quantum-computing era. A particular interesting approach to implement QKD is continuous-variable QKD making use of the quadrature amplitudes of the electromagnetic field. Continuous-variables allow to heavily make use of the digital domain by exploiting digital signal processing which in turn allows to minimize the hardware footprint. In this talk, I will introduce the concept of digital continuous-variable quantum key distribution and discuss some experiments demonstrating the power of this approach. I will cover both device dependent and measurement-device independent systems.

The interaction of light with a single atom has been explored in great detail, with optical cavities allowing one to select which and how each optical mode couples to the atom. Such setups have been used to investigate fundamental effects such as vacuum Rabi splitting to measure the light-atom coupling, the antibunching of photons, or Rabi oscillations as the atom periodically exchanges energy with the light fields.
The difficulty in venturing into the many-photon case stems from the interference as the light scatters over many atoms. The light mediates a long-range dipole-dipole interaction between the particles, which stimulates a cooperative emission, as illustrated by the superradiance effect. On the other hand, Anderson localization which arises from the atomic disorder rather prevents this coupling, blocking the propagation of light. The competition between local (localized) and global (super/subradiance) effects leads to a rich physics, from the single-photon to the many-photon regime. In this presentation, I will discuss the interaction of light with a large cloud of atoms, from the single-excitation regime where Anderson localization for light turns to be much richer than for scalar waves (acoustic, matter waves), to the several-excitation regime, where memory states can be generated through dipole-dipole interactions.

We explore collective effects caused by induced dipole-dipole interactions to generate quantum-correlated states in ensembles of neutral atoms. While treating the atoms as two-level systems in subwavelength arrays, we show how to achieve a photon blockade regime by targeting a single-excitation superradiant state.  In a different scenario of a freely decaying system, we further show how cooperative spontaneous emission leads the system to long-lived entangled states at late times. These subradiant modes are characterized by entanglement between all particles, independently of their geometrical configuration. Even though there is no threshold on the interaction strength necessary to entangle all particles, stronger interactions lead to longer-lived entanglement. Finally, we also go beyond the two-level approximation and compute dipolar frequency shifts accounting for the intrinsic atomic multilevel structure in standard Ramsey spectroscopy. When interrogating the transitions featuring the smallest Clebsch-Gordan coefficients, we find that a simplified two-level treatment becomes inappropriate, even in the presence of large Zeeman shifts. For these cases, we observe a net suppression of dipolar frequency shifts and the emergence of dominant non-classical effects for experimentally relevant parameters. 

In this talk I will review some recent theoretical and experimental results on high-fidelity quantum control of ion qubits by composite pulses: sequences of pulses with well-defined relative phases used as control parameters to shape the propagator in a desired manner. These include the implementation of broadband, narrowband and passpand profiles with single and multiple qubits. Applications range from qubit gates which are robust to experimental errors to precise localization and efficient elimination of cross talk.

Cavity optomechanics is a wonderful platform for fundamental studies of quantum mechanics, one aspect being the steering and control of macroscopic objects at the quantum level. We investigate this using two different experimental platforms: optomechanical coupling to bulk acoustic modes and feedback cooling of nanomechanical membrane oscillators. In this talk, I will report on recent preliminary results indicating strong coupling of optical and acoustic modes and implementation of all-optical coherent feedback cooling.
The ability to interact strongly and coherently is also very important in the context of public outreach and community building. In the last part of the talk, I will present some of our recent initiatives on creative communication of science to the public, ongoing efforts on forming a strong national quantum community, and the state of play for a Danish strategy on quantum technology.

Control over the quantum states of a massive oscillator is important for several technological applications and to test the fundamental limits of quantum mechanics. Hybrid electromechanical systems using superconducting qubits, based on electric charge mediated coupling, have been quite successful in this regard. In this talk, I shall introduce a hybrid device, consisting of a superconducting transmon qubit and a mechanical resonator coupled using the magnetic flux. Such coupling stems from the quantum-interference of the superconducting phase across the tunnel junctions. Consequently, we detect thermomechanical motion using drive corresponding to average occupancy of less than one photon. In addition, the large coupling between qubit and mechanical resonator is manifested in the observation of the Landau–Zener–Stückelberg effect. I will further mention the prospects of such a device in the dispersive limit.

We show that it is possible to ascertain the presence of a microwave pulse resonant with the second transition of a superconducting transmon circuit, while at the same time avoiding to excite the device onto the third level. In contrast to standard interaction-free measurement setups, where the dynamics involves a series of projection operations, our protocol employs a fully coherent evolution, which results, surprisingly, in a higher efficiency. Experimentally, this is done by using a series of Ramsey microwave pulses coupled into the first transition and monitoring the ground-state population.

It is an ongoing challenge to create more complex entangled states, for example, composed of macroscopic objects, or of many quantum modes. I will present experimental results from creating an entangled state of motion of a membrane, and of the precession of a collective spin [1] conducted in Copenhagen. I will also introduce the setup from the University of Warsaw, where we created Bell pairs of photons using a quantum memory in 500 modes in parallel [2].

[1] R. A. Thomas et al., Nature Physics 17, 228 (2021).

[2] M. Lipka et al., Communications Physics 4, 46 (2021).

Single-photon detection of Raman scattered photons can be a useful tool for observing nonclassical features of both radiation and motion in several different implementations of cavity optomechanics. In this talk, I will mainly discuss recent theoretical work [1] on how to take advantage of this tool with continuously driven systems in the standard regime of linearized optomechanical interactions. We identify features in the sideband photon statistics which can be traced back to the quantum nature of a mechanical mode. Furthermore, we derive two inequalities for the sideband photon statistics that should be valid in any classical model of the system. These inequalities are shown to be violated for small average phonon occupation numbers. The proposed setup constitutes a steady-state source of nonclassical radiation. If time permits, I will also briefly discuss ideas for future projects along these lines. 

[1] K. Børkje, F. Massel, J. G. E. Harris, Phys. Rev. A 104, 063507 (2021).

The negatively charged nitrogen-vacancy (NV−) center in diamond has excellent spin properties with a long lifetime and coherence. In combination with the robustness of diamond, that set of properties enables a broad range of innovative applications, including nano-meter scale nuclear magnetic resonance, sensing of radical-pair reactions, or recording the magnetic field component induced by ionic charge in living biological tissue.

In this talk, I will address two advances recently made in our group. I will report on (a) our experiments using NV centers to recover biomagnetic signals from living tissue in vitro and (b) our recent efforts on controlling and utilizing the NV charge state for nanometer scale sensing tasks near the diamond surface.
(a) Using electrical and optogenetic stimulation, we are able to trigger and record compound action potentials from muscle tissue [1] and from the brain (corpus callosum) of mice, with high stability over the course of many hours. I will show that our diamond sensor can recover these signals in an ordinary laboratory environment, without the need for extensive shielding against background magnetic noise.
(b) From the detailed characterization of more than 30 single NV centers implanted ~5nm below the diamond surface, we observe a strong variability in the initialization probability into the negative charge state NV−. After coating the diamond with deuterated glycerol, we observe a consistent increase in charge initialization in NV-. We furthermore observe that glycerol reduces the ionization of NV−, indicating the role and importance of the local and near-surface charge environment for the stability of the NV charge state. Finally, I will address our efforts on mapping the NV- spin state to the NV charge state, and illustrate that in the context of a nanometer scale sensing task this approach has strong potential for improving the readout noise figure as compared to the conventional readout via fluorescence detection.
[1] Webb et al., Scientific Reports 11, 2412 (2021).

I will give an overview of a body of work on CV quantum information conducted in the LIP6 Paris group, largely found in the thesis of my student Ulysse Chabaud. We will introduce the Stellar representation which ranks non-Gaussian states, and methods for certifying this non-Gaussianity with heterodyne detection. We will further discuss how these techniques can be used to verify the quantum advantage in families of sampling experiments, including boson sampling. 

Remarkable advances in qubit hardware have enabled landmark experiments demonstrating small-scale quantum simulations, quantum computational tasks, and error-correction protocols. Nonetheless, achieving scalable, fault-tolerant quantum error correction (FTQEC) necessary for building useful quantum technology remains a challenging task. Firstly, it is still very hard to realize scalable hardware which operates below the maximum noise-strength that the error correction codes can tolerate, called the threshold. Moreover, with the noisy hardware available today or in the near-future, the resource overhead for FTQEC is dauntingly large. In fact, the overhead can completely overwhelm the advantage of quantum algorithms over classical ones for many practical problems. While developing low-noise quantum hardware is important to ease the requirements for FTQEC, in this talk I will focus on a complementary strategy which is based on the observation that some type of errors are less contagious and easier to correct than others. I will show how the detailed noise properties of the underlying quantum hardware can be leveraged to design high-threshold and low-overhead protocols for FTQEC. I will also discuss the opportunities for practical applications in different hardware platforms, with specific focus on superconducting circuits.

Making two photons interact efficiently is one of the dreams of nowadays quantum opticians. As carrier of information, photons can travel long distances and are a promising platform for complex quantum optical circuitry. Indeed, optical elements and solid-state emitters can be integrated on chip, for example, to deterministically generate and route single photons. Recent progress on embedded quantum emitters also enabled to achieve single-photon-level nonlinearities [1], which demand that the emitters be both efficiently coupled to photonic modes and highly coherent. The latter requirement is challenging in the solid state, and in particular in nanophotonic systems, where decoherence processes are typically enhanced by the presence of nearby interfaces. The first observation of near-lifetime-limited transitions of quantum dots embedded in nanophotonic waveguides enabled highly coherent light-matter interactions [2]. Record extinction in the transmission of light through a waveguide by a single quantum emitter was achieved and reached over 80% in photonic crystal waveguides [3]. The confirmed high coupling efficiency and coherence of our system allowed us to probe the nonlinearity of the light-matter interactions not only at the single- but also at the two-photon level, by implementing loss-robust scattering tomography protocol [3]. We could also finally experimentally demonstrate quantum nonlinear interaction between two single-photon pulses [4].

Such progress in the emerging domain of Waveguide QED [5] pave the way towards deterministic and coherent single photon nonlinear optics and to the realization of photon sorting protocols, efficient Bell measurements or also deterministic controlled-Z gates [6]. They also promise the observation of physical phenomena that have been proposed and analysed theoretically, including photonic bound states, the generation of Schrödinger cat states, and stimulated emission in the most fundamental setting of one photon stimulating one excited emitter.

[1] A. Javadi et al., Nat. Commun. 6, 8655 (2015) 

[2] H. Thyrrestrup et al., Nano Lett. 18, 1801–1806 (2018)

[3] H. Le Jeannic et al., Phys. Rev. Lett. 126, 023603 (2021)

[4] H. Le Jeannic et al., arXiv:2112.06820 (2021)

[5] D. E. Chang et al., Rev. Mod. Phys. 90, 031002 (2018)

[6] P. Lodahl, Quantum Sci. Technol. 3, 013001 (2018)

Optomechanics deals with the interaction between a laser beam and a mechanical resonator: mechanical motion changes the path followed by light, while radiation pressure can drive the mechanical resonator into motion. Applications include quantum limits in displacement sensing (such as gravitational-wave detection) and radiation-pressure cooling of macroscopic mechanical resonators down to the quantum ground state. It now takes advantage of mechanical resonators with low mass (down to the fg range) and high mechanical quality factors, inserted in very sensitive optical interferometers based on high-finesse optical cavities. I will discuss our experiments on these 2 research fronts, with both the 3-km Advanced Virgo interferometer and µg-scale resonators.

Doubly-clamped pre-stressed silicon nitride string resonators excel as high Q nanomechanical systems enabling room temperature quality factors of several 100,000 in the 10 MHz eigenfrequency range. Dielectric transduction ideally complements the silicon nitride strings, providing an all-electrical control scheme while retaining the large mechanical quality factor [1]. It is mediated by an inhomogeneous electric field created between adjacent electrodes. The resulting gradient field provides an integrated platform for actuation, displacement detection, frequency tuning as well as strong mode coupling.
Dielectrically controlled silicon nitride strings are an ideal testbed to explore a variety of dynamical phenomena ranging from multimode coupling to coherent control. The focus of this presentation will be on the nonlinear dynamics of a driven high Q string. For relatively weak driving, emergent satellite peak reminiscent of thermomechanical squeezing are understood in the framework of the cubic nonlinearity of the Duffing model [2]. For stronger driving, an abnormal response heralds dynamics beyond the Duffing model [3].
     
[1]  Q. P. Unterreithmeier et al., Universal transduction scheme for nanomechanical systems based on dielectric forces, Nature 458, 1001 (2009).
[2]  J. S. Huber, G. Rastelli, M. J. Seitner, J. Kölbl, W. Belzig, M. I. Dykman, and E. M. Weig, Spectral Evidence of Squeezing of a Weakly Damped Driven Nanomechanical Mode, Phys. Rev. X 10, 021066 (2020).
[3]  J. S. Ochs, G. Rastelli, M. J. Seitner, M. I. Dykman, and E. M. Weig, Resonant nonlinear response of a nanomechanical system with broken symmetry, Phys. Rev. B 104, 155434 (2021).

Quantum non-Gaussianity and in particular Wigner negativity has long been recognised as a genuine quantum feature from a fundamental viewpoint. From a resource-theoretic viewpoint, a framework has been derived grounded on Gaussian protocols routinely available within current technologies. This framework finds immediate application in continuous-variable quantum computation, where the ability to implement non-Gaussian operations is crucial to obtain universal control. In this context, I will illustrate schemes to generate quantum non-Gaussian states—in particular, over mechanical oscillators and super-conducting circuits—and show the latter can be inter-converted by using resource-less operations alone. Despite their genuine quantum character, I will also show that in some circumstances these Wigner-negative resources can still be simulated efficiently with classical devices. This observation naturally leads to the concept of excess Wigner negativity, which in turn finds a useful application in quantifying magic for qubit-based quantum computation via bosonic codes.

In measurement-based quantum computing, gates and circuits are carried out by measuring in variable bases on a large-scale cluster state. With relatively simple optical means, it is possible to deterministically generate a continuous-variable cluster state consisting of thousands of entangled temporal modes and with measurements straightforwardly carried out by highly efficient homodyne detectors. This has intriguing perspectives for scalable photonic quantum computers. I will present how we generated such a state and implemented a universal Gaussian gate set on it, as well as our scheme for universal, fault-tolerant quantum computing on the platform when combined with GKP states. Finally, I will briefly present other recent work with entangled and non-Gaussian states of light.

Precision spectroscopy has been a driving force for the development of our physical understanding. In particular laser cooling and manipulation improved the achievable precision. However, only few atomic and molecular species offer suitable transitions for laser cooling. This restriction can be overcome in trapped ion systems through quantum logic spectroscopy. Coherent laser manipulation, originally developed in the context of quantum information processing, allows to combine the special spectroscopic properties of one ion species (spectroscopy ion) with the excellent control over another species (logic ion).
In my talk, I will introduce the concept of quantum logic spectroscopy and present the first implementation of a quantum logic assisted scheme for reading out the internal state of a molecular ion. In this scheme, an atomic Mg-ion is used to detect a state dependent force that acts on the molecular MgH-ion. Furthermore, a quantum-enhanced force sensing protocol is demonstrated, which can be applied to the previously described measurement, but has further applications in the general field of quantum metrology.

The prospect of using the quantum nature of light for long distance quantum communication keeps spurring the search and investigation of suitable sources of entangled photons. Semiconductor quantum dots (QDs), also dubbed “artificial atoms”, are arguably one of the most attractive, as they can generate pairs of polarization-entangled photons with high efficiency and with near-unity degree of entanglement. Despite recent advances, however, the exploitation of photons from QDs in advanced quantum communication protocols remains a major open challenge.

In this talk, I will discuss how photons generated by a GaAs quantum dot [1] can be used to implement quantum teleportation [2, 3] and entanglement swapping [4] protocols with fidelities above the classical limit. Moreover, I will present our first steps towards the construction of a quantum-dot-based quantum network for secure communication within the campus of Sapienza University of Rome [5]. A discussion on future challenges and perspectives [6, 7] will conclude the talk. 

[1] D. Huber, et al., Phys. Rev. Lett. 121, 033902 (2018).

[2] M. Reindl et al., Science Adv. 4, eaau1255 (2018).

[3] F. Basso Basset et al., npj Quantum Inf. 7, 7 (2021). 

[4] F. Basso Basset et al., Phys. Rev. Lett. 123,160501 (2019).

[5] F. Basso Basset et al., Science Adv. 7, eabe6379 (2021).

[6] M. Reindl et al., Nano Letters 17, 4090 (2017).

[7] C. Schimpf et al., Appl. Phys. Lett. 118, 100502 (2021).

The membrane-in-the-middle set up is a successful scheme for performing cavity optomechanics, where one can manipulate the quantum state of nano-mechanical modes of a membrane via the optical cavity field and vice versa. Strong coupling and new physics are possible when two (or more) membranes are placed in the cavity. Cooperative effects occur and one can have enhancement of single-photon coupling or novel nonlinear dynamical effects such as synchronization. I will review the recent results in our group and discuss future research directions.

By adapting the tools of circuit quantum electrodynamics (cQED), the field of circuit quantum acoustodynamics (cQAD) aims to further our ability to create, control, and measure the quantum states of mechanical motion. Since mechanical resonators have drastically different properties from their electromagnetic counterparts, they could potentially be used to make new circuit elements for storing, processing, and transducing quantum information. I will present a summary of the progress in realizing cQAD systems based on bulk acoustic wave resonators, including our recent work on improving the properties of these devices in order to access a greater range of protocols for quantum control of mechanical motion.

The concept of dissipation engineering has enriched the methods available for state preparation, dissipative quantum computing and quantum information processing. Combining such engineered dissipative processes with coherent dynamics allows for new effects to emerge. For example, we found that any factorisable (coherent) Hamiltonian interaction can be rendered nonreciprocal if balanced with the corresponding dissipative interaction. This powerful concept can be exploited to engineer nonreciprocal devices for quantum information processing, computation and communication protocols, e.g., to achieve control over the direction of propagation of photonic signals. In this talk I will introduce the basic concept and show that the dissipative process by itself can yield a purely unitary evolution on one subsystem.

Continuous-Variable quantum computation is emerging as a promising alternative approach to quantum computation with respect to the use of two-level systems. In this approach, typical observables have a continuous spectrum, such as for instance the real and imaginary quadratures of the quantised electromagnetic field. In this context, it is yet to be fully unveiled which processes—in terms of state preparation, evolution, and measurement—are classically efficiently simulatable, and which processes are instead resourceful, i.e., they have the potential to offer quantum speed-up for computation. On the one hand, I will present some of our recent results addressing this question for specific families of quantum circuits involving bosonic codes. On the other hand, some quantum states have been known for decades to be resourceful, i.e., to promote a set of classically efficiently simulatable operations and measurement to universal quantum computation, such as the cubic phase state. So far, efforts for generating these quantum states have been undertaken in quantum optics, however it has not yet been possible to generate these states. I will present two proposals for achieving the generation of the cubic phase state with microwave technology and argue that their experimental implementation with that technology is possible.

Squeezed light is a major resource for quantum interferometric sensing below the shot-noise limit. However, standard squeezed interferometry methods suffer from two severe limitations: First, the detection bandwidth of squeezing-enhanced interferometry is inherently narrow because of the slow response (MHz to GHz) of photodetectors, which critically prevents efficient utilization of the optical bandwidth (tens of THz and more) for quantum applications; and second, current quantum sensing requires near ideal photo-detectors with unity efficiency, prohibiting real-life applications, where ideal detection is not available. To overcome these limitations , a paradigm shift is required in terms of broadband quantum sources, detection schemes, and interferometric design, which will enable an orders-of-magnitude enhancement in the sensing throughput.

I will present a set of new methods for sub-shot-noise sensing, based on nonlinear interferometry, which overcome these limitations. By placing the phase object in question between two parametric amplifiers in series, the first amplifier generates broadband squeezed light to interrogate the object and the second amplifier acts as an ideal broadband quantum detector to measure the object’s response. This technique is robust to detection inefficiency and provides an unprecedented optical bandwidth for quantum measurement, exceeding the possibilities of photodetectors by several orders of magnitude.

I will discuss in detail two specific examples of ultra broadband parametric-homodyne measurement [1] and of squeezing-enhanced Raman spectroscopy [2].

[1] Y. Shaked, Y. Michael, R. Vered, L. Bello, M. Rosenbluh and A. Pe’er, “Lifting the Bandwidth Limit of Optical Homodyne Measurement”, Nature Communications 9, 609 (2018).

[2] Y. Michael, L. Bello, M. Rosenbluh, and A. Pe'er, “Squeezing-enhanced Raman Spectroscopy”, npj Quantum Information 5, 81 (2019).

Wigner functions that take negative values are considered to be a crucial resource for achieving a quantum computational advantage with continuous variables. In quantum optics, the subtraction (or addition) of a photon from a squeezed state is a common method to generate such Wigner negativity [1]. But this process has to be made mode-dependent with a multimode environment to prove useful for quantum information. For instance, it was shown that photon subtraction in one mode induces non-Gaussian properties in the modes that are correlated to it [2].

Here we first study theoretically what are the conditions under which photon subtraction in one mode creates Wigner negativity in a correlated mode [3]. Then, we generate a multimode Gaussian state from time/frequency modes of an optical frequency comb. Non-Gaussian quantum states, and Wigner negativity, are demonstrated removing a single photon in a mode-selective manner from the multimode environment [4]. We explore the interplay between non-Gaussianity and quantum entanglement and demonstrate large-scale non-Gaussianity with great flexibility along with an ensured compatibility with quantum information protocols. 

[1] J. Wenger, et al. Phys. Rev. Lett. 92, 153601 (2004); V. Parigi, et al. Science 317, 1890 (2007).

[2] M. Walschaers, et al. Phys Rev Lett 121, 220501 (2018).

[3] M. Walschaers, et al., PRX Quantum 1, 020305 (2020).

[4] Y.-S. Ra et al, Nature Physics 11, 1 (2019).