# Seminar

Regular talks by renowned experts on various topics in quantum physics and related disciplines. Presentations by distinguished external speakers and local experts take place on Tuesday at 11:30 in the seminar room 4.064 at the Department of Optics. All interested—including undergraduate students—are welcome to attend.

## Upcoming seminars

## Past seminars

### Experimental realization of coherent interaction-free detection with a superconducting circuit

Gheorghe-Sorin Paraoanu

Aalto University, Finland

24 May 2022, 11:30

Seminar room 4.064

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.

### Creating entangled states of atoms, photons, and optomechanics

Michał Parniak

University of Warsaw, Poland

17 May 2022, 11:30

Seminar room 4.064

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).

### Signatures of nonclassicality in optomechanical systems

Kjetil Børkje

University of South-Eastern Norway, Kongsberg, Norway

10 May 2022, 11:30

Online

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).

### Quantum sensing of bio-magnetic fields using defect centres in diamond

Alexander Huck

Technical University of Denmark, Lyngby, Denmark

3 May 2022, 11:30

Seminar room 4.064

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).

### Stellar representation for continuous variables and its applications

Damian Markham

Sorbonne University and CNRS, Paris, France

19 April 2022, 11:30

Online

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.

### Quantum Error Correction for Next-Generation Qubit Technologies

Shruti Puri

Yale University, USA

12 April 2022, 16:00

online

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.

### Photon-photon interactions induced by a single quantum dot in a photonic waveguide

Hanna Le Jeannic

University of Bordeaux, France

5 April 2022, 11:30

Seminar room 4.064

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 with Planck-mass resonators

Pierre-François Cohadon

Laboratoire Kastler Brossel, Ecole Normale Supérieure, Sorbonne Université, Collège de France, CNRS, Paris, France

7 December 2021, 13:30

Online

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.

### Nonlinear and noise-induced dynamics of high Q nanomechanical resonators

Eva Weig

Technical University of Munich, Germany

Tuesday, 30 November 2021, 14:30

Online

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).

### Generation, conversion, and simulation of quantum non-Gaussian resources

Alessandro Ferraro

Queen's University Belfast, UK

Tuesday, 9 November 2021, 11:30

Online

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.

### An optical platform for measurement-based quantum computing and other entangled adventures

Jonas Schou Neergaard-Nielsen

Technical University of Denmark, Lyngby, Denmark

Tuesday, 26 October 2021, 11:30

Seminar room 4.064

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.

### Motional quantum state engineering for quantum logic spectroscopy

Fabian Wolf

Physikalisch-Technische Bundesanstalt, Brunswick, Germany

Tuesday, 12 October 2021, 11:30

Seminar room 4.064

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.

### Towards quantum communication with entangled photons from quantum dots

Rinaldo Trotta

Department of Physics, Sapienza University of Rome, Italy

Thursday, 30 September 2021, 9:00

Seminar room 4.064

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).

### Two-membrane cavity optomechanics

David Vitali,

University of Camerino, Italy

Tuesday, 1 June 2021, 11:30

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.

### Circuit quantum acoustodynamics with bulk acoustic wave resonators

Yiwen Chu

Swiss Federal Institute of Technology in Zurich, Switzerland

Tuesday, 4 May 2021, 11:30

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.

### Interplay of dissipative and coherent processes in engineered quantum systems

Anja Metelmann

Free University Berlin, Germany

Tuesday, 27 April 2021, 11:30

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.

### Resources for continuous-variable quantum computation

Giulia Ferrini

Chalmers University of Technology, Gothenburg, Sweden

Tuesday, 13 April 2021, 11:30

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.

### Quantum sensing with unlimited optical bandwidth

Avi Pe'er

Bar-Ilan University, Ramat Gan, Israel

Tuesday, 6 April 2021, 11:30

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).

### Non-Gaussian quantum states of a multimode light field

Nicolas Treps

Kastler–Brossel Laboratory, Paris, France

Tuesday, 30 March 2021, 11:30

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).