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

Invited speakers for winter semester 2021

Pierre-François Cohadon, Sorbonne University, France

Alessandro Ferraro, Queen's University Belfast, UK

Jonas Schou Neergaard-Nielsen, Technical University of Denmark, Lyngby, Denmark

Gheorghe Sorin Paraoanu, Aalto University, Finland *

Rinaldo Trotta, Sapienza University of Rome, Italy

Eva Weig, Technical University of Munich, Germany

Fabian Wolf, Physikalisch-Technische Bundesanstalt, Brunswick, Germany

* to be confirmed

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).

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

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

Past seminars

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).