25. January 2021

Flaminia Giacomini (Perimeter Institute Canada)
Einstein's Equivalence principle for superpositions of gravitational fields

The Principle of Equivalence, stating that all laws of physics take their special-relativistic form in any local inertial frame, lies at the core of General Relativity. Because of its fundamental status, this principle could be a very powerful guide in formulating physical laws at regimes where both gravitational and quantum effects are relevant. However, its formulation implicitly presupposes that reference frames are abstracted from classical systems (rods and clocks) and that the spacetime background is well defined. Here, we we generalise the Einstein Equivalence Principle to quantum reference frames (QRFs) and to superpositions of spacetimes. We build a unitary transformation to the QRF of a quantum system in curved spacetime, and in a superposition thereof. In both cases, a QRF can be found such that the metric looks locally flat. Hence, one cannot distinguish, with a local measurement, if the spacetime is flat or curved, or in a superposition of such spacetimes. This transformation identifies a Quantum Local Inertial Frame. These results extend the Principle of Equivalence to QRFs in a superposition of gravitational fields. Verifying this principle may pave a fruitful path to establishing solid conceptual grounds for a future theory of quantum gravity.

14. December 2020

Otfried Guehne (Siegen University)
An Invitation to Quantum Steering

Quantum steering was originally introduced by Schrödinger in order to capture the essence of the EPR argument. In the modern formulation, steering is a type of quantum correlations which lies between entanglement and the violation of Bell inequalities.

In this talk, I will first give an introduction into the topic. Then, several results on quantum steering shall be discussed: First, I will explain the connection to joint measurability of generalized measurements. Second, an algorithmic approach to characterize the quantum states that can be used for steering will be introduced. With this, one can solve the problem of steerability for two-qubit states. Finally, it can be shown that the number of outcomes of a measurement is also relevant for being useful for steering.

30. November 2020

Klaus Jöns (Paderborn University)
Hybrid quantum photonic devices

I will present our recent results on hybrid quantum photonic devices as building blocks for integrated photonic quantum technologies, as shown schematically in figure 1. The hybrid approach allows us to harvest synergy effects between previously independent research fields, providing new functionalities and insights in the fascinating world of photonic quantum sciences.

I will first introduce solid-state quantum emitters, their unique properties, as well as some of their advantages and remaining challenges. Afterwards I will discuss our approach to integrate quantum light sources in complex on-chip quantum circuits. Currently, the most promising on-demand single photon sources are based on III/V semiconductor quantum dots [1]. However, complex photonic circuitry is mainly achieved in silicon photonics due to the tremendous technological challenges in circuit fabrication. We take the best of both worlds by developing a hybrid nanofabrication technique [2], allowing to integrate III/V semiconductor nanowire quantum dots [3] into silicon-based photonics. I will present on-chip generation, spectral filtering, and routing of single-photons from selected single and multiple quantum emitters all deterministically integrated in a CMOS compatible silicon nitride photonic circuit [4]. Furthermore, I will introduce a heterogeneous integration of micro-electromechanical systems with superconducting single photon detectors as an alternative active circuit element for quantum photonic integrated circuits [5].

In the second part I will discuss our efforts to fabricate large-scale quantum photonic integrated circuits using 2D materials as the on-chip quantum light source. 2D materials such as WSe2 have gotten substantial attention after the discovery of single-photon emission and are currently widely explored as novel quantum emitters. On the one hand, we take advantage of the strain-induced creation of these quantum emitters to couple single photons to photonic circuits [6]. On the other hand, a new technique using He-ion bombardment of MoS2 enables the deterministic creation of single photon sources with unprecedented position accuracy [7]. Our approaches eliminate the need for off-chip components, opening up new possibilities for largescale quantum photonic devices with different kinds of on-chip single- and entangled-photon sources.

References:
[1] L. Schweickert et al., Appl. Phys. Lett. 112, 093106 (2018).
[2] I. Esmaeil Zadeh et al., Nano Letters 16(4), 2289-2294 (2016).
[3] M. A. M. Versteegh et al., Nature Communications 5, 5298 (2014).
[4] A. W. Elshaari et al., Nature Communications 8, 379 (2017).
[5] S. Gyger et al., arXiv:2007.06429 (2020).
[6] C. Errando Herranz et al., arXiv:2002.07657 (2020).
[7] J. Klein et al., arXiv:2002.08819 (2020) and K. Barthelmi et al., Appl. Phys. Lett. 117, 070501 (2020).

23. November 2020

Vlatko Vedral (University of Oxford)
Local Quantum Physics

We describe the quantum interference of a single photon in the Mach-Zehnder interferometer using the Heisenberg picture. Our purpose is to show that the description is perfectly local just like in the case of the classical electromagnetic field, the only difference being that the electric and the magnetic fields are, in the quantum case, operators (quantum observables). We then consider a single-electron Mach-Zehnder interferometer and explain what the appropriate Heisenberg picture treatment is in this case. Interestingly, the parity superselection rule forces us to treat the electron differently to the photon. A model using only local quantum observables of different fermionic modes, such as the current operator, is nevertheless still possible. We show how to extend this local analysis to coupled fermionic and bosonic fields and how to treat the Aharonov-Bohm effect within the same local formalism of quantum electrodynamics as formulated in the Heisenberg picture.

9. November 2020

Markus Müller (IQOQI Vienna)
Black boxes in space and time: from quantum reconstructions to protocols

Why is all physics quantum, i.e. described by the counterintuitive formalism of complex state vectors and operators? How is quantum theory ultimately combined with spacetime physics? These questions are of fundamental importance in the foundations of physics, including quantum information theory and quantum gravity. In this talk, I describe a research program that aims at tackling aspects of both questions at once. I begin with results that derive quantum theory from simple information-theoretic principles, starting from a large class of probabilistic theories. Not only does this shed light on the first question above, but it also puts a remarkable insight center stage: that the structures of spacetime and of abstract quantum theory are surprisingly intertwined. Here, I explain how this relation can be analyzed in simple thought experiments, show that it can give us an exact characterization of the (2,2,2)-quantum Bell correlations, and argue that it holds the promise for new quantum information protocols and experimental tests of quantum theory.