Kai Müller

Photonic Quantum Engineering

Technical University of Munich

Walter Schottky Institut

Am Coulombwall 4

85748 Garching

Tel. +49 89 289 12772


Group Webpage

Entanglement is amazing, let’s make use of it.


Research focus: photonic quantum technologies - from fundamentals to applications

Since photons travel with the speed of light and experience low losses, they are excellent carriers for quantum information and enable a broad range of photonic quantum technologies. Our group explores light-matter interactions at the nanoscale to realize all key ingredients which are essential for the generation, manipulation and detection of quantum states of light. Examples for such quantum states are single photons or photons entangled with other photons or spin-qubits. In future applications, the individual building blocks will either be integrated in a single chip to realize fully-integrated quantum photonic circuits or form modular building blocks for distributed quantum technologies.

Examples for these building blocks are non-classical light sources, spin-photon interfaces, quantum memories and single-photon detectors. Since every quantum system has specific advantages and disadvantages we investigate a breath of systems including semiconductor quantum dots, color centers in diamond, two-dimensional transition metal dichalcogenides and rare-earth ions embedded in complex oxide crystals. Our research spans the full range from fundamentals to applications. This includes for example the investigation of novel quantum materials, development of quantum optical techniques, design and fabrication of nanophotonic structures and quantum engineering of building blocks and devices. Examples for targeted applications include quantum communication, distributed quantum networks and quantum simulation and metrology based on photons.

Selected publications

  • L. Hanschke, K. A. Fischer, S. Appel, D. Lukin, J. Wierzbowski, S. Sun, R. Trivedi, J. Vuckovic, J. J. Finley, K. Müller, “Quantum dot single photon sources with ultra-low multi-photon probability”, npj Quantum Information 4, 43 (2018).
  • K. A. Fischer, L. Hanschke, J. Wierzbowski, T. Simmet, C. Dory, J. J. Finley, J. Vuckovic, K. Müller, “Signatures of two-photon pulses from a quantum two-level system”, Nature Physics 13, 649–654 (2017).
  • K. Müller, K. A. Fischer, C. Dory, T. Sarmiento, K. G. Lagoudakis, A. Rundquist, Y. Kelaita, J. Vuckovic, “Self-homodyne-enabled generation of indistinguishable photons”, Optica 3, 931-936 (2016).
  • K. Müller, K. A. Fischer, A. Rundquist, C. Dory, K. G. Lagoudakis, T. Sarmiento, Y. A. Kelaita, V. Borish, J. Vuckovic, “Ultrafast Polariton-Phonon Dynamics of Strongly Coupled Quantum Dot-Nanocavity Systems”, Physical Review X 5, 031006 (2015).
  • K. Müller, A. Rundquist, K. A. Fischer, T. Sarmiento, K. G. Lagoudakis, Y. A. Kelaita, C. Sánchez Muñoz, E. del Valle, F. P. Laussy, J. Vuckovic, “Coherent Generation of Nonclassical Light on Chip via Detuned Photon Blockade”, Physical Review Letters 114, 233601 (2015).



Optomechanical wave mixing by a single quantum dot

M. Weiß, D. Wigger, M. Nägele, K. Müller, J.J. Finley, T. Kuhn, P. Machnikowski, H.J. Krenner

Optica 8 (3), 291-300 (2021).

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Wave mixing is an archetypical phenomenon in bosonic systems. In optomechanics, the bidirectional conversion between electromagnetic waves or photons at optical frequencies and elastic waves or phonons at radio frequencies is building on precisely this fundamental principle. Surface acoustic waves (SAWs) provide a versatile interconnect on a chip and thus enable the optomechanical control of remote systems. Here we report on the coherent nonlinear three-wave mixing between the coherent fields of two radio frequency SAWs and optical laser photons via the dipole transition of a single quantum dot exciton. In the resolved sideband regime, we demonstrate fundamental acoustic analogues of sum and difference frequency generation between the two SAWs and employ phase matching to deterministically enhance or suppress individual sidebands. This transfer between the acoustic and optical domains is described by theory that fully takes into account direct and virtual multiphonon processes. Finally, we show that the precision of the wave mixing is limited by the frequency accuracy of modern radio frequency electronics.

DOI: 10.1364/OPTICA.412201

Raman spectrum of Janus transition metal dichalcogenide monolayers WSSe and MoSSe

M.M. Petric, M. Kremser, M. Barbone, Y. Qin, Y. Sayyad, Y.X. Shen, S. Tongay, J.J. Finley, A.R. Botello-Mendez, K. Mueller

Physical Review B 103 (3), 035414 (2021).

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Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolved Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.

DOI: 10.1103/PhysRevB.103.035414

High-resolution spectroscopy of a quantum dot driven bichromatically by two strong coherent fields

C. Gustin, L. Hanschke, K. Boos, J.R.A. Müller, M. Kremser, J. J. Finley, S. Hughes, K. Müller

Physical Review Research 3, 13044 (2021).

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We present spectroscopic experiments and theory of a quantum dot driven bichromatically by two strong coherent lasers. In particular, we explore the regime where the drive strengths are substantial enough to merit a general nonperturbative analysis, resulting in a rich higher-order Floquet dressed-state energy structure. We show high-resolution spectroscopy measurements with a variety of laser detunings performed on a single InGaAs quantum dot, with the resulting features well explained with a time-dependent quantum master equation and Floquet analysis. Notably, driving the quantum dot resonance and one of the subsequent Mollow triplet sidepeaks, we observe the disappearance and subsequent reappearance of the central transition and transition resonant with detuned laser at high detuned-laser pump strengths and additional higher-order effects, e.g., emission triplets at higher harmonics and signatures of higher-order Floquet states. For a similar excitation condition but with an off-resonant primary laser, we observe similar spectral features but with an enhanced inherent spectral asymmetry.

DOI: 10.1103/PhysRevResearch.3.013044

Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures

D.B. Trivedi, G. Turgut, Y. Qin, M.Y. Sayyad, D. Hajra, M. Howell, L. Liu, S.J. Yang, N.H. Patoary, H. Li, M.M. Petric, M. Meyer, M. Kremser, M. Barbone, G. Soavi, A.V. Stier, K. Mueller, S.Z. Yang, I.S. Esqueda, H.L. Zhuang, J.J. Finley, S. Tongay

Advanced Materials 32 (50), 2006320 (2020).

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Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2. However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

DOI: 10.1002/adma.202006320

Origin of Antibunching in Resonance Fluorescence

L. Hanschke, L. Schweickert, J.C.L. Carreno, E. Scholl, K.D. Zeuner, T. Lettner, E.Z. Casalengua, M. Reindl, S.F.C. da Silva, R. Trotta, J.J. Finley, A. Rastelli, E. del Valle, F.P. Laussy, V. Zwiller, K. Muller, K.D. Jons

Physical Review Letters 125 (17), 170402 (2020).

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Resonance fluorescence has played a major role in quantum optics with predictions and later experimental confirmation of nonclassical features of its emitted light such as antibunching or squeezing. In the Rayleigh regime where most of the light originates from the scattering of photons with subnatural linewidth, antibunching would appear to coexist with sharp spectral lines. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. Using an epitaxial quantum dot for the two-level system, we independently confirm the single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our observation is explained by antibunching originating from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state. This prefigures schemes to achieve simultaneous subnatural linewidth and antibunched emission.

DOI: 10.1103/PhysRevLett.125.170402

Generation of Non-Classical Light Using Semiconductor Quantum Dots

R. Trivedi, K. A. Fischer, J. Vučković, K. Müller

Advanced Quantum Technologies 3, 1900007 (2020).

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Sources of non-classical light are of paramount importance for future applications in quantum science and technology such as quantum communication, quantum computation and simulation, quantum sensing, and quantum metrology. This Review is focused on the fundamentals and recent progress in the generation of single photons, entangled photon pairs, and photonic cluster states using semiconductor quantum dots. Specific fundamentals which are discussed are a detailed quantum description of light, properties of semiconductor quantum dots, and light–matter interactions. This includes a framework for the dynamic modeling of non-classical light generation and two-photon interference. Recent progress is discussed in the generation of non-classical light for off-chip applications as well as implementations for scalable on-chip integration.

DOI: 10.1002/qute.201900007

Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation

J. Klein, M. Lorke, M. Florian, F. Sigger, J. Wierzbowski, J. Cerne, K. Müller, T. Taniguchi, K. Watanabe, U. Wurstbauer, M. Kaniber, M. Knap, R. Schmidt, J. Finley, A. Holleitner.

Nature Communications 10, Article number: 2755 (2019).

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Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron–hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

DOI: 0.1038/s41467-019-10632-z

Resonance Fluorescence of GaAs Quantum Dots with Near-Unity Photon Indistinguishability

E. Scholl, L. Hanschke, L. Schweickert, K.D. Zeuner, M. Reindl, S.F.C. da Silva, T. Lettner, R. Trotta, J.J. Finley, K. Müller, A. Rastelli, V. Zwiller, K.D. Jons

Nano Letters 19 (4), 2404-2410 (2019).

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Photonic quantum technologies call for scalable quantum light sources that can be integrated, while providing the end user with single and entangled photons on demand. One promising candidate is strain free GaAs/A1GaAs quantum dots obtained by aluminum droplet etching. Such quantum dots exhibit ultra low multi-photon probability and an unprecedented degree of photon pair entanglement. However, different to commonly studied InGaAs/GaAs quantum dots obtained by the Stranski-Krastanow mode, photons with a near-unity indistinguishability from these quantum emitters have proven to be elusive so far. Here, we show on-demand generation of near-unity indistinguishable photons from these quantum emitters by exploring pulsed resonance fluorescence. Given the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots, we show single photon indistinguishability with a raw visibility of V-raw = (95.0(-6.1)(+5.0))%, without the need for Purcell enhancement. Our results represent a milestone in the advance of GaAs quantum dots by demonstrating the final missing property standing in the way of using these emitters as a key component in quantum communication applications, e.g., as quantum light sources for quantum repeater architectures.

DOI: 10.1021/acs.nanolett.8b05132

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