Quantum Emitters
| Staff members: | Niko Nikolay, Flavie Davidson-Marquis, Florian Böhm |
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Our research is distributed among a variety of quantum emitters, which have specific advantages for a variety of applications.
The main focus lies on the following quantum emitters with the respective topics:
1. h-BN
This system consists of single photon emitters that are hosted in a two-dimensional Van-der-Waals crystal, hexagonal Boron Nitride. It comes with a simple growth and transfer procedure and therefore stacking large layers of h-BN on photonic structures or in between other 2D materials has already been shown [1-3]. This opens a world of possibilities to create new functionalities with less experimental effort compared to some other systems.
The single photon emitters in this material show zero-phonon-lines that are spread over a wide range of wavelengths, i.e. 540 nm to 720 nm [4]. Their Debye-Waller factor is usually high, their emission is highly polarized, short living (~ 3 ns excited state lifetime) and dependent on the external magnetic field. Moreover, their emission is particularly bright. All these properties render these emitters interesting to the scientific community.
What drives the research specifically conducted in our labs on that topic is the yet unknown atomic arrangement and therefore the energy level diagram associated with these emitters. By the investigation of their fundamental properties, we aim to assemble each individual piece of the puzzle to gain full understanding of this novel system. Therefore, we measured the temperature dependent coherence of their emission which gives insight to the broadening effects and therefore the atomic surrounding and the symmetry of the defect [5]. By measuring the emitter's response to strong electrostatic fields, i.e. the Stark shift of its emission, we could measure its polarizability and its dipole moment which eventually determines the charge separation distance that the emitter is made of [6]. In our latest experiment on that topic, we could clarify a part of the underlying level structure. With quantum efficiency measurements we could show that non-radiative deexcitation paths exist. Also, the splitting ratio between radiative and non-radiative paths was revealed together with species dependent differences, which give room for interpretation [7].
In our laboratory we use a rare combination of a confocal scanning microscope coupled to an atomic force microscope. Some experiments mentioned previously were made possible by this ensemble, along with other experiments mentioned here in the other sections.
Figure 1.1: The atomic arrangement of hexagonal boron nitride (red) (blue) is shown on the left. On the right-hand side is the experimental apparatus used in our laboratories - a combination of light microscope and atomic force microscope (AFM).
[1] Noh, G., Choi, D., Kim, J. H., Im, D. G., Kim, Y. H., Seo, H., & Lee, J. (2018). "Stark tuning of single-photon emitters in hexagonal boron nitride". Nano letters, 18(8), 4710-4715.
[2] Proscia, N. V., Shotan, Z., Jayakumar, H., Reddy, P., Cohen, C., Dollar, M., ... & Menon, V. M. (2018). "Near-deterministic activation of room-temperature quantum emitters in hexagonal boron nitride". Optica, 5(9), 1128-1134.
[3] Li, C., Mendelson, N., Ritika, R., Chen, Y., Xu, Z. Q., Toth, M., & Aharonovich, I. (2021). "Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride". Nano Letters.
[4] Hayee, F., Yu, L., Zhang, J. L., Ciccarino, C. J., Nguyen, M., Marshall, A. F., ... & Dionne, J. A. (2020). "Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy". Nature materials, 19(5), 534-539.
[5] Sontheimer, B., Braun, M., Nikolay, N., Sadzak, N., Aharonovich, I., & Benson, O. (2017). "Photodynamics of quantum emitters in hexagonal boron nitride revealed by low-temperature spectroscopy". Physical Review B, 96(12), 121202.
[6] Nikolay, N., Mendelson, N., Sadzak, N., Böhm, F., Tran, T. T., Sontheimer, B., ... & Benson, O. (2019). "Very large and reversible Stark-shift tuning of single emitters in layered hexagonal boron nitride". Physical Review Applied, 11(4), 041001.
[7] Nikolay, N., Mendelson, N., Özelci, E., Sontheimer, B., Böhm, F., Kewes, G., ... & Benson, O. (2019). "Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride". Optica, 6(8), 1084-1088.
Contact: Niko Nikolay
2. Molecules
On-demand sources of non-classical light are a prerequisite to the development of photonic quantum technologies. To this end, several families of quantum emitters have been proposed and are currently being investigated (see, for example, the other sections of this page). Organic dyes are one of these promising candidates [1,2]. Paired with a suitable host matrix, they give rise to photostable sources of single photons. In particular, Dibenzoterrylene (DBT) in an Anthracene (Ac) host is a proven pair which provides a good insertion site of the guest (DBT) into the host (Ac) thus giving rise to a strong and narrow zero-phonon line (ZPL) at cryogenic temperatures. Furthermore, the system’s ZPL is robust against spectral diffusion processes and has a low intersystem crossing (ISC) yielding to a high fluorescence rate [3,4]. These properties tied with an emission wavelength in the near-infrared (785 nm) and a straightforward crystal growth makes these molecules particularly attractive for scalable photon-based quantum information processing and communication devices.
At room temperature, single molecules of DBT in ultrathin (20 nm) crystalline films of anthracene have been shown to reach count rates up to 106 photons per seconds [5]. Below liquid helium temperatures – when nothing else then the intrinsic lifetime broadening influences the spectral lineshape of the emitter – it is possible to spectrally resolve molecules individually. This arises from the mechanical stress experienced by each molecule in their immediate vicinity due to the host crystal, therefore producing a fingerprint-emission unique to each [4].
To increase the collection efficiency, non-classical light sources profit from coupling to photonic structures [6]. At room temperature, single DBT molecules have in fact been successfully integrated to architectures such as – amongst others – dielectric microstructures [7], direct laser written polymeric chips [8], plasmonic guides [9,10] and been casted on a solid immersion lenses [2,8]. One of our imminent goals is to extend this to cryogenic applications for all advantages previously mentioned and produce a source of Fourier limited photons on-chip as a hybrid device. The hybrid integration of single quantum emitters with photonic platforms is further discussed in the closing section of this page.
Figure 2.1: (a) Benzene rings forming anthracene (C14H10) and dibenzoterrylene (C38H20) molecules respectively. (b) Electronic structure of DBT in Ac. DBT has a three-level electronic structure. The upper vibrational mode S1 is excited by a 767 nm beam. At this stage, a rapid decay onto the vibrational ground state occurs. The excited state decays to the zero-phonon line (785 nm) after 2-4 ns or branches into one of the many vibrational modes of the ground states. Diagram (b) taken from [11].
[1] Schofield, R. C., Major, K. D., Grandi, S., et al., "Efficient excitation of dye molecules for single photon generation", J. Phys. Commun. 2 115027 (2018), DOI: 1088/2399-6528/aaf09a
[2] Trebbia, J.-B., Tamarat, P. and Lounis, B., "Indistinguishable near-infrared single photons from an individual organic molecule", Phys. Rev. A 82(6), 063803 (1-5) (2010), DOI: 10.1103/PhysRevA.82.063803
[3] Nicolet, A., Hofmann, C., Kol’chenko, M., et al., "Single Dibenzoterrylene Molecules in an Anthracene Crystal: Spectroscopy and Photophysics", ChemPhysChem, 8(8), 1215-1220 (2007), DOI: 10.1002/cphc.200700091
[4] Nicolet, A., P. Bordat, Hofmann, C. et al., "Single Dibenzoterrylene Molecules in an Anthracene Crystal: Main Insertion Sites", ChemPhysChem, 8, 1929-1936 (2007), DOI: 10.1002/cphc.200700340
[5] Toninelli, C., Early, K., Bremi, J. et al., "Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature", Optics Express 18(7) 6577 (2010), DOI: 10.1364/oe.18.006577
[6] Benson, O. "Assembly of hybrid photonic architectures from nanophotonic constituents", Nature 480 193–199 (2011), DOI : 10.1038/nature10610
[7] Lombardi, P., Ovvyan, A. P., Pazzagli, S. et al., "Photostable Molecules on Chip: Integrated Sources of Nonclassical Light", ACS Photonics, 5 (2018) DOI: 10.1021/acsphotonics.7b00521
[8] Colautti, M., Lombardi, P., Trapuzzano, M. et al., "A 3D Polymeric Platform for Photonic Quantum Technologies", Advanced Quantum Technologies 3(7) 2000004 (2020), DOI: 10.1002/qute.202000004
[9] Kewes, G., Schoengen, M., Neitzke, O. et al., "A realistic fabrication and design concept for quantum gates based on single emitters integrated in plasmonic-dielectric waveguide structures", Scientific Reports 6(1), 1-10 (2016), DOI: 10.1038/srep28877
[10] Kumar, S., Leißner, T., Boroviks, S. et al., "Efficient Coupling of Single Organic Molecules to Channel Plasmon Polaritons Supported by V-Grooves in Monocrystalline Gold", ACS Photonics 7(8) 2211-2218 (2020), DOI: 10.1021/acsphotonics.0c00738
[11] Neitzke, O. "On the Integration of Single Quantum Emitters in Solids and Photonic Nano-Structures", Dr. Rer. Nat dissertation, Humboldt University Berlin (2018)
Contact: Flavie Davidson-Marquis
3. Nitrogen-Vacancy Centers in Diamond
The nitrogen-vacancy (NV) center in diamond is a solid-state defect qubit which is formed by a substitutional nitrogen atom with an adjacent vacancy in the diamond lattice (see Figure 3.1a) and can be addressed individually. The negatively charged NV center forms multiple levels in the bandgap of diamond with a zero-phonon line emission about 637 nm (see Figure 3.1b). The electron and nuclear spins associated with the NV defect center show long coherence times and can be initialized and read-out optically at room temperature [1].
These and other properties make the NV center a promising candidate to act, e.g. as the fundamental building block in a quantum repeater segment [2]. Within the joint project quantum link extension (Q.Link.X), which aims towards the extension of quantum communication beyond point to point connections we are working on enhancing the light collection efficiency from NV centers in bulk diamond substrates. Due to the high contrast of the index of refraction in air and diamond only a small fraction of the fluorescence of the NV center can escape the diamond to be collected (see Figure 3.1a). One way to circumvent this refraction from the diamond-air interface is a solid immersion lens [3], fabricated directly over pre-selected NV emitters (see Figure 3.1c). This fabrication is done in a multi-step process where first suitable emitters are located and characterized, after which the fabrication is done via focussed ion beam milling. In Figure 3.1c an image of the processed diamond surface can be found.
Furthermore we are working on coherent control schemes to gain insight into more advanced control possibilities which the NV center offers. We investigate the application of multifrequency microwave control schemes involving all three spin ground states of the NV center, such as stimulated Raman adiabatic passage (STIRAP) in the microwave regime [4]. This enables to directly drive the spin-forbidden transition between the two electron spin states -1 and +1 (see Figure 1b) by application of adiabatic pulses, enabling more advanced control of the NV center and the development of new sensing schemes.
Figure 3.1: The nitrogen-vacancy (NV) center in diamond. a) Sketch of the atomic structure of a single NV center and its associated spins. b) Energy level scheme of the NV center, showing the three electron spin states 0, -1 and +1, which can be initialized and read-out optically and manipulated with microwave radiation at ~3 GHz. The data plot shows the population evolution of the NVs ground states subject to microwave stimulated Raman adiabatic passage (STIRAP) driving fields [4]. c) Sketch of the NV center in bulk diamond, showing total internal reflection for a planar surface and the advantage of a solid immersion lens (SIL). On the bottom a scanning microscope image of a fabricated SIL.
[1] Aharonovich, I., Castelletto, S., Simpson, D. A., Su, C. H., Greentree, A. D., & Prawer, S. (2011). Reports on progress in Physics, 74(7), 076501.
[2] van Loock, P., Alt, W., Becher, C., Benson, O., Boche, H., Deppe, C., ... & Weinfurter, H. (2020). Advanced Quantum Technologies, 3(11), 1900141.
[3] Jamali, M., Gerhardt, I., Rezai, M., Frenner, K., Fedder, H., & Wrachtrup, J. (2014). Review of Scientific Instruments, 85(12), 123703.
[4] Böhm, F., Nikolay, N., Neinert, S., Nebel, C. E., & Benson, O. (2021). arXiv preprint arXiv:2103.13788.
Contact: Niko Nikolay, Florian Böhm
4. Hybrid integration of single quantum emitters with photonic platforms
One important building block for future integrated nanophotonic devices is the scalable on-chip interfacing of single photon emitters and quantum memories with single optical modes in combination with passive optical structures such as cavities and couplers [1]. One fundamental requirement for this is the efficient collection and routing of single photons, without generating unwanted background fluorescence, degrading the single quantum emitter’s fluorescence.
We are working on the deterministic hybrid integration of single solid-state qubits, e.g. defect centers in diamond or molecules with a variety of photonic platforms. One example is single mode waveguides made of pure silicon dioxide (SiO2) grown thermally on a Si substrate (see Figure 4.1). The platform stands out by its ultra-low fluorescence and the ability to produce various passive structures such as high-Q microresonators and mode-size converters [2]. We were already able to show the coupling of a single NV center to this photonic platform and the efficient on-chip excitation and detection of the single NV center [3], but in principle this platform can be extended to all of the before mentioned single quantum emitter systems.
Figure 4.1: Hybrid integration. a) Scanning electron micrograph (top-view) of the coupling region of two SiO2 waveguides (highlighted in blue). b) Illustration of an SiO2 waveguide and the field profile of the guided optical mode. Also the deterministic positioning process with a diamond-nanocrystal containing a single NV center with an atomic force microscope (AFM) tip, is shown.
[1] Kimble, H. J. (2008). Nature, 453(7198), 1023-1030.
[2] Pyrlik, C., Schlegel, J., Böhm, F., Thies, A., Krüger, O., Benson, O., ... & Tränkle, G. (2019). IEEE Photonics Technology Letters, 31(6), 479-482.
[3] Böhm, F., Nikolay, N., Pyrlik, C., Schlegel, J., Thies, A., Wicht, A., ... & Benson, O. (2019). New Journal of Physics, 21(4), 045007.
Contact: Niko Nikolay, Florian Böhm