Doctoral School of Physics 

TTK, Department of Physics

Supervisor: Dr. Simon Ferenc

Telecom Compatible Quantum Communication

Introducing the research area

In quantum computing, the basic unit of information is the quantum bit (qubit) which is a two-level quantum-mechanical system. Nowadays, communication is optics-based, therefore it is advisable to create quantum information networks where the qubits (being the nodes of these systems) can be manipulated by optical means. Nitrogen vacancy (NV) centers created in diamond can be excited (initialized) by green light (500–550 nm) and their luminescence is in the 600–700 nm wavelength range. In my research, I focus on the magneto-optical properties of such systems by performing photoluminescence spectroscopy, optically detected magnetic resonance and electron spin resonance measurements. Using the same experimental techniques, I also investigate the electronic levels of carbon nanotubes (CNT), because by combining the two systems, a bridge could be created between the NV centers manipulable in the visible spectral range and the optical network working in the near infrared range.

Brief introduction of the research place

The research takes place at the ESR Laboratory of the Department of Physics at the BUTE, under the supervision of Ferenc Simon. In our research group, the experimental research is focused on the creation of carbon based structures (graphene, carbon nanotubes) and on the investigation and modification of their electronic, magnetic and optical properties. Collaboration with Hungarian (BME NTI, Wigner FK, ATOMKI) and cross-border research groups (K. Holczer, UCLA; L. Forró EPFL, T. Pichler, Univ. Vienna; N. M. Nemes, UCM) offers us a many opportunities in both sample preparation and measurements.

History and context of the research

Quantum communication is a buzzword of our century, the promise of fast and unbreakable (quantum cryptography) information transfer. Achieving this requires the improvement on existing technology and radically novel approaches. The information is carried not by the classical but the quantum bit (qubit), which can be implemented in several competing ways. There are many concepts for storing and manipulating quantum information, but to pass that information only (the polarization of) light seems to be a suitable candidate. This highlights qubits that can be manipulated by light. My aim is to create qubits that can be integrated into optical networks. The complex defects in solid state matter, such as nitrogen-vacancy (NV) centers created in diamonds, are excellent examples of qubits that can be optically initialized [2], but there is a problem of telecom compatibility, namely that the optical fibers used as channel of information exhibit their lowest absorption ratio in the near infrared (NIR) spectral range (1300 nm, 1550 nm) which favors point defects emitting light in this regime. Another way of thinking is to create color centers emitting outside of the NIR range and integrate them with a system that can convert photons into the NIR range. Carbon nanotubes (CNT) are a practical choice as they can be excited with visible light and emit photons in the NIR range thus building a bridge between qubits manipulated with visible light and optical communication networks working in the near infrared spectral range.

The research goals, open questions

My aim is to create nitrogen-vacancy centers in single crystal diamonds and to examine the possibility of integrating these systems into a communication network by conducting optical and microwave spectroscopic measurements. The creation and classification of NV centers with a high yield but also in a controlled manner is important for potential large-scale industrial application.

To achieve the photoconversion (information transfer) between NV centers and carbon nanotubes it is first necessary to study the two isolated subsystems, therefore I perform measurements both on NV centers and CNTs.

Fig. 1.: Mechanism of photoconversion with carbon nanotubes between nitrogen-vacancy centers and optical fiber (source in order of pictures: [1], [S2], Toptica photonics)

 

Methods

Sample preparation:

Diamond samples created by high pressure high temperature (HPHT) method contain substantial amounts of substitutional nitrogen sitting at the carbon lattice sites. In cooperation with the Institute of Nuclear Techniques (BME NTI), these crystals are exposed to neutron irradiation to create vacancies that can be mobilized by subsequent thermal annealing. During their diffusion, the vacancies pass by nitrogen atoms in the diamond lattice and become trapped in a bound state, forming nitrogen-vacancy centers.

Fig. 2.: Diamond sample containing nitrogen atoms before (left, source: E6 Ltd.) and after neutron irradiation and subsequent annealing (right). Thermal annealing is conducted at 900°C in a closed system under a varigon atmosphere (95% Ar, 5% H₂).

 

Characterization:

After annealing, the samples are placed in a commercial Bruker Elexsys E500 spectrometer and their electron spin resonance (ESR) spectra are acquired, from which the amount of NV centers and the nitrogen to NV conversion ratio are determined based on the signal intensity. Optical measurements such as photoluminescence and absorption (performed in cooperation) provide supporting information, but I found that for high density ensembles of NV centers, optical methods fail to correctly estimate the amount of NV centers due to graphitization of the samples, which does not affect the ESR measurements [S5].

Spin relaxation time:

When measuring the luminescent intensity of NV centers, one finds that the optical signal changes under the influence of resonant microwave irradiation. This is the base of optically detected magnetic resonance (ODMR), which is a more sensitive method compared to ESR. ODMR measurements enable us to determine the spin relaxation time of NV centers, which is crucial in understanding the interaction of the NV-CNT system.

Fig. 3.: ODMR intensity of NV centers as a function of microwave modulation frequency measured at room temperature (left) and at 77 K (right). As the sample cools, the spin relaxation time gets longer, as indicated by the downshift of the maximum position of the (red) signal.

 

Results

In recent years, all the necessary instrumental developments have been completed, and we produced carbon nanotube samples and nitrogen-vacancy centers in diamond. We gained useful information in both the instrumental development and the sample preparation for the broader research community.

Technical details of the ODMR spectrometer tailored to study carbon nanotubes [S1] and the physical information (singlet triplet gap, spin relaxation time) gained with this setup [S2] were published (IF 1.428 and 14.588; number of independent citations 3 and 6 respectively). The spectrometer was significantly modified to study NV centers[S4]. In both systems, we strongly rely on our high-stability tunable laser system, which we have adapted to work in pulsed mode [S3] to perform time-resolved measurements. Thanks to our collaborations, we have created NV centers in outstanding concentration. Our manuscript describing the sample preparation and the characterization will be published soon [S5] and some of the results I have already presented at conferences [S7-8]. The high density of NV centers is advantageous in metrological applications (magnetometry [3-4], electrometry [5] and thermometry [6]) as the signal-to-noise ratio of the detected signal scales with the square root of the number of NV centers in the measured volume. There is another manuscript in preparation[S6], in which we investigate the possible application of NV centers in the terahertz frequency range, also known as the 6G network, based on our light enhanced electron spin resonance measurements carried out in high magnetic field (16 Tesla).

 

Expected impact and further research

We intend to publish the two above mentioned manuscripts in high impact journals. We continuously produce NV centers, refine methods, and there is an ongoing collaboration with the Institute of Nuclear Techniques where the application of diamond in dosimetry is investigated based on the correlation between the amount of the created NV centers and the applied neutron energy and flux.

The NV center concentration in some of our samples exceeds 10 ppm (part per million). In this regime, the interaction between NV centers becomes significant as they are close enough to each other. This hinders their application as qubits, but this interacting spin system is worth studying as it is an experimental realization of many-body systems and it paves the way for studying theoretically well-described fundamental physics phenomena. [7]

In a new collaboration, we can study the ODMR of chemically modified (functionalized) carbon nanotubes. The necessary instrumental development is in progress and the measurements can hopefully start in autumn.

Our long-term aim is to integrate these nanotubes and our NV centers in one system and verify the photoconversion by photoluminescent spectroscopy and ODMR measurements.

Publications, references, links

List of corresponding own publications: (cumulated impact factor: 19.104)

[S1] M. Negyedi, J. Palotás, B. Gyüre, S. Dzsaber, S. Kollarics, P. Rohringer, T. Pichler, F. Simon. An optically detected magnetic resonance spectrometer with tunable laser excitation and wavelength resolved infrared detection Rev Sci Instrum 88. (2017) IF: 1.428

[S2] J. Palotás, M. Negyedi, S. Kollarics, A. Bojtor, P. Rohringer, T. Pichler, F Simon submitted, Incidence of Quantum Confinement on Dark Triplet Excitons in Carbon Nanotubes ACS Nano 2020, 14, 9, 11254–11261          IF: 14.588 (2019)

[S3] S. Kollarics, J. Palotás, A. Bojtor, B. G. Márkus, P. Rohringer, T. Pichler, F. Simon Improved laser-based photoluminescence on single-walled carbon nanotubes Phys. Status Solidi B, 256: 1900235. (2019) IF: 1.544

[S4] S. Kollarics, A. Bojtor, K. Koltai, B. G. Márkus, K. Holczer, J. Volk, G. Klujber, M. Szieberth, F. Simon Optical-microwave pump-probe studies of electronic properties in novel materials Phys. Status Solidi B,257:2000298. (2020) IF: 1.544 (2019)

[S5] S. Kollarics, F. Simon, A. Bojtor, K. Koltai, G. Klujber, M. Szieberth, B. G. Márkus, D. Beke, K. Kamarás, A. Gali, D. Amirari, R. Berry, S. Boucher, D. Gavryushkin, G. Jeschke, J. P. Cleveland, S. Takahashi, P. Szirmai, L. Forró, E. Emmanouilidou, R. Singh, K. Holczer: Ultrahigh nitrogen-vacancy center concentration in diamond, in preparation

[S6] S. Kollarics, B. G. Márkus, L. Forró, K. Holczer, F. Simon Light enhanced high field electron spin resonance on nitrogen-vacancy centers, manuscript in preparation

[S7] Kollarics S., Bojtor A., Holczer K., Gali Á., Beke D., Klujber G., Szieberth M., Simon F. Nitrogén-vakancia centrumok vizsgálata elektronspin-rezonancia és fotolumineszcens spektroszkópiai módszerekkel Magyar Fizikus Vándorgyűlés 2019

[S8] S. Kollarics, A. Bojtor, K. Holczer, Á. Gali, D. Beke, G. Klujber, M. Szieberth, F. Simon Experimental study of diamond with high density of nitrogen-vacancy centers

International Winter School on Electronic Properties of Novel Materials 2020

 

 

Table of links.

quantum computing

quantum bit

BME TTK Spin Spectroscopy Group

graphene

carbon nanotubes

Quantum communication

quantum cryptography

nitrogen-vacancy (NV) center

optical fibers

Toptica photonics

6G

 

List of references.

[1] M. Fujiwara, R. Tsukahara, Y .Sera, H. Yukawa, Y. Baba, S. Shikata, H. Hashimoto Monitoring spin coherence of single nitrogen-vacancy centers in nanodiamonds during pH changes in aqueous buffer solutions RSC Advances 9 (2019) 12606–12614.

[2] F. Jelezko, T. Gaebel, I. Popa, M. Domhan, A. Gruber, J. Wrachtrup, Observation

295 of coherent oscillation of a single nuclear spin and realization of a two-qubit

conditional quantum gate, Phys. Rev. Lett. 93 (2004) 130501.

[3] J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro,

L. Jiang, M. V. G. Dutt, E. Togan, A. S. Zibrov, A. Yacoby, R. L. Walsworth,

M. D. Lukin, Nanoscale magnetic sensing with an individual electronic spin in

300 diamond, Nature 455 (7213) (2008) 644–647.

[4] L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, V. Jacques, Magnetometry with nitrogen-vacancy defects in diamond, Rep. Prog. Phys. 77 (5)

320 (2014) 056503.

[5] E. H. Chen, H. A. Clevenson, K. A. Johnson, L. M. Pham, D. R. Englund, P. R.

Hemmer, D. A. Braje, High-sensitivity spin-based electrometry with an ensemble

of nitrogen-vacancy centers in diamond, Phys. Rev. A 95 (2017) 053417.

[6] P. Neumann, I. Jakobi, F. Dolde, C. Burk, R. Reuter, G. Waldherr, J. Honert,

305 T. Wolf, A. Brunner, J. H. Shim, D. Suter, H. Sumiya, J. Isoya, J. Wrachtrup,

High-precision nanoscale temperature sensing using single defects in diamond,

Nano Lett. 13 (6) (2013) 2738–2742.

[7] S. Choi, J. Choi, R. Landig, G. Kucsko, H. Zhou, J. Isoya, F. Jelezko, S. Onoda,

330 H. Sumiya, V. Khemani, C. von Keyserlingk, N. Y. Yao, E. Demler, M. D. Lukin,

Observation of discrete time-crystalline order in a disordered dipolar many-body

system, Nature 543 (7644) (2017) 221–225.