Keynotes

Long-Distance Quantum Communication and Internet: Fundamentals and Practical Challenges

The world is looking for a safer communication method due to the increase in hacker and eavesdropper, trying to steal transmitted confidential data. Quantum key distribution (QKD) system is proven approach for secure information transfer, especially that involve financial, banking and tele-medicine. Quantum network will also be needed to connect quantum devices such as the emerging quantum computers/simulators, with potentiality towards quantum internet.

There have been a lot of experiment and research paper on entanglement process, which is a fundamental asset for quantum technology. The essential components include entangled photon pairs normally produced by type II SPDC process(Couteau, 2018), quantum teleportation and entanglement swapping of quantum information. This is due to the two principle of quantum mechanics which states that any measurement done on the system will perturb the system thus alerting the original communicators (Assche, 2006) and the “no cloning theorem”(Humble, 2013). To build a sufficiently long distance QKD communication system, entanglement swapping needs quantum repeaters interface composed of quantum memory. This allows us to overcome high photon attenuation problem in the propagating medium.

Many quantum memory schemes involve Raman transitions for writing and reading of quantum information. This includes the DLCZ (Duan 2001 ) protocol which has been shown to be robust against operational noise and imperfections. The working principle of DLCZ involves the control of correlated photons using double Raman scheme(Moiseev 2001, van der Wal 2003, Balic 2005 ) as promising quantum memory process for storing (writing) quantum information of photon into atomic states and retrieving (reading) it back into photon at later time ’on demand’. High performance quantum memory is necessary to maintain high level of entanglement between distant quantum bits (qubits) and to teleport the qubits between arbitrarily separated partners with high fidelity. A number of quantum memory materials and schemes have been proposed and demonstrated, each has its own strengths and weaknesses.

The early QKD schemes like BB84 and E91 have been experimentally tested and improved by MDI-QKD (Lo 2012 ) and TF-QKD. Current challenges to long-distance QKD is the realization of robust quantum memory schemes, with performances such as low noise, efficient, high fidelity and sufficiently large information capacity. Various quantum memory schemes involving Raman systems using atomic ensembles, trapped ions, nitrogen vacancy diamonds and rare-earth ion doped crystals have been studied. Proper theoretical framework that incorporates quantum noise is crucial to assess these quantum memory schemes and materials, particularly the nonclassical correlations between the write and read photons (Ooi 2007 ). Besides fundamental aspects, the challenge behind this technology is practicality.

References

Couteau, C. (2018) Spontaneous parametric down-conversion, Contemporary Physics, 59(3), pp. 291-304.

Assche, G. Van (2006) Quantum Cryptography and Secret-Key Distillation. 1st edn. New York: Cambridge University press.

Humble, T. S. (2013) ’Quantum security for the physical layer’, IEEE Communications Magazine, 51(8), pp. 56-62

L.-M. Duan, Lukin, M., Cirac, J. I. & Zoller, P. (2001) Long-distance quantum communication with atomic ensembles and linear optics. Nature Nature 414, 413418

S. A. Moiseev and S. Kr ̈oll, Complete Reconstruction of the Quantum State of a Single-Photon Wave Packet Absorbed, Phys Rev Lett.87,173601 (2001)

van der Wal et. al., Atomic Memory for Correlated Photon States, Science 301, 196 (2003).

V. Balic, D. A. Braje, P. Kolchin, G. Y. Yin, and S. E. Harris, Phys. Rev. Lett. 94, 183601 (2005).

H.-K. Lo, M. Curty, and B. Qi, Phys. Rev. Lett. 108, 130503 (2012).

C. H. Raymond Ooi, Q. Sun, M. S. Zubairy and M. O. Scully, Correlation of Photon Pairs from Double Raman Amplifier:

Generalized Analytical Quantum Langevin Theory, Phys. Rev. A 75, 013820 (2007).

Integration of Radio and Photonics Systems Toward 6G and Beyond

High capacity and low latency communication systems are highly demanded IoT-enabled cyber-physical systems, realizing smart factories, smart cities, smart agriculture, and so on, to implement Society 5.0. To connect the sensors and actuators from/to the processors, integration of radio (wireless) and photonics (wireline) networks plays an important role in Beyond 5G era from the viewpoint of both latency management and the reduction of energy consumption. In this scenario, a combination of centralized and distributed processing will be key for realizing the future network: just the Beyond 5G/6G network architecture.

In this network, important and heavy signals are processed in the centralized processors, and light and low-latency-required processing are distributed. In the access network, wireless communication will be dominant for the enhancement of the user experiences. In this case, millimeter-wave and terahertz-wave radio will be implemented as a high-capacity communication link. For the reception of high-frequency and high-capacity wireless signals to the wireline networks seamlessly, the conversion technology between the radio and photonic signals should be developed [1, 2].

In this talk, we introduce the demands and requirements for integrating the radio and photonics systems, one of their solutions of integrated photonic/radio chips, and the seamless conversions of these signals with radio over fiber techniques. We will also show the recent activities and prospects of the R&Ds toward 6G and beyond.

Distributed millimeter-wave radar system connected to the photonic network is shown as a demonstrator for the integration of the communication and sensing systems.

References:

[1] A. Kanno et al., Journal of Infrared, Millimeter, and Terahertz Waves 36 (2), 180-197 (2015).

[2] P. T. Dat et al., Journal of Lightwave Technology 37 (2), 592-605 (2018).

EUV light source as an example of compact laser-produced quantum beam source

Low density materials can control plasma properties of laser absorption, which can enhance quantum beam generation. [1] The recent practical extreme ultraviolet light (EUV) is the first industrial example of laser plasma source with low density targets. Here we introduce some examples of EUV generation by focusing laser.

An easy-handling target source is based on a hollow sub-millimeter microcapsule fabricated from polyelectrolyte cationic and anionic surfactant on air bubbles. The lightweight microcapsules acted as a scaffold for surface coating by tin (II) oxide nanoparticles (22-48%), and then dried. As a proof of concept study, the microcapsules were ablated with a Nd:YAG laser (7.1x1010 W/cm2, 1 ns) to generate 13.5 nm EUV relatively directed to laser incidence. The laser conversion efficiency (CE) at 13.5 nm 2% bandwidth from the tin-coated microcapsule (0.8%) was competitive compared with bulk tin (1%). We propose that microcapsule aggregates could be utilized as a potential small scale/compact EUV source, and future quantum beam sources by changing the coating to other elements. [2]

Another example is also easy-handling laser targets of a liquid gallium-tin alloy (Ga:Sn) for 13.5 nm light generation. Alloys had 30°C freezing point alloys according to DSC cooling process. A Nd:YAG laser (1064 nm, 1 ns, 7.1x1010 W/cm2) ablated the alloys, obtaining a similar EUV emission intensity to Sn in spite of small quantity of Sn. Finally, we demonstrate a liquid metal alloy jet and droplets using a plastic nozzle for high-repetition target supply. [3]

References:

[1] K. Nagai, C S A Musgrave, W. Nazarov Phys. Plasmas., 25 (3), 030501 (2018); doi: 10.1063/1.5009689.

[2] C S A Musgrave, S. Shoji, K. Nagai, Sci. Rep., 10, 5906, (2020). DOI. /10.1038/s41598-020-62858-3

[3] C S A Musgrave, N. Lu, R. Sato, K. Nagai, RSC Adv., 9, (24) 13927-13932 (2019). DOI: 10.1039/c9ra01905

[4] https://www.titech.ac.jp/english/news/2020/046774