# Speakers & Abstracts

*The list is in order of presentation

**Denis Konstantinov **(Professor, Quantum Dynamics Unit)

*Quantum computing with electron spins on superfluid helium*

*Quantum computing with electron spins on superfluid helium*

Electrons trapped on the surface of superfluid helium present the cleanest two-dimensional (2D) charged particle system known in nature. For many years it was considered as a unique classical complement to the quantum-degenerate 2D electron gas in semi-conductors. In fact, we find even more similarities between electrons on helium (e@He) and some well-studied atomic systems, such as trapped ions and Rydberg atoms. Disorder-free environment void of magnetic impurities provides relatively long coherence of the motional states of e@He and extremely long coherence of their spin states, making this system very attractive for realization of qubits. However, the detection and coherent control of spin states of electrons is challenging and has not been realized yet. Recently, we proposed to introduce the spin-orbit coupling between motional Rydberg states of a trapped electron and its spin to accomplish the spin-state readout, fast spin-state operations, as well as long-range interaction between distant spin-qubits for constructing a two-qubit gate. A potential advantage of electrons on helium comparing to other systems is scalability to a very large number of qubits. In this short talk I aim to outline the most essential parts of our proposal and briefly mention our current experimental efforts towards the sensitive spin-state detection in this system.

**Akihito Soeda** (Associate Professor, Principles of Informatics Research Division, National Institute of Informatics (NII))

*Input-agnostic inversion of unitary processes*

*Input-agnostic inversion of unitary processes*

The possibilities of quantum information processing are determined by our ability to manipulate quantum systems. Quantum computers can also be used to convert pre-existing quantum processes/dynamics. Typically, the quantum process that we wish to manipulate is assumed to be well-understood, in advance. Should this not be the case, process tomography is a versatile option that can be applied to identify the input process, but can be costly. In this work, we present a probabilistic quantum circuit that transforms k uses of a general d-dimensional unitary process into its inverse. The input process is modelled as a quantum gate. The circuit is “input-agnostic”, namely, that it does not depend on the details of the input unitary process other than its dimension. The probability of failure of our circuit decays exponentially in the number of uses, which implies an exponential improvement over conventional tomographic approaches. The latter would only achieve an approximate conversion, while ours is exact. We also find that any quantum circuit performing exact unitary inversion of a general unitary operation requires multiple uses of the input unitary process, in fact, at least d-1 uses. Our circuit incorporates the input process in a sequential manner. We show that such a sequential use is necessary, since the failing probability of circuits which make k parallel uses decays linearly or worse. We extend our analysis to circuits without a definite causal order and find that the requirement of k=d-1 still holds. We also execute numerical optimizations based on semidefinite programming and find instances of causally indefinite circuits, whose probability of inverting a general unitary operation is strictly larger than causally ordered ones.

**Victor Bastidas **(Senior Research Scientist in the Theoretical Quantum Physics Research Group & the Research Center for Theoretical Quantum Physics, NTT BRL.)

**Quantum Simulation with Noisy Intermediate-Scale Quantum (NISQ) devices**

In recent years we have witnessed a fast development in quantum technologies. This is not only of interest for practical applications of quantum devices, but it also allows us to understand fundamental concepts in statistical mechanics, nonequilibrium disordered systems and manybody physics [1,2,3]. In this talk, I will discuss one of the most promising applications of superconducting devices as quantum simulators [4], which opens a new avenue for practical applications of near-term quantum devices. I will discuss our research results in this area within the context of the QLEAP project. In particular, I will discuss our theoretical and experimental results on the robustness of localized states when coupled to quantum reservoir [5,6]. After that, I will discuss our recent experiment on quantum walks in two-dimensional superconducing processor with 62 functional qubits. At the end of my talk, I will discuss future perspectives of our work.

[1] D. Basko, I. Aleiner, and B. Altshuler, Ann. Phys. (NY) 321, 1126 (2006).

[2] R. Nandkishore and D. A. Huse, Annu. Rev. Condens. Matter, Phys. 6, 15 (2015).

[3] M. P. Estarellas, T. Osada, V. M. Bastidas, B. Renoust, K. Sanaka, W. J. Munro, Kae Nemoto, Science Advances 6, eaay8892 (2020).

[4] P. Roushan, C. Neill, J. Tangpanitanon, V. M. Bastidas, A. Megrant, R. Barends, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, et al., Science 358, 1175 (2017).

[5] V. M. Bastidas, B. Renoust, Kae Nemoto, W. J. Munro, Phys. Rev. B 98, 224307 (2018).

[6] C. Zha, V. M. Bastidas, M. Gong, Y. Wu, H. Rong, R. Yang, Y. Ye, S. Li. Q. Zhu, S. Wang, et al., Phys. Rev. Lett. 125, 170503 (2020).

[7] M. Gong, et al., Science 372, 948 (2021).

**Nic Shannon** (Professor, Theory of Quantum Matter Unit)

*From quantum spin liquids to quantum computers*

*From quantum spin liquids to quantum computers*

Quantum effects lie at the heart of many of the interesting phenomena observed in nature, including superfluidity and superconductivity. They also underpin all of magnetism, and in particular the high-entangled "quantum spin liquid" phases found in certain magnetic materials at low temperature. In this talk I briefly review work carried out in the Theory of Quantum Matter Unit on quantum spin liquids, as well as more recent departures into machine learning and quantum computing.

**Rodney Van Meter** (Professor, Faculty of Environment and Information Studies, Keio University) ->__ this talk has been cancelled__

*A Quantum Internet Architecture*

*A Quantum Internet Architecture*

Entangled quantum communication is advancing rapidly, with laboratory and metropolitan testbeds under development, but to date there is no unifying Quantum Internet architecture. We propose a Quantum Internet architecture centered around the Quantum Recursive Network Architecture (QRNA), using RuleSet-based connections established using a two-pass connection setup. Scalability and internetworking (for both technological and administrative boundaries) are achieved using recursion in naming and connection control. In the near term, this architecture will support end-to-end, two-party entanglement on minimal hardware, and it will extend smoothly to multi-party entanglement and the use of quantum error correction on advanced hardware in the future. For a network internal gateway protocol, we recommend (but do not require) qDijkstra with seconds per Bell pair as link cost for routing; the external gateway protocol is designed to build recursively.

The strength of our architecture is shown by assessing extensibility and demonstrating how robust protocol operation can be confirmed using the RuleSet paradigm. https://arxiv.org/abs/2112.07092

Rodney Van Meter received a B.S. in engineering and applied science from the California Institute of Technology in 1986, an M.S. in computer engineering from the University of Southern California in 1991, and a Ph.D. in computer science from Keio University in 2006.

His current research centers on quantum computer architecture, quantum networking and quantum education. He is the author of the book _Quantum Networking_. Other research interests include storage systems, networking, and post-Moore's Law computer architecture. He is now a Professor of Environment and Information Studies at Keio University's Shonan Fujisawa Campus. He is the Vice Center Chair of Keio's Quantum Computing Center, co-chair of the Quantum Internet Research Group, a leader of the Quantum Internet Task Force, and a board member of the WIDE Project. Dr. Van Meter is a member of AAAS, ACM, APS, and IEEE. He is currently Editor in Chief of IEEE Transactions on Quantum Engineering, but this talk is 100% personal opinions.

**Artur Ekert **(Adjunct Professor, Quantum Information Security Unit)

*Bell inequalities: from curiosity to security *

*Bell inequalities: from curiosity to security*

**Akitada Sakurai** (Postdoctoral Scholar, Quantum Information Science and Technology Unit)

*Utilizing quantum dynamics for machine learning*

*Utilizing quantum dynamics for machine learning*

As quantum technology developed, it has been possible to realize quantum systems with tens to hundreds of qubits with quantum systems. Because qubits are noisy, such quantum systems are called noise intermediate-scale quantum (NISQ) devices. Due to its size restriction, we cannot implement quantum computation in a fault-tolerant manner in these NISQ devices. However, they have enough qubits to generate complex dynamics and have been used to simulate new physics arising from many-body effects. It also has attracted attention and efforts worldwide to discover a way to utilize such quantum complexity to solve practical problems. In this talk, we will introduce a new quantum neural network (QNN) to run practical problems such as image classification problems. This model is based on

two classical computational models, reservoir computation and extreme learning machine recently proposed, and only requires simple quantum hardware. We then explain how we encode classical information into the QNN and how the encoding affects its performance in solving practical problems. We also discuss the advantages of the QNN model through an example with the hand-written digit data set (MNIST).

**Shohei Watabe** (Associate Professor, Shibaura Institute of Technology (SIT))

*Quantum Spatial Search on Complex Network*

*Quantum Spatial Search on Complex Network*

The complex network is ubiquitous in many real-world systems, including the World-Wide-Web, social and biological networks, such as the actors network, protein-protein interaction network, and cellular network. These complex networks show the small world network property, the scal-free property as well as the fractal property. If a quantum network system is spread in the world, notions of a scaling law or a universality will be helpful to understand characteristics of the complex quantum network, because such a quantum network becomes huge that cannot be simulated by classical computers. In this talk, I will talk about the scaling law of the quantum walk in complex networks, such as the small world network, the scale free network and the fractal network.

**Shota Nagayama **( Research Associate Professor, Graduate school of Media and Governance, Keio University)

*Quantum Network Testbed in Japan *

*Quantum Network Testbed in Japan*

The Quantum Network Testbed Project in Japan is introduced. A quantum networking project for distributed quantum computers supported by JST Moonshot has been started and that network is going to be extended to wide-area quantum networks, namely, the Quantum Internet, headed by Quantum Internet Task Force. We are focusing on the future in which quantum data is communicated world-wide and computed freely like digital data today.

**Yuimaru Kubo **(Principal Investigator, Hybrid Quantum Device Team, STG)

*Thermally induced negative temperature in diamond*

*Thermally induced negative temperature in diamond*

A thermally pumped "negative temperature," i.e., population inversion, is realized in the nitrogen-vacancy center (NV-center) spins in a diamond crystal placed in a microwave resonator. In the three levels of the NV electron spin system (S = 1), the relaxation from the upper to the middle levels is accelerated with the assistance of fast relaxing spin systems upon abrupt heating-up and cooling-down of the device. In contrast, the relaxation from the middle to the bottom levels remains as long as ~ 10^4 seconds. Consequently, population inversion is established between the middle and bottom levels, manifested by a gain of a weak probe microwave tone applied to the microwave resonator. Our result is a modified version of the quantum heat engine proposed by Scovil and Schulz-DuBois.

[1] Scovil and Schulz-DuBois, Phys. Rev. Lett. 2, 262 (1959).