China

Quantum 2020

Proposal of Variational Quantum Eigensolver method using Genetic Algorithm then Local Search

Quantum sensing is inevitably one of the elegant examples for supremacy of quantum technologies over their classical counterparts. One of the desired endeavor of quantum metrology is ac field sensing. Here, by the means of analytical and numerical analysis, we show that many-body systems can be exploited efficiently for detecting the amplitude of an ac field. Due to the periodicity of the dynamics, any local block of the system saturates to a steady state which allows achieving precision for sensing the amplitude well beyond the classical limit. We associate the enhanced
quantum precision to closing of the Floquet gap, resembling the features of quantum sensing in the ground state of critical systems. Remarkably, the scaling of precision with respect to the number of resources becomes close to the Heisenberg limit, along the line where Floquet gap vanishes. This shows the potential of many-body systems for providing quantum enhanced precision in ac field sensing.

Many-body steady-state AC field quantum sensing

Quantized confined energy takes big values when the dimensions of the confining system are nanoscales dimensions. Therefore, it is important to study the effect of confined energy on the electron blockade phenomena.in this work, We will employ the spherical quantum dot as a topical nanostructure system after defining its electronic structure to investigate the effect of confined energy on Coulomb blockade phenomena, this effect is known as a quantum coulomb blockade phenomena . Then. We will employ orthodox theory (master equation approach) to study electron transport through quantum coulomb blockade system. Finally, we will investigate the effect of the QD’s radius, tunneling rate, and temperature on SET current. Also, we will study the effect of gate voltage on the SET current by taken different values of gate voltage in quantum coulomb blockade region, and we will define a two-point where the transistor is work in one case (transistor off) if the condition of maximum source-drain voltage was respected.

Quantum coulomb blockade phenomena and electron transport in single electron devices

An important property of a Quantum Key Distribution (QKD) is the ability of the two communicating users to detect the presence of any third party trying to gain knowledge of the key. A fundamental requirement in the process is the secrecy of the outcome. This work presents some results of a contactless eavesdropping of the electric activities of a Single Photon Avalanche Diode (SPAD) detector for obtaining its response without affecting the quantum state of the system.
Three different types of well-known commercial SPAD have been used in the experiments, which are referenced herein as SPAD1, SPAD2 and SPAD3.
This work shows how the measurements on Bob’s computer can be compromised by the eavesdropper.


Quantum Communication vulnerabilities in a QKD system

"Trapped ions are among the best-controlled quantum systems. However, for molecules, a similar degree of control currently lacks due to their complex energy-level structure. Quantum-logic protocols in which atomic ions serve as probes for molecular ions are a promising route for achieving this level of control. Here, I will describe our experimental results in achieving >99% fidelity in the quantum-nondemolition state detection of the nitrogen ion's electronic, vibration, and rotation ground state [1], thus making a crucial step towards the coherent manipulation of molecular quantum states. We further exploited our quantum-logic protocol for hyperfine and Zeeman resolved state identification and preparation in a complex region of the molecular spectrum, mimicking the situation encountered with polyatomic molecules [2]. The quantum control of rotation and vibration of molecules will open up opportunities for creating new time standards in the THz domain [3], searching new physics, improving molecular calculations, performing state-to-state controlled chemistry, and encoding quantum information in molecular qubits in quantum-computation and communication applications.

[1] Sinhal, Meir, Najafian, Hegi, Willitsch, Science 367, 1213 (2020).
[2] Najafian, Meir, Sinhal, Willitsch, arXiv:2004:05306 (2020).
[3] Najafian, Meir, Willitsch, arXiv:2007.11097 (2020)."

Quantum-logic state detection and preparation of rovibrational molecular states

"Protecting qubits from errors is mandatory to realize scalable quantum computing platforms. However, standard quantum error correction (QEC) codes based on multi-qubit encoding yield a large increase in the number of physical qubits and operations, thus making the control of such a platform very demanding.

Here we show that Molecular Nanomagnets can be used to easily overcome this limitation, by naturally embedding quantum error corrected units in a single multi-level molecule.

This is made possible by the rich and chemically tunable spectrum of these systems, characterized by many accessible levels with long coherence times. We perform a perturbative expansion of the Lindbald equation to design a scheme, based on such molecular qudits, which efficiently protects them from real errors, induced by pure dephasing. We then derive the sequence of pulses actually implementing QEC on a real spin system and demonstrate the effectiveness of our approach by numerical simulations.

We already get a remarkable gain (up to an order of magnitude) in the final error with a simple existing molecule consisting of a  nuclear qudit coupled to an electronic ancilla, used for error detection. This gain is further improved by scaling to larger spins, as we quantitatively analyze."

Molecular Nanomagnets as Qubits with Embedded Quantum-Error Correction

Global scale quantum communication links will form the backbone of the quantum internet. However, exponential loss in optical fibres precludes any realistic application beyond few hundred kilometres. Quantum repeaters (QR) and space-based systems offer to overcome this limitation. However, the most ambitious land based QR architecture is limited to around 4000 km beyond which meaningful secret generation becomes impossible. On the other hand, majority of the satellite-based quantum communication architectures proposed so far are either limited to the line-of-sight distance of the orbit or they rely on satellites as trusted nodes. Here, we analyse the use of quantum memory (QM)-equipped satellites for quantum communication focussing on global range repeaters and Measurement-Device-Independent (MDI) QKD. We show that a network consisting of satellites equipped with QMs could offer orders of magnitude faster entanglement distribution rates across the whole globe than the existing hybrid, space-ground protocols. We further analyse how entanglement distribution performance depends on memory characteristics, determine benchmarks to assess performance of different tasks, and propose various architectures for light-matter interfaces. Our work provides a practical roadmap to realise unconditionally secure quantum communications over global distances with current technologies.



Sponsored Session - Hai Zhong: The Qnami ProteusQ - A scanning NV (nitrogen-vacancy) microscope for the analysis of magnetic materials at the atomic scale - Qnami

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