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We propose a unifying theory for estimating unknown quantum states using observed data, both before (past) and after (future) the estimation time. State estimation using past-future observed data has been studied even in the classical cases; for example via the state smoothing technique, which has been shown to perform better than the state filtering (using only the past information). In the quantum regime, there are mainly three existing formalisms that take into account the information both before and after the estimation time, i.e., the weak-value formalism [Phys. Rev. Lett. 60 1351 (1988)], the quantum most-likely path [Phys. Rev. A 88 042110 (2013)], and the quantum state smoothing [Phys. Rev. Lett. 115, 180407 (2015)]. Considering a partially observed quantum system, in which there exist both observed and unobserved records from continuous monitoring of the system, we give a common formulation that establishes the connection among three existing formalisms. The state estimators are calculated based on the expected cost minimization, either in the state space or the unknown record space. Our theory not only unifying existing formalisms for quantum state estimation, but also suggest new estimators that can be applied in practical scenarios.

Quantum 2020

Unifying theory of quantum state estimation using past and future information

state smoothing

cost function

quantum state estimation

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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MMM 2020

technical paper

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.

Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. 


Rare earth ions in solids are broadly utilized in quantum information processing regime by resolving the fluorescence spectrum and absorption spectrum. In our group we have reported the ability to address a single erbium atom in a silicon nanotransistor by a hybrid approach which combined the optical excitation and charge relaxation processes . 
Here I would report our most recent results of two coupled erbium atoms in a silicon transistor by using the same technique. We saw two kinds of avoid crossings at low magnet field from Zeeman splitting pattern. Those avoid crossings could be explained by magnetic dipole interaction and optical interaction. By fitting the spectrum, we resolved the Hamiltonian of the coupled erbium atoms system and a rough distance of these two erbium ions in silicon lattice.
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The absorption spectrum of two coupled erbium ions in silicon

Since 2014 there has been a massive increase in the number of patent applications. This presentation reviews the increase in the filing rate and will look at the geographical areas in which the most patent applications are generated as well as the major players patenting innovation in quantum computing.

Quantum Patents - Looking at the Intellectual Property Landscape

Modeling quantum devices is to find a model according to quantum theory that can explain the result of experiments in a quantum device. We find that usually we cannot correctly identify the model describing the actual physics of the device regardless of the experimental effort given a limited set of operations. According to sufficient conditions that we find, correctly reconstructing the model requires either a particular set of pure states and projective measurements or a set of evolution operators that can generate all unitary operators.

Quantum Devices Modeling and Verification

We study the possibility of using dark-solitons in quasi one-dimensional Bose-Einstein condensates to produce two or three-level systems (qudits) by exploiting the intrinsic nonlinear and the coherent nature of the matter waves. More precisely, we intend to create the dark soliton qudits by using trapped impurities. The decoherence induced by the quantum fluctuations (phonons) produces a finite lifetime due to their intrinsic slow-time dynamics. Remarkably, the qubit lifetime is estimated to be of the order of a few seconds, being only limited by the dark-soliton death due to quantum evaporation. Further, we explore the spontaneous generation of phononic entanglement between dark soliton qubits in the dissipative process of spontaneous emission. By driving the qubits with the help of oscillating magnetic field gradient, we observe the formation of long distance steady-state concurrence. Our results suggest that dark-soliton qubits are a good candidates for quantum information protocols based purely on matter-wave phononics.

Dark Soliton Qudit and Entanglement Dynamics

Quantum entanglement is essential to the development of quantum computation, communications, and technology. The controlled SWAP test, widely used for state comparison, can be adapted to an efficient and general test for entanglement of a pure state. We show that the test can evidence the presence of entanglement (and further, genuine n-qubit entanglement), can distinguish entanglement classes, and generates the concurrence in the case of a 2-qubit state. We also propose a multipartite degree of entanglement, related to the test's probability outputs. The average number of measurements required to detect entanglement increases with decreased entanglement. Maximally entangled states require fewer measurements the larger the system, two on average for many (n≥8) qubits. Furthermore, the results are robust to second order when typical small errors are introduced to the state under investigation.

The controlled SWAP test for determining quantum entanglement


Simulation of a quantum many-body system at finite temperatures is crucially important but quite challenging. Here we present an experimentally feasible quantum algorithm assisted with continuous-variable for simulating quantum systems at finite temperatures. Our algorithm has a time complexity scaling polynomially with the inverse temperature and the desired accuracy.
We demonstrate the quantum algorithm by simulating finite temperature phase diagram of the quantum Ising and Kitaev models. It is found that the important crossover phase diagram of the Kitaev ring can be accurately simulated by a quantum computer with only a few qubits and thus the algorithm may be readily implemented on current quantum processors. We further propose a protocol implementable with superconducting or trapped ion quantum computers.

Continuous-variable assisted thermal quantum simulation

In this study, we investigated the academic literature on quantum technologies (QT) using bibliometric tools. We used a set of 49,823 articles obtained from the Web of Science (WoS) database using a search query constructed through expert opinion. Analysis of this revealed that QT is deeply rooted in physics, and the majority of the articles are published in physics journals. Keyword analysis revealed that the literature could be clustered into three distinct sets, which are (i) quantum communication/cryptography, (ii) quantum computation, and (iii) physical realizations of quantum systems. We performed a burst analysis that showed the emergence and fading away of certain key concepts in the literature. This is followed by co-citation analysis on the highly cited articles provided by the WoS, using these we devised a set of core corpus of 34 publications. Comparing the most highly cited articles in this set with respect to the initial set we found that there is a clear difference in most cited subjects. Finally, we performed co-citation analyses on country and organization levels to find the central nodes in the literature. Overall, the analyses of the datasets allowed us to cluster the literature into three distinct sets, construct the core corpus of the academic literature in QT, and to identify the key players on country and organization levels, thus offering insight into the current state of the field.

The Landscape of Academic Literature in Quantum Technologies

Solid state quantum emitters, like color centers in diamond and systems alike, offer appealing prerequisites for practicable devices that exploit quantum effects. For example, optically addressable quantum sensors, based on the ubiquitous nitrogen-vacancy defect, have been recently commercialized in magnetometers offering unprecedented spatial resolution. Another class of defects in diamond, based on group-IV elements, is currently attracting considerable interest due to potentially further improved properties. Thereby, the vast majority of experimental studies relies on identification, characterization, and manipulation of these quantum emitters by optical means - that is, in practice, by employing suitable laser light sources. Yet, the experimental demands are challenging. In frequency domain experiments, the characterization of single color defects often necessitates quasi-continuous laser frequency tuning over wide spectral ranges, while high finesse spectral resonances require sufficiently narrow linewidths. In turn, experiments that interrogate temporal dynamics, like initialization and read out of spin states, demand fast intensity modulation capability with high on/off ratios. In this context, we present principles, design challenges, and performance highlights of state-of-the-art laser light sources tailored to meet requirements typically encountered in quantum technology research. Throughout the presentation we keep an eye on emerging applications and present recent experimental data in an illustrative manner.

Novel continuous-wave lasers tailored for identification, characterization, and manipulation of single color centers in diamond

Nonlinear optical interaction at a single photon level plays a key role in quantum optical information processing. One of its important applications is optical switching with extremely low power consumption. Although optical switching controlled by a few photons has been demonstrated using strongly coupled quantum dot-photonic crystal cavity systems, the strong coupling regime brings significant fabrication challenges as high cavity quality factor and accurate QD positioning are required. This becomes more problematic for integrated photonic circuits where the cavity Q-factor is strongly degraded due to the waveguide coupling. In this work, we show that low-photon number (~2 photons) optical switching can be realized with a QD weakly coupled to a low-Q (only 600) cavity in the high Purcell factor regime. Our scheme significantly relaxes the fabrication requirement while achieving high switching speed (22.7 ps), high switching contrast (39%), and large tunability. Our work paves the way towards the practical quantum optical switch, especially for on-chip photonic circuits.

Ultrafast few-photon switching with a coherently driven quantum dot in the high Purcell factor regime

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

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The absorption spectrum of two coupled erbium ions in silicon

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