China

&quot;Computing the electronic structure of molecules is a central problem. Despite the enormous success of approximate methods, facing this problem exactly with conventional computers is still a tremendous challenge. This has triggered theoretical and experimental efforts to solve chemistry problems using quantum computers, with proof-of-concept results using a few qubits. An appealing alternative to this digital approach is analog quantum simulation, which relies on a highly controllable device to mimic the system of interest and gain information about the desired solution, and does not require a fully operative computer.

Here we propose how it is possible to simulate quantum chemistry problems using ultra-cold atoms as a simulator [1]. In this simulator, fermionic atoms hopping in an optical potential play the role of electrons, additional optical potentials provide the nuclear attraction, and an additional atomic species mediates the electronic Coulomb repulsion. We analyze the impact of discretization and finite size effects of the lattice, and provide the working conditions required for the precise determination of the electronic energy of simple molecules. We also show how simplified models (e.g. two-dimensional [2]) can lead to the first experiments in this analog simulation approach.

[1] Nature 574 (7777), 215-218 (2019). 
[2] Phys. Rev. Research. (Accepted, 2020).&quot;

Quantum 2020

Quantum simulation of quantum chemistry in optical lattices

quantum chemistry

ultracold atoms

quantum simulation

**WELCOME TO THE 2021 CAP VIRTUAL CONGRESS UNDERLINE PLATFORM**

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CAP Virtual Congress 2021

"Computing the electronic structure of molecules is a central problem. Despite the enormous success of approximate methods, facing this problem exactly with conventional computers is still a tremendous challenge. This has triggered theoretical and experimental efforts to solve chemistry problems using quantum computers, with proof-of-concept results using a few qubits. An appealing alternative to this digital approach is analog quantum simulation, which relies on a highly controllable device to mimic the system of interest and gain information about the desired solution, and does not require a fully operative computer.

Here we propose how it is possible to simulate quantum chemistry problems using ultra-cold atoms as a simulator [1]. In this simulator, fermionic atoms hopping in an optical potential play the role of electrons, additional optical potentials provide the nuclear attraction, and an additional atomic species mediates the electronic Coulomb repulsion. We analyze the impact of discretization and finite size effects of the lattice, and provide the working conditions required for the precise determination of the electronic energy of simple molecules. We also show how simplified models (e.g. two-dimensional [2]) can lead to the first experiments in this analog simulation approach.

[1] Nature 574 (7777), 215-218 (2019). 
[2] Phys. Rev. Research. (Accepted, 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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If you have a ticket and are experiencing problems in accessing the event, please contact us. 

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. 

All elementary particles in nature can be classified as fermions with half-integer spin and bosons with integer spin. Within quantum electrodynamics (QED), even though the spin of the Dirac particle is well defined, there exist open questions on the quantized description of spin of the gauge field particle---the photon. Here, we discover the quantum operators for the spin angular momentum (SAM) $\boldsymbol{S}_{M}=(1/c)\int d^{3}x\boldsymbol{\pi}\times\boldsymbol{A}$ and orbital angular momentum (OAM) $\boldsymbol{L}_{M}=-(1/c)\int d^{3}x\pi^{\mu}\boldsymbol{x}\times\boldsymbol{\nabla}A_{\mu}$ of the photon, where $\pi^{\mu}$ is the conjugate canonical momentum of the gauge field $A_{\mu}$. Using relativistic field theory, we reveal a fundamental gauge-hiding mechanism that identifies the missing link between the complete photon spin operator and experimental observables -- the transverse-field photon spin and helicity. Our work resolves the long-standing issues on the decomposition of orbital and spin angular momentum of the photon with applications in quantum optics, topological photonics as well as nanophotonics and also has important ramifications for the spin structure of nucleons.

Quantum spin operator of the photon

Number-resolving single-photon detectors represent a key technology for a host of quantum optics protocols, but despite significant efforts, state-of-the-art devices are limited to few photons. In contrast, state-dependent atom counting in arrays can be done with extremely high fidelity up to hundreds of atoms. We show that in waveguide QED, the problem of photon counting can be reduced to atom counting, by entangling the photonic state with an atomic array in the collective number basis. This is possible as the incoming photons couple to collective atomic states and can be achieved by engineering a second decay channel of an excited atom to a metastable state. Our scheme is robust to disorder and finite Purcell factors, and its fidelity increases with the atom number. Analyzing the state of the re-emitted photons, we further show that if the initial atomic state is a symmetric Dicke state, dissipation engineering can be used to implement a nondestructive photon-number measurement, in which the incident state is scattered into the waveguide unchanged. Our results generalize to related platforms, including superconducting qubits.

Nondestructive photon counting in waveguide QED

We list three fundamental features of relativistic quantum mechanics equations (RQMEs). 1) A free particle has positive kinetic energy (PKE) and negative kinetic energy (NKE) branches. 2) For Dirac equation, the probability current of particles with PKE is opposite to that of NKE. 3) In each RQME, when potential takes minus sign, the eigen values also take minus sign.
A fundamental postulation is that the features RQMEs should remain in low momentum motion. However, Schrödinger equation does not hold the above three feature. Or these features are lost in low momentum approximation.
Starting from RQMEs and taking low momentum approximation, we derive NKE Schrödinger equation beside familiar Schrödinger equation which is of PKE. With the help of NKE Schrödinger equation, the lost formulas are recovered. The negative branch, the feature 1) above, is retained by NKE Schrödinger equation. In the region where the potential is greater than the energy of a particle, the particle is of NKE. NKE should be treated on an equal footing as PKE.
Klein-Gordon equation is decoupled to be two: PKE one and NKE. In this way, the feature 2) is held by decoupled Klein-Gordon equations. The so-called difficulty of negative probability of Klein-Gordon equation is solved, for the correct expression for the probability current of this equation is given. 
A (PKE) NKE particle can have stationary motion only when it is subject to an attractive (repulsive) potential, which is guaranteed by Virial theorem. 
Two NKE electrons in a potential can constitute a stable system, a new kind of possible mechanism for electron paring. 
The whole discussion stems from RQMEs with no any new postulation. 
We suggest some experiments which may verify that there are indeed NKE electrons.


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Quantum simulation of quantum chemistry in optical lattices

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Quantum spin operator of the photon

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