China

The power of a quantum circuit is determined through the number of two-qubit entangling gates that can be performed within the coherence time of the system. In the absence of parallel quantum gate operations, this would make the quantum simulators limited to shallow circuits. Here, we propose a protocol to parallelize the
implementation of two-qubit entangling gates between multiple users which are spatially separated and use a commonly sheared spin chain data-bus. Our protocol works through inducing effective interaction between each pair of qubits without disturbing the others, therefore, it increases the rate of gate operations without creating crosstalk. This is achieved by tuning the Hamiltonian parameters appropriately, described in the form of two different strategies. The tuning of the parameters makes different bilocalized eigenstates responsible for the realization of the entangling gates between different pairs of distant qubits. Remarkably, the performance of our protocol is robust against increasing the length of the data-bus and the number of users. Moreover, we show that
this protocol can accomplish two-way communication and standout the decoherence effects.

Quantum 2020

Parallel entangling gate operations in spin chins

average gate fidelity

spin chain

entangling gate operations

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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poster

The preparation of a high-quality single-photon source at the mid-infrared (MIR) wavelength range (approximately 2-20 µm) is of great importance for the next-generation technology of quantum sensing, imaging, and communications. The current research on single-photon sources mainly focuses on the near-infrared band. The high-brightness, spectrally pure, and easy-to-operate quantum photon source at the MIR range remains an unsettled problem. To solve this problem, we design a scheme based on a machine learning framework to optimize the poling period of LN crystal at the MIR range. Calculation results show that the spectral purity of the heralded single photons can over 0.999 for such a custom poled LN (cpLN). This high-quality source could be very useful for applications in quantum sensing, imaging, and communications at the MIR band.

Mid-infrared spectrally pure photons generation from LN optimized by machine learning

The mid-infrared (MIR) wavelength range (approximately 2-20 μm) is of great interest to a variety of applications. However, the advantages of the MIR range have not been fully exploited using quantum technology. It is still lack of high-quality quantum sources for the MIR range. In this work, we theoretically study the preparation of pure-state single-photon source from 23 nonlinear optical crystals, including birefringent crystals CMTC, LIIO3, AAS, HGS, CGA, TAS, AGS, AGSE, GASE, LIS, LISE, LGS, and LGSE; periodic poling crystals LT, LN, KTP, KN, CSP, ZGP, BATIO3, MGBAF4, OP-ZNSE, and PMN-0.38PT. Calculation results show that these crystals satisfy three kinds of group-velocity-matching (GVM) conditions. The down-converted photons wavelength range is from 1298 to 7384 nm for the GVM1 condition, from 2312 to 11650 nm for the GVM2 condition, and from 1658 to 9380 nm for the GVM3 condition. The spectral purities for three kinds of GVM conditions are around 0.97, 0.97, and 0.82, respectively. We also calculate their performance in the Hong-Ou-Mandel interference from independent sources, and it can also achieve visibility as high as 97%. This study may provide high-quality heralded single-photon sources and entangled photon sources for quantum sensing, imaging, and communications at the MIR wavelength range.

Mid-infrared spectrally-pure single-photon states generation from 23 nonlinear optical crystals

We theoretically investigate the preparation of spectrally-uncorrelated biphotons from a spontaneous parametric down-conversion process using doped PPLN crystals, including MgO, ZnO, and In2O3 doped PPLN with a doping ratio from 0 to 7 mol%. Calculation results show that these crystals satisfy three kinds of group-velocity-matching (GVM) conditions at mid-infrared (MIR) wavelengths. Under the GVM1, GVM2, and GVM3 conditions, the wavelengths are at around 2.5μm, 4.0μm, and 3.2μm, and the corresponding spectral purities are around 0.97, 0.97, and 0.82, respectively. We also calculate the thermal properties of the doped PPLN crystals and their performance in Hong-Ou-Mandel interference. It can be concluded that the doping ratio can be utilized as a degree of freedom to manipulate the biphoton state. The spectrally uncorrelated biphotons can be used to prepare pure single-photon source and entangled photon source, which may have promising applications for quantum-enhanced sensing, imaging, and communications at the MIR range.

Mid-infrared spectrally-uncorrelated biphotons generation from doped PPLN

The orbital angular momentum (OAM) is an important degree of freedom for photons because the OAM state can be utilized as a high dimensional information carrier for large-capacity quantum communications. How to recognizing a single -photon-level OAM state with high accuracy in a noisy environment is a very important problem in this scenario. In this research, we use a convolutional neural 
network (CNN) technique to recognize the Laguerre-Gaussian (LG) modes at the single-photon level. Simulation and experimental results show that with the help of deep learning techniques, recognizing accuracy can be greatly improved. This study may shed new light on the OAM based quantum sensing and quantum communications in the future.

Recognizing the OAM state of a single photon using deep learning technique

Metal porphyrazines (Pz) and their analogies for example tetrathiadiazoporphyrazines (TTDDPz) attracted attention due to successful application of these compounds in different field. 
Simulation of the  electronic spectra were carried out  on the base of calculations by TTDFT method using functional B3LYP with pcseg-2 basis set.
 Analyzing the simulated electronic spectra of molecules of the form MClPz and MClTTDPz, we can observe a unsplit Q-band, which is explained by the degeneracy of LUMO and an increase in the symmetry of metalloporphyrazines to D4h relative to their protonated analogs with D2h symmetry. Thus, the molecules under consideration correspond to the Gouterman four-orbital model.
The maximum of the Q-band for MClPz molecules is in the region with a wavelength of 509-511 nm, while for MClTTDPz it is located in the region of 582-590 nm. This can be explained by the influence of the electron-withdrawing substituent (thiadiazole).

Simulation of molecular spectrums of MeClPz and MeClTTDPZ (Me=La, Lu, Y)

Continuous-time quantum computation is a method for solving computational problems that uses qubits which evolve according to a continuous-time Hamiltonian. Adiabatic quantum computing and continuous-time quantum walks are both forms of continuous-time quantum computing which have been found to be especially useful for solving optimization problems. We apply these methods of computation to an optimization problem known as the maximum-2-satisfiability problem (MAX 2-SAT). Although being NP-complete, modern classical algorithms for MAX 2-SAT have been refined to the extent that they are very effective in practice. We explore how the hardness of MAX 2-SAT instances differs for a classical algorithm and the two quantum algorithms. The results we obtain for quantum walks follow what was found previously for the problem of finding spin glass ground states, indicating that there is nothing special about the spin glass problem. We discuss a technique for producing a faster quantum algorithm using rapid quenches [see Callison et al poster & arXiv:2007.11599]. Although the classical algorithm still performs better in this case, there is potential for further improvements that could be made to the quantum algorithms.

Comparing Quantum and Classical Algorithms for MAX 2-SAT

Like their classical counterparts, today’s quantum computers are also easily affected by errors. It is crucial to develop error correction procedures to reduce the effects of the noise in the system. However, the current mainstream error correction codes, like the surface code and toric code, often require a large number of physical qubits, which is not feasible on the NISQ (noisy intermediate-scale quantum) devices where the number of available qubits is limited (< 100 qubits). This work proposed a variational quantum circuit based scheme to discover device-specific error correction codes for the NISQ computing systems in which the encode and recovery (syndrome) layers of our error correction circuit are trained together. Various choices of local optimizers are explored, and performances were compared with each other. We adopted shallow and simple-structured circuits in both encode and recovery layers to alleviate the barren plateau problem in the training of parameterized quantum circuits. The error correction circuit was trained against different kinds of error models, including depolarizing, phase and amplitude damping errors. We adopted a stronger loss constraint. We adopted a stronger loss constraint which, unlike previous attempts, enabled the error correction circuit to achieve higher fidelity on the encoding qubits.

Quantum Error Correction with Shallow Variational Quantum Circuits

We studied the morphology of quantum vortex surfaces in positron-atom ionization collisions. By conducting a detailed calculation of the position of vortices in the final momentum space with no kinematic restrictions, we could analyze the characteristics of the full four-dimensional vortex structure. This is the first time that these structures are fully calculated for positron-atom collisions. Quantum vortices have been scrutinized both theoretically and experimentally over the last decade for the ionization of atomic and molecular targets by the impact of proton, electron, and positron projectiles. Because they are submanifolds of co-dimension 2 on the phase space of the T-matrix element of the collision, they appear as points in two dimensions, lines in three dimensions, and surfaces in four dimensions. A previous exhaustive study of the ionization of atoms for different positron impact energies allowed us to show how these structures emerge in a collinear geometry as pairs of isolated points. In a subsequent work, we proved that many structures that appeared isolated when studied in two dimensions were part of a single vortex ring. In the current study, we were able to trace the full vortex structure surface.

Vortex surfaces in the ionization of atoms by positron impact

Quantum defects in solids have become a leading candidate for potential qubits. This is due to their long coherence times, the ability to read the spin state through monitoring of luminescence, and spin manipulation through microwave irradiation. These defects can be thought of as artificial molecules, as they share similar properties to fluorescent molecules containing transition metal ions. We are investigating single molecule magnets to see if we can manipulate and detect their spin by monitoring their luminescence. This is similar to the nitrogen vacancy center (NV) defect in diamond; however, we would like to find a single molecule which exhibits this behavior. The other avenue of work we are taking is use the NV center as a means of detecting molecules which do not exhibit this spin dependent luminescence. This would be done by measuring the relaxation rate of the NV center in the presence of these molecules while under an externally applied magnetic field. This relaxation would increase when the energy transitions in the molecule and NV center are on resonance from the Zeeman splitting.

Single Molecule Magnets for Spintronics and Quantum Information Processing

We analyse single hole spin interactions of disorder based quantum dots in p-type silicon MOSFET devices at a temperature of T = 2 K. Through the formation of a double-quantum dot system exhibiting Pauli spin-blockade, we characterize the magnetic field dependence of the leakage current to estimate a spin-orbit length of l_so = 300 nm. This provides support for fast electrical control of single hole spin via a strong spin orbit interaction using a readily scalable platform such as industry silicon MOSFET technology for future quantum information processing applications.


Parallel entangling gate operations in spin chins

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Mid-infrared spectrally pure photons generation from LN optimized by machine learning

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