China

We consider the transport properties of a single-electron transistor consisting of a diffusive normal-metal island and superconducting electrodes. We show that the interplay of a dissipative impedance of the gate electrode, a weak magnetic field, and a charging energy of the island causes a suppression of the critical supercurrent of the transistor. When the device is voltage biased, pumping of single electrons can be achieved by a periodic modulation of the gate voltage [1-3]. We derive the analytic expressions for the error rates and show that they are suppressed by the applied magnetic field [4].

[1] J. P. Pekola et al, Nat. Phys. 4 (2008) 120.
[2] D. V. Averin and J. P. Pekola, Phys. Rev. Lett. 101 (2008) 066801.
[3] S. Nakamura et al, Phys. Rev. Appl. 7 (2017) 054021.
[4] V. Bubanja, Phys. Rev. B 97 (2018) 224516.

Quantum 2020

Effect of quantum fluctuations on the critical supercurrent through a mesoscopic normal-metal island

electron transport in mesoscopic systems

quantum fluctuations

josephson effects

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CAP Virtual Congress 2021

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. 

The recent miniaturization of heat engines to the nanoscale introduces the possibility of engines that harness quantum resources. The analysis of quantum engines provides important insight into how their efficiency compares to classical analogues and deepens our understanding of thermodynamic mechanisms at the quantum scale. We examine a quantum Otto engine with a harmonic working medium consisting of two particles to explore the use of wave function symmetry as an accessible resource. It is shown that a bosonic working medium displays enhanced performance when compared to distinguishable particles, while a fermionic working medium displays reduced performance. To this end, we explore the trade-off between efficiency and power output and the parameter regimes under which the system functions as engine, refrigerator, or heater. Remarkably, the bosonic system operates under a wider parameter space both when operating as an engine and as a refrigerator.

Bosons Outperform Fermions: The Thermodynamic Advantage of Symmetry

Variational quantum algorithms are believed to be promising for solving computationally hard problems on noisy intermediate-scale quantum (NISQ) systems. Gaining computational power from these algorithms critically relies on the mitigation of errors during their execution, which for coherence-limited operations is achievable by reducing the gate count. Here, we demonstrate an improvement of up to a factor of 3 in algorithmic performance for the quantum approximate optimization algorithm (QAOA) as measured by the success probability, by implementing a continuous hardware-efficient gate set using superconducting quantum circuits. This gate set allows us to perform the phase separation step in QAOA with a single physical gate for each pair of qubits instead of decomposing it into two CZ gates and single-qubit gates. With this reduced number of physical gates, which scales with the number of layers employed in the algorithm, we experimentally investigate the circuit-depth-dependent performance of QAOA applied to exact-cover problem instances mapped onto three and seven qubits, using up to a total of 399 operations and up to nine layers. Our results demonstrate that the use of continuous gate sets may Q2 be a key component in extending the impact of near-term quantum computers.

Improving the Performance of Deep Quantum Optimization Algorithms with Continuous Gate Sets

The theory of the asymptotic manipulation of pure bipartite quantum systems can be considered completely understood: the rates at which bipartite entangled states can be asymptotically transformed into each other are fully determined by a single number each, the respective entanglement entropy. In the multipartite setting, similar questions of the optimally achievable rates of transforming one pure state into another are notoriously open. This seems particularly unfortunate in the light of the revived interest in such questions due to the perspective of experimentally realizing multipartite quantum networks. In this talk, we report substantial progress by deriving simple upper and lower bounds on the rates that can be achieved in asymptotic multipartite entanglement transformations. These bounds are based on ideas of entanglement combing and state merging. We identify cases where the bounds coincide and hence provide the exact rates. As an example, we bound rates at which resource states for the cryptographic scheme of quantum secret sharing can be distilled from arbitrary pure tripartite quantum states. This result provides further scope for quantum internet applications, supplying tools to study the implementation of multipartite protocols over quantum networks.

Rates of Multipartite Entanglement Transformations

Quantum simulation, being one of the most promising applications of a quantum computer, has been realised on current noisy intermediate-scale quantum (NISQ) devices using different algorithms. Among these, the variational quantum eigensolver is extensively used for finding the ground state energy of molecular Hamiltonians. The feasibility and performance of this algorithm depend critically on the number of variational parameters in the wavefunction ansatz and on the depths of the state preparation circuits that create the ansatz on the quantum processor. Recently, an algorithm termed ADAPT-VQE was introduced to build system-adapted ansätze with substantially fewer variational parameters compared to other approaches. However, deep state preparation circuits remain a challenge. Here, we present a hardware-efficient variant of this algorithm called qubit-ADAPT. By numerical simulations on H4, LiH and H6, we show that with a well-designed operator pool, qubit-ADAPT can reduce the circuit depth by one order of magnitude while maintaining the same accuracy as the original ADAPT-VQE. Addressing the high measurement cost which is proportional to the size of the operator pool, we show the possibility to construct a sufficient pool with size growing linearly in the number of qubits. This result highlights the promise of adaptive simulation algorithms on near-term quantum devices.

qubit-ADAPT-VQE: An adaptive algorithm for constructing hardware-efficient ansätze on a quantum processor

The possibility of readout and transferring information through qubits are crucial issue in the development of quantum computers. One of the promising ways to construct qubits is based on single phosphorus atoms doped in crystalline silicon. In present work Quantum Espresso software package, based on the density functional theory and the pseudopotential method, was used to non-collinear study of the equilibrium spin orientation of P atom in silicon matrix and its manipulation possibility. The total magnetization of the Si:P system is determined by the presence of an unpaired electron of the phosphorus atom. Excess electron density is not localized only near the phosphorus atom, but is spatially distributed in some area, so magnetization also has a spatial distribution. We had calculated the local magnetic field B(r), the local magnetization m(r) and the spin current density created by the excess electron of the P atom. It had been shown that the spin current densities has different directions for the |0> and |1> quantum states.

The work was supported in accordance with the State assignment for research work implementation from the Ministry of Science and Higher Education of the Russian Federation (project No 0818-2020-0005).

Ab initio study of possibility of phosphorus atom spin manipulation in doped P:Si crystal

The multiphoton Jaynes-Cummings model is investigated and applications in quantum information science are explored. Considering the strong atom-field coupling regime and an N-photon interaction, a nonlinear driving field can perform an arbitrary rotation in the Fock space of a bosonic mode involving the vacuum and an M-Fock state, with M < N. In addition, driving a bosonic mode with a linear coherent field (superposition of many Fock states), only the cavity states within the Fock subspace |0>; |1>;…; |N − 1> can be populated; i.e., we show how to implement a Fock state filter, or quantum scissor, that restricts the dynamics of a given bosonic mode to a limited Hilbert space. Such a device can be employed as a generator of finite-dimensional quantum-optical states and also as a quantum-optical intensity limiter, allowing as a special case the generation of single-photon pulses. On the other hand, our system also provides a very rich physics in the weak atom-field coupling regime, in particular, multiphoton electromagnetically induced transparencylike phenomena, inducing a narrow (controllable) reflectivity window for nonlinear probe fields. These results are useful for applications in quantum information processing and also motivate further investigations, e.g., the use of an N-photon Jaynes-Cummings system as a qudit with harmonic spectrum and the exploration of multiphoton quantum interference.

Multiphoton Jaynes-Cummings Model: Arbitrary Rotations in Fock Space and Quantum Filters

The efficient and reliable characterization of quantum states plays a vital role in most, if not all, quantum information processing tasks. In this work, we present a universally optimal protocol for verifying entangled states by employing the so-called quantum non-demolition measurements, such that the verification efficiency is equivalent to that of the optimal global strategy. Instead of being probabilistic as the standard verification strategies, our protocol is constructed sequentially, which is thus more favorable for experimental realizations. In addition, the target states are preserved in the protocol after each measurement, so can be reused in any subsequent tasks. We demonstrate the power of our protocol for the optimal verification of Bell states, arbitrary two-qubit pure states, and stabilizer states. We also prove that our protocol is able to perform tasks including fidelity estimation and state preparation.

Universally optimal verification of entangled states with non-demolition measurements

The famous Kibble-Zurek mechanism offers us a significant clue to study quantum phase transitions out of equilibrium. Here, we investigate an intriguing phenomenon of a Spin-Orbit coupled Bose-Einstein condensate by quenching the Raman coupling strength from high symmetry phase (non-magnetic phase) to low symmetry phase (magnetic phase). When crossing the critical point,the fluctuation of momentum distribution leads to delayed bifurcation structures. Simultaneously,the domain information emerges in momentum space. Moreover, the universal scalings of spatiotemporal dynamics are extracted from the fluctuations and domains, which manifests homogeneous and inhomogeneous Kibble-Zurek power-laws at different time scale. Our work demonstrates a paradigmatic study on inhomogeneous Kibble-Zurek mechanism.

Exploring inhomogeneous Kibble-Zurek mechanism in a spin-orbit coupled Bose-Einstein condensate

In quantum mechanics, physical states are represented by rays in Hilbert space, which is a vector space imbued by an inner product whose physical meaning arises as the overlap between a pure state (description of preparation) and a projective measurement. However, quantum mechanics in principle does not formally address the consequences of preparation and measurement being at different spacetime sites. Following concepts of PT symmetry, we introduce purity-preserving transport, which can be interpreted as a changing inner product. We show that such a change in inner product is consistent with standard quantum mechanics, and is observable in a tomographic sense. Mathematically, we show that changing the inner product can be realized by a quantum channel corresponding to a completely positive trace-nonincreasing map, and observing a changing inner product is equivalent to process tomography on this channel. Further, we show how to realize experimentally a changing inner product for a qubit Hilbert space in terms of a qutrit protocol by using only unitary dynamics.

Changing the Hilbert-space Inner Product


Recent experimental advances allow one to engineer topological models in bosonic systems like photonic crystals, coupled microwave resonators, or optical lattices experiments, among other platforms [1]. In parallel, experimental progress is also being made to couple quantum emitters to such platforms in order to obtain (or simulate) light-matter Hamiltonians [2-4] with topological photons. The emergent behaviour and applications of such topological quantum optics models, however, remain mostly unexplored.


In this talk, I will discuss the emergence of unconventional quantum dynamics and interactions when quantum emitters are coupled to topological 1D waveguides [5] and 3D photonic Weyl environments [6]. We will show how the photon-mediated interactions induced by these topological photons give rise to the emergence of spin models with exotic and tunable long-range interactions, that can be used for quantum simulation purposes, e.g., to simulate quantum chemistry problems [7].



References
[1] Rev. Mod. Phys. 91, 015006 (2019)
[2] Nature 508, 241–244 (2014), Nature Communications 5, 3808 (2014), Rev. Mod. Phys. 87, 347 (2015), Science 359, 666 (Feb 2018)
[3] Nature Physics 13 (1), 48-52 (2017), Nature Communications, 9, 3706 (2018)
[4] Nature 559, 589–592 (2018)
[5] Science Advances 5, eaaw0297 (2019)
[6] arXiv:1903.07513
[7] Nature 574 , 215-218 (2019)

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Effect of quantum fluctuations on the critical supercurrent through a mesoscopic normal-metal island

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