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The existence of fundamentally identical particles represents a foundational distinction between classical and quantum mechanics. Due to their exchange symmetry, identical particles can appear to be entangled -- another uniquely quantum phenomenon with far-reaching practical implications. However, a long-standing debate has questioned whether identical particle entanglement is physical or merely a mathematical artefact. In this work, we provide such particle entanglement with a consistent theoretical description as a quantum resource in processes frequently encountered in optical and cold atomic systems. This leads to a plethora of applications of immediate practical impact. On one hand, we show that the metrological advantage for estimating phase shifts in systems of identical bosons amounts to a measure of their particle entanglement, with a clearcut operational meaning. On the other hand, we demonstrate in general terms that particle entanglement is the property resulting in directly usable mode entanglement when distributed to separated parties, with particle conservation laws in play. Application of our tools to an experimental implementation with Bose-Einstein condensates leads to the first quantitative estimation of identical particle entanglement. Further connections are revealed between particle entanglement and other resources such as optical nonclassicality and quantum coherence. Overall, this work marks a resolutive step in the ongoing debate by delivering a unifying conceptual and practical understanding of entanglement between identical particles.

Quantum 2020

Entanglement between identical particles is a useful and consistent resource

bosons

resource theory

entanglement

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. 

technical paper

The classical Gibbs paradox concerns the change in entropy upon mixing two gases. Whether or not an observer assigns an entropy increase to the process depends on their ability to distinguish the gases. A resolution is provided by realising that an ``ignorant'' observer, who cannot distinguish the gases with devices in their lab, has no way of extracting work by mixing them. Moving the thought experiment into the quantum realm, we reveal new and surprising behaviour. We show that the ignorant observer can in fact extract work from mixing different gases, even if the gases could not be directly distinguished. Moreover, in a macroscopic limit that classically recovers the ideal gas from statistical mechanics, there is a marked divergence in the quantum case: as much work can be extracted as if the gases had been fully distinguishable. Our analysis reveals that the ignorant observer assigns more microstates to the system than are found by naive state-counting in semiclassical statistical mechanics. This effect demonstrates the importance of carefully accounting for the level of knowledge of an observer, and its implications for genuinely quantum modifications to thermodynamics.

Extracting work from mixing indistinguishable systems: A quantum Gibbs "paradox''

CCP-QC is an EPSRC-funded network linking computational scientists with quantum computing scientists and engineers, to develop some of the first useful applications of quantum computers.
An important set of computational tasks in materials, chemistry,physics, 
biology, and engineering is developed by communities supported by 
collaborative computational projects (CCPs).
CCP-QC will network across these CCPs and the quantum computing 
community, to enable the CCP communities to enhance their computations 
by using quantum computers. It will do this by organising joint meetings, holding training days to teach computational scientists about quantum computing, supporting small projects to develop proof-of principle code and demonstrations on early quantum computing hardware, and providing an online information resource on early quantum computing applications. This contribution introduces CCPs in general, CCP-QC in particular, and explains how to get involved.

Collaborative Computational Project - Quantum Computing

The development of quantum technologies requires reliable processing of quantum information on large quantum systems. This is highly challenging as quantum systems are more sensitive to perturbations with increasing system size. In the last years, out-of-time-order correlations (OTOC) have gained special interest for measuring sensitivity to perturbations of many-body systems. These OTOCs can be experimentally measured with time reversion of quantum evolutions, on which the reversion is considered perfect. However, in practice the time reversion always contains imperfections. We study the spreading of information from a local state by monitoring OTOC dynamics. Using quantum simulations with solid-state Nuclear Magnetic Resonance, we engineer a ½-spin system Hamiltonian and we add a variable perturbation. When the perturbation interaction Hamiltonian is not reverted in time, we observe a localized spreading of information by the OTOC dynamic. The OTOC measures the effective cluster size of spins that were correlated by the ideal -control- Hamiltonian. If the perturbation is included in the time reversion, the localization effects are not manifested, and the information spreads over the system indefinitely. These results provide a new tool to determine the maximum system size that can be controlled in realistic experimental setups with unavoidable imperfect control operations.

Quantum information spreading measured by out-of-time-order correlations with magnetic resonance

How to characterize the interacting topological states is of particular importance in understanding the interplay between topology and interaction [1]. The main obstacle is that single-particle Bloch states are destroyed by interactions and fail to define topological invariant. Here, we use multi-particle Bloch states due to co-translational symmetry to define topological invariant for the first time [2, 3, 4]. Owing to the multi-particle topological band theory, in spin systems we have revealed topological bound edge states [2], and predicted interaction-induced topological phase transitions [4]; In bosonic systems we prove that Chern number of the multi-particle Bloch band is related to the shift of multi-particle Wannier state [3]; In fermionic systems we show interaction-induced topological bound states and Thouless pumping [5], where interaction plays an essential role. Our work creates a paradigm for multi-particle topological effects and provides a way to detect topological states in interacting multi-particle systems.

[1] X.-G. Wen, Rev. Mod. Phys. 89, 041004 (2017)
[2] X. Qin, et. al., Phys. Rev. B 96, 195134 (2017)
[3] Y Ke, et. al., Phys. Rev. A 95, 063630 (2017)
[4] X. Qin, et. al., New J. Phys. 20, 013003 (2018)
[5] L. Lin, et. al., Phys. Rev. A 101, 023620 (2020)

Theory of topological multi-particle bands

Photon correlations are central to quantum applications with light. We show that the photon-statistics of the light emitted by optical systems under coherent and resonant excitation can be modulated from Sub- to Super-Poissonian due to a self-homodyne interference. This scheme can be further controlled and/or optimized at the N-photon level by purposely tuning the coherent excitation. As one application of our theory, we design a perfectly antibunched and subnatural-linewidth (monochromatic) source, based on resonance fluorescence (a qubit weakly excited). Furthermore, the study highlights the close relation between two relevant quantum attributes of light, photon antibunching and quadrature squeezing, which were so far studied separately. This paradigm is not only interesting for single-mode correlations but can also be extended to multi-frequency spectroscopy. When the laser-qubit detuning is large, a perfect circle of antibunching between photons with different frequencies arises due to quantum interferences, revealing exotic multi-mode squeezing properties of great potential for applications.

Bringing together squeezing and antibunching of light

All-optical quantum teleportation and entanglement swapping are essential to the operation of quantum networks. Both rely on properties of entangled states of light that, in the prospect of real-life applications, should be encoded on photon pairs on demand [1]. Despite recent advances [2], however, the goal has proved elusive. In this work, we show that entangled photon pairs deterministically generated by a single quantum dot can successfully be used to perform multi-photon experiments. 
We demonstrate photonic entanglement swapping relying on a quantum emitter [3], as well as quantum teleportation with fidelity violating the classical limit for any arbitrary input state [4]. The experimental results are quantitatively predicted based on entanglement fidelity and photon indistinguishability. The role of the Bell state measurement implementation and spectral filtering are investigated within the same theoretical framework. Moving from these considerations, we envision the next steps for quantum dots towards implementing fundamental quantum protocols in practical quantum networks.
[1] D. Huber, et al., Phys. Rev. Lett. 121, 33902 (2018).
[2] J. Nilsson, et al., Nat. Phot. 7, 311 (2013).
[3] F. Basso Basset, M. Rota, C. Schimpf, D. Tedeschi, et al., Phys. Rev. Lett. 123, 160501 (2019).
[4] F. Basso Basset., et al., arXiv:2006.02733 (2020).

All-photonic quantum teleportation and entanglement swapping using on-demand solid-state quantum emitters

Reversible entanglement transfer between light and matter is a crucial requisite for the ongoing developments of quantum information technologies. Quantum networks and their envisioned applications, e.g., secure communications beyond direct transmission, distributed quantum computing or enhanced sensing, rely on entanglement distribution between nodes. Although entanglement transfer has been demonstrated, a current roadblock is the limited efficiency of this process that can compromise the scalability of multi-step architectures. Here we demonstrate the efficient transfer of heralded single-photon entanglement into and out-of two quantum memories based on large ensembles of cold cesium atoms. We achieve an overall storage-and-retrieval efficiency of 85% together
with a preserved suppression of the two-photon component of about 10% of the value for a coherent state. Our work constitutes an important capability that is needed towards large scale networks and increased functionality

Efficient reversible entanglement transfer between light and quantum memories

The Density Matrix Renormalisation Group (DMRG) is the state of the art for accurate simulation of quantum systems on a 1D lattice. It works by efficiently truncating the Hilbert space based on the local entanglement, producing important physical insights which have spawned a growing field of tensor network techniques. However, several key experiments operate in the continuum, and attempts to generalise the DMRG to continuous systems have proved difficult and challenging. Here we introduce an intuitive and practical method for extending the tools of traditional DMRG to the continuum by using a flexible set of local basis functions that are subject to a continuity constraint. We benchmark our approach against the Bethe Ansatz for interacting bosons in a box trap, and demonstrate improved scaling over simple discretisation that can be controlled by the basis set. We then apply our technique to find ground states of bosons in a sinusoidal trap, recovering known limits and showing a superfluid to Mott transition in a shallow lattice.

Density Matrix Renormalisation Group for continuous systems

Quantum computers are expected to outperform conventional computers for a range of important problems, once they can be scaled up to large numbers of qubits, typically millions. For most solid-state qubit technologies, e.g. those using superconducting circuits or semiconductor spins, scaling poses a significant challenge as every additional qubit increases the heat generated, while the cooling power of dilution refrigerators is severely limited at their operating temperature below 100 mK. Here we demonstrate operation of a scalable silicon quantum processor unit cell, comprising two qubits confined to quantum dots (QDs) at ∼1.5 kelvin. We achieve this by isolating the QDs from the electron reservoir, initialising and reading the qubits solely via tunnelling of electrons between the two QDs. We coherently control the qubits using electrically-driven-spin-resonance in isotopically enriched silicon 28Si, attaining single-qubit gate fidelities of 98.6% and coherence time T2* = 2μs during `hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures. Our work indicates that a spin-based quantum computer could be operated at elevated temperatures in a simple pumped 4He system, offering orders of magnitude higher cooling power than dilution refrigerators, potentially enabling classical control electronics to be integrated with the qubit array.

Operation of a silicon quantum processor unit cell above one kelvin

A hybrid system consisting of a quantum dot (QD) based single-photon source and a rubidium-memory is a promising platform for implementing quantum networks. For this purpose, GaAs QDs in AlGaAs are interesting since they can emit single photons frequency-matched to rubidium atoms. However, GaAs QDs suffered so far from strong telegraph noise due to an unstable charge environment. Here, we present ultra-low noise and charge-tunable GaAs QDs. We place the QDs inside a p-i-n diode and show deterministic electrical tuning of the charge-state and the emission frequency of the QDs. For a typical QD, the resonance fluorescence has an ultra-narrow linewidth of 640 MHz, just slightly above the lifetime-limit (590 MHz). The QDs are extremely stable and do not show telegraph-noise (blinking). Under resonant excitation, we achieve a high single-photon purity in combination with high photon indistinguishability (HOM-visibility up to 98%). We additionally demonstrate high fidelity optical spin initialization on an electron- as well as a hole-spin. We find that the spin has a long lifetime of up to 50μs. Together with the excellent photonic properties, the new QDs form a promising platform for a spin-photon interface.

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Entanglement between identical particles is a useful and consistent resource

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Extracting work from mixing indistinguishable systems: A quantum Gibbs "paradox''

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