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If nature allowed nonlocal correlations other than those predicted by quantum mechanics, would that contradict some physical principle? Various approaches have been put forward in the past two decades in an attempt to single out quantum nonlocality. However, none of them can explain the set of quantum correlations arising in the simplest scenarios. In [Sci. Adv. 5, eaav8370] we have shown that generalized uncertainty relations, as well as a specific notion of locality, give rise to both familiar and new characterizations of quantum correlations. In particular, we have identified a condition, relativistic independence, which states that uncertainty relations are local in the sense that they cannot be influenced by other experimenters’ choices of measuring instruments. We proved that theories with nonlocal correlations stronger than the quantum ones do not satisfy this notion of locality, and therefore, they either violate the underlying generalized uncertainty relations or allow experimenters to nonlocally tamper with the uncertainty relations of their peers. In this talk I will discuss several implications of this work to multiplicative Bell inequalities, causality and networks. Finally, I will support the claim that locally consistent indeterminism should be understood as the enabler of most quantum phenomena (and possibly all of them).

Quantum 2020

Relativistic independence and the immense explanatory power of quantum uncertainty

quantum nonlocality

quantum entanglement

quantum uncertainty

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

Randomized benchmarking (RB) protocols represent an efficient tool to compare different qubit technologies and architectures. Despite traditional methods of characterizing gate fidelity that involve quantum process tomography, RB possesses some advantages in terms of the robustness against the state preparation and measurement (SPAM) errors, beyond the fact that it scales efficiently with the system size, requiring fewer resources. RB experiments have been performed in many different physical contexts for quantum technologies, ranging from superconducting qubits, trapped ions or semiconducting qubits, i.e. nitrogen-vacancy centers in diamond, quantum dots and donors (e.g. phosphorous) atoms in silicon. In this study, we simulated randomized benchmarking protocols in a semiconducting all-electrical three-electron double-quantum dot qubit, i.e. hybrid qubit, under different error models, that include quasi-static Gaussian and the more realistic 1/f noise model, for the input controls. The average error of specific quantum computational gates is extracted through interleaved randomized benchmarking obtained including Clifford gates between the gate of interest. Quantitative results are reported that on one hand allow to predict the fidelity when specific error parameters are included, on the other provide a complete picture of the hybrid qubit operations in view of realization of more complex circuits, such as quantum error correction circuits.

Randomized benchmarking protocols in the all-electrical quantum dot qubit with 1/f noise

The concept of topological phases is a powerful framework to characterize many-body systems beyond the paradigm of symmetry breaking. In addition to studying topological phases in condensed-matter systems, a large effort is underway to realize topological matter in quantum simulators. Various platforms have already succeeded in designing non-trivial topological single-particle modes [Ozawa2018,Cooper2018]; however, these modes have only been populated with non-interacting particles, where properties of the many-body states are solely determined by the single-particle modes. A current challenge is to introduce strongly interacting particles in these systems and search for emergent properties of the many-body states. 

Here, we use a quantum simulator based on atoms trapped in optical tweezers and excited to Rydberg levels [Browaeys2020] to realize a symmetry-protected topological (SPT) phase of interacting bosons in a 1D-lattice hosting non-trivial topological modes [deLéséleuc2019]. I will present signatures of the single-particle modes and of the many-body SPT phase obtained by half-filling the lattice with hard-core bosons. Especially, I will show that some perturbations affect single-particle modes, but not the many-body phase: in striking contrast from what occurs in non-interacting topological phases.

[Ozawa2018,Cooper2018] RMP 91, 015005 and 015006 (2018)
[Browaeys2020] Nat. Phys. 16, 132 (2020)
[deLéséleuc2019] Science 365, 775 (2019)


Observation of a symmetry-protected topological phase of interacting bosons with Rydberg atoms

Open classical and quantum systems with effective parity-time (PT) symmetry have shown tremendous promise for advances in sensing, lasers and non-reciprocal devices. And yet, how such effective PT-symmetric non-Hermitian models emerge out of Hermitian quantum mechanics is not well understood. An active quantum PT-symmetric system, which requires a precisely controllable gain medium, is yet to be realized. Starting from a fully Hermitian microscopic Hamiltonian description, we show that a non-Hermitian Hamiltonian emerges naturally in an already state-of-the-art double-quantum-dot-circuit-QED (DQD-circuit-QED) set-up, which can be controllably tuned to an active PT-symmetric point. This effective Hamiltonian governs the dynamics of two coupled circuit-QED cavities with a voltage-biased DQD in one of them, which acts as the tunable gain medium. Our analysis also reveals the effect of quantum fluctuations on the PT-symmetric system. As simple applications of the PT-transition in this set-up, we show that loss-induced lasing and loss-induced trapping of photons can be observed in the coupled cavities. Our results pave the way for an on-chip realization of a potentially scalable non-Hermitian system with a gain medium in quantum regime, as well as its potential applications for quantum technology.

Emergent PT symmetry in a double-quantum-dot circuit QED set-up

We present a fine-structure entanglement classification under stochastic local operation and classical communication (SLOCC) for multiqubit pure states. To this end, we employ specific algebraic-geometry tools that are SLOCC invariants, secant varieties, to show that for n-qubit systems there are ⌈2n/(n+1)⌉ entanglement families. By using another invariant, ℓ-multilinear ranks, each family can be further split into a finite number of subfamilies. Not only does this method facilitate the classification of multipartite entanglement, but it also turns out to be operationally meaningful as it quantifies entanglement as a resource.

Fine-Structure Classification of Multiqubit Entanglement by Algebraic Geometry

An excited electron can relax by emitting a photon. In a radiative Auger process, this optical decay leads to the simultaneous excitation of another particle (shake-up). On a single quantum emitter, radiative Auger has not been observed. Here, we report radiative Auger for trions in individual quantum dots. For the trion, just one electron is left after the optical recombination. The radiative Auger process promotes this Auger electron to a higher shell of the quantum dot, and the emitted photon is red-shifted. We show that the energy splitting between this red-shifted photon and the resonance fluorescence directly measures the single-particle splittings of the quantum dot [1], which is otherwise difficult to acquire. We prove the radiative Auger mechanism by measuring the photon statistics and the magnetic field dispersion of the emission. Going beyond the original work in the X-ray spectrum of atoms, we apply quantum optics techniques to the radiative Auger photons. We show how quantum optics gives access to the single-electron dynamics, notably the relaxation and tunneling rates. All these properties of radiative Auger can be exploited on other semiconductor nanostructures and quantum emitters in the solid-state.

[1] M. C. Löbl et al., Nat. Nanotechnol. 15, 558–562 (2020)

Radiative Auger process in the single-photon limit

We propose an efficient, scalable and deterministic scheme[1] to generate up to hundreds of indistinguishable photons over multiple channels, on demand. Our design relies on multiple single-photon sources, each coupled to a waveguide, and all of them interacting with a common cavity mode. The cavity synchronizes and triggers the simultaneous emission of one photon by each source, which are collected by the waveguides. For a state-of-the-art circuit QED implementation, this scheme supports the creation of single photons with purity, indistinguishability, and efficiency of $99\%$ at rates of $\sim $MHz. We also discuss conditions to create a device to produce 30-photon simultaneously with efficiency above $70\%$ at a rate of hundreds of kHz. This is several orders of magnitude more efficient than previous demultiplexed sources for boson sampling, and enables the realization of deterministic multi-photon sources and scalable quantum information processing with photons.

[1] Li, M., et al., arXiv:2009.02382 (2020).

Synchronization of single-photon sources via a cavity

The nonclassical properties of quantum states are of tremendous interest due to their potential applications in future technologies. It has recently been realized that the concept of a resource theory is a powerful approach to quantifying and understanding nonclassicality. To realize the potential of this approach, one must first find resource theoretic measures of nonclassicality that are operational, meaning that they also quantify the ability of quantum states to provide enhanced performance for specific tasks, such as precision sensing.

In this talk, I will discuss some interesting results we have achieved in this endeavor by presenting the first operational resource theoretic measure [1]. In addition to satisfying the requirements of a resource measure, I will show that our measure has the closest possible relationship to the quantum enhancement provided by a nonclassical state for measuring phase-space displacement: It is equal to this enhancement for pure states and has a tight upper bound on it for mixed states. I will further show that a lower bound on this measure can be obtained experimentally using a simple Mach-Zehnder interferometer.

[1] W. Ge, K. Jacobs, S. Asiri, M. Foss-Feig, and M. Suhail Zubairy Phys. Rev. Research 2, 023400 (2020)

Quantifying Nonclassicality via the Precision in Quantum Metrology

Quantum metrology is the interdisciplinary of investigating how to utilize the principles of quantum mechanics to perform parameter estimation and improve the measurement precision by quantum effects. Attributed to well-developed techniques of quantum control, one can prepare several exotic non-Gaussian multi-particle entangled states in the ensembles of ultracold atoms. Based on many-body quantum interferometry, and using these non-Gaussian entangled states as probe, the high-precision measurement beyond the standard quantum limit can be realized . Utilizing suitable operations of quantum control, the non-Gaussian entangled states can be used for realizing Heisenberg-limited sensors, such as Sagnac interferometers, magnetometers, and many-body lock-in amplifiers. In this presentation, we will introduce the backgrounds and advances of this field.

Quantum metrology with non-Gaussian entangled states

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.

Entanglement between identical particles is a useful and consistent resource

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.

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