Exploring the properties and applications of topological quantum states is essential to better understand topological matter. Here, we theoretically study a quasi-one-dimensional topological atom array. In the low-energy regime, the atom array is equivalent to a topological superatom. Driving the superatom in a cavity, we study the interaction between light and topological quantum states. We find that the edge states exhibit topology-protected quantum coherence, which can be characterized from the photon transmission. This quantum coherence helps us to find a superradiance-subradiance transition, and we also study its finite-size scaling behavior. The superradiance-subradiance transition also exists in symmetry-breaking systems. More importantly, it is shown that the quantum coherence of the subradiant edge state is robust to random noises, allowing the superatom to work as a topologically protected quantum memory. We suggest a relevant experiment with three-dimensional circuit QED. Our study may have applications in quantum computation and quantum optics based on topological edge states.

The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. The quantum sensor does so with unprecedented DC sensitivity at micron resolution from room-to-cryogenic temperatures. An additional advantage of the SQCRAMscope is the preservation of optical access to the sample: Magnetometry imaging of, e.g., electron transport may be performed in concert with other imaging techniques. This multimodal imaging capability can be brought to bear with great effect in the study of nematicity in iron-pnictide high-temperature superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface. This enables simultaneous and spatially resolved detection of both bulk and near-surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials.

Finding the right material platform for engineering efficient photonic interfaces to solid state emitters has been a long-standing bottleneck for scaling up solid state quantum systems. In this work, we demonstrate that nitrogen-rich silicon nitride, with its low background auto-fluorescence at visible wavelengths, is a viable quantum photonics platform by showing that nitrogen vacancy centres embedded in nanodiamonds preserve both their quantum optical and spin properties post-encapsulation. Given the variety of high-performance photonic components already demonstrated in silicon nitride, our work opens up a new avenue for building integrated photonic circuits using solid state emitters.

We propose a novel scheme to prepare a macroscopic mechanical oscillator in a cat-like state, close to a coherent state superposition. The mechanical oscillator, coupled by radiation-pressure interaction to a field in an optical cavity, is first prepared close to a squeezed vacuum state using a reservoir engineering technique. The system is then probed using a short optical pulse tuned to the lower motional sideband of the cavity resonance, realizing a photon-phonon swap interaction. A photon number measurement of the photons emerging from the cavity then conditions a phonon-subtracted cat-like state with a negative Wigner distribution exhibiting separated peaks and multiple interference fringes. We show that this scheme is feasible using state-of-the-art photonic crystal optomechanical system.

Triple quantum dots (TQDs) are promising semiconductor spin qubits because of their all-electrical control via fast, tunable exchange interactions and immunity to global magnetic fluctuations. These qubits couple to photons in the resonant exchange (RX) regime, when exchange is simultaneously active on both qubit axes. However, most theoretical work has been based on phenomenological Fermi-Hubbard models, which may not fully capture the complexity of the qubit spin-charge states in this regime. Here we investigate exchange in Si/SiGe and GaAs TQDs using full configuration interaction (FCI) calculations which better describe practical device operation. We show that high exchange operation in general, and the RX regime in particular, can differ significantly from simple models, presenting new challenges and opportunities for spin-photon coupling. We highlight the impact of device electrostatics and effective mass on exchange and identify a new operating point (XRX) where strong spin-photon coupling is most likely to occur in Si/SiGe TQDs. Based on our numerical results, we analyze the feasibility of a remote entanglement cavity iSWAP protocol and discuss design pathways for improving fidelity. Our analysis provides insight into the requirements for TQD spin-photon transduction and demonstrates more generally the necessity of accurate modeling of exchange in spin qubits.

We present how basic logic gates including NAND, NOR and XOR gates can be implemented counterfactually. The two inputs (Bob and Charlie) and the output (Alice) of the proposed counterfactual logic gate are not within the same station but rather separated in three different locations. We show that there is no need to pre-arrange entanglement for the gate, and more importantly, there is no real physical particles traveling among Alice, Bob and Charlie during the information processing. Bob and Charlie only need to independently control the blocking and unblocking of the transmission channels that connect them to Alice. In this way, they can completely determine the state of a real photon at Alice's end, thereby leading to implement a counterfactual logic gate. The functionality of a particular counterfactual logic gate is determined only by an appropriate design of Alice's optical device. Furthermore, by utilizing the proposed counterfactual logic gates, we demonstrate how to counterfactually prepare the Greenberger-Horne-Zeilinger state and W state with three remote quantum objects, which are in a superposition state of blocking and unblocking the transmission channel.

Light-pulse atom interferometers constitute powerful quantum sensors for inertial forces. They are based on delocalised spatial superpositions and the combination with internal transitions directly links them to atomic clocks. Since classical tests of the gravitational redshift are based on a comparison of two clocks localised at different positions under gravity, it is promising to explore whether the aforementioned interferometers constitute a competitive alternative for tests of general relativity. Here we present a specific geometry which together with state transitions leads to a scheme that is concurrently sensitive to both violations of the universality of free fall and gravitational redshift, two premises of general relativity. The proposed interferometer does not rely on a superposition of internal states, but merely on transitions between them, and therefore generalises the concept of physical atomic clocks and quantum-clock interferometry. An experimental realisation seems feasible with already demonstrated techniques in state-of-the-art facilities.

We present an information-theoretic interpretation of quantum formalism based on a Bayesian framework and devoid of any extra axiom or principle. Quantum information is merely construed as a technique of Bayesian inference for analyzing a logical system subject to classical constraints. This sole procedure leads to completely recover the standard quantum information theory and thus provides an information-theoretic rationale to its technical rules. In this framework, the great challenges of quantum mechanics become simple platitudes: Why is the theory probabilistic? Why is the theory linear? Where does the Hilbert space come from? In addition, most of the paradoxes, such as uncertainty principle, entanglement, contextuality, nonsignaling correlation, measurement problem, etc., become straightforwards features, while the amount of information conveyed by a wave vector is clarified. In the end, our major conclusion is that quantum information is nothing but classical information processed by Bayesian inference techniques and as such, consubstantial with Aristotelian logic.

In quantum mechanics, photonic de Broglie waves have been understood as a unique property of quantum mechanics satisfying the complementarity between particle and wave natures of light, where the photonic de Broglie wavelength is inversely proportional to the number of entangled photons acting on a beam splitter. Very recently, the heart of nonclassical feature of photon bunching on a beam splitter was newly interpreted using pure wave nature of coherence optics [arXiv:1911.07174v2], paving a road to unconditionally secured classical key distribution [arXiv:1807.04233v3]. Here, Mach-Zehnder interferometer-based deterministic photonic de Broglie waves are studied in a coherence regime for both fundamental physics and potential applications of coherence-quantum metrology.

Generalized electrostatic quantum swap gate implemented in the chain of 2 double coupled quantum dots using single electron in semiconductor is presented in tight-binding simplistic model specifying both analytic and numerical results. The anticorrelation principle coming from Coulomb electrostatic repulsion is exploited in single electron devices. The formation of quantum entanglement is specified and supported by analytical results. The difference between classical and quantum picture is given. The correlations between geometry of quantum structures and entanglement dynamics are shown in analytical way. Effective wavefunction of single electrons is extracted from 2 electron electrostatic interactions. The presented results have its significance in cryogenic CMOS quantum technologies that gives perspective of implementation of semiconductor quantum computer on massive scale.

Keyword: electrostatic quantum swap gate, entanglement, anticorrelation, tight-binding model, two-body wavefunction, correlation-anticorrelation crossover, density matrix of 2 electron system, single electron devices, coupled quantum dots

What can one infer about the dynamical evolution of quantum systems just by symmetry considerations? For Markovian dynamics in finite dimensions, we present a simple construction that assigns to each symmetry of the generator a family of scalar functions over quantum states that are monotonic under the time evolution. The aforementioned monotones can be utilized to identify states that are non-reachable from an initial state by the time evolution and include all constraints imposed by conserved quantities, providing a generalization of Noether's theorem for this class of dynamics. As a special case, the generator itself can be considered a symmetry, resulting in non-trivial constraints over the time evolution, even if all conserved quantities trivialize. The construction utilizes tools from quantum information-geometry, mainly the theory of monotone Riemannian metrics. We analyze the prototypical cases of dephasing and Davies generators.

Isothermal transformations are minimally dissipative but slow processes, as the system needs to remain close to thermal equilibrium along the protocol. Here, we show that smoothly modifying the system-bath interaction can significantly speed up such transformations. In particular, we construct protocols where the overall dissipation $W_{\rm diss}$ decays with the total time $\tau_{\rm tot}$ of the protocol as $W_{\rm diss} \propto \tau_{\rm tot}^{-2\alpha-1}$, where each value $\alpha > 0$ can be obtained by a suitable modification of the interaction, whereas $\alpha=0$ corresponds to a standard isothermal process where the system-bath interaction remains constant. Considering heat engines based on such speed-ups, we show that the corresponding efficiency at maximum power interpolates between the Curzon-Ahlborn efficiency for $\alpha =0$ and the Carnot efficiency for $\alpha \to \infty$. We confirm our analytical results with two numerical examples where $\alpha = 1/2$, namely the time-dependent Caldeira-Leggett and resonant-level models, with strong system-environment correlations taken fully into account.

Entangled states, such as the Bell and GHZ states, are generated from separable states using matrices known to satisfy the Yang-Baxter equation and its generalization. This remarkable fact hints at the possibility of using braiding operators as quantum entanglers, and is part of a larger speculated connection between topological and quantum entanglement. We push the analysis of this connection forward, by showing that supersymmetry algebras can be used to construct large families of solutions of the spectral parameter-dependent generalized Yang-Baxter equation. We present a number of explicit examples and outline a general algorithm for arbitrary numbers of qubits. The operators we obtain produce, in turn, all the entangled states in a multi-qubit system classified by the Stochastic Local Operations and Classical Communication protocol introduced in quantum information theory.

Entanglement, while being critical in many quantum applications, is difficult to characterize experimentally. While entanglement witnesses based on the fidelity to the target entangled state are efficient detectors of entanglement, they in general underestimate the amount of entanglement due to errors during state preparation and measurement. Therefore, to detect entanglement more robustly in the presence of such control errors, we introduce a 'subspace' witness that detects a broader class of entangled states with strictly larger violation than the conventional state-fidelity witness at the cost of additional measurements, while remaining more efficient with respect to state tomography. We experimentally demonstrate the advantages of the subspace witness by generating and detecting entanglement with a hybrid, two-qubit system composed of electronic spins in diamond. We further extend the notion of subspace witness to specific genuine multipartite entangled (GME) states such as GHZ, W, and Dicke states, and motivate the choice of the metric based on quantum information tasks such as entanglement-enhanced sensing. In addition, as the subspace witness identifies the many-body coherences of the target entangled state, it facilitates (beyond detection) lower-bound quantification of entanglement via generalized concurrences. We expect the straightforward and efficient implementation of subspace witnesses would be beneficial in detecting specific GME states in noisy, intermediate-scale quantum processors with a hundred qubits.

In this work we discuss the failure of the principle of truth functionality in the quantum formalism. By exploiting this failure, we import the formalism of N-matrix theory and non-deterministic semantics to the foundations of quantum mechanics. This is done by describing quantum states as particular valuations associated to infinite non-deterministic truth tables. This allows us to introduce a natural interpretation of quantum states in terms of a non-deterministic semantics. We also provide a similar construction for arbitrary probabilistic theories based in orthomodular lattices, allowing to study post-quantum models using logical techniques.

In this paper we report a continuous-variable quantum key distribution protocol using multimode coherent states generated on subcarrier frequencies of the optical spectrum. To detect the quadrature components of bosonic field we propose a coherent detection scheme where power from a carrier wave is used as a local oscillator. We compose a mathematical model of the proposed scheme and perform its security analysis in the finite-size regime using fully quantum asymptotic equipartition property technique. We calculate a lower bound on the secret key rate for the system under the assumption that the quantum channel noise is negligible compared to detector dark counts, and an eavesdropper is restricted to collective attacks. Our calculation shows that the current realistic system implementation would allow distributing secret keys over channels with losses up to 9 dB.

Author(s): P. R. Berman

A theory is presented that describes the atom and field dynamics for two atoms in a chiral waveguide. A source-field approach is used that enables one to identify the various physical processes contributing to these dynamics. Each atom is prepared in an arbitrary state at t=0 and the field intensity...

[Phys. Rev. A 101, 013830] Published Tue Jan 28, 2020

Author(s): Valery E. Lobanov, Nikita M. Kondratiev, Artem E. Shitikov, and Igor A. Bilenko

We studied numerically the generation of the coherent frequency combs at second-harmonic generation in χ(2) microresonators via the conventional frequency scan method. It was demonstrated that under particular conditions it is possible to generate two-color flat-top solitonic pulses, platicons, usin...

[Phys. Rev. A 101, 013831] Published Tue Jan 28, 2020

Author(s): Aonan Zhang, Jie Xie, Huichao Xu, Kaimin Zheng, Han Zhang, Yiu-Tung Poon, Vlatko Vedral, and Lijian Zhang

The accurate and reliable description of measurement devices is a central problem in both observing uniquely nonclassical behaviors and realizing quantum technologies from powerful computing to precision metrology. To date quantum tomography is the prevalent tool to characterize quantum detectors. H...

[Phys. Rev. Lett. 124, 040402] Published Tue Jan 28, 2020

Author(s): Yijian Zou, Ashley Milsted, and Guifre Vidal

At a quantum critical point, the low-energy physics of a quantum spin chain is described by conformal field theory (CFT). Given the Hamiltonian of a critical spin chain, we propose a variational method to build an approximate lattice representation ϕα of the corresponding primary CFT operators ϕαCFT...

[Phys. Rev. Lett. 124, 040604] Published Tue Jan 28, 2020