Employing the volume of quantum steering ellipsoids (QSEs) as a measure onthe fifteen-dimensional convex set of two-qubit states, we estimate the ratioof the integral of the measure over the separable states to its integral overall (separable and entangled) states to be 0.0288. This can be contrasted withthe considerably larger separability ratios (probabilities) of $\frac{8}{33} =\frac{2^3}{3 \cdot 11} \approx 0.242424$ and $\frac{25}{341}=\frac{5^2}{11\cdot 31} \approx 0.0733138$ that various forms of evidence point to with theuse of the prominent Hilbert-Schmidt and Bures measures, respectively. Thequestions of whether the ratio in the QSE setting can be more preciselyobtained or even exactly computed, as well as whether a metric can beconstructed, the volume element of which yields the measure, remain to beaddressed. We also investigate related issues pertaining to absoluteseparability. Further, we examine the behavior of the separabilityprobability--constant in the Hilbert-Schmidt case and decreasing in theBures--as a function of the Bloch vector norm in the QSE instance. It appearsto increase approaching the pure state boundary.
Are neural quantum states good at solving non-stoquastic spin Hamiltonians?Variational Monte Carlo with neural network quantum states has proven to be apromising avenue for evaluating the ground state energy of spin Hamiltonians.Based on anecdotal evidence, it has been claimed repeatedly in the literaturethat neural network quantum state simulations are insensitive to the signproblem. We present a detailed and systematic study of restricted Boltzmannmachine (RBM) based variational Monte Carlo for quantum spin chains, resolvingexactly how relevant stoquasticity is in this setting. We show that in mostcases, when the Hamiltonian is phase connected with a stoquastic point, thecomplex RBM state can faithfully represent the ground state, and localquantities can be evaluated efficiently by sampling. On the other hand, weidentify a number of new phases that are challenging for the RBM Ansatz,including non-topological robust non-stoquastic phases as well as stoquasticphases where sampling is nevertheless inefficient. Our results suggest thatgreat care needs to be taken with neural network quantum state basedvariational Monte Carlo when the system under study is highly frustrated.
A Bayesian analysis of classical shadowsThe method of classical shadows heralds unprecedented opportunities forquantum estimation with limited measurements [H.-Y. Huang, R. Kueng, and J.Preskill, Nat. Phys. 16, 1050 (2020)]. Yet its relationship to establishedquantum tomographic approaches, particularly those based on likelihood models,remains unclear. In this article, we investigate classical shadows through thelens of Bayesian mean estimation (BME). In direct tests on numerical data, BMEis found to attain significantly lower error on average, but classical shadowsprove remarkably more accurate in specific situations -- such as high-fidelityground truth states -- which are improbable in a fully uniform Hilbert space.We then introduce an observable-oriented pseudo-likelihood that successfullyemulates the dimension-independence and state-specific optimality of classicalshadows, but within a Bayesian framework that ensures only physical states. Ourresearch reveals how classical shadows effect important departures fromconventional thinking in quantum state estimation, as well as the utility ofBayesian methods for uncovering and formalizing statistical assumptions.
Tests of Fundamental Quantum Mechanics and Dark Interactions with Low Energy Neutrons -- Extended VersionAmong the known particles, the neutron takes a special position, as itprovides experimental access to all four fundamental forces and a wide range ofhypothetical interactions. Despite being unstable, free neutrons live longenough to be used as test particles in interferometric, spectroscopic, andscattering experiments probing low-energy scales. As was already recognized inthe 1970s, fundamental concepts of quantum mechanics can be tested in neutroninterferometry using silicon perfect-single-crystals. Besides allowing fortests of uncertainty relations, Bell inequalities and alike, neutrons offer theopportunity to observe the effects of gravity and hypothetical dark forcesacting on extended matter wave functions. Such tests gain importance in thelight of recent discoveries of inconsistencies in our understanding ofcosmology as well as the incompatibility between quantum mechanics and generalrelativity. Experiments with low-energy neutrons are thus indispensable toolsfor probing fundamental physics and represent a complementary approach tocolliders. In this review we discuss the history and experimental methods usedat this low-energy frontier of physics and collect bounds and limits on quantummechanical relations and dark energy interactions.
Generalised expression of the noise figure of phase sensitive amplifiers for an arbitrary number of modesPhase sensitive amplifiers (PSA), contrary to usual phase insensitiveamplifiers (PIA), are in principle capable to achieve noiseless amplification,i.e. exhibit a quantum-limited noise figure (NF) of 0 dB. When implementedusing four-wave mixing (FWM) in a nonlinear fiber, extra waves can be generatedby undesired FWM processes, which may introduce extra input ports for vacuumfluctuations, thus potentially degrading the NF. In this situation, we givehere a general analytical quantum derivation of the PSA NF, valid for anarbitrary number of nonlinearly coupled modes. This expression is usable assoon as a linear input-output relation can be found for the annihilation andcreation operators of the involved modes. It predicts that the noise leveldepends on the number of interacting waves. We illustrate the usefulness ofthis expression in the case of six waves, corresponding to four interactingquantum modes. In this example the signal NF is degraded by 0.4 dB, compared to10 dB obtained for PIA operation of the same scheme.
Constructing quantum circuits with global gatesThere are various gate sets that can be used to describe a quantumcomputation. A particularly popular gate set in the literature on quantumcomputing consists of arbitrary single-qubit gates and 2-qubit CNOT gates. ACNOT gate is however not always the natural multi-qubit interaction that can beimplemented on a given physical quantum computer, necessitating a compilationstep that transforms these CNOT gates to the native gate set. A particularlyinteresting case where compilation is necessary is for ion trap quantumcomputers, where the natural entangling operation can act on more than 2 qubitsand can even act globally on all qubits at once. This calls for an entirelydifferent approach to constructing efficient circuits. In this paper we studythe problem of converting a given circuit that uses 2-qubit gates to one thatuses global gates. Our three main contributions are as follows. First, we findan efficient algorithm for transforming an arbitrary circuit consisting ofClifford gates and arbitrary phase gates into a circuit consisting ofsingle-qubit gates and a number of global interactions proportional to thenumber of non-Clifford phases present in the original circuit. Second, we finda general strategy to transform a global gate that targets all qubits into onethat targets only a subset of the qubits. This approach scales linearly withthe number of qubits that are not targeted, in contrast to the exponentialscaling reported in (Maslov & Nam, N. J. Phys. 2018). Third, we improve on thenumber of global gates required to synthesise an arbitrary n-qubit Cliffordcircuit from the 12n-18 reported in (Maslov & Nam, N. J. Phys. 2018) to 6n-8.
Noisy quantum input loophole in measurement-device-independent entanglement witnessesEntanglement witnesses form an effective method to locally detectentanglement in the laboratory without having the prior knowledge of the fulldensity matrix. However, separable states can be erroneously indicated asentangled in such detections in the presence of wrong measurements or loss indetectors. Measurement-device-independent entanglement witnesses (MDI-EWs)never detect fake entanglement even under wrong measurements and for aparticular kind of lossy detectors. A crucial assumption in the case offaithful detection of entanglement employing MDI-EWs is that the preparationdevices producing "quantum inputs" - which are inputs additional to the quantumstate whose entanglement is to be detected - are perfect and there is no noiseduring their transmission. Here, we relax these assumptions and provide ageneral framework for studying the effect of noise on the quantum inputs,invoking uniform and non-uniform noise models. We derive sufficient conditionson the uniform noisy map for retaining the characteristic of MDI-EWs. We findthat in the context of non-uniform and entangling noise, fake entanglementdetection is possible even by MDI-EWs. We also investigate various paradigmaticmodels of local noise and find conditions of revealing entanglement in theclass of Werner states.
Dynamically Corrected Nonadiabatic Holonomic Quantum GatesThe key for realizing fault-tolerant quantum computation lies in maintainingthe coherence of all the qubits so that high-fidelity and robust qubitmanipulations can be achievable. One of the promising approaches is to usegeometric phases in the construction of universal quantum gates, due to theintrinsic robustness against operational errors, i.e., X errors. However, dueto the implementation limitations, previous schemes for nonadiabatic holonomicquantum computation (NHQC) do not possess the noise-resilience feature. Here,combining with the dynamical correction technique, we propose a generalprotocol for universal NHQC, which can greatly suppress X errors, retaining themerit of geometric quantum operations. Numerical simulation shows that our gateperformance can be much better than previous protocols. Remarkably, whenincorporate a minimum resource decoherence-free subspace encoding, our schemecan also be robust against dephasing error, i.e., the Z error. In addition, wealso outline the physical implementation of the protocol that is insensitive toboth X and z errors. Therefore, our protocol provides a promising strategy forscalable fault-tolerant quantum computation.
CMOS Quantum Computing: Toward A Quantum Computer System-on-ChipQuantum computing is experiencing the transition from a scientific to anengineering field with the promise to revolutionize an extensive range ofapplications demanding high-performance computing. Many implementationapproaches have been pursued for quantum computing systems, where currently themain streams can be identified based on superconducting, photonic, trapped-ion,and semiconductor qubits. Semiconductor-based quantum computing, specificallyusing CMOS technologies, is promising as it provides potential for theintegration of qubits with their control and readout circuits on a single chip.This paves the way for the realization of a large-scale quantum computingsystem for solving practical problems. In this paper, we present an overviewand future perspective of CMOS quantum computing, exploring developedsemiconductor qubit structures, quantum gates, as well as control and readoutcircuits, with a focus on the promises and challenges of CMOS implementation.
Adiabatic Sensing Technique for Optimal Temperature Estimation using Trapped IonsWe propose an adiabatic method for optimal phonon temperature estimationusing trapped ions which can be operated beyond the Lamb-Dicke regime. Thequantum sensing technique relies on a time-dependent red-sideband transition ofphonon modes, described by the non-linear Jaynes-Cummings model in general. Aunique feature of our sensing technique is that the relevant information of thephonon thermal distributions can be transferred to the collective spin-degreeof freedom. We show that each of the thermal state probabilities isadiabatically mapped onto the respective collective spin-excitationconfiguration and thus the temperature estimation is carried out simply byperforming a spin-dependent laser fluorescence measurement at the end of theadiabatic transition. We characterize the temperature uncertainty in terms ofthe Fisher information and show that the state projection measurement saturatesthe fundamental quantum Cramér-Rao bound for quantum oscillator at thermalequilibrium.
Optical demonstration of quantum fault-tolerant thresholdA major challenge in practical quantum computation is the ineludible errorscaused by the interaction of quantum systems with their environment.Fault-tolerant schemes, in which logical qubits are encoded by several physicalqubits, enable correct output of logical qubits under the presence of errors.However, strict requirements to encode qubits and operators render theimplementation of a full fault-tolerant computation challenging even for theachievable noisy intermediate-scale quantum technology. Here, we experimentallydemonstrate the existence of the threshold in a special fault-tolerantprotocol. Four physical qubits are implemented using 16 optical spatial modes,in which 8 modes are used to encode two logical qubits. The experimentalresults clearly show that the probability of correct output in the circuit,formed with fault-tolerant gates, is higher than that in the correspondingnon-encoded circuit when the error rate is below the threshold. In contrast,when the error rate is above the threshold, no advantage is observed in thefault-tolerant implementation. The developed high-accuracy optical system mayprovide a reliable platform to investigate error propagation in more complexcircuits with fault-tolerant gates.
Self-consistency of optimizing finite-time Carnot engines with the low-dissipation modelThe efficiency at the maximum power (EMP) for finite-time Carnot enginesestablished with the low-dissipation model, relies significantly on theassumption of the inverse proportion scaling of the irreversible entropygeneration $\Delta S^{(\mathrm{ir})}$ on the operation time $\tau$, i.e.,$\Delta S^{(\mathrm{ir})}\propto1/\tau$. The optimal operation time of thefinite-time isothermal process for EMP has to be within the valid regime of theinverse proportion scaling. Yet, such consistency was not tested due to theunknown coefficient of the $1/\tau$-scaling. In this paper, using a two-levelatomic heat engine as an illustration, we reveal that the optimization of thefinite-time Carnot engines with the low-dissipation model is self-consistentonly in the regime of $\eta_{\mathrm{C}}\ll1$, where $\eta_{\mathrm{C}}$ is theCarnot efficiency. In the large-$\eta_{\mathrm{C}}$ regime, the operation timefor EMP obtained with the low-dissipation model is not within the valid regimeof the $1/\tau$-scaling, and the exact EMP is found to surpass the well-knownbound $\eta_{+}=\eta_{\mathrm{C}}/(2-\eta_{\mathrm{C}})$
The influence of Aharonov-Casher effect on the generalized Dirac oscillator in the cosmic string space-timeIn this manuscript we investigate the generalized Dirac oscillator in thesimplest topological defect described by the cosmic string space-time under theeffect of the external electromagnetic fields. The radial wave equation andenergy eigenvalue of the Dirac oscillator considered as the Cornell potentialfunction are derived via the Nikifornov-Uvarov method, we start with theinitial analysis of the Aharonov-Casher frequency and phase, deficit angle, andpotential parameters on energy spectrum. We also give two specific cases thatDirac oscillator with the Coulomb and Linear potential in this system. Notethat the Coulomb strength N1 has non-negligible effect on the studied system.
Programmable Quantum Annealers as Noisy Gibbs SamplersDrawing independent samples from high-dimensional probability distributionsrepresents the major computational bottleneck for modern algorithms, includingpowerful machine learning frameworks such as deep learning. The quest fordiscovering larger families of distributions for which sampling can beefficiently realized has inspired an exploration beyond established computingmethods and turning to novel physical devices that leverage the principles ofquantum computation. Quantum annealing embodies a promising computationalparadigm that is intimately related to the complexity of energy landscapes inGibbs distributions, which relate the probabilities of system states to theenergies of these states. Here, we study the sampling properties of physicalrealizations of quantum annealers which are implemented through programmablelattices of superconducting flux qubits. Comprehensive statistical analysis ofthe data produced by these quantum machines shows that quantum annealers behaveas samplers that generate independent configurations from low-temperature noisyGibbs distributions. We show that the structure of the output distributionprobes the intrinsic physical properties of the quantum device such aseffective temperature of individual qubits and magnitude of local qubit noise,which result in a non-linear response function and spurious interactions thatare absent in the hardware implementation. We anticipate that our methodologywill find widespread use in characterization of future generations of quantumannealers and other emerging analog computing devices.
Observations of on-demand quantum correlation using Poisson-distributed photon pairsComplementarity or wave-particle duality has been the basis of quantummechanics over the last century. Since the Hanbury Brown and Twiss experimentsin 1956, the particle nature of single photons has been intensively studied forvarious quantum phenomena such as anticorrelation and Bell inequalityviolation. Regarding the fundamental question on quantumness ornonclassicality, however, no clear answer exists for what quantum entanglementshould be and how to generate it. Here, we experimentally demonstrate thesecrete of quantumness using the wave nature of single photons.
Quantum estimation of coupling strengths in driven-dissipative optomechanicsWe exploit local quantum estimation theory to investigate the measurement oflinear and quadratic coupling strengths in a driven-dissipative optomechanicalsystem. For experimentally realistic values of the model parameters, we findthat the linear coupling strength is considerably easier to estimate than thequadratic one. Our analysis also reveals that the majority of information aboutthese parameters is encoded in the reduced state of the mechanical element, andthat the best estimation strategy for both coupling parameters is wellapproximated by a direct measurement of the mechanical position quadrature.Interestingly, we also show that temperature does not always have a detrimentaleffect on the estimation precision, and that the effects of temperature aremore pronounced in the case of the quadratic coupling parameter.
Securing practical quantum cryptosystems with optical power limitersControlling the energy of unauthorized light signals in a quantumcryptosystem is an essential task for implementation security. Here, we proposea passive optical power limiter device based on thermo-optical defocusingeffects, which provides a reliable power limiting threshold that can be readilyadjusted to suit various quantum applications. In addition, the device isrobust against a wide variety of signal variations (e.g. wavelength, pulsewidth), which is important for implementation security. To show its practicalutility for quantum cryptography, we demonstrate and discuss three potentialapplications: (1) measurement-device-independent quantum key distribution withenhanced security against a general class of Trojan-horse attacks, (2) usingthe power limiter as a countermeasure against bright illumination attacks, and(3) the application of power limiters to potentially enhance the implementationsecurity of plug-and-play quantum key distribution.
Quantum scars from zero modes in an Abelian lattice gauge theoryWe consider the spectrum of a $U(1)$ quantum link model where gauge fieldsare realized as $S=1/2$ quantum spins and demonstrate a new mechanism forgenerating quantum many-body scars (high-energy eigenstates that violate theeigenstate thermalization hypothesis) in a constrained Hilbert space. Many-bodydynamics with local constraints has attracted much attention due to the recentdiscovery of non-ergodic behavior in quantum simulators based on Rydberg atoms.Lattice gauge theories provide natural examples of constrained systems sincephysical states must be gauge-invariant. In our case, the Hamiltonian $H={\calO}_{\rm kin}+\lambda {\cal O}_{\rm pot}$, where ${\cal O}_{\rm pot}$ (${\calO}_{\rm kin}$) is diagonal (off-diagonal) in the electric flux basis, containsexact mid-spectrum zero modes at $\lambda=0$ whose number grows exponentiallywith system size. This massive degeneracy is lifted at any non-zero $\lambda$but some special linear combinations that simultaneously diagonalize ${\calO}_{\rm kin}$ and ${\cal O}_{\rm pot}$ survive as quantum many-body scars,suggesting an "order-by-disorder" mechanism in the Hilbert space. We giveevidence for such scars and show their dynamical consequences on ladders withup to $56$ spins, which may be tested using available proposals of quantumsimulators.
Fisher information as a probe of spacetime structure: Relativistic quantum metrology in (A)dSRelativistic quantum metrology studies the maximal achievable precision forestimating a physical quantity when both quantum and relativistic effects aretaken into account. We study the relativistic quantum metrology of temperaturein (3+1)-dimensional de Sitter and anti-de Sitter space. Using Unruh-DeWittdetectors coupled to a massless scalar field as probes and treating them asopen quantum systems, we compute the Fisher information for estimatingtemperature. We investigate the effect of acceleration in dS, and the effect ofboundary condition in AdS. We find that the phenomenology of the Fisherinformation in the two spacetimes can be unified, and analyze its dependence ontemperature, detector energy gap, curvature, interaction time, and detectorinitial state. We then identify estimation strategies that maximize the Fisherinformation and therefore the precision of estimation.
High-Power Near-Concentric Fabry-Perot Cavity for Phase Contrast Electron MicroscopyTransmission electron microscopy (TEM) of vitrified biological macromolecules(cryo-EM) is limited by the weak phase contrast signal that is available fromsuch samples. Using a phase plate would thus substantially improve thesignal-to-noise ratio. We have previously demonstrated the use of a high-powerFabry-Perot cavity as a phase plate for TEM. We now report improvements to ourlaser cavity that allow us to achieve record continuous-wave intensities ofover 450 GW/cm$^{2}$, sufficient to produce the optimal 90° phase shiftfor 300 keV electrons. In addition, we have performed the first cryo-EMreconstruction using a laser phase plate, demonstrating that the stability ofthis laser phase plate is sufficient for use during standard cryo-EM datacollection.
Negative Thermal Hall Conductance in Two-Dimer Shastry-Sutherland Model with π-flux Dirac TriplonWe introduce an effective 2-dimer tight-binding model for the family ofShastry-Sutherland models with geometrically tunable triplon excitations. TheRashba pseudospin-orbit coupling induced by the tilted external magnetic fieldleads to elementary excitations having nontrivial topological properties with{\pi}-Berry flux. The interplay between the in-plane and out-of-plane magneticfield thus allows us to effectively engineer the band structure in this bosonicsystem. In particular, the in-plane magnetic field gives rise to Berrycurvature hotspot near the bottom of the triplon band, and at the same timesignificantly increases the critical magnetic field for the topological triplonband. We calculate explicitly the experimental signature of the thermal Halleffect of triplons in SrCu2(BO3)2, and show a pronounced and tunabled transportsignals within the accessible parameter range, particularly with a change ofsign of the thermal Hall conductance. The tilted magnetic field is also usefulin reducing the bandwidth of the lowest triplon band. We show it can thus be aflexible theoretical and experimental platform for the correlated bosonictopological system.
Ab initio Ultrafast Spin Dynamics in SolidsSpin relaxation and decoherence is at the heart of spintronics and spin-basedquantum information science. Currently, no theoretical approaches canaccurately predict spin relaxation of solids including necessary scatteringpathways for required ns to ms simulation time. We present a first-principlesreal-time density-matrix approach based on Lindblad dynamics to simulateultrafast spin dynamics including various scattering processes for generalsolid-state systems. Through the complete theoretical descriptions of pump,probe and scattering processes including electron-phonon, electron-impurity andelectron-electron scatterings, our method can directly simulate the ultrafastpump-probe measurements for coupled spin and electron dynamics at anytemperatures and doping levels. We apply this method to a prototypical systemGaAs and obtain excellent agreement with experiments. We found that therelative contributions of different scattering mechanisms and phonon modes varyconsiderably between spin and carrier relaxation processes. Importantly, insharp contrast to previous work based on model Hamiltonians, we point out thatat low temperatures the electron-electron scattering becomes very important forspin relaxation. Most importantly, we examine the applicable conditions of thecommonly-used phenomenological D'yakonov-Perel' relation, which may break downfor individual scattering processes. Our work provides a predictivecomputational platform for spin relaxation in solids, which has unprecedentedpotentials for designing new materials ideal for spintronics and quantuminformation technology.
Topology and Quantized Response in Complex Spectral EvolutionThe spectral winding on the complex energy plane is a unique topology fornon-Hermitian systems under the periodic boundary condition (PBC). Despiteconsiderable efforts devoted to non-Hermitian topological systems, theimplications of different nonzero winding numbers of such point-gap topologyfor realistic lattice systems are still unknown. By looking into the complexPBC-OBC spectral evolution as a result of continuous PBC-OBC interpolation, weuncover a physically appealing correspondence between the spectral windingnumber and response quantization. Specifically, the ratio of the change in onequantity depicting signal amplification to the variation in one impurityparameter is found to display fascinating plateaus, with their quantized valuesgiven by the spectral winding numbers.
Numerical Implementation of Just-In-Time Decoding in Novel Lattice Slices Through the Three-Dimensional Surface CodeWe build on recent work by B. Brown (Sci. Adv. 6, eaay4929 (2020)) to developand simulate an explicit recipe for a just-in-time decoding scheme in three 3Dsurface codes, which can be used to implement a transversal (non-Clifford)$\overline{CCZ}$ between three 2D surface codes in time linear in the codedistance. We present a fully detailed set of bounded-height lattice slicesthrough the 3D codes which retain the code distance and measurement-errordetecting properties of the full 3D code and admit a dimension-jumping processwhich expands from/collapses to 2D surface codes supported on the boundaries ofeach slice. At each timestep of the procedure the slices agree on a common setof overlapping qubits on which $CCZ$ should be applied. We use these slices tosimulate the performance of a simple JIT decoder against stochastic $X$ andmeasurement errors and find evidence for a threshold $p_c \sim 0.1\%$ in allthree codes. We expect that this threshold could be improved by optimisation ofthe decoder.
Classification and Generation of Light Sources Using Gamma FittingIn general, the typical approach to discriminate antibunching, bunching orsuperbunching categories make use of calculating the second-order coherencefunction ${g^{(2)}}(\tau )$ of light. Although the classical light sourcescorrespond to the specific degree of second-order coherence ${g^{(2)}}(0)$, itdoes not alone constitute a distinguishable metric to characterize anddetermine light sources. Here we propose a new mechanism to directly classifyand generate antibunching, bunching or superbunching categories of light, aswell as the classical light sources such as thermal and coherent light, byGamma fitting according to only one characteristic parameter $\alpha$ or$\beta$. Experimental verification of beams from four-wave mixing process is inagreement with the presented mechanism, and the in fluence of temperature $T$and laser detuning $\Delta$ on the measured results are investigated. Theproposal demonstrates the potential of classifying and identifying light withdifferent nature, and the most importantly, provides a convenient and simplemethod to generate light sources meeting various application requirementsaccording to the presented rules. Most notably, the bunching and superbunchingare distinguishable in super-Poissonian statistics using our mechanism.
On black hole Schrodinger equation and "gravitational fine structure constant"The Schrodinger equation of the Schwarzschild black hole (SBH) has beenrecently found by the Author and collaborators. By following with caution theanalogy between this SBH Schrodinger equation and the traditional Schrodingerequation of the s states (l=0) of the hydrogen atom, the SBH Schrodingerequation can be solved and discussed. The approach also permits to find thequantum gravitational quantities which are the gravitational analogous of thefine structure constant and of the Rydberg constant. Remarkably, suchquantities are not constants. Instead, they are dynamical quantities havingwell defined discrete spectra. In particular the spectrum of the "gravitationalfine structure constant" is exactly the set of non-zero natural numbers\mathbb{N}-\left\{ 0\right\} . Therefore, one argues the interestingconsequence that the SBH results in a perfect quantum gravitational system,which obeys Schrodinger's theory: the "gravitational hydrogen atom".
Electric probe for the toric code phase in Kitaev materials through the hyperfine interactionThe Kitaev model is a remarkable spin model with gapped and gapless spinliquid phases, which are potentially realized in iridates and$\alpha$-RuCl$_3$. In the recent experiment of $\alpha$-RuCl$_3$, the signatureof a nematic transition to the gapped toric code phase, which breaks the $C_3$symmetry of the system, has been observed through the angle dependence of theheat capacity. We here propose a mechanism by which the nematic transition canbe detected electrically. This is seemingly impossible because$J_\textrm{eff}=1/2$ spins do not have an electric quadrupole moment (EQM).However, in the second-order perturbation the virtual state with a nonzero EQMappears, which makes the nematic order parameter detectable by nuclear magneticresonance and Mössbauer spectroscopy. The purely magnetic origin of EQM isdifferent from conventional electronic nematic phases, allowing the directdetection of the realization of Kitaev's toric error-correction code.
Laser stabilization to a cryogenic fiber ring resonatorThe frequency stability of lasers is limited by thermal noise instate-of-the-art frequency references. Further improvement requires operationat cryogenic temperature. In this context, we investigate a fiber-based ringresonator. Our system exhibits a first-order temperature-insensitive pointaround $3.55$ K, much lower than that of crystalline silicon. The observed lowsensitivity with respect to vibrations ($<5\cdot{10^{-11}}\,\text{m}^{-1}\text{s}^{2}$), temperature ($-22(1)\cdot{10^{-9}}\,\text{K}^{-2}$) andpressure changes ($4.2(2)\cdot{10^{-11}}\,\text{mbar}^{-2}$) makes our approachpromising for future precision experiments.
Coherent time-dependent oscillations and time-correlations in triangular triple quantum dotsThe fluctuation behavior of triple quantum dots has, so far, largely focusedon current cumulants in the long-time limit via full counting statistics. Giventhat triple quantum dots are non-trivial open quantum systems with manyinteresting features, such as Kondo-enhanced scattering, Aharonov-Bohminterference, and coherent population blocking, new fluctuating-timestatistics, such as the waiting time distribution (WTD), may provide moreinformation than just the current cumulants alone. In this paper, consequently,we use a Born-Markov master equation to calculate the standard and higher-orderWTDs for coherently-coupled triple quantum dots arrayed in triangular ringgeometries for several transport regimes. In all cases we find that the WTDdisplays coherent oscillations that correspond directly to individualtime-dependent dot occupation probabilities, a result also reported recently inRef.[1]. Our analysis, however, goes beyond the single-occupancy and singlewaiting time regimes, investigating waiting time behavior for triple quantumdots occupied by multiple electrons and with finite electron-electroninteractions. We also demonstrate that in these regimes of higher occupancy,quantum coherent effects introduce correlations between successive waitingtimes, which we characterize both with an averaged approach via the Pearsoncorrelation coefficient, and a quantity $W(\tau_{1},\tau_{2})$ that describescorrelations between each pair of successive waiting times $\tau_{1}$ and$\tau_{2}$.
High-Resolution Imaging of Cold Atoms through a Multimode FiberWe developed an ultra-compact high-resolution imaging system for cold atoms.Its only in-vacuum element is a multimode optical fiber with a diameter of$230\,\mu$m, which simultaneously collects light and guides it out of thevacuum chamber. External adaptive optics allow us to image cold Rb atoms with a$\sim 1\,\mu$m resolution over a $100 \times 100\,\mu$m$^2$ field of view.These optics can be easily rearranged to switch between fast absorption imagingand high-sensitivity fluorescence imaging. This system is particularly suitedfor hybrid quantum engineering platforms where cold atoms are combined withoptical cavities, superconducting circuits or optomechanical devicesrestricting the optical access.
Experimental Fock-State Capability of Non-Ideal Single-Photon StatesAdvanced quantum technologies, as well as fundamental tests of quantumphysics, crucially require the interference of multiple single photons inlinear-optics circuits. This interference can result in the bunching of photonsinto higher Fock states, leading to a complex bosonic behaviour. Thesechallenging tasks timely require to develop collective criteria to benchmarkmany independent initial resources. Here we determine whether n independentimperfect single photons can ultimately bunch into the Fock state $|n\rangle$.We thereby introduce an experimental Fock-state capability for single-photonsources, which uses phase-space interference for extreme bunching events as aquantifier. In contrast to autocorrelation functions, this operational approachtakes into account not only residual multi-photon components but also vacuumadmixture and the dispersion of the individual photon statistics. We apply thisapproach to high-purity single photons generated from an optical parametricoscillator and show that they can lead to a Fock-state capability of at least14. Our work demonstrates a novel collective benchmark for single-photonsources and their use in subsequent stringent applications.
Stabilization via feedback switching for quantum stochastic dynamicsWe propose a new method for fast pure-state preparation in quantum systems,which employs the output of a continuous measurement process and switchingdissipative control to improve convergence speed, as well as robustness withrespect to the initialization of the quantum stochastic filter. In particular,we prove that the proposed control law makes the desired state globallyasymptotically stable both in mean and in probability, and we compare thisnovel measurement-based switching rule against a time-based and a state-basedswitching control law. Numerical simulations show that the proposed methodconverges to the target state faster than the other two methods, showingsignificant improvements in the case of faulty initialization.
Trapped bosons, thermodynamic limit and condensation: a study in the framework of resolvent algebrasThe virtues of resolvent algebras, compared to other approaches for thetreatment of canonical quantum systems, are exemplified by infinite systems ofnon-relativistic bosons. Within this framework, equilibrium states of trappedand untrapped bosons are defined on a fixed C*-algebra for all physicallymeaningful values of the temperature and chemical potential. Moreover, thealgebra provides the tools for their analysis without having to rely on 'adhoc' prescriptions for the test of pertinent features, such as the appearanceof Bose-Einstein condensates. The method is illustrated in case ofnon-interacting systems in any number of spatial dimensions and sheds new lighton the appearance of condensates. Yet the framework also covers interactionsand thus provides a universal basis for the analysis of bosonic systems.
Design of a W-band Superconducting Kinetic Inductance Qubit (Kineticon)Superconducting qubits are widely used in quantum computing research andindustry. We describe a superconducting kinetic inductance qubit (Kineticon)operating at W-band frequencies with a nonlinear nanowire section that providesthe anharmonicity required for two distinct quantum energy states. Operatingthe qubits at higher frequencies relaxes the dilution refrigerator temperaturerequirements for these devices and paves the path for multiplexing a largenumber of qubits. Millimeter-wave operation requires superconductors withrelatively high Tc, which implies high gap frequency, 2$\Delta$/h, beyond whichphotons break Cooper pairs. For example, NbTiN with Tc=16K has a gap frequencynear 1.4 THz, which is much higher than that of aluminum (90 GHz), allowing foroperation throughout the millimeter-wave band. Here we describe a design andsimulation of a W-band Kineticon qubit embedded in a 3-D cavity.
Effects of Magnetic And Aharanov-Bohm (AB) Fields on the Energy Spectra of the Yukawa PotentialIn this article, the Yukawa potential is scrutinized taking intoconsideration the effects of magnetic and AB flux fields within thenon-relativistic regime using the factorization method. The energy equation andwave function of the system are obtained in close form. We find that theall-encompassing effects result in a strongly attractive system andconsequently there is a significant upward shift in the bound state energy ofthe system. We also find that to achieve a low-energy medium for the Yukawapotential, weak magnetic field is required, however the AB flux field can beused as a controller. The magnetic and AB fields eliminates the degeneracy fromthe spectra. From our findings, it could be concluded that to manipulate theenergy spectra of this system, the AB-flux and magnetic field will do sogreatly. The results from this study can be applied in condensed matterphysics, atomic and molecular physics.
Non-Abelian gauge invariance from dynamical decouplingLattice gauge theories are fundamental to such distinct fields as particlephysics, condensed matter or quantum information theory. The recent progress inthe control of artificial quantum systems already allows for studying Abelianlattice gauge theories in table-top experiments. However, the realization ofnon-Abelian models remains challenging. Here, we employ a coherent quantumcontrol scheme to enforce non-Abelian gauge invariance, and discuss this ideain detail for a one dimensional SU(2) lattice gauge system. Finally, we commenton how to extend our scheme to other non-Abelian gauge symmetries and higherspatial dimensions, which summarized, provides a promising route for thequantum simulation of non-Abelian lattice gauge theories.
Improved DIQKD protocols with finite-size analysisThe security of finite-length keys is essential for the implementation ofdevice-independent quantum key distribution (DIQKD). Presently, there areseveral finite-size DIQKD security proofs, but they are mostly focused onstandard DIQKD protocols and do not directly apply to the recent improved DIQKDprotocols based on noisy preprocessing, random key measurements, and modifiedCHSH inequalities. Here, we provide a general finite-size security proof thatcan simultaneously encompass these approaches, using tighter finite-size boundsthan previous analyses. In doing so, we develop a method to compute tight lowerbounds on the asymptotic keyrate for any such DIQKD protocol with binary inputsand outputs. With this, we show that positive asymptotic keyrates areachievable up to depolarizing noise values of $9.26\%$, exceeding allpreviously known noise thresholds. We also develop a modification torandom-key-measurement protocols, using a pre-shared seed followed by a "seedrecovery" step, which yields substantially higher net key generation rates byessentially removing the sifting factor. Some of our results may also improvethe keyrates of device-independent randomness expansion.
Optimization of Quantum-dot Qubit Fabrication via Machine LearningPrecise nanofabrication represents a critical challenge to developingsemiconductor quantum-dot qubits for practical quantum computation. Here, wedesign and train a convolutional neural network to interpret in-line scanningelectron micrographs and quantify qualitative features affecting devicefunctionality. The high-throughput strategy is exemplified by optimizing amodel lithographic process within a five-dimensional design space and bydemonstrating a new approach to address lithographic proximity effects. Thepresent results emphasize the benefits of machine learning for developingrobust processes, shortening development cycles, and enforcing quality controlduring qubit fabrication.
Higher-dimensional Hong-Ou-Mandel effect and state redistribution with linear-optical multiportsWe expand the two-photon Hong-Ou-Mandel (HOM) effect onto ahigher-dimensional set of spatial modes and introduce an effect that allowscontrollable redistribution of quantum states over these modes usingdirectionally unbiased linear-optical four-ports without post-selection. Theoriginal HOM effect only allows photon pairs to exit in two directions inspace. But when accompanied by beam splitters and phase shifters, the result isa directionally controllable two-photon HOM effect in four spatial modes, withdirection controlled by changing the phases in the system. This controllablequantum amplitude manipulation also allows demonstration of a "delayed" HOMeffect by exploiting phase shifters in a system of two connected multiportdevices. By this means, both spatial and temporal control of the propagation ofthe two-photon superposition state through a network can be achieved.