Full Counting Statistics Research Articles

Exact full counting statistics for the staggered magnetization and the domain walls in the XY spin chain.

We calculate exactly cumulant generating functions (full counting statistics) for the transverse, staggered magnetization, and the domain walls at zero temperature for a finite interval of the XY spin chain. In particular, we also derive a universal interpolation formula in the scaling limit for the full counting statistics of the transverse magnetization and the domain walls which is based on the solution of a Painlevé V equation. By further determining subleading corrections in a large interval asymptotics, we are able to test the applicability of conformal field theory predictions at criticality. As a by-product, we also obtain exact results for the probability of formation of ferromagnetic and antiferromagnetic domains in both the σ^{z} and σ^{x} basis in the ground state. The analysis hinges upon asymptotic expansions of block Toeplitz determinants, for which we formulate and check numerically a different conjecture.

Suppression of shot noise in a spin-orbit coupled quantum dot.

We study theoretically the transport properties of electrons in a quantum dot system with spin–orbit coupling. By using the quantum master equation approach, the shot noise and skewness of the transport electrons are calculated. We obtain super-Poisson noise behaviour by investigating the full counting statistics of the transport system. We discover super-Poisson behaviour is more obvious with the spin polarization increasing. More importantly, we discover the suppression of shot noise induced by spin–orbit coupling. The value of shot noise is gradually decreasing when spin–orbit coupling strength increases.

Fermionic criticality is shaped by Fermi surface topology: a case study of the Tomonaga-Luttinger liquid

We perform a unitary renormalization group (URG) study of the 1D fermionic Hubbard model. The formalism generates a family of effective Hamiltonians and many-body eigenstates arranged holographically across the tensor network from UV to IR. The URG is realized as a quantum circuit, leading to the entanglement holographic mapping (EHM) tensor network description. A topological Θ-term of the projected Hilbert space of the degrees of freedom at the Fermi surface are shown to govern the nature of RG flow towards either the gapless Tomonaga-Luttinger liquid or gapped quantum liquid phases. This results in a nonperturbative version of the Berezenskii-Kosterlitz-Thouless (BKT) RG phase diagram, revealing a line of intermediate coupling stable fixed points, while the nature of RG flow around the critical point is identical to that obtained from the weak-coupling RG analysis. This coincides with a phase transition in the many-particle entanglement, as the entanglement entropy RG flow shows distinct features for the critical and gapped phases depending on the value of the topological Θ-term. We demonstrate the Ryu-Takyanagi entropy bound for the many-body eigenstates comprising the EHM network, concretizing the relation to the holographic duality principle. The scaling of the entropy bound also distinguishes the gapped and gapless phases, implying the generation of very different holographic spacetimes across the critical point. Finally, we treat the Fermi surface as a quantum impurity coupled to the high energy electronic states. A thought-experiment is devised in order to study entanglement entropy generated by isolating the impurity, and propose ways by which to measure it by studying the quantum noise and higher order cumulants of the full counting statistics.

Revealing strong correlations in higher-order transport statistics: A noncrossing approximation approach

We present a method for calculating the full counting statistics of a nonequilibrium quantum system based on the propagator noncrossing approximation (NCA). This numerically inexpensive method can provide higher order cumulants for extended parameter regimes, rendering it attractive for a wide variety of purposes. We compare NCA results to Born-Markov quantum master equations (QME) results to show that they can access different physics, and to numerically exact inchworm quantum Monte-Carlo data to assess their validity. As a demonstration of its power, the NCA method is employed to study the impact of correlations on higher order cumulants in the nonequilibrium Anderson impurity model. The four lowest order cumulants are examined, allowing us to establish that correlation effects have a profound influence on the underlying transport distributions. Higher order cumulants are therefore demonstrated to be a proxy for the presence of Kondo correlations in a way that cannot be captured by simple QME methods.

Full counting statistics of the particle currents through a Kitaev chain and the exchange fluctuation theorem.

Exchange fluctuation theorems (XFTs) describe a fundamental symmetry relation for particle and energy exchange between several systems. Here we study the XFTs of a Kitaev chain connected to two reservoirs at the same temperature but different bias. By varying the parameters in the Kitaev chain model, we calculate analytically the full counting statistics of the transport current and formulate the corresponding XFTs for multiple current components. We also demonstrate the XFTs with numerical results. We find that due to the presence of the U(1) symmetry breaking terms in the Hamiltonian of the Kitaev chain, various forms of the XFTs emerge, and they can be interpreted in terms of various well-known transport processes.

Coherences and the thermodynamic uncertainty relation: Insights from quantum absorption refrigerators.

The thermodynamic uncertainty relation, originally derived for classical Markov-jump processes, provides a tradeoff relation between precision and dissipation, deepening our understanding of the performance of quantum thermal machines. Here, we examine the interplay of quantum system coherences and heat current fluctuations on the validity of the thermodynamics uncertainty relation in the quantum regime. To achieve the current statistics, we perform a full counting statistics simulation of the Redfield quantum master equation. We focus on steady-state quantum absorption refrigerators where nonzero coherence between eigenstates can either suppress or enhance the cooling power, compared with the incoherent limit. In either scenario, we find enhanced relative noise of the cooling power (standard deviation of the power over the mean) in the presence of system coherence, thereby corroborating the thermodynamic uncertainty relation. Our results indicate that fluctuations necessitate consideration when assessing the performance of quantum coherent thermal machines.

Quantum thermal transport via a canonically transformed Redfield approach

We develop a non-Markovian approach to study quantum dissipation and thermal transport in the nonequilibrium two-level system based on a canonical transformation and full-counting statistics. Specifically, we analytically study dissipative dynamics of population difference, quantum coherence, and heat current in terms of the canonical transformed Redfield approach. It demonstrates that the dynamics of both the population difference and the energy flow show monotonic decaying behaviors with the identical decay rate, whereas the quantum coherence is dominated by the oscillation showing the non-Markovian effect. Moreover, through tuning the temperature bias of thermal baths, negative differential thermal conductance is clearly exhibited beyond the weak system-bath coupling limit and Markovian approximation. The results may provide guidance for the efficient control of energy and the information transport in nanodevices.

Pumped heat and charge statistics from Majorana braiding

We examine the heat and charge transport of a driven topological superconductor. Our particular system of interest consists of a Y-junction of topological superconducting wires, hosting non-Abelian Majorana zero modes at their edges. The system is contacted to two leads which act as continuous detectors of the system state. We calculate, via a scattering matrix approach, the full counting statistics of the driven heat transport, between two terminals contacted to the system, for small adiabatic driving and characterise the energy transport properties as a function of the system parameters (driving frequency, temperature). We find that the geometric, dynamic contribution to the pumped heat statistics results in a correction to the Gallavotti-Cohen type fluctuation theorem for quantum heat transfer. Notably, the correction term to the fluctuation theorem extends to cycles which correspond to topologically protected braiding of the Majorana zero modes. This geometric correction to the fluctuation theorem differs from its analogs in previously studied systems in that (i) it is non-vanishing for adiabatic cycles of the system's parameters, without the need for cyclic driving of the leads and (ii) it is insensitive to small, slow fluctuations of the driving parameters due to the topological protection of the braiding operation.

Large deviations in one-dimensional random sequential adsorption.

In random sequential adsorption (RSA), objects are deposited randomly, irreversibly, and sequentially; attempts leading to an overlap with previously deposited objects are discarded. The process continues until the system reaches a jammed state when no further additions are possible. We analyze a class of lattice RSA models in which landing on an empty site in a segment is allowed when at least b neighboring sites on the left and the right are unoccupied. For the minimal model (b=1), we compute the full counting statistics of the occupation number. We reduce the determination of the full counting statistics to a Riccati equation that appears analytically solvable only when b=1. We develop a perturbation procedure which, in principle, allows one to determine cumulants consecutively, and we compute the variance of the occupation number for all b.

Observing the emergence of a quantum phase transition shell by shell.

Many-body physics describes phenomena that cannot be understood by looking only at the constituents of a system1. Striking examples are broken symmetry, phase transitions and collective excitations2. To understand how such collective behaviour emerges as a system is gradually assembled from individual particles has been a goal in atomic, nuclear and solid-state physics for decades3-6. Here we observe the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The transition is signalled by the softening of the mode associated with amplitude vibrations of the order parameter, usually referred to as a Higgs mode7. We achieve fine control over ultracold fermions confined to two-dimensional harmonic potentials and prepare closed-shell configurations of 2, 6 and 12 fermionic atoms in the ground state with high fidelity. Spectroscopy is then performed on our mesoscopic system while tuning the pair energy from zero to a value larger than the shell spacing. Using full atom counting statistics, we find the lowest resonance to consist of coherently excited pairs only. The distinct non-monotonic interaction dependence of this many-body excitation, combined with comparison with numerical calculations allows us to identify it as the precursor of the Higgs mode. Our atomic simulator provides a way to study the emergence of collective phenomena and the thermodynamic limit, particle by particle.

Full counting statistics of phonon transport in disordered systems

The coherent potential approximation (CPA) within full counting statistics (FCS) formalism is shown to be a suitable method to investigate average electric conductance, shot noise as well as higher order cumulants in disordered systems. We develop a similar FCS-CPA formalism for phonon transport through disordered systems. As a byproduct, we derive relations among coefficients of different phonon current cumulants. We apply the FCS-CPA method to investigate phonon transport properties of graphene systems in the presence of disorders. For binary disorders as well as Anderson disorders, we calculate up to the $8$-th phonon transmission moments and demonstrate that the numerical results of the FCS-CPA method agree very well with that of the brute force method. The benchmark shows that the FCS-CPA method achieves $20$ times more speedup ratio. Collective features of phonon current cumulants are also revealed.

Full counting statistics for electron transport in periodically driven quantum dots

Time-dependent driving influences the quantum and thermodynamic fluctuations of a system, changing the familiar physical picture of electronic noise which is an important source of information the about microscopic mechanism of quantum transport. Giving access to all cumulants of the current, the full counting statistics (FCS) is the powerful theoretical method to study fluctuations in nonequilibrium quantum systems. In this paper, we propose the application of FCS to consider periodic driven junctions. The combination of Floquet theory for time dynamics and nonequilibrium counting-field Green's functions enables the practical formulation of FCS for the system. The counting-field Green's functions are used to compute the moment generating function, allowing for the calculation of the time-averaged cumulants of the electronic current. The theory is illustrated using different transport scenarios in model systems.

Linear Response Theory and Entropic Fluctuations in Repeated Interaction Quantum Systems

We study Linear Response Theory and Entropic Fluctuations of finite dimensional non-equilibrium repeated interaction systems (RIS). More precisely, in a situation where the temperatures of the probes can take a finite number of different values, we prove analogs of the Green-Kubo fluctuation-dissipation formula and Onsager reciprocity relations on energy flux observables. Then we prove a Large Deviation Principle, or Fluctuation Theorem, and a Central Limit Theorem on the full counting statistics of entropy fluxes. We consider two types of non-equilibrium RIS: either the temperatures of the probes are deterministic and arrive in a cyclic way, or the temperatures of the probes are described by a sequence of i.i.d. random variables with uniform distribution over a finite set.

Self-equilibration theorem in quantum-point contacts of interacting electrons: Time-dependent quantum fluctuations of tunnel transport beyond the Levitov–Lesovik scattering approach

Equilibration to the steady state for a wide class of Luttinger liquid ballistic weakly linked tunnel contacts is extensively studied. Quantum fluctuations of tunnel current are considered in all orders in tunnel coupling and out of the equilibrium in the time domain. Especially, two important mathematical statements: Self-equilibration (SE-)theorem and Self-equilibration (SE-)lemma on the exact re-exponentiation of thermal average from the Keldysh-contour-ordered evolution operator for arbitrary weakly linked Luttinger liquid tunnel contact are proven. Demonstrated proof of SE-theorem and SE-lemma represents first evidence of a novel emergent phenomenon of ”self-equilibration” in the dynamics of quantum fluctuations of electron transport through ballistic tunnel junctions. This phenomenon and all related real-time full counting statistics are shown to be much more general as compared to the usual Levitov–Lesovik scattering approach for the non-interacting electrons, though SE-theorem also contains known results obtained within the Levitov–Lesovik scattering approach as corresponding limiting case for lowest order cumulants at g=1. As the result, corresponding differential equation of ”self-equilibration” for time-dependent Keldysh partition function of tunnel contact is derived and explained. It is shown that proven SE-theorem can be considered also as the dynamical version of well-known Jarzynski equality for the tunnel electron transport out of the equilibrium. As the result, for arbitrary weakly linked tunnel junction model of interest the exact time-dependent cumulant generating functional is derived being valid at arbitrary electron–electron repulsion in the Luttinger liquid leads of the junction, arbitrary temperature and arbitrary bias voltage. Respective general formula in its long-time asymptote turns into a non-perturbative Luttinger liquid generalization of the Levitov–Lesovik type of detailed balance formula for cumulant generating function. As the consequence of obtained results, a universal character of self-equilibration of tunnel current in one-dimensional weakly linked tunnel junctions is revealed and studied on the level of non-equilibrium Fano factor. As well, a new measure of disequilibrium in such systems – the ”steady flow” rate – is also introduced and discussed.

Fractional corner charges in a two-dimensional superlattice Bose-Hubbard model

We study a two dimensional super-lattice Bose-Hubbard model with alternating hoppings in the limit of strong on-site interactions. We evaluate the phase diagram of the model around half-filling using the density matrix renormalization group method and find two gapped phases separated by a gapless superfluid region. We demonstrate that the gapped states realize two distinct higher order symmetry protected topological phases, which are protected by a combination of charge conservation and $C_4$ lattice symmetry. The phases are distinguished in terms of a quantized fractional corner charge and a many-body topological invariant that is robust against arbitrary, symmetry preserving edge manipulations. We support our claims by numerically studying the full counting statistics of the corner charge, finding a sharp distribution peaked around the quantized values. These results are experimentally observable in ultracold atomic settings using state of the art quantum gas microscopy.

Full counting statistics of spin-flip and spin-conserving charge transitions in Pauli-spin blockade

We investigate the full counting statistics (FCS) of spin-conserving and spin-flip charge transitions in Pauli-spin blockade regime of a GaAs double quantum dot. A theoretical model is proposed to evaluate all spin-conserving and spin-flip tunnel rates, and to demonstrate the fundamental relation between FCS and waiting time distribution. We observe the remarkable features of parity effect and a tail structure in the constructed FCS, which do not appear in the Poisson distribution, and are originated from spin degeneracy and coexistence of slow and fast transitions, respectively. This study is potentially useful for elucidating the spin-related and other complex transition dynamics in quantum systems.

Signatures of the Majorana spin in electrical transport through a Majorana nanowire

In this paper, we investigate the transport properties of spinful electrons tunnel coupled to a finite-length Majorana nanowire on one end which is further tunnel coupled to a quantum dot (QD) at the other end. Using a full counting statistics approach, we show that Andreev reflection can happen in two separate channels that can be associated with the two spin states of the tunneling electrons. In a low-energy model for the nanowire that is represented by two overlapping Majorana bound states (MBSs) localized at the ends of the wire, analytical formulas for conductance and noise reveal their crucial dependence on the spin-canting angle difference of the two MBSs in the absence of the QD if the spinful lead couples to both MBSs. We further investigate the influence of a finite temperature on the observation of the coupling to both MBSs. In the presence of the QD, the interference of different tunneling paths gives rise to Fano resonances and the symmetry of those provide decisive information about the coupling to both MBSs. We contrast the low-energy model with a tight-binding model of the Majorana nanowire and treat the Coulomb interaction on the QD with a self-consistent mean field approach. Using the scattering matrix approach, we thereby extend the transport results obtained in the low-energy model including also higher excited states in the nanowire.

Exact out-of-equilibrium steady states in the semiclassical limit of the interacting Bose gas

We study the out-of-equilibrium properties of a classical integrable non-relativistic theory, with a time evolution initially prepared with a finite energy density in the thermodynamic limit. The theory considered here is the Non-Linear Schrödinger equation which describes the dynamics of the one-dimensional interacting Bose gas in the regime of high occupation numbers. The main emphasis is on the determination of the late-time Generalised Gibbs Ensemble (GGE), which can be efficiently semi-numerically computed on arbitrary initial states, completely solving the famous quench problem in the classical regime. We take advantage of known results in the quantum model and the semiclassical limit to achieve new exact results for the momenta of the density operator on arbitrary GGEs, which we successfully compare with ab-initio numerical simulations. Furthermore, we determine the whole probability distribution of the density operator (full counting statistics), whose exact expression is still out of reach in the quantum model.

Symmetry resolved entanglement entropy of excited states in a CFT

We report a throughout analysis of the entanglement entropies related to different symmetry sectors in the low-lying primary excited states of a conformal field theory (CFT) with an internal U(1) symmetry. Our findings extend recent results for the ground state. We derive a general expression for the charged moments, i.e. the generalised cumulant generating function, which can be written in terms of correlation functions of the operator that define the state through the CFT operator-state correspondence. We provide explicit analytic computations for the compact boson CFT (aka Luttinger liquid) for the vertex and derivative excitations. The Fourier transform of the charged moments gives the desired symmetry resolved entropies. At the leading order, they satisfy entanglement equipartition, as in the ground state, but we find, within CFT, subleading terms that break it. Our analytical findings are checked against free fermions calculations on a lattice, finding excellent agreement. As a byproduct, we have exact results for the full counting statistics of the U(1) charge in the considered excited states.

Bunching-antibunching crossover in a double S/N/N capacitively coupled single-electron transistor

We present a Schwinger-Keldysh scheme theory for full counting statistics applicable to the solid-state entangler, which consists of two coupled superconducting/normal-conducting/normal-conducting capacitively coupled single-electron transistors (double S/N/N C-SET). We focus on the case when the superconducting gap energy is larger than the charging energy since flexible control of entanglement information can be expected by the charging effect for various bias conditions, and we study the double S/N/N C-SET with and without dissipative environment. We derive the cumulant generation function for the double S/N/N C-SET under current continuity conditions, from which arbitrary order of current noise cumulants are obtained. We explicitly show that the synchronized Coulomb oscillation, a celebrated experimental finding by Hofstetter et al. [Nature (London) 461, 960 (2009)], originate from the crossed Andreev current conveying quantum entanglement. We also investigate current in the double S/N/N C-SET, its current components, and the resulting cross-correlation of current noise ${S}_{\mathrm{LR}}(\ensuremath{\omega}=0)$ in the superconducting subgap region. It is shown that the bunching-antibunching nature strongly depends on the relative location of the Coulomb gap regions for the relevant current components since the bunching-antibunching nature is determined by the competition between the contributions of relevant current components. Depending on the bias conditions for each of the two C-SETs, ${S}_{\mathrm{LR}}(\ensuremath{\omega}=0)$ exhibits a sign crossover from positive (bunching) to negative (antibunching), which is followed by restoration to bunching correlations. The effect of the dissipative environment tends to reduce ${S}_{\mathrm{LR}}(\ensuremath{\omega}=0)$ because of a reduction in relevant currents. Although crossover and restoration become less conspicuous, the way the bunching-antibunching nature appears is essentially the same as in the case of the inductive environment.