Coherent Quantum Process Research Articles

Toward Highly Efficient Multimode Superconducting Quantum Memory

Microwave quantum memory promises advanced capabilities for noisy intermediate-scale superconducting quantum computers. Existing approaches to microwave quantum memory lack complete combination of high efficiency, long storage time, noiselessness and multi-qubit capacity. Here we report an efficient microwave broadband multimode quantum memory. The memory stores two spectral modes of single photon level microwave radiation in on-chip system of eight coplanar superconducting resonators. Single mode storage shows a power efficiency of up to $60\pm 3\%$ at single photon energy and more than $73\pm 3\%$ at higher intensity. The demonstrated efficiency is an order of magnitude larger than the previously reported multimode microwave quantum memory. The noiseless character of the storage is confirmed by coherent state quantum process tomography. The demonstrated results pave the way to further increase in efficiency and hence building a practical multimode microwave memory for superconducting quantum circuits.

Recent advances in quantum nanophotonics: plexcitonic and vibro-polaritonic strong coupling and its biomedical and chemical applications

Abstract The fundamental understanding of molecular quantum electrodynamics via the strong light–matter interactions between a nanophotonic cavity and quantum emitters opens various applications in quantum biology, biophysics, and chemistry. However, considerable obstacles to obtaining a clear understanding of coupling mechanisms via reliable experimental quantifications remain to be resolved before this field can truly blossom toward practical applications in quantitative life science and photochemistry. Here, we provide recent advancements of state-of-the-art demonstrations in plexcitonic and vibro-polaritonic strong couplings and their applications. We highlight recent studies on various strong coupling systems for altering chemical reaction landscapes. Then, we discuss reports dedicated to the utilization of strong coupling methods for biomolecular sensing, protein functioning studies, and the generation of hybrid light–matter states inside living cells. The strong coupling regime provides a tool for investigating and altering coherent quantum processes in natural biological processes. We also provide an overview of new findings and future avenues of quantum biology and biochemistry.

Direct observation of ultrafast singlet exciton fission in three dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Ultrastrong Coupling of Band‐Nested Excitons in Few‐Layer Molybdenum Disulphide

AbstractThe 2D transition‐metal dichalcogenides (2D TMDCs) are an intriguing platform for studying strong light–matter interactions because they combine the electronic properties of conventional semiconductors with the optical resonances found in organic systems. However, the coupling strengths demonstrated in strong exciton–polariton coupling in the 2D TMDCs remain much lower than those found in organic systems. In this paper, a new approach is taken by utilizing the large oscillator strength of the above‐band gap C exciton in few‐layer molybdenum disulphide (FL‐MoS2). A k‐space Rabi splitting of 293 meV is shown when coupling FL‐MoS2 C excitons to surface plasmon polaritons at room temperature. This value is 11% of the uncoupled exciton energy (2.67 eV or 464 nm), ≈2× what is typically seen in the TMDCs, placing the system in the ultrastrong coupling regime. The results take a step toward finally achieving the efficient quantum coherent processes of ultrastrong coupling in a CMOS‐compatible system—the 2D TMDCs—in the visible spectrum.

Quantum contact process on scale-free networks

The quantum contact process (QCP), a combination of the quantum coherent and classical incoherent processes, exhibits a quantum absorbing phase transition (QAPT). Most studies of the QCP have focused on the case in which active states are induced by the s-excited state of Rydberg atoms. Thus, quantum coherence is induced by short-range interactions. In this case, a QAPT in one dimension is second-order. However, when active states are induced by the d-excited state, long-range interactions must be considered. In this case, even in one dimension, a discontinuous transition occurs. Thus, the type of QAPT depends on interaction range. Here, we aim to investigate how the QAPT depends on embedded structure and thus consider the QCP model on scale-free (SF) networks, in which the degree distribution follows a power law Pd(k) ~ k−λ and the mean distance between two nodes depends on system size N logarithmically. Thus, we can find how the QAPT depends on interaction range and the heterogeneity of the number of connections simultaneously. We find analytically that various types of QAPTs emerge depending on the degree exponent λ. Finally, we compare the properties of the PT of the QCP with those of its classical counterparts.

Phase diagram of quantum generalized Potts-Hopfield neural networks

We introduce and analyze an open quantum generalization of the q-state Potts-Hopfield neural network (NN), which is an associative memory model based on multi-level classical spins. The dynamics of this many-body system is formulated in terms of a Markovian master equation of Lindblad type, which allows to incorporate both probabilistic classical and coherent quantum processes on an equal footing. By employing a mean field description we investigate how classical fluctuations due to temperature and quantum fluctuations effectuated by coherent spin rotations affect the ability of the network to retrieve stored memory patterns. We construct the corresponding phase diagram, which in the low temperature regime displays pattern retrieval in analogy to the classical Potts-Hopfield NN. When increasing quantum fluctuations, however, a limit cycle phase emerges, which has no classical counterpart. This shows that quantum effects can qualitatively alter the structure of the stationary state manifold with respect to the classical model, and potentially allow one to encode and retrieve novel types of patterns.

Class of quasiprobability distributions of work with initial quantum coherence

The work is a concept of fundamental importance in thermodynamics. An open question is how to describe the work fluctuation for quantum coherent processes in the presence of initial quantum coherence in the energy basis. With the aim of giving a unified description, here we introduce and study a class of quasiprobability distributions of work, which give an average work equal to the average energy change of the system and reduce to the two-projective-measurement scheme for an initial incoherent state. Moreover, we characterize the work with the help of fluctuation relations. In particular, by considering the joint distribution of work and initial quantum coherence, we find a fluctuation theorem involving quantum coherence, from which follows a second law of thermodynamics in the case of initial thermal populations.

Realization of a complete Stern-Gerlach interferometer: Toward a test of quantum gravity.

The Stern-Gerlach effect, found a century ago, has become a paradigm of quantum mechanics. Unexpectedly, until recently, there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Several theoretical studies have explained why a Stern-Gerlach interferometer is a formidable challenge. Here, we provide a detailed account of the realization of a full-loop Stern-Gerlach interferometer for single atoms and use the acquired understanding to show how this setup may be used to realize an interferometer for macroscopic objects doped with a single spin. Such a realization would open the door to a new era of fundamental probes, including the realization of previously inaccessible tests at the interface of quantum mechanics and gravity.

Thermoelectric figure of merit enhancement in dissipative superlattice structures

Utilizing the non-coherent quantum transport formalism, we investigate thermoelectric (TE) performance across dissipative superlattice (SL) configurations in the linear regime of operation. Using the dissipative non-equilibrium Green’s function formalism coupled self-consistently with the Poisson’s equation, we report an enhanced figure of merit zT in the multi-barrier device designs. The proposed enhancement, we show, is a result of a drastic reduction in the electronic thermal conductance triggered via non-coherent transport. We show that a maximum zT value of 18 can be achieved via the inclusion of momentum relaxation processes. There is also a reasonable enhancement in the Seebeck coefficient, with a maximum of 1000 µV K−1, which we attribute to an enhancement in electronic filtering arising from the non-coherent transport. Distinctly the thermal conduction is drastically reduced as the length of the SL scales up, although the power factor shows an overall degradation. While the presence of interfaces is known to kill phonon thermal conduction, our analysis shows that momentum relaxation processes in SL structures can effectively kill electronic thermal conduction also. We believe that the analysis presented here could set the stage to understand better the interplay between relaxation processes and coherent quantum processes in the electronic engineering of heterostructure TE devices.

Long-Range Ordered Water Correlations between A–T/C–G Nucleotides

Summary The interactions between complementary base pairs are crucial to the helical duplex structure of DNA. Although base-base interactions have been widely reported, the study of long-range interactions in the base-pairing processes is still a major challenge in this field. Here, we first study the long-range interactions between the base pairs by atomic force microscopy-based single-molecule force spectroscopy (SMFS). The SMFS results of A–T/C–G imply that there are weak long-range interactions between them, which have a range of 15–25 nm. Raman spectroscopy results imply that these weak interactions can be attributed to multiplex hydrogen bond interaction of ordered water structure between A–T/C–G nucleotides. In addition, the theoretical calculations deduce that the ideal structure of these water molecules exhibits a specific helical structure. Our findings might open up a new understanding of biological assembly processes and provide helpful strategies for bio-nanotechnology.

Pulsed two-photon coherent control of channelrhodopsin-2 photocurrent in live brain cells.

Channelrhodopsin-2 (ChR2) is an ion channel activated by the absorption of light. A recent experiment demonstrated that the current emanating from neurons in live brain cells expressing ChR2 can be controlled using two-photon phase control. Here, we propose an experimentally testable coherent control mechanism for this phenomenon. Significantly, we describe how femtosecond, quantum coherent processes arising from weak-field ultrafast excitation are responsible for the reported control of the millisecond classical dynamics of the neuronal current.

Coherent and dissipative quantum process tensor reconstructions in two-dimensional electronic spectroscopy.

A major goal of time-resolved spectroscopy is to resolve the dynamical processes that follow photoexcitation. This amounts to identifying all the quantum states involved and the rates of population transfer between them. Unfortunately, such quantum state and kinetic reconstructions are ambiguous using one-dimensional methods such as transient absorption even when all the states of the system are fully resolved. Higher-dimensionality methods like two-dimensional spectroscopy lift some of the ambiguity, but unless the spectral features are well-separated, current inversion methods generally fail. Here, we show that, using both coherence and population signals of the nonlinear response, it is indeed possible to accurately extract both static and dynamic information from the 2D spectrum even when features are highly congested. Coherences report on the positions of the vibronic states of the system, providing a useful constraint for extracting the full kinetic scheme. We model time-resolved 2D photon echo spectra using a sum-over-states approach and show in which regimes the Hamiltonian and kinetic schemes may be recovered. Furthermore, we discuss how such algorithms may be applied to experimental data and where some of the underlying assumptions may fail. The ability to systematically extract the maximal information content of multidimensional spectroscopic data is an important step toward utilizing the full power of these techniques and elucidating the structure and dynamics of increasingly complex molecular systems.

Ultrafast Dynamics of High-Harmonic Generation in Terms of Complex Floquet Spectral Analysis

We studied the high-harmonic generation (HHG) of a two-level-system (TLS) driven by an intense monochromatic phase-locked laser based on complex spectral analysis with the Floquet method. In contrast with phenomenological approaches, this analysis deals with the whole process as a coherent quantum process based on microscopic dynamics. We have obtained the time-frequency resolved spectrum of spontaneous HHG single-photon emission from an excited TLS driven by a laser field. Characteristic spectral features of the HHG, such as the plateau and cutoff, are reproduced by the present model. Because the emitted high-harmonic photon is represented as a superposition of different frequencies, the Fano profile appears in the long-time spectrum as a result of the quantum interference of the emitted photon. We reveal that the condition of the quantum interference depends on the initial phase of the driving laser field. We have also clarified that the change in spectral features from the short-time regime to the long-time regime is attributed to the interference between the interference from the Floquet resonance states and the dressed radiation field.

Quantum work for sudden quenches in Gaussian random Hamiltonians.

In the context of nonequilibrium quantum thermodynamics, variables like work behave stochastically. A particular definition of the work probability density function (pdf) for coherent quantum processes allows the verification of the quantum version of the celebrated fluctuation theorems, due to Jarzynski and Crooks, that apply when the system is driven away from an initial equilibrium thermal state. Such a particular pdf depends basically on the details of the initial and final Hamiltonians, on the temperature of the initial thermal state, and on how some external parameter is changed during the coherent process. Using random matrix theory we derive a simple analytic expression that describes the general behavior of the work characteristic function G(u), associated with this particular work pdf for sudden quenches, valid for all the traditional Gaussian ensembles of Hamiltonians matrices. This formula well describes the general behavior of G(u) calculated from single draws of the initial and final Hamiltonians in all ranges of temperatures.

Effects of quantum coherence on work statistics

In the conventional two-point measurement scheme of quantum thermodynamics, quantum coherence is destroyed by the first measurement. But as we know the coherence really plays an important role in the quantum thermodynamics process, and how to describe the work statistics for a quantum coherent process is still an open question. In this paper, we use the full counting statistics method to investigate the effects of quantum coherence on work statistics. First, we give a general discussion and show that for a quantum coherent process, work statistics is very different from that of the two-point measurement scheme, specifically the average work is increased or decreased and the work fluctuation can be decreased by quantum coherence, which strongly depends on the relative phase, the energy level structure and the external protocol. Then, we concretely consider a quenched 1-D transverse Ising model, and show that quantum coherence has a more significant influence on work statistics in the ferromagnetism regime compared with that in the paramagnetism regime, so that due to the presence of quantum coherence the work statistics can exhibit the critical phenonmenon even at high temperature.

Symmetry breaking by quantum coherence in single electron attachment

Quantum coherence-induced effects in atomic and molecular systems are the basis of several proposals for laser-based control of chemical reactions. So far, these rely on coherent photon beams inducing coherent reaction pathways that may interfere with one another, to achieve the desired outcome. This concept has been successfully exploited for removing the inversion symmetry in the dissociation of homonuclear diatomic molecules, but it remains to be seen if such quantum coherent effects can also be generated by the interaction of incoherent electrons with such molecules. Here we show that resonant electron attachment to H2 and the subsequent dissociation into H (n = 2) + H− is asymmetric about the inter-nuclear axis, whereas the asymmetry in D2 is far less pronounced. We explain this observation as due to attachment of a single electron resulting in a coherent superposition of two resonances of opposite parity. In addition to exemplifying a new quantum coherent process, our observation of coherent quantum dynamics involves the active participation of all three electrons and two nuclei, which could provide new tools for studying electron correlations as a means to control chemical processes, and demonstrates the role of coherent effects in electron-induced chemistry. Resonant electron attachment and subsequent dissociation of diatomic molecules is shown to exhibit spatial asymmetry as a consequence of coherent excitation and subsequent interference between reaction pathways.

Quantum Spin Lenses in Atomic Arrays

We propose and discuss `quantum spin lenses', where quantum states of delocalized spin excitations in an atomic medium are `focused' in space in a coherent quantum process down to (essentially) single atoms. These can be employed to create controlled interactions in a quantum light-matter interface, where photonic qubits stored in an atomic ensemble are mapped to a quantum register represented by single atoms. We propose Hamiltonians for quantum spin lenses as inhomogeneous spin models on lattices, which can be realized with Rydberg atoms in 1D, 2D and 3D, and with strings of trapped ions. We discuss both linear and non-linear quantum spin lenses: in a non-linear lens, repulsive spin-spin interactions lead to focusing dynamics conditional to the number of spin excitations. This allows the mapping of quantum superpositions of delocalized spin excitations to superpositions of spatial spin patterns, which can be addressed by light fields and manipulated. Finally, we propose multifocal quantum spin lenses as a way to generate and distribute entanglement between distant atoms in an atomic lattice array.

No-Go Theorem for the Characterization of Work Fluctuations in Coherent Quantum Systems.

An open question of fundamental importance in thermodynamics is how to describe the fluctuations of work for quantum coherent processes. In the standard approach, based on a projective energy measurement both at the beginning and at the end of the process, the first measurement destroys any initial coherence in the energy basis. Here we seek extensions of this approach which can possibly account for initially coherent states. We consider all measurement schemes to estimate work and require that (i)the difference of average energy corresponds to average work for closed quantum systems and that (ii)the work statistics agree with the standard two-measurement scheme for states with no coherence in the energy basis. We first show that such a scheme cannot exist. Next, we consider the possibility of performing collective measurements on several copies of the state and prove that it is still impossible to simultaneously satisfy requirements (i) and (ii). Nevertheless, improvements do appear, and in particular, we develop a measurement scheme that acts simultaneously on two copies of the state and allows us to describe a whole class of coherent transformations.

Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots

Multiple exciton generation (MEG) is a process in which more than one electron hole pair is generated per absorbed photon. It allows us to increase the efficiency of solar energy harvesting. Experimental studies have shown the multiple exciton generation yield of 1.2 in isolated colloidal quantum dots. However real photoelectric devices require the extraction of electron hole pairs to electric contacts. We provide a systematic study of the corresponding quantum coherent processes including extraction and injection and show that a proper design of extraction and injection rates enhances the yield significantly up to values around 1.6.

Electric conductivity of nanocluster PbTe structures with controlled topology: Manifestation of macroscopic quantum effects

Methods of laser modification allow us to observe macroscopic quantum phenomena in nanostructured (cluster) materials. The laser synthesis of nanoparticles/nanoclusters with different topologies in semiconductor PbTe samples is performed via direct laser modification of thin films under the action of continuous laser radiation with a wavelength of 1.06 μm and a power density of ~105 W/cm2. Nanoparticles with bimodal distribution in lateral dimensions are obtained on the surfaces of the samples. The electrophysical properties of such structures can be controlled as desired by modifying their topology. Variations in electric properties depending on the particle location density are demonstrated. The results are interpreted based on the existence of quantum coherent processes with tunneling transitions and hopping conductivity. This approach is promising for the fabrication of elements and devices in optoelectronics and photonics based on new physical principles, and of different hybrid optoelectrical schemes.