Quantum Coherent Control Research Articles

Coherent quantum control of nitrogen-vacancy center spins near 1000 kelvin

Quantum coherence control usually requires low temperature environments. Even for nitrogen-vacancy center spins in diamond, a remarkable exception, the coherence signal is limited to about 700 K due to the quench of the spin-dependent fluorescence at a higher temperature. Here we overcome this limit and demonstrate quantum coherence control of the electron spins of nitrogen-vacancy centers in nanodiamonds at temperatures near 1000 K. The scheme is based on initialization and readout of the spins at room temperature and control at high temperature, which is enabled by pulse laser heating and rapid diffusion cooling of nanodiamonds on amorphous carbon films. Using the diamond magnetometry based on optically detected magnetic resonance up to 800 K, we observe the magnetic phase transition of a single nickel nanoparticle at about 615 K. This work enables nano-thermometry and nano-magnetometry in the high-temperature regime.

Experimental preparation of Greenberger-Horne-Zeilinger states in an Ising spin model by partially suppressing the nonadiabatic transitions

The creation of multipartite entangled states is a key task of quantum information processing. Among the various implementations, shortcut to adiabaticity (STA) offers a fast and robust means for generating entanglement. The traditional counterdiabatic driving, as a conventional and simple method for STA, suppresses transitions with an auxiliary Hamiltonian, but its complex interactions in many-body systems may hamper the feasibility of experimental implementation. To avoid this drawback, a flexible and efficient way was proposed theoretically by Chen et al. [Phys. Rev. A 93, 052109 (2016)] by substituting the counterdiabatic terms. Inspired by this work, we devise a practical protocol for preparing the Greenberger-Horne-Zeilinger state on the Ising spin model via STA by partial suppression of the nonadiabatic transitions, which can obviously reduce the complexity in experiments compared with the original method. We also experimentally demonstrate the viability of our scheme with a nuclear magnetic resonance quantum simulator. This work provides an alternative method to realize fast coherent quantum control for a multiqubit system in experiments.

Quantum coherent control of H3 + formation in strong fields.

Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two- and three-photon optical excitation of atoms and molecules. Here, we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H3 + from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H3 + when a negative 34 π phase step is applied near the low energy region of the laser spectrum and when a positive 34 π phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO+ fragment, we found the enhancement exceeded the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.

Complex Quantum State Generation and Coherent Control Based on Integrated Frequency Combs

The investigation of integrated frequency comb sources characterized by equidistant spectral modes was initially driven by considerations toward classical applications, seeking a more practical and miniaturized way to generate stable broadband sources of light. Recently, in the context of scaling the complexity of optical quantum circuits, these on-chip approaches have provided a new framework to address the challenges associated with non-classical state generation and manipulation. For example, multi-photon and high-dimensional states were to date either inaccessible, lacked scalability, or were difficult to manipulate, requiring elaborate approaches. The emerging field of quantum frequency combs studying spectral multimode sources based on the judicious excitation of (typically) third-order nonlinear optical micro-cavities has begun to address these issues. Several quantum sources based on this concept have already been demonstrated, among them are combs of correlated photons, cross-polarized photon pairs, entangled photon pairs, multi-photon states, and high-dimensional entangled states. While sources have achieved increasing complexity, so have coherent state processing operations, demonstrated in a practical manner using standard telecommunications components. Here, we review our recent work in the development of this framework, with a focus on multi-photon and high-dimensional states. The integrated frequency comb platform thus demonstrates significant potential for the development of meaningful quantum optical technologies.

Observation of Quantum Interference and Coherent Control in a Photochemical Reaction.

Coherent control of reactants remains a long-standing challenge in quantum chemistry. In particular, we have studied laser-induced molecular formation (photoassociation) in a Raman-dressed spin-orbit-coupled ^{87}Rb Bose-Einstein condensate, whose spin quantum state is a superposition of multiple bare spin components. In contrast to the notably different photoassociation-induced fractional atom losses observed for the bare spin components of a statistical mixture, a superposition state with a comparable spin composition displays the same fractional loss on every spin component. We interpret this as the superposition state itself undergoing photoassociation. For superposition states induced by a large Raman coupling and zero Raman detuning, we observe a nearly complete suppression of the photoassociation rate. This suppression is consistent with a model based upon quantum destructive interference between two photoassociation pathways for colliding atoms with different spin combinations. This model also explains the measured dependence of the photoassociation rate on the Raman detuning at a moderate Raman coupling. Our work thus suggests that preparing atoms in quantum superpositions may represent apowerful new technique to coherently control photochemical reactions.

A wavelength-convertible quantum memory: Controlled echo

Quantum coherence control is reinvestigated for a new physical insight in quantum nonlinear optics and applied for a wavelength-convertible quantum memory in a solid ensemble whose spin states are inhomogeneously broadened. Unlike typical atomic media whose spin decays are homogeneous, a spin inhomogeneously broadened solid ensemble requires a counter-intuitive quantum coherence control to avoid spontaneous emission-caused quantum noises. Such a quantum coherence control in a solid ensemble satisfying both near perfect retrieval efficiency and ultralong photon storage offers a solid framework to quantum repeaters, scalable qubit generations, quantum cryptography, and highly sensitive magnetometry. Here, the basic physics of the counter-intuitive quantum coherence control is presented not only for a fundamental understanding of collective ensemble phase control but also for a coherence conversion mechanism between optical and spin states involving Raman rephasing.

Conditional quantum dynamics and nonlocal states in dimeric and trimeric arrays of organic molecules

Arrays of covalently bound organic molecules possess potential for light-harvesting and energy transfer applications due to the strong coherent dipole-dipole coupling between the transition dipole moments of the molecules involved. Here, we show that such molecular systems, based on perylene-molecules, can be considered as arrays of qubits that are amenable for laser-driven quantum coherent control. The perylene monomers exhibit dephasing times longer than four orders of magnitude a typical gating time, thus allowing for the execution of a large number of gate operations on the sub-picosecond timescale. Specifically, we demonstrate quantum logic gates and entanglement in bipartite (dimer) and tripartite (trimer) systems of perylene-based arrays. In dimers, naturally entangled states with a tailored degree of entanglement can be produced. The nonlocality of the molecular trimer entanglement is demonstrated by testing Mermin's (Bell-like) inequality violation.

Preservation of Quantum Coherence for Gaussian-State Dynamics in a Non-Markovian Process**Supported by the National Natural Science Foundation of China under Grant Nos 11405100, 11404377 and 11674360, the Natural Science Basic Research Plan of Shaanxi Province of China under Grant No 2015JM1032, and the Doctoral Research Fund of Shaanxi University of Science and Technology of China under Grant No 2018BJ-02.

Coherence is a key resource in quantum information science. Exactly understanding and controlling the variation of coherence are vital for implementation in realistic quantum systems. Using P-representation of density matrix, we obtain the analytical solution of the master equation for the classical states in the non-Markovian process and investigate the coherent dynamics of Gaussian states. It is found that quantum coherence can be preserved in such a process if the coupling strength between system and environment exceeds a threshold value. We also discuss the characteristic function of the Gaussian states in the non-Markovian process, which provides an inevitable bridge for the control and operation of quantum coherence.

Conditions for the Absence of Local Extrema in Problems of Quantum Coherent Control

We consider a terminal control problem for quantum systems which is formulated as the problem of maximizing the objective functional at some fixed finite time. Within the framework of this problem, we discuss known results on the local maxima of the objective functional that are not global. This question is important for quantum control, since such local maxima could make it difficult to find the global maximum by local search in numerical optimization or under laboratory conditions.

Quantum coherent control of the photoelectron angular distribution in bichromatic-field ionization of atomic neon

We investigate the coherent control of the photo\-electron angular distribution in bichromatic atomic ionization. Neon is selected as target since it is one of the most popular systems in current gas-phase experiments with free-electron lasers (FELSs). In particular, we tackle practical questions, such as the role of the fine-structure splitting, the pulse length, and the intensity. Time-dependent and stationary perturbation theory are employed, and we also solve the time-dependent Schr\"odinger equation in a single-active electron model. We consider neon ionized by a FEL pulse whose fundamental frequency is in resonance with either $2p-3s$ or $2p-4s$ excitation. The contribution of the non\-resonant two-photon process and its potential constructive or destructive role for quantum coherent control is investigated.

Bilinear Hamiltonian interactions between linear quantum systems via feedback

A number of recent works employ bilinear Hamiltonian interactions between Linear Quantum Stochastic Systems (LQSSs). To the authors’ knowledge, implementation schemes for such interactions exist only between single-mode systems. In this work, we propose a general method for the implementation of an arbitrary bilinear Hamiltonian interaction between two multi-mode LQSSs via a feedback interconnection. As an application, we show that the direct interaction realization of a certain coherent quantum control architecture is very useful for design and optimization.

Fast and robust quantum control based on Pauli blocking

Coherent quantum control over many-particle quantum systems requires high fidelity dynamics. One way of achieving this is to use adiabatic schemes where the system follows an instantaneous eigenstate of the Hamiltonian over timescales that do not allow transitions to other states. This, however, makes control dynamics very slow. Here we introduce another concept that takes advantage of preventing unwanted transitions in fermionic systems by using Pauli blocking: excitations from a protected ground state to higher-lying states are avoided by adding a layer of buffer fermions, such that the protected fermions cannot make a transition to higher lying excited states because these are already occupied. This allows to speed-up adiabatic evolutions of the system. We do a thorough investigation of the technique, and demonstrate its power by applying it to high fidelity transport, trap expansion and splitting in ultracold atoms systems in anharmonic traps. Close analysis of these processes also leads to insights into the structure of the orthogonality catastrophe phenomenon.

A numerical approach to optimal coherent quantum LQG controller design using gradient descent

This paper is concerned with coherent quantum linear quadratic Gaussian (CQLQG) control. The problem is to find a stabilizing measurement-free quantum controller for a quantum plant so as to minimize a mean square cost for the fully quantum closed-loop system. The plant and controller are open quantum systems interconnected through bosonic quantum fields. In comparison with the observation–actuation structure of classical controllers, coherent quantum feedback is less invasive to the quantum dynamics. The plant and controller variables satisfy the canonical commutation relations (CCRs) of a quantum harmonic oscillator and are governed by linear quantum stochastic differential equations (QSDEs). In order to correspond to such oscillators, these QSDEs must satisfy physical realizability (PR) conditions in the form of quadratic constraints on the state-space matrices, reflecting the CCR preservation in time. The symmetry of the problem is taken into account by introducing equivalence classes of coherent quantum controllers generated by symplectic similarity transformations. We discuss a modified gradient flow, which is concerned with norm-balanced realizations of controllers. A line-search gradient descent algorithm with adaptive stepsize selection is proposed for the numerical solution of the CQLQG control problem. The algorithm finds a local minimum of the LQG cost over the parameters of the Hamiltonian and coupling operators of a stabilizing coherent quantum controller, thus taking the PR constraints into account. A convergence analysis of the algorithm is presented. Numerical examples of designing locally optimal CQLQG controllers are provided in order to demonstrate the algorithm performance.

Coherent Features of Resonance-Mediated Two-Photon Absorption Enhancement by Varying the Energy Level Structure, Laser Spectrum Bandwidth and Central Frequency**Supported by the National Natural Science Foundation of China under Grant Nos 51132004, 11474096 and 11604199, the Science and Technology Commission of Shanghai Municipality under Grant No 14JC1401500, and the Higher Education Key Program of He’nan Province under Grant Nos 17A140025 and

The femtosecond pulse shaping technique has been shown to be an effective method to control the multi-photon absorption by the light–matter interaction. Previous studies mainly focused on the quantum coherent control of the multi-photon absorption by the phase, amplitude and polarization modulation, but the coherent features of the multi-photon absorption depending on the energy level structure, the laser spectrum bandwidth and laser central frequency still lack in-depth systematic research. In this work, we further explore the coherent features of the resonance-mediated two-photon absorption in a rubidium atom by varying the energy level structure, spectrum bandwidth and central frequency of the femtosecond laser field. The theoretical results show that the change of the intermediate state detuning can effectively influence the enhancement of the near-resonant part, which further affects the transform-limited (TL)-normalized final state population maximum. Moreover, as the laser spectrum bandwidth increases, the TL-normalized final state population maximum can be effectively enhanced due to the increase of the enhancement in the near-resonant part, but the TL-normalized final state population maximum is constant by varying the laser central frequency. These studies can provide a clear physical picture for understanding the coherent features of the resonance-mediated two-photon absorption, and can also provide a theoretical guidance for the future applications.

Quantum and classical resources for unitary design of open-system evolutions

A variety of tasks in quantum control, ranging from purification and cooling to quantum stabilisation and open-system simulation, rely on the ability to implement a target quantum channel over a specified time interval within prescribed accuracy. This can be achieved by engineering a suitable unitary dynamics of the system of interest along with its environment, which, depending on the available level of control, is fully or partly exploited as a coherent quantum controller. After formalising a controllability framework for completely positive trace-preserving quantum dynamics, we provide sufficient conditions on the environment state and dimension that allow for the realisation of relevant classes of quantum channels, including extreme channels, stochastic unitaries or simply any channel. The results hinge on generalisations of Stinespring’s dilation via a subsystem principle. In the process, we show that a conjecture by Lloyd on the minimal dimension of the environment required for arbitrary open-system simulation, albeit formally disproved, can in fact be salvaged, provided that classical randomisation is included among the available resources. Existing measurement-based feedback protocols for universal simulation, dynamical decoupling and dissipative state preparation are recast within the proposed coherent framework as concrete applications, and the resources they employ discussed in the light of the general results.

Radio-frequency measurement in semiconductor quantum computation

Semiconductor quantum dots have attracted wide interest for the potential realization of quantum computation. To realize efficient quantum computation, fast manipulation and the corresponding readout are necessary. In the past few decades, considerable progress of quantum manipulation has been achieved experimentally. To meet the requirements of high-speed readout, radio-frequency (RF) measurement has been developed in recent years, such as RF-QPC (radio-frequency quantum point contact) and RF-DGS (radio-frequency dispersive gate sensor). Here we specifically demonstrate the principle of the radio-frequency reflectometry, then review the development and applications of RF measurement, which provides a feasible way to achieve high-bandwidth readout in quantum coherent control and also enriches the methods to study these artificial mesoscopic quantum systems. Finally, we prospect the future usage of radio-frequency reflectometry in scaling-up of the quantum computing models.

Chiral topological excitons in the monolayer transition metal dichalcogenides

We theoretically investigate the chiral topological excitons emerging in the monolayer transition metal dichalcogenides, where a bulk energy gap of valley excitons is opened up by a position dependent external magnetic field. We find two emerging chiral topological nontrivial excitons states, which exactly connects to the bulk topological properties, i.e., Chern number = 2. The dependence of the spectrum of the chiral topological excitons on the width of the magnetic field domain wall as well as the magnetic filed strength is numerically revealed. The chiral topological valley excitons are not only important to the excitonic transport due to prevention of the backscattering, but also give rise to the quantum coherent control in the optoelectronic applications.

Coherent perfect absorption and transmission of a generalized three-mode cavity optico-mechanical system

With the rapid development of nano-physics and quantum optics, optico-mechanical coupling system is developing toward the miniaturization and lightweight. The physical characteristics of optical cavity and applications of optic-mechanical devices have received much attention. In this paper, a generalized three-mode cavity optico-mechanical system is presented, the steady-state responses of the system to the characteristics of weak detection of light absorption and dispersion in several different coherent driving modes are studied. Situated in the middle of system is a portable total reflection mechanical oscillator with a reflectance of 100%, and located on each side is a fixed optical cavity mirror with partial transmittance, Three-mode cavity optical mechanical system consists of fixed-mirror, removable-vibrator, fixed-mirror structure. in which the two optical cavities are coupled by coupling a stronger control field and weak probe light with the same mechanical oscillator. Analysis and numerical results show that under the mechanism with different parameters, due to nonlinear effect of pressure, in the three-mode cavity optical mechanical system, there appear some interesting quantum coherent phenomena such as coherent perfect absorption, coherent perfect transmission and coherent perfect synthesis. When coherent perfect absorption occurs, the mutual conversion between input signal power full-field energies and oscillator vibration of internal coherence can be realized, and the law of conservation of energy is satisfied. When relaxation rate due to mechanical oscillator is very small, the coherent perfect transmission is completely transmitted from the system side of the input field to the other side in the case of no loss of energy. And mechanical relaxation rate of the oscillator approaches to zero in the middle, which can ensure that the perfect transmission of the detection field takes place on one side, and the field total reflection and coherent perfect synthesis happen on the other side of. In addition, we alsofind that the adjustment of coupling between cavity and cavity can change the intensity of the probe field of quantum coherent control thereby realizing that the output of the detection field is transformed between coherent perfect absorption and coherence transmission; through simple phase modulation the output direction and input direction of detection field for left cavity-right cavity can swap mutually. So, these dynamic controls in quantum information networks can be used to construct some optical devices with special functions, such as photon switch, photo router, photon exchange machine, etc.

Resonant laser spectroscopy of localized excitons in monolayer WSe_2

Coherent quantum control and resonance fluorescence of few-level quantum systems is integral for quantum technologies. Here we perform resonance and near-resonance excitation of three-dimensionally confined excitons in monolayer WSe2 to reveal near-ideal single-photon fluorescence with count rates up to 3 MHz. Using high-resolution photoluminescence excitation spectroscopy of the localized excitons, we uncover a weakly fluorescent exciton state ∼5 meV blue shifted from the ground-state exciton, providing important information to unravel the precise nature of quantum states. Successful demonstration of resonance fluorescence paves the way to probe the localized exciton coherence in two-dimensional semiconductors.

Time-delayed quantum coherent Pyragas feedback control of photon squeezing in a degenerate parametric oscillator

Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength.