Coherent Control Of Quantum Systems Research Articles

Tuning quantum pathway interference in two-color laser photoemission using DC bias

Coherent control of quantum systems depends on the manipulation of quantum interference through external fields. In this work, we investigate the effects of DC bias field on coherent control of quantum pathways in two-color laser photoemission using exact analytical solutions of the one-dimensional time dependent Schrödinger equation. Increasing DC bias lowers and narrows the surface potential barrier, shifting the dominant emission to lower order multiphoton photoemission, photo-assisted tunneling and then direct tunneling. Those lower order photon absorption processes result in fewer possible pathways, and therefore modulation of photoemission current can be suppressed as DC field increases. It is shown that a maximum modulation depth of 99.4% can be achieved for a gold emitter at local DC bias F 0 = 0.5 V nm−1, fundamental (800 nm) laser field F 1 = 2.6 V nm−1 and second harmonic laser field F 2 = 0.25 V nm−1 . For a given set of input parameters, the total photoemission consists of different k-photon processes, each of which has their own different multiple possible pathways and interference effects. However, the quantum pathways and their interference for the dominant k-photon process and for the total photoemission probability show the same trends. This study demonstrates strong flexibility in tuning two-color lasers induced photoemission using a DC bias and provides insights into coherent control schemes of general quantum systems.

Flux-controllable and fast state inversion in a Laudau–Zener system of superconducting charge qubit

Coherent control of quantum systems in an optimized manner is of significance to information processing and state engineering. In this paper, an effective scheme is proposed for implementing rapid state inversion in a Laudau–Zener (LZ) system of superconducting charge qubit. By linearly adjusting time-dependent gate charge, the system with a given tunneling splitting of energy can be described by the LZ model. By means of the applied flux capable of inducing desired level spacing, qubit state inversion with high probability can be performed in a short time. We further address the criterion to ensure system evolution with negligible non-adiabatic excitation. With the accessible decoherence rates, high-fidelity operations can be obtained numerically. Without adding auxiliary driving, the present strategy could perform the shortcut-like accelerated operation, which paves a promising avenue towards optimized information processing with superconducting qubits.

Controlling quantum systems with modulated electron beams

Coherent control of quantum transitions -- indispensable in quantum technology -- generally relies on the interaction of quantum systems with electromagnetic radiation. Here, we theoretically demonstrate that the non-radiative electromagnetic near-field of a temporally modulated free-space electron beam can be utilized for coherent control of quantum systems. We show that such manipulation can be performed with only classical control over the electron beam itself, and is readily realizable with current technology. This approach may provide a pathway towards spectrally selective quantum control with nano-scale spatial resolution, harnessing the small de Broglie wavelength of electrons.

Space-Time Coupling: Current Concept and Two Examples from Ultrafast Optics Studied Using Exact Solution of EM Equations

Current approach to space-time coupling (STC) phenomena is given together with a complementary version of the STC concept that emphasizes the finiteness of the energy of the considered pulses. Manifestations of STC are discussed in the framework of the simplest exact localized solution of Maxwell’s equations, exhibiting a “collapsing shell”. It falls onto the center, continuously deforming, and then, having reached maximum compression, expands back without losing energy. Analytical solutions describing this process enable to fully characterize the field in space-time. It allowed to express energy density in the center of collapse in the terms of total pulse energy, frequency and spectral width in the far zone. The change of the pulse shape while travelling from one point to another is important for coherent control of quantum systems. We considered the excitation of a two-level system located in the center of the collapsing EM (electromagnetic) pulse. The result is again expressed through the parameters of the incident pulse. This study showed that as it propagates, a unipolar pulse can turn into a bipolar one, and in the case of measuring the excitation efficiency, we can judge which of these two pulses we are dealing with. The obtained results have no limitation on the number of cycles in a pulse. Our work confirms the productivity of using exact solutions of EM wave equations for describing the phenomena associated with STC effects. This is facilitated by rapid progress in the search for new types of such solutions.

Chirp control of tunable terahertz synchrotron radiation.

It is of scientific significance to explore the terahertz radiation source with the performances of high power, tunable frequency, and controllable chirp for the realization of coherent control of quantum systems. How to realize frequency chirp control of terahertz synchrotron radiation is the last puzzle to be completed. In this Letter, we propose a method to control the radiation frequency chirp with precision. A novel photomixing scheme is presented to generate a longitudinally modulated laser pulse with non-uniform time intervals between the adjacent micro-peaks, which means that there is a chirp in the modulation frequency, and this chirp can be continuously tuned. The interaction is made to occur between an electron beam and the modulated laser pulse in a modulator (an undulator tuned at the laser wavelength), then terahertz synchrotron radiation with the same spectrum characteristics as the modulated laser will be generated when the electron beam passes through the following bending magnet. We expect that this method will open a new way for the coherent control of quantum systems in the terahertz regime.

Thermoelectric graphene photodetectors with sub-nanosecond response times at terahertz frequencies

AbstractUltrafast and sensitive (noise equivalent power <1 nW Hz−1/2) light-detection in the terahertz (THz) frequency range (0.1–10 THz) and at room-temperature is key for applications such as time-resolved THz spectroscopy of gases, complex molecules and cold samples, imaging, metrology, ultra-high-speed data communications, coherent control of quantum systems, quantum optics and for capturing snapshots of ultrafast dynamics, in materials and devices, at the nanoscale. Here, we report room-temperature THz nano-receivers exploiting antenna-coupled graphene field effect transistors integrated with lithographically-patterned high-bandwidth (∼100 GHz) chips, operating with a combination of high speed (hundreds ps response time) and high sensitivity (noise equivalent power ≤120 pW Hz−1/2) at 3.4 THz. Remarkably, this is achieved with various antenna and transistor architectures (single-gate, dual-gate), whose operation frequency can be extended over the whole 0.1–10 THz range, thus paving the way for the design of ultrafast graphene arrays in the far infrared, opening concrete perspective for targeting the aforementioned applications.

Composite pulses with errant phases

Composite pulses --- sequences of pulses with well defined relative phases --- are an efficient, robust and flexible technique for coherent control of quantum systems. Composite sequences can compensate a variety of experimental errors in the driving field (e.g. in the pulse amplitude, duration, detuning, chirp, etc.) or in the quantum system and its environment (e.g. inhomogeneous broadening, stray electric or magnetic fields, unwanted couplings, etc.). The control parameters are the relative phases between the constituent pulses in the composite sequence, an accurate control over which is required in all composite sequences reported hitherto. In this paper, we introduce two types of composite pulse sequences which, in addition to error compensation in the basic experimental parameters, compensate systematic errors in the composite phases. In the first type of such composite sequences, which compensate pulse area errors, relative phase errors of up to 10% can be tolerated with reasonably short sequences while maintaining the fidelity above the 99.99% quantum computing benchmark. In the second type of composite sequences, which compensate pulse area and detuning errors, relative phase errors of up to 5% can be compensated.

Multiphoton Raman transitions and Rabi oscillations in driven spin systems

In the framework of the non-secular perturbation theory based on the Bogoliubov averaging method, the coherent dynamics of multiphoton Raman transitions in a two-level spin system driven by an amplitude-modulated microwave field is studied. Closed-form expressions for the Rabi frequencies of these transitions have been obtained beyond the rotating wave approximation for the low-frequency driving component. It is shown that spin states dressed by the high-frequency component of the driving field are shifted due to the Bloch-Siegert-like effect caused by antiresonant interactions with the strong low-frequency driving. We predict that with increasing the order of the Raman transition the Rabi frequency decreases and the contribution of the Bloch-Siegert shift to this frequency becomes dominant. It is found that the amplitude and phase of the Rabi oscillations strongly depend on the initial phase of the low-frequency field as well as on detuning from multiphoton resonance. The recent experimental data for the second- and third-order Raman transitions observed for nitrogen-vacancy center in diamond [Z. Shu, et al., arXiv:1804. 10492] are well described in the frame of our approach. Our results provide new possibilities for coherent control of quantum systems.

Coherent control of quantum systems as a resource theory

Control at the interface between the classical and the quantum world is fundamental in quantum physics. In particular, how classical control is enhanced by coherence effects is an important question both from a theoretical as well as from a technological point of view. In this work, we establish a resource theory describing this setting and explore relations to the theory of coherence, entanglement and information processing. Specifically, for the coherent control of quantum systems, the relevant resources of entanglement and coherence are found to be equivalent and closely related to a measure of discord. The results are then applied to the DQC1 protocol and the precision of the final measurement is expressed in terms of the available resources.

Controlling Quantum Devices with Nonlinear Hardware

To achieve high-fidelity, coherent control of quantum systems via imperfect classical hardware, the authors provide a general framework for designing control sequences that account for distortions---even those that are nonlinear and noninvertible. These sequences are robust to both a distribution of device Hamiltonians (such as for spatially varying fields) and uncertainties in the description of the control hardware itself, greatly extending the bounds of quantum device operation.

Assessments of macroscopicity for quantum optical states

With the slow but constant progress in the coherent control of quantum systems, it is now possible to create large quantum superpositions. There has therefore been an increased interest in quantifying any claims of macroscopicity. We attempt here to motivate three criteria which we believe should enter in the assessment of macroscopic quantumness: The number of quantum fluctuation photons, the purity of the states, and the ease with which the branches making up the state can be distinguished.

Waveform-Controlled Terahertz Radiation from the Air Filament Produced by Few-Cycle Laser Pulses

Waveform-controlled terahertz (THz) radiation is of great importance due to its potential application in THz sensing and coherent control of quantum systems. We demonstrated a novel scheme to generate waveform-controlled THz radiation from air plasma produced when carrier-envelope-phase (CEP) stabilized few-cycle laser pulses undergo filamentation in ambient air. We launched CEP-stabilized 10 fs-long (~1.7 optical cycles) laser pulses at 1.8 μm into air and found that the generated THz waveform can be controlled by varying the filament length and the CEP of driving laser pulses. Calculations using the photocurrent model and including the propagation effects well reproduce the experimental results, and the origins of various phase shifts in the filament are elucidated.

Optimized control of multistate quantum systems by composite pulse sequences

We introduce a technique for derivation of high-fidelity composite pulse sequences for two types of multistate quantum systems: systems with the SU(2) and Morris-Shore dynamic symmetries. For the former type, we use the Majorana decomposition to reduce the dynamics to an effective two-state system, which allows us to find the propagator analytically and use the pool of available composite pulses for two-state systems. For the latter type of multistate systems, we use the Morris-Shore decomposition, which reduces the multistate dynamics to a set of two-state systems. We present examples which demonstrate that the multistate composite sequences open a variety of possibilities for coherent control of quantum systems with multiple states.

Coherently controlling metamaterials

Two independent significant developments have challenged our understanding of light-matter interaction, one, involves the artificially structured materials known as metamaterials, and the other, relates to the coherent control of quantum systems via the quantum interference route. We theoretically demonstrate that one can engineer the electromagnetic response of composite metamaterials using coherent quantum interference effects. In particular, we predict that these composite materials can show a variety of effects ranging from dramatic reduction of losses to switchable ultraslow-to-superluminal pulse propagation. We propose parametric control of the metamaterials by active tuning of the capacitance of the structures, which is most efficiently engineered by embedding the metamaterial structures within a coherent atomic/molecular medium. This leads to dramatic frequency dependent features, such as significantly reduced dissipation accompanied by enhanced filling fraction. For a Split-ring resonator medium with magnetic properties, the associated splitting of the negative permeability band can be exploited for narrow band switching applications at near infrared frequencies involving just a single layer of such composite metamaterials.

Interference-induced transparency and coherent control of quantum systems by frequency-chirped pulses

A selective excitation technique based on light interference is proposed to control quantum systems by frequency-chirped laser fields. Interference of two identical, delayed and phase-shifted pulses is used to modulate the laser spectrum and project it onto the time domain. By adjusting the delay and phase shift, selected transitions can be brought into the ``holes'' of the spectrum and thus remain nonexcited. The possibility to selectively manipulate or even ``shut down'' resonant transitions, making the medium transparent to the field, is shown for the Rb atom.

Femtosecond pulse shaping based on spectral hole burning

In a series of experiments we explore various aspects of femtosecond pulse shaping by coherent filters engraved in a persistent spectral hole burning (PSHB) material. We have in mind the coherent control of quantum systems, with high resolution and large number of addressable spectral points. We examine the phase correction efficiency of this approach, relying on Fourier transform spectral interferometry techniques to characterize the coherent filter. Two different PSHB materials are considered, with two different setups, involving different values of repetition rate, fluence, wavelength, spectral width and pulse duration.

Using feedback for coherent control of quantum systems

A longstanding goal in chemical physics has been the control of atoms and molecules using coherent light fields. This paper provides a brief overview of the field and discusses experiments that use a programmable pulse shaper to control the quantum state of electronic wavepackets in Rydberg atoms and electronic and nuclear dynamics in molecular liquids. The shape of Rydberg wavepackets was controlled by using tailored ultrafast pulses to excite a beam of caesium atoms. The quantum state of these atoms was measured using holographic techniques borrowed from optics. The experiments with molecular liquids involved the construction of an automated learning machine. A genetic algorithm directed the choice of shaped pulses which interacted with the molecular system inside a learning control loop. Analysis of successful pulse shapes that were found by using the genetic algorithm yield insight into the systems being controlled.

Physicists Honored by APS

Physicists Honored by APS