Decoherence Rate Research Articles

Quantum Decoherence in Dense Media: A Few Examples

Decoherence is a relatively recent concept in the field of quantum mechanics. Although the pioneers of the field must have understood that loss of phase coherence in quantum superpositions is the reason underlying the appearance of definite outcomes in the quantum measurement problem, the latter was not treated in terms of decoherence until sixty years after the formulation of quantum mechanics as described in a report by Joos and Zeh in 1983 [1]. However, soon after, the theory was developed in further detail, and experiments to measure the actual decoherence rates in various systems commenced. Today, decoherence is the main concern for another reason, i.e., the fact that superposition states must be maintained undisturbed in the quantum communication systems and decoherence presents a strong limitation for their practical application. Decoherence appears in open quantum systems, where the basic system under consideration interacts relatively strongly with an environment. Decoherence times may be as long as seconds for small atomic systems in extreme vacua, although below what is presently measurable (i.e., less than a fraction of a femtosecond) in most liquids and solids, where coupling to the surrounding molecules or atomic arrangements is strong. The relatively slow decoherence in well-isolated particle systems has been described in several recent articles on quantum optics, while the rapid decoherence in liquids and solids has not received this kind of attention. It is the purpose of the present review to fill this gap.

Spin mechanics with levitating ferromagnetic particles

We propose and demonstrate first steps towards schemes where the librational mode of levitating ferromagnets is strongly coupled to the electronic spin of Nitrogen-Vacancy (NV) centers in diamond. Experimentally, we levitate ferromagnets in a Paul trap and employ magnetic fields to attain oscillation frequencies in the hundreds of kHz range with Q factors close to $10^4$. These librational frequencies largely exceed the decoherence rate of NV centers in typical CVD grown diamonds offering prospects for sideband resolved operation. We also prepare and levitate composite diamond-ferromagnet particles and demonstrate both coherent spin control of the NV centers and read-out of the particle libration using the NV spin. Our results will find applications in ultra-sensitive gyroscopy and bring levitating objects a step closer to spin-mechanical experiments at the quantum level.

Phase Covariant Qubit Dynamics and Divisibility

Phase covariant qubit dynamics describes an evolution of a two-level system under simultaneous action of pure dephasing, energy dissipation, and energy gain with time-dependent rates $\gamma_z(t)$, $\gamma_-(t)$, and $\gamma_+(t)$, respectively. Non-negative rates correspond to completely positive divisible dynamics, which can still exhibit such peculiarities as non-monotonicity of populations for any initial state. We find a set of quantum channels attainable in the completely positive divisible phase covariant dynamics and show that this set coincides with the set of channels attainable in semigroup phase covariant dynamics. We also construct new examples of eternally indivisible dynamics with $\gamma_z(t) < 0$ for all $t > 0$ that is neither unital nor commutative. Using the quantum Sinkhorn theorem, we for the first time derive a restriction on the decoherence rates under which the dynamics is positive divisible, namely, $\gamma_{\pm}(t) \geq 0$, $\sqrt{\gamma_+(t) \gamma_-(t)} + 2 \gamma_z(t) > 0$. Finally, we consider phase covariant convolution master equations and find a class of admissible memory kernels that guarantee complete positivity of the dynamical map.

QUANTUM ENTANGLEMENT RECOVERY USING SPEED GRADIENT OVER NOISY QUANTUM CHANNELS

Decoherence of entanglement is one of the main challenges in quantum communication. It is caused by quantum systems interacting with their environment and limiting the distribution of entangled qubits over quantum channels for a few kilo- meters. Preserving and recovering the initial entangled states is a critical issue to permit designed tasks (e.g. quantum key distribution, quantum teleportation, and quantum dense coding) to be fulfilled. This study investigates the adaptive control (speed gradient) method to recover initial entangled state after exposure to decoherence effects. The results show that the entanglement of the initial state can be maximized and recovered over the variant decoherence rates of the quantum channel noise.

Dark matter spin–spin interaction through the pseudo-scalar vacuum field

We suggest that the pseudo-scalar vacuum (PSV) field in the dark matter (DM) sector of the Universe may be as important as the electromagnetic vacuum field in the baryonic sector. In particular, the spin–spin interaction between the DM fermions, mediated by PSV, may represent the strongest interaction between the DM fermions due to the absence of the electric charge and the magnetic dipole moment. Based on this assumption, we consider the influence of the spin–spin interaction, mediated by PSV, on the spin precession of the DM fermions (e.g. neutralino). In the secular approximation, we obtain the exact expression describing the frequency of the precession and estimate the decoherence rate.

Optimal Dense Coding in a Two-qubit Heisenberg XXZ Model with Decoherence

Optimal dense coding of a two-qubit anisotropic Heisenberg XXZ model under decoherence is investigated with the Werner state as the initial state. With the time evolution, the dense coding capacity χ oscillates firstly, and then reaches a stable value 1 in the long time limit, whatever the values of other parameters are. Our results imply that the purity r of the initial state have a strong influence on the initial value of the optimal dense coding capacity. When the purity r of the initial state increase, the initial value of dense coding capacity increase. Besides, the coupling coefficient and Dzyaloshinski-Moriya(DM) both have strong influence on the frequency of the oscillation and the area of t that the optimal dense coding is feasible(tf). Interestingly, the decoherence rate γ can strongly affect the dense coding capacity. With the decreasing of γ, the area of tf(which will make the dense coding capacity greater than 1) becomes more wider. When γ = 0, it will still allows a valid dense coding. So we can adjust the parameters to get a optimal dense coding.

Quantum Fisher information and coherence of an atom in a dissipative cavity

In this work, we investigate quantum Fisher information (QFI) and quantum coherence (QC) of an atom in a dissipative cavity. In a zero-temperature reservoir and with one excitation number, we obtain the analytical solutions of QFI and QC as well as their relationship for Ohmic and Lorentzian reservoirs, respectively. The results show that both the atom–cavity coupling and the cavity–reservoir coupling can effectively protect QFI and QC. In particular, QFI and QC will tend to their stable values when the atom–cavity coupling or cavity–reservoir coupling is larger than a certain value. QC can augment QFI and can effectively improve the quantum metrology. In addition, we give a physical explanation of the dynamic behavior of QFI and QC using the decoherence rate.

Speeding up the Generation of Entangled State between a Superconducting Qubit and Cavity Photons via Counterdiabatic Driving

AbstractOptimal generation of entangled states is of critical significance for robust quantum information processing. An effective scheme is presented for speeding up the generation of an entangled state between a superconducting qubit and microwave photons via counterdiabatic driving. At a magic bias point, the first three levels of a charge‐phase quantum circuit constitute an effective qutrit. An entangled state based on adiabatic population transfer is first achieved. By the technique of shortcuts to adiabaticity, a counterdiabatic driving is applied to the qutrit, which then accelerates the entanglement generation significantly. Moreover, with the accessible decoherence rates, the rapid operations in a shortcut way are highly robust when compared with adiabatic manipulations. The scheme could offer a promising approach toward optimal preparation of entangled states with superconducting artificial atoms in circuit quantum electrodynamics, experimentally.

Magnetic pulses enable multidimensional optical spectroscopy of dark states.

The study and manipulation of low dipole moment quantum states have been challenging due to their inaccessibility by conventional spectroscopic techniques. Controlling the spin in such states requires unfeasible strong magnetic fields to overcome typical decoherence rates. However, the advent of terahertz technology and its application in magnetic pulses opens up a new scenario. In this article, we focus on an electron-hole pair model to demonstrate that it is possible to control the precession of the spins and to modify the transition rates to different spin states. Enhancing transitions from a bright state to a dark state with different spins means that the latter can be revealed by ordinary spectroscopy. We propose a modification of the standard two-dimensional spectroscopic scheme in which a three pulse sequence is encased in a magnetic pulse. Its role is to drive transitions between a bright and a dark spin state, making the latter susceptible to spectroscopic investigation.

Anti-Zeno-based dynamical control of the unfolding of quantum Darwinism

We combine the collisional picture for open system dynamics and the control of the rate of decoherence provided by the quantum (anti-)Zeno effect to illustrate the temporal unfolding of the redundant encoding of information into a multipartite environment that is at the basis of Quantum Darwinism, and to control it. The rate at which such encoding occurs can be enhanced or suppressed by tuning the dynamical conditions of system-environment interaction in a suitable and remarkably simple manner. This would help the design of a new generation of quantum experiments addressing the elusive phenomenology of Quantum Darwinism and thus its characterization.

Decoherence due to XPM-assisted Raman amplification for polarization or wavelength offset pulses in all-normal dispersion supercontinuum generation

We report the importance of cross-phase modulation (XPM) on the coherence of a low-energy probe pulse co-propagating with a high-energy pump pulse that generates incoherent supercontinuum in all-normal dispersion (ANDi) fiber due to Raman amplification of quantum noise. By investigating numerous fiber and pulse parameters, we show consistently that for weak probe pulses, the XPM from the pump is the dominant influence on the degradation of the probe coherence. We show that the faster decoherence at the pump leading edge means that the probe coherence is reduced more significantly when the probe has a higher group velocity, i.e., when an orthogonally polarized probe is aligned to the fast (lower refractive index) axis of the fiber or when a co-polarized probe has a longer central wavelength. Simulations show that this effect occurs for both polarization-maintaining (PM) and non-PM ANDi fibers and can result in a probe decoherence rate that is higher than that of the pump. These previously unreported results extend our earlier scalar simulations showing incoherent supercontinuum within a single pulse.

Decoherence in Conformal Field Theory

Noise sources are ubiquitous in Nature and give rise to a description of quantum systems in terms of stochastic Hamiltonians. Decoherence dominates the noise-averaged dynamics and leads to dephasing and the decay of coherences in the eigenbasis of the fluctuating operator. For energy-diffusion processes stemming from fluctuations of the system Hamiltonian the characteristic decoherence time is shown to be proportional to the heat capacity. We analyze the decoherence dynamics of entangled CFTs and characterize the dynamics of the purity, and logarithmic negativity, that are shown to decay monotonically as a function of time. The converse is true for the quantum Renyi entropies. From the short-time asymptotics of the purity, the decoherence rate is identified and shown to be proportional to the central charge. The fixed point characterizing long times of evolution depends on the presence degeneracies in the energy spectrum. We show how information loss associated with decoherence can be attributed to its leakage to an auxiliary environment and discuss how gravity duals of decoherence dynamics in holographic CFTs looks like in AdS/CFT. We find that the inner horizon region of eternal AdS black hole is highly squeezed due to decoherence.

Transferring Einstein-Podolsky-Rosen State Cross Frequency Bands via Cavity Electro-Opto-Mechanical Converters

In order to transfer two-mode quantum state from optical input to microwave output, we propose a scheme based on double cavity electro-opto-mechanical converters. Principles and operation modes of cavity electro-opto-mechanical converter are introduced in detail, and theoretical model of transferring Einstein-Podolsky-Rosen state via our scheme is built. The dynamics and corresponding scattering matrixes of our scheme are analyzed carefully. Meanwhile, Ulhmann fidelity is utilized to quantify the similarity of initial state and final state, which also stands for the performance of our scheme. Influences of coupling rates and optical input bandwidth on fidelity are well discussed, and some significant factors are also taken into consideration, such as transmission spectrum, interaction rates and decoherence rates. Furthermore, Wigner distributions of initial state and final state as well as the von Neumann entropy are obtained to verify the performance of our scheme. The results show that our scheme could transfer the Einstein-Podolsky-Rosen state from optical input to microwave output with high fidelity (0.74), and the entropy ratio between initial and final states is about 0.78, which is quite close with the fidelity. Besides, Wigner distributions also show that initial state and final state are slightly different in phase structures. In case of strong coupling rates, ultralow temperature and narrow input bandwidth, the performance of our scheme could be further enhanced.

Nonadiabatic sunlight harvesting.

Experimental and theoretical evidence points out the crucial role of specific intramolecular vibrational modes resonant with excitonic splittings in the interpretation of long-lived coherences observed in the two-dimensional spectra of some natural and synthetic light harvesting complexes. For the natural situation of illumination by incoherent (sun)light, the relevance of these vibrations is analyzed here for light-harvesting vibronic prototype dimers. The detailed analysis of the density matrix dynamics reveals that the inclusion of intramolecular vibrational modes reinforces the exciton coherence up to one order of magnitude and may increase the populations of lowest energy single exciton states, as well as populations and coherences in the site basis. In sharp contrast to the case of initial-state preparation by coherent (laser)light-sources, the initial thermal state of the local vibrational modes, as well as that of the anticorrelated vibrational mode, evolves devoid of non-classical correlations as confirmed by the absence of negative values of its phase-space quasi-probability distribution at all times. Therefore, not only the long-lived coherences observed in the two-dimensional spectra are induced by the coherent character of pulsed laser sources, but it is unambiguously shown here that the non-classical character generally assigned to the anticorrelated vibrational mode also comes as the result of the preparation of the initial state by coherent pulsed laser sources.

Deterministic remote preparation of arbitrary single-qubit state via one intermediate node in noisy environment

We propose a remote state preparation scheme working in noisy environment via one intermediate node. Firstly, we report the remote preparation of an arbitrary single-qubit state with one intermediate node in ideal environment. Afterwards, we investigate the influence of four kinds of representative noises that are commonly encountered in practical applications of quantum communication protocols. Detailed analysis indicates that the fidelities of the output state rely on the coefficients of the prepared state and the decoherence rate. The turning point is λ=0.5 in both the bit-flip and phase-flip noisy channels. The fidelity in the depolarizing channel is related with the decoherence rate, and independent with the phase parameters.

Ultrastrong Parametric Coupling between a Superconducting Cavity and a Mechanical Resonator.

We present a new optomechanical device where the motion of a micromechanical membrane couples to a microwave resonance of a three-dimensional superconducting cavity. With this architecture, we realize ultrastrong parametric coupling, where the coupling not only exceeds the dissipation in the system but also rivals the mechanical frequency itself. In this regime, the optomechanical interaction induces a frequency splitting between the hybridized normal modes that reaches 88% of the bare mechanical frequency, limited by the fundamental parametric instability. The coupling also exceeds the mechanical thermal decoherence rate, enabling new applications in ultrafast quantum state transfer and entanglement generation.

Transition between continuous and discrete spectra in dynamical-decoupling noise spectroscopy

Here, we explore the prospects of carrying out the single qubit spectroscopy of environmental noise when the resolution of the frequency filters produced by the dynamical decoupling pulse sequences can be set high enough to reveal the discrete structure of the noise spectral density. The standard form of spectroscopy is applicable when the filter is unable to resolve the discrete spectral lines and only coarse grained approximation of the spectrum is recorded in the qubit's decoherence rate. When the discrete structure becomes accessible, the qubit probe evolves in a qualitatively different manner, and the procedure for recovering spectral density has to be redesigned.

Influence of the Coupling between Two Qubits in an Open Coherent Cavity: Nonclassical Information via Quasi-Probability Distributions

In this paper, we investigate the dynamics of two coupled two-level systems (or qubits) that are resonantly interacting with a microwave cavity. We examine the effects of the intrinsic decoherence rate and the coupling between the two qubits on the non-classicality of different system partitions via quasi-probability functions. New definitions for the partial Q-function and its Wehrl entropy are used to investigate the information and the quantum coherence of the phase space. The amount of the quantum coherence and non-classicality can be appropriately tuned by suitably adopting the rates of the intrinsic-decoherence and the coupling between the two qubits. The intrinsic decoherence has a pronounced effect on the negativity and the positivity of the Wigner function. The coupling between the two qubits can control the negativity and positivity of the quasi-probability functions.

Local quantum uncertainty and interferometric power for a two-qubit system under decoherence channels with memory

Via local quantum uncertainty (LQU) and interferometric power (IP) as a measure of nonclassical correlations, we study the dynamics of quantum correlations for two non-interacting qubits passing through decoherence channels where the channels with successive uses are correlated. The effect of three different types of decoherence channels, such as amplitude damping, phase damping, and depolarizing channels, on quantum correlations has been discussed in detail. The explicit analytical expressions of LQU and IP for a class of Bell-diagonal states under these decoherence channels have been derived. Our results show that the behaviors of both LQU and IP exhibit similar variation under the amplitude damping and phase damping channels, with monotonic decay from an increasing decoherence rate, but decay more slowly when the memory coefficient of the channel increases. Compared to the phenomenon of LQU under a depolarizing channel, we find that there exists a sudden change of LQU under both amplitude damping and phase damping channels by choosing proper initial parameters. Finally, we compare the dynamics of LQU with that of IP under decoherence channels. Our results also confirm the amount of quantum correlations measured by LQU is smaller than IP.

Environment-assisted Quantum-enhanced Sensing with Electronic Spins in Diamond

The performance of solid-state quantum sensors based on electronic spin defects is often limited by the presence of environmental spin impurities that cause decoherence. A promising approach to improve these quantum sensors is to convert environment spins into useful resources for sensing. Here we demonstrate the efficient use of an unknown electronic spin defect in the proximity of a nitrogen-vacancy center in diamond as both a quantum sensor and a quantum memory. We first experimentally evaluate the improvement in magnetic field sensing provided by mixed entangled states of the two electronic spins. Our results critically highlight the tradeoff between the advantages expected from increasing the number of spin sensors and the typical challenges associated with increasing control errors, decoherence rates, and time overheads. Still, by taking advantage of the spin defect as both a quantum sensor and a quantum memory whose state can be repetitively measured to improve the readout fidelity, we can achieve a gain in performance over the use of a single-spin sensor. These results show that the efficient use of available quantum resources can enhance quantum devices, pointing to a practical strategy towards quantum-enhanced sensing and information processing by exploiting environment spin defects.