Back Action Research Articles

Quantum coherence dynamics of displaced squeezed thermal state in a non-Markovian environment

The dynamical behavior of quantum coherence of a displaced squeezed thermal state in contact with an external bath is discussed in the present work. We use a Fano-Anderson type of Hamiltonian to model the environment and solve the quantum Langevin equation. From the solution of the quantum Langevin equation we obtain the Green's functions which are used to calculate the expectation value of the quadrature operators which are in turn used to construct the covariance matrix. We use a relative entropy based measure to calculate the quantum coherence of the mode. The single mode squeezed thermal state is studied in the Ohmic, sub-Ohmic and the super-Ohmic limits for different values of the mean photon number. In all these limits, we find that when the coupling between the system and the environment is weak, the coherence decays monotonically and exhibit a Markovian nature. When the system and the environment are strongly coupled, we observe that the evolution is initially Markovian and after some time it becomes non-Markovian. The non-Markovian effect is due to the environmental back action on the system. Finally, we also present the steady state dynamics of the coherence in the long time limit in both low and high temperature regime. We find that the qualitative behavior remains the same in both the low and high temperature limits. But quantitative values differ because the coherence in the system is lower due to thermal decoherence.

Proposal for a solid-state magnetoresistive Larmor quantum clock

We propose a solid-state implementation of the Larmor clock that exploits tunnel magnetoresistance to distill information on how long itinerant spins take to traverse a barrier embedded in it. Keeping in mind that the tunnelling time innately involves pristine pre-selection and post-selection, our proposal takes into account the detrimental aspects of multiple reflections by incorporating multiple contacts, multiple current measurements and suitably defined magnetoresistance signals. Our analysis provides a direct mapping between the magnetoresistance signals and the tunneling times and aligns well with the interpretation in terms of generalized quantum measurements and quantum weak values. By means of an engineered pre-selection in one of the ferromagnetic contacts, we also elucidate how one can make the measurement "weak" by minimizing the back-action, while keeping the tunneling time unchanged. We then analyze the resulting interpretations of the tunneling time and the measurement back action in the presence of phase breaking effects that are intrinsic to solid state systems. We unravel that while the time-keeping aspect of the Larmor clock is reasonably undeterred due to momentum and phase relaxation processes, it degrades significantly in the presence of spin-dephasing. We believe that the ideas presented here also open up a fructuous solid state platform to encompass emerging ideas in quantum technology such as quantum weak values and its applications, that are currently exclusive to quantum optics and cold atoms.

Spontaneous emergence of a spin state for an emitter in a time-varying medium

Time-varying media can dramatically modify the emission of embedded sources by producing time reversed waves refocusing on the source. Here, we show that such a back action can create an angular momentum by setting the source in a spontaneous spin state. We experimentally implement this coupling using self-propelled bouncing droplets sources coupled to the surface waves they emit on a parametrically excited bath. The spin state dynamics result from a self-organized interplay between the source motion and the time reversed waves. The discrete stability analysis agrees with the experimental observations. In addition, we show that these spin states provide a unique opportunity for an experimental access to parameters enabling comparison and calibration of the various existing models.

Back action evading electro-optical transducer

Electro-optical transducers are utilized for upconversion of radio frequency (RF) signals to the optical frequency domain to study the RF signals with optical tools. The transducers frequently impact the RF system and introduce additional noise, including optical shot noise as well as quantum back action noise, limiting the measurement accuracy. In this paper, we theoretically study a technique based on a high efficiency electro-optical phase modulation effect that allows back action evading detection of an RF field quadrature. The main idea of the proposed method is independent homodyne detection of two (Stokes and anti-Stokes) optical modes. It allows subsequent postprocessing of the detected signals, which, in turn, results in a broadband back action evading measurement leading to the high sensitivity evaluation of the RF signals.

Rotating and spiraling spatial dissipative solitons of light and cold atoms

Clouds of cold neutral atoms driven by a coherent light beam in a ring cavity exhibit self-structured states transversely with respect to the beam axis due to optomechanical forces and the back action of the atomic structures on the beam. Below the instability threshold for extended hexagonal structures, localized soliton-like excitations can be stable. These constitute peaks or holes of atom density, depending on the linear susceptibility of the cloud. Complex rotating and spiralling motion of coupled atom-light solitons, and hence atomic transport, can be achieved via phase gradients in the input field profile. We also discuss the stability of rotating soliton chains in view of soliton-soliton interactions. The investigations are performed in a cavity scheme but expected to apply to other longitudinally pumped schemes with diffractive coupling.

Optical measurement of electron spins in quantum dots: quantum Zeno effects.

We describe theoretically the effects of the quantum back action under the continuous optical measurement of electron spins in quantum dots. We consider the system excitation by elliptically polarized light close to the trion resonance, which allows for simultaneous spin orientation and measurement. We microscopically demonstrate that the nuclei-induced spin relaxation can be both suppressed and accelerated by the continuous spin measurement due to the quantum Zeno and anti-Zeno effects, respectively. Our theoretical predictions can be directly compared with the future experimental results and straightforwardly generalized for pump-probe experiments.

Broadband quantum back action evading measurements of a resonant force

Quantum back action imposes fundamental sensitivity limits to the majority of quantum measurements. The effect results from the unavoidable contamination of the measured parameter with the quantum noise of a meter. Back action evading measurements take advantage of the quantum correlations introduced by the system under study to the meter and allow overcoming the fundamental limitations. The measurements are frequently restricted in their bandwidth due to a finite response time of the system components. Here we show that probing a mechanical oscillator with a dichromatic field with frequencies separated by the oscillator frequency enables independent detection in two outputs and complete subtraction of the measurement noise associated with the quantum back action.

Stochastic-field approach to the quench dynamics of the one-dimensional Bose polaron

We consider the dynamics of a quantum impurity after a sudden interaction quench into a one-dimensional degenerate Bose gas. We use the Keldysh path integral formalism to derive a truncated Wigner like approach that takes the back action of the impurity onto the condensate into account already on the mean-field level and further incorporates thermal and quantum effects up to one-loop accuracy. This framework enables us not only to calculate the real space trajectory of the impurity but also the absorption spectrum. We find that quantum corrections and thermal effects play a crucial role for the impurity momentum at weak to intermediate impurity-bath couplings.Furthermore, we see the broadening of the absorption spectrum with increasing temperature.

Broadband dichromatic variational measurement

Standard Quantum Limit (SQL) of a classical mechanical force detection results from quantum back action impinged by the meter on a probe mechanical transducer perturbed by the force of interest. In this paper we introduce a technique of continuous \vy{broadband} back action avoiding measurements for the case of a resonant signal force acting on a linear mechanical oscillator supporting one of mirrors of an optical Michelson-Sagnac Interferometer (MSI). The interferometer with the movable mirror is an opto-mechanical transducer able to support polychromatic probe field. The method involves a dichromatic optical probe resonant with the MSI modes and having frequency separation equal to the mechanical frequency. We show that analyzing each of the harmonics of the probe reflected from the mechanical system separately and postprocessing the measurement results allows excluding the back action in a broad frequency band and measuring the force with sensitivity better than SQL.

Collective self-trapping of atoms in a cavity

We study experimentally the optical dipole trapping of a cloud of cold atoms in a high-finesse cavity in the parameter regime where the atomic back-action on the cavity mode is significant. Back-action based effects lead to state selective optical manipulation schemes. We identify a parameter range where the collective back action of the atoms is needed for the trapping, i.e. a single atom would not be trapped under the same laser drive conditions. The collective self-trapping is demonstrated by the observation of a significant increase of the trapping time as a function of the atom number. The atomic back action on the cavity field gives rise to a simultaneous real-time monitoring of the number of trapped atoms. This is used to show a non-exponential collapse of the atom trap.

Uncertainty limits of the information exchange between a quantum system and an external meter

It is not possible to obtain information about the observable properties of a quantum system without a physical interaction between the system and an external meter. This physical interaction is described by a unitary transformation of the joint quantum state of the system and the meter, which means that the information transfer from the system to the meter depends on the initial quantum coherence of the meter. In the present paper, we analyze the measurement interaction in terms of the changes of the meter state caused by the interaction with the system. The sensitivity of the meter can then be defined by evaluating the distinguishability of meter states for different values of the target observable. It is shown that the sensitivity of the meter requires quantum coherences in the generator observable that determines the magnitude of the back action of the meter on the system. The trade-off between measurement resolution and back action is decided by the relation between sensitivity and quantum coherent uncertainty in the external meter. No matter how macroscopic the device used as a meter is, it must necessarily be quantum coherent in the degrees of freedom that interact with the system being measured.

Quantum Euler Relation for Local Measurements.

In classical thermodynamics the Euler relation is an expression for the internal energy as a sum of the products of canonical pairs of extensive and intensive variables. For quantum systems the situation is more intricate, since one has to account for the effects of the measurement back action. To this end, we derive a quantum analog of the Euler relation, which is governed by the information retrieved by local quantum measurements. The validity of the relation is demonstrated for the collective dissipation model, where we find that thermodynamic behavior is exhibited in the weak-coupling regime.

Einstein–de Haas phase shifts in surface acoustic waves

We demonstrate the phase shift of the Rayleigh-type surface acoustic wave (RSAW) in a ferromagnetic ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ film based on the gyromagnetic Einstein--de Haas (EdH) torque that can be attributed to the back action of spin-wave excitation. The EdH torque modulates the transverse velocity of the bulk acoustic wave that causes the bipolar phase shift of RSAW. When compared with a magnetoelastic torque, the phase shift can be more prominently enhanced by increasing the frequency. The gyromagnetic torque in the RSAW paves the way for controlling spin-mechatronics devices without magnetoelastic torque which is specific to a certain material.

Resurgence, refusal, and reconciliation through food movement organizations: A case study of Food Secure Canada’s 2018 Assembly

Indigenous food systems have been sites of deliberate and sustained disruption in the service of the settler colonial project on Turtle Island. The revitalization of traditional foodways is a powerful and popular means through which Indigenous Peoples are practicing cultural and political resurgence. We are at a crucial moment of societal reckoning reinforced by recent anti-racist uprisings and Indigenous Land Back actions. In this context, food movements have an important role to play in addressing ongoing colonial impacts on Indigenous food systems by supporting Indigenous Food Sovereignty as a way to advance reconciliation between settlers and Indigenous Peoples. Since its founding in 2005, Food Secure Canada (FSC) has become a national leader in food movements in Canada and its biennial Assembly is arguably the largest food movement event in the country. Despite its sustained engagement with Indigenous Peoples and significant efforts toward inclusion, its 2018 Assembly saw Indigenous people, Black people, and other people of color expressing important concerns, culminating in a walk-out on the last day. To understand how these events might guide transformative reconciliation in and through food movements, we analyzed 124 post-Assembly qualitative questionnaires, held 10 interviews, and analyzed organizational archives, in addition to conducting participant observation throughout the following year. This research portrays the actions taken at the Assembly to be a refusal of settler structures and processes, and the creation of a caucus space for Indigenous people, Black people, and other people of color as an act of resurgence. Engagement with FSC by a number of those involved with the protests throughout the year that followed, and the resultant commitment to center decolonization in FSC’s work, reveal the intimate connection between resurgence and reconciliation. These acts of generative refusal and resurgence are an essential part of efforts toward reconciliation without assimilation, aligned in a shared struggle toward the decolonized futures at the heart of food sovereignty for all.

Quantum limits for stationary force sensing

State-of-the-art sensors of force, motion and magnetic fields have reached the sensitivity where the quantum noise of the meter is significant or even dominant. In particular, the sensitivity of the best optomechanical devices has reached the Standard Quantum Limit (SQL), which directly follows from the Heisenberg uncertainty relation and corresponds to balancing the measurement imprecision and the perturbation of the probe by the quantum back action of the meter. The SQL is not truly fundamental and several methods for its overcoming have been proposed and demonstrated. At the same time, two quantum sensitivity constraints which are more fundamental are known. The first limit arises from the finiteness of the probing strength (in the case of optical interferometers - of the circulating optical power) and is known as the Energetic Quantum Limit or, in a more general context, as the Quantum Cram\'{e}r-Rao Bound (QCRB). The second limit arises from the dissipative dynamics of the probe, which prevents full efficacy of the quantum back action evasion techniques developed for overcoming the SQL. No particular name has been assigned to this limit; we propose the term Dissipative Quantum Limit (DQL) for it. Here we develop a unified theory of these two fundamental limits by deriving the general sensitivity constraint from which they follow as particular cases. Our analysis reveals a phase transition occurring at the boundary between the QCRB-dominated and the DQL regimes, manifested by the discontinuous derivatives of the optimal spectral densities of the meter field quantum noise. This leads to the counter-intuitive (but favorable) finding that quantum-limited sensitivity can be achieved with certain lossy meter systems. Finally, we show that the DQL originates from the non-autocommutativity of the internal thermal noise of the probe and that it can be overcome in non-stationary measurements.

Surface contribution to the electric dipole moment near an interface, and its effect on power emission.

When a small particle is located near an interface, its electric dipole moment can be induced by laser irradiation. Since the laser light reflects at the interface, this leads to an interference pattern, and the dipole moment is determined by this total field. In addition, the dipole radiation emitted by the particle reflects at the interface, and this field adds to the external field. In this fashion, the dipole moment is altered by the field it emits, and as such the emitted radiation modifies its own source. We have derived a simple expression to take this back action into account. We introduce two resolvent functions, Υ‖(h) and Υ⊥(h), which depend on the dimensionless distance h between the particle and the interface. These functions exhibit resonance features due to the underlying back-action mechanism. It is shown that two, one, or no resonance peaks appear in the induced dipole moment. Whether these peaks are present depends on the parameters under consideration. The power emitted by the particle depends on h due to interference between the source radiation and the reflected radiation. With the surface induced contribution to the dipole moment included, an additional h dependence appears. This dependence shows the resonance peaks, which may be amenable to experimental observation.

Coupling light to a nuclear spin gas with a two-photon linewidth of five millihertz.

Nuclear spins of noble gases feature extremely long coherence times but are inaccessible to optical photons. Here, we realize a coherent interface between light and noble-gas spins that is mediated by alkali atoms. We demonstrate the optical excitation of the noble-gas spins and observe the coherent back action on the light in the form of high-contrast two-photon spectra. We report on a record two-photon linewidth of 5 ± 0.7 mHz above room temperature, corresponding to a 1-min coherence time. This experiment provides a demonstration of coherent bidirectional coupling between light and noble-gas spins, rendering their long-lived spin coherence accessible for manipulations in the optical domain.

Low-Autocorrelation Random Number Generator Based on Adiabatic Quantum-Flux-Parametron Logic

We propose a true random number generator (TRNG) based on adiabatic quantum-flux-parametron (AQFP) logic. This AQFP TRNG corresponds to an AQFP buffer chain with the input current removed, which enables the generation of random bits via thermal fluctuation. To realize ultralow autocorrelation, several isolation inductors are inserted between adjacent buffers in the AQFP TRNG, thereby reducing the impact of the back action between buffers. We design and fabricate an AQFP TRNG using the AIST 10 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Nb high-speed standard process and evaluate the quality of the generated random bit streams. For a 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -bit random stream obtained from the AQFP TRNG at 100 kHz, the autocorrelation function is 5.00 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Furthermore, we evaluate a 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> -bit random stream from the AQFP TRNG using a statistical test suite provided by the National Institute of Standards and Technology. The bit stream passed 14 out of 15 tests, which suggests that the proposed AQFP TRNG can be used for applications such as stochastic computing and stochastic local search algorithms.

Aerobics Movement Decomposition Action Teaching System Based on Intelligent Vision Sensor

With the development of the times, teaching has not only stayed between people, but also gradually developed into the teaching interaction between man and machine. In the past, the teaching form was relatively single and old. Based on the intelligent visual sensor, this paper develops an auxiliary teaching system for the decomposition of aerobics action and reasonably uses the Internet and algorithms to catalog a series of aerobics action systems into the system. The DTW dynamic motion matching algorithm of the system will recognize human actions more accurately. The system will feed back human actions to the system in real time based on human feature recognition. Then, after comparison, the system will display the standard posture of this action and the aerobics posture in the next step. Therefore, this system develops teaching not only in class, but everywhere. The system not only improves the teaching quality of aerobics, but also strengthens the physical quality of teenagers. It has a new understanding of the standardization of aerobics teaching. After the function of the system is complete, the system will be distributed to aerobics learners. In many feedback information, the average use satisfaction has reached about 80%, which is a good performance index for the performance of the system itself.

New bounds on macroscopic scalar-field topological defects from nontransient signatures due to environmental dependence and spatial variations of the fundamental constants

We point out that in models of macroscopic topological defects composed of one or more scalar fields that interact with standard model fields via scalar-type couplings the back action of ambient matter on the scalar field(s) produces an environmental dependence of the fundamental constants of nature as well as spatial variations of the fundamental constants in the vicinity of dense bodies such as Earth due to the formation of a ``bubblelike'' defect structure surrounding the dense body. In sufficiently dense environments, spontaneous symmetry breaking may be inhibited altogether for ${\ensuremath{\phi}}^{2}$ interactions, potentially delaying the cosmological production of topological defects to rather late times. We derive bounds on nontransient variations of the fundamental constants from torsion-pendulum experiments that search for equivalence-principle-violating forces, experiments comparing the frequencies of ground- and space-based atomic clocks as well as ground-based clocks at different heights in the recent Tokyo Skytree experiment, and measurements comparing atomic and molecular transition frequencies in terrestrial and low-density astrophysical environments. Our results constrain the present-day mass-energy fraction of the Universe due to a network of infinite domain walls produced shortly after the big bang nucleosynthesis or cosmic microwave background epochs to be ${\mathrm{\ensuremath{\Omega}}}_{\text{walls},0}\ensuremath{\ll}{10}^{\ensuremath{-}10}$ for the symmetron model with a ${\ensuremath{\phi}}^{4}$ potential and ${\ensuremath{\phi}}^{2}$ interactions, improving over cosmic microwave background quadrupolar temperature anisotropy bounds by at least 5 orders of magnitude. Our newly derived bounds on domain walls with ${\ensuremath{\phi}}^{2}$ interactions via their effects of nontransient variations of the fundamental constants are significantly more stringent than previously reported clock- and cavity-based limits on passing domain walls via transient signatures (regardless of their possible degradation due to previously neglected effects of the strongly repulsive potential generated by Earth on the passing domain walls) and previous bounds from different types of nontransient signatures (by about 10 orders of magnitude for wall thicknesses comparable to the size of Earth), under the same set of assumptions.