Decoherence Rate Research Articles

Decoherence and classicalization of continuous-time quantum walks on graphs

We address decoherence and classicalization of continuous-time quantum walks (CTQWs) on graphs. In particular, we investigate three different models of decoherence and employ the quantum-classical (QC) dynamical distance as a figure of merit to assess whether, and to which extent, decoherence classicalizes the CTQW, i.e. turns it into the analogue classical process. We show that the dynamics arising from intrinsic decoherence, i.e. dephasing in the energy basis, do not fully classicalize the walker and partially preserves quantum features. On the other hand, dephasing in the position basis, as described by the Haken–Strobl master equation or by the quantum stochastic walk (QSW) model, asymptotically destroys the quantumness of the walker, making it equivalent to a classical random walk. We also investigate how fast is the classicalization process and observe a larger rate of convergence of the QC-distance to its asymptotic value for intrinsic decoherence and the QSW models, whereas in the Haken–Strobl scenario, larger values of the decoherence rate induce localization of the walker.

Generation of entangled states between two transmon qubits via invariant-based shortcuts with constant drivings

Entanglement generation via optimal drivings is of significance for quantum information science and engineering. Here, an efficient scheme is proposed for creating entangled states between two transmon qubits via shortcuts to adiabaticity with constant Rabi drivings. Two three-level atoms of Cooper-pair box circuits are coupled to a common cavity field of transmission-line resonator and individually driven by adjustable classical microwaves. In the regime of large detuning, we obtain an effective three-state system consisting of qubits and cavity photons. By the invariant-based shortcuts with time-independent Rabi drivings, two-qubit entangled states can be rapidly induced from an easily prepared product state. Compared to the entanglement creation with time-dependent Rabi drivings, our protocol not only becomes more feasible in experiment but also needs shorter times. Moreover, high-fidelity quantum operations can be realized with the accessible decoherence rates. The scheme could pave a promising avenue towards optimal generation of entangled states between superconducting qubits.

Quantum dynamical decoupling by shaking the close environment

Quantum dynamical decoupling is a procedure to cancel the effective coupling between two systems by applying sequences of fast actuations, under which the coupling Hamiltonian averages out to leading order(s). One of its prominent uses is to drive a target system in such a way as to decouple it from a less protected one. The present manuscript investigates the dual strategy: acting on a noisy “environment” subsystem, such as to decouple it from a target system. The target system only has a Hamiltonian coupling to this close environment which itself undergoes Lindbladian decoherence. The potential advantages are that actions on the environment commute with system operations, and that imprecisions in the decoupling actuation are harmless to the target. We consider two versions of environment-side decoupling: adding an imprecise Hamiltonian drive which stirs the environment components; and, increasing the decoherence rates on the environment. The latter can be viewed as driving the environment with pure noise and our conclusions establish how, maybe counterintuitively, isolating the environment from noise sources as much as possible is often not the best option. In this setting, we can explicitly analyze the induced decoherence on the target system, and focusing on the example of a two-level environment subsystem, we establish how the induced decoherence is influenced by the parameters in both cases. The analysis combines Lindbladian derivation, adiabatic elimination, and Floquet modeling in a way that may be of independent interest.

Dark-exciton giant Rabi oscillations with no external magnetic field

Multi-phonon physics is an emerging field that serves as a test bed for fundamental quantum physics and several applications in metrology, on-chip communication, among others. Quantum acoustic cavities or resonators are devices that are being used to study multi-phonon phenomena both theoretically and experimentally. In particular, we study a system consisting of a semiconductor quantum dot pumped by a driving laser, and coupled to an acoustic cavity. This kind of systems has proven to yield interesting multi-phonon phenomena, but the description of the quantum dot has been limited to a two-level system. This limitation restrains the complexity that a true semiconductor quantum dot can offer. Instead, in this work we consider a model where the quantum dot can have both bright and dark excitons, the latter being particularly useful due to their lower decoherence rates, because they do not present spontaneous photon emission. In this setup, we demonstrate that by fine-tuning the driving laser frequency, one is able to realise giant Rabi oscillations between the vacuum state and a dark exciton state with $N$-phonon bundles. From this, we highlight two outstanding features: first, we are able to create dark states excitations in the quantum dot without the usual external magnetic field needed to do so; and second, in a dissipative scenario where the acoustic cavity and the quantum dot suffer from losses, the system acts as a phonon gun able to emit $N$-phonon bundles.

Dense Coding and Quantum Memory Assisted Entropic Uncertainty Relations in a Two‐Qubit State Influenced by Dipole and Symmetric Cross Interactions

AbstractOptimal dense coding and entropic uncertainty relation of quantum memory (EUR‐QM) of the two‐qubit Heisenberg chain model influenced by atomic dipole and Kaplan–Shekhtman–Entin–Wohlman–Aharony interactions are studied. The temporal evolution of the two functions at the phase decoherence environment with the Werner state as the initial state is discussed. The results imply that the dense coding and EUR‐QM have a contrary relationship, and an increase in the phase decoherence rate reduces the behavior of the two functions. The growth in coupling dipole interaction plays a role in oscillating and increasing the minimum bounds of the dense coding capacity. On the other hand, the effect of a thermal bath on the two functions at finite temperatures is investigated. The coding capacity suffers from sudden death as the temperature increases. The possibility of restraining the sudden death induced by the temperature may be enhanced by increasing the strength of the symmetric cross interaction. As the coupling of dipole increases, the EUR‐QM decreases while the coding capacity rises to its maximum value.

In situ Tuning of the Electric-Dipole Strength of a Double-Dot Charge Qubit: Charge-Noise Protection and Ultrastrong Coupling

Semiconductor quantum dots in which electrons or holes are isolated via electrostatic potentials generated by surface gates are promising building blocks for semiconductor-based quantum technology. Here, we investigate double-quantum-dot (DQD) charge qubits in GaAs capacitively coupled to high-impedance superconducting quantum interference device array and Josephson-junction array resonators. We tune the strength of the electric-dipole interaction between the qubit and the resonator in situ using surface gates. We characterize the qubit-resonator coupling strength, the qubit decoherence, and the detuning noise affecting the charge qubit for different electrostatic DQD configurations. We find all quantities to be systematically tunable over more than one order of magnitude, resulting in reproducible decoherence rates Γ2/2π<5 MHz in the limit of high interdot capacitance. In the opposite limit, by reducing the interdot capacitance, we increase the DQD electric-dipole strength and, therefore, its coupling to the resonator. Employing a Josephson-junction array resonator with an impedance of approximately 4 kΩ and a resonance frequency of ωr/2π∼5.6 GHz, we observe a coupling strength of g/2π∼630 MHz, demonstrating the possibility to operate electrons hosted in a semiconductor DQD in the ultrastrong-coupling regime (USC). The presented results are essential for further increasing the coherence of quantum-dot-based qubits and investigating USC physics in semiconducting QDs.6 MoreReceived 26 October 2021Revised 3 April 2022Accepted 26 May 2022DOI:https://doi.org/10.1103/PhysRevX.12.031004Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum information architectures & platformsQuantum information processingQuantum information with hybrid systemsQuantum information with solid state qubitsQuantum Information

Holonomic quantum manipulation in the Weyl disk

It has been shown that a Weyl point in a superconducting nanostructure may give rise to a Weyl disk where two quantum states are almost degenerate in a two-dimensional manifold in the parametric space. This opens up the possibility of a holonomic quantum manipulation: A transformation of the wave function upon an adiabatic change of the parameters within the degenerate manifold. In this paper, we investigate in detail the opportunities for holonomic manipulation in Weyl disks. We compute the connection at the manifold in quasiclassical approximation to show it is Abelian and can be used for a phase gate. To provide a closed example of quantum manipulation that includes a state preparation and readout, we augment the holonomic gate with a change of parameters that brings the system out of the degenerate subspace. For numerical illustrations, we use a finite value of quasiclassical parameter and exact quantum dynamics. We investigate the fidelity of an example gate for different execution times. We evaluate the decoherence rate and show it can be made small to ensure a wide frequency range where an adiabatic manipulation remains coherent.

Hypercontractivity and Logarithmic Sobolev Inequality for Non-primitive Quantum Markov Semigroups and Estimation of Decoherence Rates

We generalize the concepts of weak quantum logarithmic Sobolev inequality (LSI) and weak hypercontractivity (HC), introduced in the quantum setting by Olkiewicz and Zegarlinski, to the case of non-primitive quantum Markov semigroups (QMS). The originality of this work resides in that this new notion of hypercontractivity is given in terms of the so-called amalgamatedmathbb {L}_p norms introduced recently by Junge and Parcet in the context of operator spaces theory. We make three main contributions. The first one is a version of Gross’ integration lemma: we prove that (weak) HC implies (weak) LSI. Surprisingly, the converse implication differs from the primitive case as we show that LSI implies HC but with a weak constant equal to the cardinal of the center of the decoherence-free algebra. Building on the first implication, our second contribution is the fact that strong LSI and therefore strong HC do not hold for non-trivially primitive QMS. This implies that the amalgamated mathbb {L}_p norms are not uniformly convex for 1le p le 2. As a third contribution, we derive universal bounds on the (weak) logarithmic Sobolev constants for a QMS on a finite dimensional Hilbert space, using a similar method as Diaconis and Saloff-Coste in the case of classical primitive Markov chains, and Temme, Pastawski and Kastoryano in the case of primitive QMS. This leads to new bounds on the decoherence rates of decohering QMS. Additionally, we apply our results to the study of the tensorization of HC in non-commutative spaces in terms of the completely bounded norms (CB norms) recently introduced by Beigi and King for unital and trace preserving QMS. We generalize their results to the case of a general primitive QMS and provide estimates on the (weak) constants.

Engineering shortcut-based operations on a nitrogen-vacancy spin qubit and microwave photons via optimized drivings

Optimal control of quantum systems is of significance to information processing and state engineering. Here an efficient scheme is proposed for engineering shortcut-based operations on a nitrogen-vacancy spin qubit and microwave photons via simplified drivings. The spin qubit dispersively coupled to a cavity field constitutes a composite system. Within a three-state subspace, we treat an effective interaction between the composite system and two classical drivings. By the technique of invariant-based shortcuts to adiabaticity, a state swap via Rabi pulses with sine- and cosine-typed waveforms and the generation of entangled states using constant Rabi pulses can be constructed, respectively. Moreover, with the accessible rates of qubit decoherence and photon decay, robust operations could be achieved by numerically simulating noise effects. The strategy could offer simple yet versatile avenues to implement the shortcut-based operations on the composite spin–photon system experimentally.

Quantum anonymous veto: a set of new protocols

We propose a set of protocols for quantum anonymous veto (QAV) broadly categorized under the probabilistic, iterative, and deterministic schemes. The schemes are based upon different types of quantum resources. Specifically, they may be viewed as single photon-based, bipartite and multipartite entangled states-based, orthogonal state-based and conjugate coding-based. The set of the proposed schemes is analyzed for all the requirements of a valid QAV scheme (e.g., privacy, verifiability, robustness, binding, eligibility and correctness). The proposed schemes are observed to be more efficient in comparison to the existing QAV schemes and robust up to the moderate decoherence rate. In addition, a trade-off between correctness and robustness of the probabilistic QAV schemes is observed. Further, the multipartite dense coding based deterministic QAV scheme is most efficient scheme among the set of schemes proposed here. A bipartite entanglement based iterative scheme employing dense coding is yet another efficient and practical scheme. The intrinsic connections between dining cryptographer-net with anonymous veto-net is also explored in the process of designing new protocols.

Perimeter Modes of Nanomechanical Resonators Exhibit Quality Factors Exceeding 109 at Room Temperature

Systems with low mechanical dissipation are extensively used in precision measurements such as gravitational wave detection, atomic force microscopy and quantum control of mechanical oscillators via opto- and electromechanics. The mechanical quality factor ($Q$) of these systems determines the thermomechanical force noise and the thermal decoherence rate of mechanical quantum states. While the dissipation rate is typically set by the bulk acoustic properties of the material, by exploiting dissipation dilution, mechanical $Q$ can be engineered through geometry and increased by many orders of magnitude. Recently, soft clamping in combination with strain engineering has enabled room temperature quality factors approaching one billion ($10^9$) in millimeter-scale resonators. Here we demonstrate a new approach to soft clamping which exploits vibrations in the perimeter of polygon-shaped resonators tethered at their vertices. In contrast to previous approaches, which rely on cascaded elements to achieve soft clamping, perimeter modes are soft clamped due to symmetry and the boundary conditions at the polygon vertices. Perimeter modes reach $Q$ of 3.6 billion at room temperature while spanning only two acoustic wavelengths -- a 4-fold improvement over the state-of-the-art mechanical $Q$ with 10-fold smaller devices. The small size of our devices makes them well-suited for near-field integration with microcavities for quantum optomechanical experiments. Moreover, their compactness allows the realization of phononic lattices. We demonstrate a one-dimensional Su-Schrieffer-Heeger chain of high-$Q$ perimeter modes coupled via nearest-neighbour interaction and characterize the localized edge modes.

Non-Markovianity criteria for mixtures of noninvertible Pauli dynamical maps

We analyze the connections between the non-Markovianity degree of the most general phase-damping qubit maps and their legitimate mixtures. Using the results for image non-increasing dynamical maps, we formulate the necessary and sufficient conditions for the Pauli maps to satisfy specific divisibility criteria. Next, we examine how the non-Markovianity properties for (in general noninvertible) Pauli dynamical maps influence the properties of their convex combinations. Our results are illustrated with instructive examples. For P-divisible maps, we propose a legitimate time-local generator whose all decoherence rates are temporarily infinite.

Dynamical approach to improving Majorana qubits and distinguishing them from trivial bound states

We study a series of dynamical protocols which involve periodically driving a quantum dot coupled to a putative nanowire hosting Majorana zero modes (MZMs) to i) reduce the hybridization between MZMs, ii) improve the coherence of the Majorana qubit with respect to $1/f$ dephasing noise and quasiparticle poisoning, and iii) provide a definitive test to differentiate Andreev Bound states (ABSs) from MZMs. The protocols are based on the notion of $draiding$ - exchanging a pair of Majoranas twice, repeatedly, at high frequency [1]. In this process, the exchanged Majorana operators acquire a robust minus sign such that terms in the Hamiltonian, linear in either operator, vanish on average. The four protocols proposed implement draiding by coupling quantum dot(s) to the end(s) of the nanowire. They are treated using Floquet theory and numerical simulations. The hybridization energy and decoherence rate are shown to be reduced by several orders of magnitude, in accordance with theoretical expectations, when the protocols are implemented on nanowires described by experimentally relevant parameters. The tunneling conductance computed in this Floquet setting reveals zero bias peaks (ZBPs) that become more centered at zero voltage bias. When these protocols are implemented on nanowires supporting ABSs that mimic MZMs in ZBP measurements, the qubit coherence $deteriorates$, in stark contrast to the case where the nanowire supports MZMs and the coherence drastically improves, thus serving as a dynamical test to distinguish MZMs from trivial bound states.

Increasing the decoherence rate of Rydberg polaritons due to accumulating dark Rydberg atoms

We experimentally observed an accumulative type of nonlinear attenuation and distortion of slow light, i.e., Rydberg polaritons, with the Rydberg state $|32D_{5/2}\rangle$ in the weak-interaction regime. The present effect of attenuation and distortion cannot be explained by considering only the dipole-dipole interaction (DDI) between Rydberg atoms in $|32D_{5/2}\rangle$. Our observation can be attributed to the atoms in the dark Rydberg states other than those in the bright Rydberg state, i.e., $|32D_{5/2}\rangle$, driven by the coupling field. The dark Rydberg states are all the possible states, in which the population decaying from $|32D_{5/2}\rangle$ accumulated over time, and they were not driven by the coupling field. Consequently, the DDI between the dark and bright Rydberg atoms increased the decoherence rate of the Rydberg polaritons. We performed three different experiments to verify the above hypothesis, to confirm the existence of the dark Rydberg states, and to measure the decay rate from the bright to dark Rydberg states. In the theoretical model, we included the decay process from the bright to dark Rydberg states and the DDI effect induced by both the bright and dark Rydberg atoms. All the experimental data of slow light taken at various probe Rabi frequencies were in good agreement with the theoretical predictions based on the model. This study pointed out an additional decoherence rate in the Rydberg-EIT effect, and provides a better understanding of the Rydberg-polariton system.

Qubit decoherence and symmetry restoration through real-time instantons

A parametrically driven quantum oscillator, stabilized by a nonlinear dissipation, exhibits a spontaneous breaking of the parity symmetry. It results in the quantum bi-stability, corresponding to a Bloch sphere of dark states. This makes such a driven-dissipative system an attractive candidate for a qubit. The parity symmetry breaking is exact both on the classical level and within the quantum mechanical perturbation theory. Here we show that non-perturbative quantum effects lead to the symmetry restoration and result in exponentially small but finite qubit decoherence rate. Technically the symmetry restoration is due to real time instanton trajectories of the Keldysh path integral, which represents the Lindbladian evolution of the driven-dissipative oscillator.

Near-Field Generation and Control of Ultrafast, Multipartite Entanglement for Quantum Nanoplasmonic Networks.

For a quantum Internet, one needs reliable sources of entangled particles that are compatible with measurement techniques enabling time-dependent, quantum error correction. Ideally, they will be operable at room temperature with a manageable decoherence versus generation time. To accomplish this, we theoretically establish a scalable, plasmonically based archetype that uses quantum dots (QD) as quantum emitters, known for relatively low decoherence rates near room temperature, that are excited using subdiffracted light from a near-field transducer (NFT). NFTs are a developing technology that allow rasterization across arrays of qubits and remarkably generate enough power to strongly drive energy transitions on the nanoscale. This eases the fabrication of QD media, while efficiently controlling picosecond-scale dynamic entanglement of a multiqubit system that approaches maximum fidelity, along with fluctuation between tripartite and bipartite entanglement. Our strategy radically increases the scalability and accessibility of quantum information devices while permitting fault-tolerant quantum computing using time-repetition algorithms.

Improving resilience of quantum-gravity-induced entanglement of masses to decoherence using three superpositions

Recently, a protocol called quantum-gravity-induced entanglement of masses (QGEM) that aims to test the quantum nature of gravity using the entanglement of two qubits was proposed. The entanglement can arise only if the force between the two spatially superposed masses is occurring via the exchange of a mediating virtual graviton. In this paper we examine a possible improvement of the QGEM setup by introducing a third mass with an embedded qubit so that there are now three qubits to witness the gravitationally generated entanglement. We compare the entanglement generation for different experimental setups with two and three qubits and find that a three-qubit setup where the superpositions are parallel to each other leads to the highest rate of entanglement generation within $\ensuremath{\tau}=5\phantom{\rule{4pt}{0ex}}\mathrm{s}$. We show that the three-qubit setup is more resilient to the higher rate of decoherence. The entanglement can be detected experimentally for the two-qubit setup if the decoherence rate $\ensuremath{\gamma}$ is $\ensuremath{\gamma}&lt;0.11\phantom{\rule{4pt}{0ex}}\mathrm{Hz}$ compared to $\ensuremath{\gamma}&lt;0.16\phantom{\rule{4pt}{0ex}}\mathrm{Hz}$ for the three-qubit setup. However, the introduction of an extra qubit means that more measurements are required to characterize entanglement in an experiment. We conduct experimental simulations and estimate that the three-qubit setup would allow detecting the entanglement in the QGEM protocol at a $99.9%$ certainty with $O({10}^{4})--O({10}^{5})$ measurements when $\ensuremath{\gamma}\ensuremath{\in}[0.1,0.15]\phantom{\rule{4pt}{0ex}}\mathrm{Hz}$. Furthermore, we find that the number of needed measurements can be reduced to $O({10}^{3})--O({10}^{5})$ if the measurement schedule is optimized using joint Pauli basis measurements. For $\ensuremath{\gamma}&gt;0.06\phantom{\rule{4pt}{0ex}}\mathrm{Hz}$ the three-qubit setup is favorable compared to the two-qubit setup in terms of the minimum number of measurements needed to characterize the entanglement. Thus, the proposed setup here provides a promising avenue for implementing the QGEM experiment.

Environmental radiation impact on lifetimes and quasiparticle tunneling rates of fixed-frequency transmon qubits

Quantum computing relies on the operation of qubits in an environment as free of noise as possible. Assessing the quality of this environment is a key aspect of ensuring high-fidelity implementations based on superconducting qubits. Relaxation, decoherence, dephasing, and quasiparticle tunneling rates have been measured for various shielding configurations used in the measurement environment for state-of-the-art transmon qubits. An ensemble of approximately 120 control devices was used for this study, with five different capacitor pad designs. The shielding elements varied in the configuration included an indium gasket at the qubit can's lid, Cryoperm magnetic shielding, the mixing chamber shield of the dilution refrigerator, the inclusion of a vacuum pump-out port, and capping unused subminiature version A connectors at the top of the measurement can's lid. It was found that the qubit lifetimes T1, T2, and Tϕ are robust to the all of configuration changes tried until the mixing chamber shield was removed, significantly increasing blackbody radiation levels in the qubit measurement space, where in that limit it was found that tapering the qubit pads reduced the amount of loss. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configuration changes tested. Consistent with earlier reports [McEwen et al., arXiv:2104.05219 (2021); Cardani et al., Nat. Commun. 12, 2733 (2021); Wilen et al., Nature 594, 369–373 (2021); Ristè et al. Nat. Commun. 4, 1913 (2013)], the findings from this study indicate that non-equilibrium quasiparticles do not currently limit the lifetimes of well-shielded transmon qubits.

The entanglement and second-order coherence function in a two-atom nonlinear Jaynes-Cummings model

The influence of the Kerr nonlinearity on the two two-level atoms entanglement is investigated in Jaynes–Cummings model (JCM), so that one of atoms is inside the cavity field and the other atom is outside it. We show that the entanglement of atoms increases with the decrease of the phase decoherence rate or Kerr coefficient, when considering the effect of external environment. Then due to the statistic properties of the anti-bunching effect of photons are related to entanglement, the nonlinear JCM is examined theoretically to analyze the bunching or anti-bunching effect by using the second-order coherence function. We find that the second-order coherence function decreases with the Kerr coefficient. Our results show that the field reflects the stronger bunching effect with the bigger average photon number.

Quantum speed limit of the double quantum dot in pure dephasing environment under measurement

The quantum speed limit (QSL) of the double quantum dot (DQD) system has been theoretically investigated by adopting the detection of the quantum point contact (QPC) in the pure dephasing environment. The Mandelstam–Tamm (MT) type of the QSL bound which is based on the trace distance has been extended to the DQD system for calculating the shortest evolving time. The increase of decoherence rate can weaken the capacity for potential speedup (CPS) and delay the evolving process due to the frequently measurement localizing the electron in the DQD system. The system needs longer time to evolve to the target state as the enhancement of dephasing rate, because the strong interaction between pure dephasing environment and the DQD system could vary the oscillation of the electron. Increasing the dephasing rate can sharp the QSL bound, but the decoherence rate would weaken the former effect and vice versa. Moreover, the CPS would be raised by increasing the energy displacement, while the enhancement of the coupling strength between two quantum dots can diminish it. It is interesting that there has an inflection point, when the coupling strength is less than the value of the point, the increasing effect of the CPS from the energy displacement is dominant, otherwise the decreasing tendency of the CPS is determined by the coupling strength and suppress the action of the energy displacement if the coupling strength is greater than the point. Our results provide theoretical reference for studying the QSL time in a semiconductor device affected by numerous factors.