Decoherence Of States Research Articles

Effect of Etching Methods on Dielectric Losses in Transmons

Superconducting qubits are considered as a promising platform for implementing a fault tolerant quantum computing. However, surface defects of superconductors and the substrate leading to qubit state decoherence and fluctuations in qubit parameters constitute a significant problem. The amount and type of defects depend both on the chip materials and fabrication procedure. In this work, transmons produced by two different methods of aluminum etching: wet etching in a solution of weak acids and dry etching using a chlorine-based plasma are experimentally studied. The relaxation and coherence times for dry-etched qubits are more than twice as long as those for wet-etched ones. Additionally, the analysis of time fluctuations of qubit frequencies and relaxation times, which is an effective method to identify the dominant dielectric loss mechanisms, i-ndicates a significantly lower impact of two-level systems in the dry-etched qubits compared to the wet-etched ones.

Inherent Thermal-Noise Problem in Addressing Qubits

Qubit-specific measurement in a superconducting quantum processor requires physical interconnects that traverse 4 orders of magnitude in temperature from 293 K to 10 mK. Although the quantum processor can be thermalized and shielded from electromagnetic noise, the interconnects themselves introduce an unavoidable remote heat bath that causes decoherence of quantum states. In the present work, we report quantitative and device-independent measurements of the power radiated to the quantum processor from its control lines. Our results have been obtained using a calibrated bolometer that operates within a millikelvin environment with time-resolved measurement capability. In the limit of zero applied power, the noise power emitted to the quantum processor is equivalent to that of a blackbody with temperature 63–71 mK for the prototypical drive lines in the study. Experimentally, we increase the applied power of a simulated control signal to map out the resulting temperature rise and thermal time constant of five prototypical drive-line varieties. We input the data to an open quantum system model to demonstrate the trade-off between dissipated signal power, transmon-qubit lifetime, pure dephasing, gate fidelity, and the implied decoherence rates due to self-heating during microwave operations. Beyond explaining dephasing rates observed in the literature, our work sets the stage for accurate noise modeling in novel quantum computer interfacing methods due to our device-agnostic approach. Published by the American Physical Society 2024

A solvable model for graph state decoherence dynamics

We present an exactly solvable toy model for the continuous dissipative dynamics of permutation-invariant graph states of NN qubits. Such states are locally equivalent to an NN-qubit Greenberger-Horne-Zeilinger (GHZ) state, a fundamental resource in many quantum information processing setups. We focus on the time evolution of the state governed by a Lindblad master equation with the three standard single-qubit jump operators, the Hamiltonian part being set to zero. Deriving analytic expressions for the expectation values of observables expanded in the Pauli basis at all times, we analyze the nontrivial intermediate-time dynamics. Using a numerical solver based on matrix product operators, we simulate the time evolution for systems with up to 64 qubits and verify a numerically exact agreement with the analytical results. We find that the evolution of the operator space entanglement entropy of a bipartition of the system manifests a plateau whose duration increases logarithmically with the number of qubits, whereas all Pauli-operator products have expectation values decaying at most in constant time.

Decoherence and nonclassicality of photon-added and photon-subtracted multimode Gaussian states

Photon addition and subtraction render Gaussian states non-Gaussian. We provide a quantitative analysis of the change in nonclassicality produced by these processes by analyzing the Wigner negativity and quadrature coherence scale (QCS) of the resulting states. The QCS is a recently introduced measure of nonclassicality [Phys. Rev. Lett. 122, 080402 (2019); Phys. Rev. Lett. 124, 090402 (2020)], which we show to undergo a relative increase under photon addition and subtraction that can be as large as 200%. This implies that the degaussification and the concomitant increase of nonclassicality come at a cost. Indeed, the QCS is proportional to the decoherence rate of the state so that the resulting states are considerably more prone to environmental decoherence. Our results are quantitative and rely on explicit and general expressions for the characteristic and Wigner functions of photon-added and -subtracted single- and multimode Gaussian states for which we provide a simple and straightforward derivation. These expressions further allow us to certify the quantum non-Gaussianity of the photon-subtracted states with positive Wigner function.

Distributed Quantum Computing with Photons and Atomic Memories

AbstractThe promise of universal quantum computing requires scalable single‐ and inter‐qubit control interactions. Currently, three of the leading candidate platforms for quantum computing are based on superconducting circuits, trapped ions, and neutral atom arrays. However, these systems have strong interaction with environmental and control noises that introduce decoherence of qubit states and gate operations. Alternatively, photons are well decoupled from the environment and have advantages of speed and timing for quantum computing. Photonic systems have already demonstrated capability for solving specific intractable problems like Boson sampling, but face challenges for practically scalable universal quantum computing solutions because it is extremely difficult for a single photon to “talk” to another deterministically. Here, a universal distributed quantum computing scheme based on photons and atomic‐ensemble‐based quantum memories is proposed. Taking the established photonic advantages, two‐qubit nonlinear interaction is mediated by converting photonic qubits into quantum memory states and employing Rydberg blockade for the controlled gate operation. Spatial and temporal scalability of this scheme is demonstrated further. These results show photon‐atom network hybrid approach can be a potential solution to universal distributed quantum computing.

On the Capacity Region of Bipartite and Tripartite Entanglement Switching

We study a quantum entanglement distribution switch serving a set of users in a star topology with equal-length links. The quantum switch, much like a quantum repeater, can perform entanglement swapping to extend entanglement across longer distances. Additionally, the switch is equipped with entanglement switching logic, enabling it to implement switching policies to better serve the needs of the network. In this work, the function of the switch is to create bipartite or tripartite entangled states among users at the highest possible rates at a fixed ratio. Using Markov chains, we model a set of randomized switching policies. Discovering that some are better than others, we present analytical results for the case where the switch stores one qubit per user, and find that the best policies outperform a time division multiplexing policy for sharing the switch between bipartite and tripartite state generation. This performance improvement decreases as the number of users grows. The model is easily augmented to study the capacity region in the presence of quantum state decoherence and associated cut-off times for qubit storage, obtaining similar results. Moreover, decoherence-associated quantum storage cut-off times appear to have little effect on capacity in our identical-link system. We also study a smaller class of policies when the switch stores two qubits per user.

On the General Entangled State and Quantum Decoherence

We study the primary entanglement effect on the decoherence of reduced-density matrices of scalar fields, which interact with other fields or independent mode functions. We study the (leading) tree-level evolution of the scalar bispectrum due to a coupling between two scalar fields. We show that the primary entanglement has a significant role in the decoherence of the given quantum state. We find that the existence of such an entanglement could couple dynamical equations coming from a Schrödinger equation. We show that if one wants to see no effect of the entanglement parameter in the decohering of the quantum system, then the ground state eigenvalues of the interaction terms in the Hamiltonian cannot be independent of each other Generally, including the primary entanglement destroys the independence of the interaction terms in the ground state. We show that the imaginary part of the entanglement parameter plays an important role in the decoherence process without posing any specific restriction to the interaction terms. Our results could be generalized to every scalar quantum field theory with a well-defined quantization of its fluctuations in a given curved space-time.

Squeezed States of Light for Future Gravitational Wave Detectors at a Wavelength of 1550nm.

The generation of strongly squeezed vacuum states of light is a key technology for future ground-based gravitational wave detectors (GWDs) to reach sensitivities beyond their quantum noise limit. For some proposed observatory designs, an operating laser wavelength of 1550nm or around 2 μm is required to enable the use of cryogenically cooled silicon test masses for thermal noise reduction. Here, we present for the first time the direct measurement of up to 11.5dB squeezing at 1550nm over the complete detection bandwidth of future ground-based GWDs ranging from 10kHz down to below 1Hz. Furthermore, we directly observe a quantum shot-noise reduction of up to (13.5±0.1) dB at megahertz frequencies. This allows us to derive a precise constraint on the absolute quantum efficiency of the photodiode used for balanced homodyne detection. These results hold important insight regarding the quantum noise reduction efficiency in future GWDs, as well as for quantum information and cryptography, where low decoherence of nonclassical states of light is also of high relevance.

Microscopic and macroscopic effects in the decoherence of neutrino oscillations

We present a generic structure (the layer structure) for decoherence effects in neutrino oscillations, which includes decoherence from quantum mechanical and classical uncertainties. The calculation is done by combining the concept of open quantum system and quantum field theory, forming a structure composed of phase spaces from microscopic to macroscopic level. Having information loss at different levels, quantum mechanical uncertainties parameterize decoherence by an intrinsic mass eigenstate separation effect, while decoherence for classical uncertainties is typically dominated by a statistical averaging effect. With the help of the layer structure, we classify the former as state decoherence (SD) and the latter as phase decoherence (PD), then further conclude that both SD and PD result from phase wash-out effects of different phase structures on different layers. Such effects admit for simple numerical calculations of decoherence for a given width and shape of uncertainties. While our structure is generic, so are the uncertainties, nonetheless, a few notable ones are: the wavepacket size of the external particles, the effective interaction volume at production and detection, the energy reconstruction model and the neutrino production profile. Furthermore, we estimate the experimental sensitivities for SD and PD parameterized by the uncertainty parameters, for reactor neutrinos and decay-at-rest neutrinos, using a traditional rate measuring method and a novel phase measuring method.

Decoherence effects break reciprocity in matter transport

The decoherence of quantum states defines the transition between the quantum world and classical physics. Decoherence or, analogously, quantum mechanical collapse events pose fundamental questions regarding the interpretation of quantum mechanics and are technologically relevant because they limit the coherent information processing performed by quantum computers. We have discovered that the transition regime enables a novel type of matter transport. Applying this discovery, we present nanoscale devices in which decoherence, modeled by random quantum jumps, produces fundamentally novel phenomena by interrupting the unitary dynamics of electron wave packets. Noncentrosymmetric conductors with mesoscopic length scales act as two-terminal rectifiers with unique properties. In these devices, the inelastic interaction of itinerant electrons with impurities acting as electron trapping centers leads to a novel steady state characterized by partial charge separation between the two leads, or, in closed circuits to the generation of persistent currents. The interface between the quantum and the classical worlds therefore provides a novel transport regime of value for the realization of a new category of mesoscopic electronic devices.

Two-qubit Heisenberg XYZ dynamics of local quantum Fisher information, skew information coherence: Dyzaloshinskii–Moriya interaction and decoherence

We theoretically demonstrate how to achieve quantum entanglement control of a dissipative anisotropic two-qubit Heisenberg XYZ model. New dynamical features of local quantum coherence, local quantum Fisher information and Bures distance entanglement are achieved by controlling the initial state setting, DM interaction and decoherence. The proposed scheme allows for the generation of sudden changes in the entanglement. We illustrate the control by manipulating the local quantum coherence, Fisher information dynamics, and the sudden appearance of the disentanglement intervals. We further consider simultaneous changes in these behaviors for different values of the system parameters. These results provide helpful significance for understanding the dynamics of the dissipative anisotropic two-qubit Heisenberg XYZ model.

Relaxation of a dense ensemble of spins in diamond under a continuous microwave driving field

Decoherence of Rabi oscillation in a two-level quantum system consists of two components, a simple exponential decay and a damped oscillation. In dense-ensemble spin systems like negatively charged nitrogen-vacancy (NV−) centers in diamond, fast quantum state decoherence often obscures clear observation of the Rabi nutation. On the other hand, the simple exponential decay (or baseline decay) of the oscillation in such spin systems can be readily detected but has not been thoroughly explored in the past. This study investigates in depth the baseline decay of dense spin ensembles in diamond under continuously driving microwave (MW). It is found that the baseline decay times of NV− spins decrease with the increasing MW field strength and the MW detuning dependence of the decay times shows a Lorentzian-like spectrum. The experimental findings are in good agreement with simulations based on the Bloch formalism for a simple two-level system in the low MW power region after taking into account the effect of inhomogeneous broadening. This combined investigation provides new insight into fundamental spin relaxation processes under continuous driving electromagnetic fields and paves ways to better understanding of this underexplored phenomena using single NV− centers, which have shown promising applications in quantum computing and quantum metrology.

A theoretical study of the group refractive index n g in a four-level inverted Y-type system formed by 87Rb atom – laser radiation interaction

We report a theoretical investigation of the dispersion resulting from electromagnetically induced transparency (EIT) and the associated group refractive index profiles n g of a four-level inverted Y-type system formed by the interaction of three optical fields (probe, pump and control) with 87Rb atoms. The density matrix equations are derived from the semi-classical Liouville’s equation and solved both numerically and analytically to study the coherent nonlinear optical properties of the medium. We first present the EIT, dispersion and corresponding group index profiles n g under the switch-on/off and on/off-resonance conditions of the pump and control lasers. In presence of both pump and control lasers, an enhancement of the EIT window, a sharp EIT spike and related steeper dispersion slopes are obtained at the line centre of the probe frequency detuning. The group index profiles with the variation of the strengths of individual applied optical fields are studied. The effect of the ground state decoherence rates on the group index profile is examined in detail. It is found that the manipulation of n g values and the corresponding group velocities ν g of the probe light can be easily controlled from subluminal to superluminal values or vice versa by changing the strengths of the applied fields and the ground state decoherence rates. Besides, the EIT-based ‘optical switching’ phenomenon in the medium is explained by studying the variation of the group index with the pump and control Rabi frequencies.

Quantifying Decoherence of Gaussian Noise Channels

Gaussian noise channels (also called classical noise channels, bosonic Gaussian channels) arise naturally in continuous variable quantum information and play an important role in both theoretical analysis and experimental investigation of information transmission. After reviewing concisely the basic properties of these channels, we introduce an information-theoretic measure for the decoherence of optical states caused by these channels in terms of averaged Wigner-Yanase skew information, explore its basic features, obtain a scaling law, and derive a complementarity relation between the decoherence and the quantum affinity. As an application of the decoherence measure, we derive a convenient and sufficient criterion for detecting optical nonclassicality. The decoherence on some typical optical states caused by Gaussian noise channels are explicitly evaluated to illustrate the concept.

On the Stochastic Analysis of a Quantum Entanglement Distribution Switch

We study a quantum entanglement switch that serves $k$ users in a star topology. We model variants of the system using Markov chains and standard queueing theory and obtain expressions for switch capacity and the expected number of qubits stored in memory at the switch. While it is more accurate to use a discrete-time Markov chain (DTMC) to model such systems, we quickly encounter practical constraints of using this technique and switch to using continuous-time Markov chains (CTMCs). Using CTMCs allows us to obtain a number of analytic results for systems in which the links are homogeneous or heterogeneous and for switches that have infinite or finite buffer sizes. In addition, we can model the effects of decoherence of quantum states fairly easily using CTMCs. We also compare the results we obtain from the DTMC against the CTMC in the case of homogeneous links and infinite buffer, and learn that the CTMC is a reasonable approximation of the DTMC. From numerical observations, we discover that decoherence has little effect on capacity and expected number of stored qubits for homogeneous systems. For heterogeneous systems, especially those operating close to stability constraints, buffer size and decoherence can have significant effects on performance metrics. We also learn that in general, increasing the buffer size from one to two qubits per link is advantageous to most systems, while increasing the buffer size further yields diminishing returns.

Selection of entanglement state in quantum repeater process

The decoherence of entanglement states stored in quantum memory is a major obstacle when implementing a quantum repeater. So far, the electron spins in quantum dots are usually utilized to construct entangled states in quantum repeater. In the quantum repeater process, the distance between quantum dots is large, so the interaction between them can be neglected. Thus the hyperfine interaction between the electron spin and its neighbor nuclear spins in the quantum dot is considered to be the main reason for the decoherence of the system. In early researches, the hyperfine interaction between the electron spin and its neighbor nuclear spins was considered as an effective magnetic field whose magnitude and direction are random and the magnitude follows the Gaussian distribution. In this paper, we simultaneously consider an applied magnetic field and the interaction between the electron spin and its neighbor nuclear spins, and investigate the decoherence of the quantum repeater of two quantum dots. We first solve the time evolution of the system by the numerical method, and the result shows that when the applied magnetic field is increased to a certain value, the four Bell states can be divided into two kinds, each with two Bell states. The system cannot transit from the Bell state in one kind to that in the other kind, but can transit between two Bell states with in the same kind. This effectively improves the fidelity of the initial state and suppresses the decoherence of the system. For a given applied magnetic field with large magnitude, we theoretically study the effect of the fluctuation of nuclear spin on the entangled state, and give an analytical expression for each of the fidelity and the decoherence time of the initial state. We show that the decoherence times of the four Bell states are the same, but the time evolutions of the Bell states belonging to different kinds are different obviously. The fidelity of two Bell states not only decays exponentially but also oscillates rapidly, so such two Bell states are difficult to be manipulated and not suggested in quantum repeater process. The results in this paper are expected to provide theoretical suggestions for selecting the entangled states in quantum repeater.

On the exact analysis of an idealized quantum switch

We study an entanglement distribution switch that serves k users in a star topology. The function of the switch is to facilitate end-to-end bipartite entangled state generation for pairs of users. We study a simple variant of this problem, wherein all links connecting the users to the switch are identical, the effects of state decoherence are negligible, and the switch can store an arbitrary number of qubits. We model the system using a discrete-time Markov chain and obtain the capacity of the switch. When the switch operates at capacity, we also present a numerical method for computing the expected number of qubits stored at the switch, which depends on the number of users k and the probability of successful entanglement generation at the link level p. We then compare the results of our exact analysis to that of a continuous-time Markov chain model of a quantum switch and argue that the latter is a reasonable approximation to the more realistic model presented in this work.

Nonperturbative Effects in the Emission Spectra of Quantum Dots Interacting with Bosonic and Fermionic Reservoirs

The problem of the description of the strong interaction of the InAs/GaAs exciton quantum dot (QD) grown by molecular-beam epitaxy with acoustic phonon and electron reservoirs is studied. A nonperturbative solution to the self-energy function of the exciton quantum dot is found. It is shown that these functions at temperatures higher than 10 K differ significantly from those obtained within in the Born approximation. The accurate calculation of the self-energy function of the exciton QD makes it possible to solve the problem of decoherence and dephasing of quantum states, which opens the way to develop single-photon sources necessary in quantum calculations and quantum communication.

Neutrino evolution and quantum decoherence

Neutrino interactions with an external environment can influence the neutrino oscillation pattern and the oscillations can be damped as a result of the neutrino quantum decoherence. In particular, the quantum decoherence of neutrino states engendered by the neutrino radiative decay accounting for the nonstandard interactions (NSI) leads to the suppression of flavor neutrino oscillations in the solar neutrino fluxes.

On the Stochastic Analysis of a Quantum Entanglement Switch

We study a quantum entanglement switch that serves k users in a star topology. We model variants of the system using continuous-time Markov chains (CTMCs) and obtain expressions for switch capacity and the expected number of qubits stored in memory at the switch. Using CTMCs allows us to obtain a number of analytic results for systems in which the links are homogeneous or heterogeneous and for switches that have infinite or finite buffer sizes. In addition, we can easily model the effects of decoherence of quantum states using this technique. From numerical observations, we discover that decoherence has little effect on capacity and expected number of stored qubits for homogeneous systems. For heterogeneous systems, especially those operating close to stability constraints, buffer size and decoherence can significantly affect performance. We also learn that, in general, increasing the buffer size from one to two qubits per link is advantageous to most systems, while increasing the buffer size further yields diminishing returns.