Identical Qubits Research Articles

Comparing coherent and incoherent models for quantum homogenization

Here we investigate the role of quantum interference in the quantum homogenizer, whose convergence properties model a thermalization process. In the original quantum homogenizer protocol, a system qubit converges to the state of identical reservoir qubits through partial-swap interactions, that allow interference between reservoir qubits. We design an alternative, incoherent quantum homogenizer, where each system-reservoir interaction is moderated by a control qubit using a controlled-swap interaction. We show that our incoherent homogenizer satisfies the essential conditions for homogenization, being able to transform a qubit from any state to any other state to arbitrary accuracy, with negligible impact on the reservoir qubits' states. Our results show that the convergence properties of homogenization machines that are important for modeling thermalization are not dependent on coherence between qubits in the homogenization protocol. We then derive bounds on the resources required to re-use the homogenizers for performing state transformations. This demonstrates that both homogenizers are universal for any number of homogenizations, for an increased resource cost. Published by the American Physical Society 2024

Directional emission and photon bunching from a qubit pair in waveguide

Waveguide quantum electrodynamics represents a powerful platform to generate entanglement and tailor photonic states. We consider a pair of identical qubits coupled to a parity invariant waveguide in the microwave domain. By working in the one- and two-excitation sectors, we provide a unified view of decay processes and show the common origin of directional single-photon emission and two-photon directional bunching. Unveiling the quantum trajectories, we demonstrate that both phenomena are rooted in the selective coupling of orthogonal Bell states of the qubits with photons propagating in opposite directions. We comment on how to use this mechanism to implement optimized post-selection of Bell states, heralded by the detection of a photon on one side of the system. Published by the American Physical Society 2024

Dynamic Cooling on Contemporary Quantum Computers

We study the problem of dynamic cooling whereby a target qubit is cooled at the expense of heating up N−1 further identical qubits by means of a global unitary operation. A standard back-of-the-envelope high-temperature estimate establishes that the target qubit temperature can be dynamically cooled by at most a factor of 1/N. Here we provide the exact expression for the minimum temperature to which the target qubit can be cooled and reveal that there is a crossover from the high initial temperature regime, where the scaling is 1/N, to a low initial temperature regime, where a much faster scaling of 1/N occurs. This slow, 1/N scaling, which was relevant for early high-temperature NMR quantum computers, is the reason dynamic cooling was dismissed as ineffectual around 20 years ago; the fact that current low-temperature quantum computers fall in the fast, 1/N scaling regime, reinstates the appeal of dynamic cooling today. We further show that the associated work cost of cooling is exponentially more advantageous in the low-temperature regime. We discuss the implementation of dynamic cooling in terms of quantum circuits and examine the effects of hardware noise. We successfully demonstrate dynamic cooling in a three-qubit system on a real quantum processor. Since the circuit size grows quickly with N, scaling dynamic cooling to larger systems on noisy devices poses a challenge. We therefore propose a suboptimal cooling algorithm, whereby relinquishing a small amount of cooling capability results in a drastically reduced circuit complexity, greatly facilitating the implementation of dynamic cooling on near-future quantum computers. Published by the American Physical Society 2024

Experimental quantum state compression from two identical qubits to a qutrit

Experimental quantum state compression from two identical qubits to a qutrit

Dynamics of entangled Greenberger — Horne — Zeilinger states in three qubits thermal Tavis — Cummings model

In this paper, we investigated the dynamics of systems of two and three identical qubits interacting resonantly with a selected mode of a thermal field of a lossless resonator. We found solutions of the quantum time-dependent Liouville equation for various three- and two-qubit entangled states of qubits. Based on these solutions, we calculated the criterion of the qubit entanglement — fidelity. The results of numerical calculations of the fidelity showed that increasing the average number of photons in a mode leads to a decrease in the maximum degree of entanglement. It is shown that the two-qubit entangled state is more stable with respect to external noise than the three-qubit entangled Greenberger — Horne — Zeilinger states (GHZ). Moreover, a genuine entangled GHZ-state is more stable to noise than a GHZ-like entangled state.

Manipulation and enhancement of the performance of Otto cycle in the presence of nonthermal reservoirs

In this work, we investigate the impact of energetic coherence in nonthermal reservoirs on the performance of the Otto cycle. We first focus on the situation where the working substance is a qubit. Due to the existence of coherence of nonthermal reservoir, various anomalous operating regimes such as the engine and refrigerator with efficiencies exceeding Carnot limits, as well as the hybrid refrigerator that can simultaneously achieve cooling and supplying work to an external agent, can occur. We demonstrate that the energetic coherence of the system’s steady state plays a significant role in determining the cycle’s functions by adding an additional stroke implementing dephasing and phase modulation operations in the cycle. The energetic coherence of the system is necessary to trigger the reservoir’s coherence to exert influences on the cycle. We decompose the thermodynamic quantities to the components arising from the populations and coherence of the system, and find that the reservoir’s coherence impacts the cycle from two aspects: one is the modification of the system’s steady-state populations or temperatures, and the other is the direct contributions to the heat in the interaction between the system and reservoirs. We then explore the scenario where the working substance is two identical qubits, and the reservoirs are common to them. We show that the degenerate coherence of the system in the steady state can enhance the performances of the cycle as different machines. Additionally, the energetic coherence of the reservoir modifies the functions of the cycle still through the energetic coherence of the system rather than their degenerate coherence.

Entanglemnt in nonlinear three-qubits Jaynes — Cummings

In this paper, we investigated the dynamics of entanglement of pairs of qubits in a system of three identical qubits that interact non-resonantly with the selected mode of a microwave resonator without loss with the Kerr medium by means of single-photon transitions. We have found solutions to the quantum time Schrodinger equation for the total wave function of the system for the initial separable, biseparable and true entangled states of qubits and the Fock initial state of the resonator field. Based on these solutions, the criterion of entanglement of qubit pairs — negativity is calculated. The results of numerical simulation of the negativity of qubit pairs have shown that the presence of disorder and Kerr nonlinearity in the case of an initial non-entangled state of a pair of qubits can lead to a significant increase in the degree of their entanglement. In the case of an initial entangled state of a pair of qubits, the disorder and the Kerr medium can lead to a significant stabilization of the initial entanglement.

Indistinguishability-assisted two-qubit entanglement distillation

Production of quantum states exhibiting a high degree of entanglement out of noisy conditions is one of the main goals of quantum information science. Here, we provide a conditional yet efficient entanglement distillation method which functions within the framework of spatially localized operations and classical communication. This method exploits indistinguishability effects due to the spatial overlap between two identical qubits in distinct sites and encompasses particle statistics imprint. We derive the general conditions for the maximum entanglement distillation out of mixed states. As applications, we give a thorough description of distilled entanglement and associated success probability starting from typical noisy states, such as thermal Gibbs states and Werner states. The influence of local temperatures and of a noise parameter is discussed, respectively, in these two cases. The proposed scheme paves the way towards quantum repeaters in composite networks made of controllable identical quantum particles.

ENTANGLEMENT BETWEEN TWO SUPERCONDUCTING CHARGE
 QUBITS

In this paper, we investigated the dynamics of entanglement of two identical charge qubits with Josephson junctions in the case when one of the qubits is exposed to a microwave field in a coherent or thermal state. We have found the exact solution of the quantum time equation of evolution of the system under consideration for the statistical operator in the case of initial separable and entangled states of qubits. The exact solution for the complete statistical operator is used to calculate the qubit entanglement criterion - concurrence. The results of numerical simulation of the time dependence of the concurrence in the case of the coherent field showed that, with a certain choice of model parameters, the system can realize long-lived entangled states. It is also shown that for the thermal state of the field and the entangled initial state of qubits, the qubits retain a certain degree of entanglement during evolution even in the case of very intensive fields. In this case, for any intensities of thermal noise, there is no effect of the sudden death of entanglement.

Robust Engineering of Maximally Entangled States by Identical Particle Interferometry

AbstractThis study proposes a procedure for the robust preparation of maximally entangled states of identical fermionic qubits, studying the role played by particle statistics in the process. The protocol exploits externally activated noisy channels to reset the system to a known state. The subsequent interference effects generated at a beam splitter result in a mixture of maximally entangled Bell states and NOON states. It also discusses how every maximally entangled state of two fermionic qubits distributed over two spatial modes can be obtained from one another by fermionic passive optical transformations. Using a pseudospin‐insensitive, non‐absorbing, parity check detector, the proposed technique is thus shown to deterministically prepare any arbitrary maximally entangled state of two identical fermions. These results extend recent findings related to bosonic qubits. Finally, it analyzes the performance of the protocol for both bosons and fermions when the externally activated noisy channels are not used and the two qubits undergo standard types of noise. The results supply further insights toward viable strategies for noise‐protected entanglement exploitable in quantum‐enhanced technologies.

Charging by Quantum Measurement

We propose a quantum charging scheme fueled by measurements on ancillary qubits serving as disposable chargers. A stream of identical qubits are sequentially coupled to a quantum battery of $N+1$ levels and measured by projective operations after joint unitary evolutions of optimized intervals. If charger qubits are prepared in excited state and measured on ground state, then their excitations (energy) can be near perfectly transferred to battery by iteratively updating the optimized measurement intervals. Starting from its ground state, the battery could be constantly charged to an even higher energy level. Starting from a thermal state, the battery could also achieve a near-unit ratio of ergotropy and energy through less than $N$ measurements, when a population inversion is realized by measurements. If charger qubits are prepared in ground state and measured on excited state, useful work extracted by measurements alone could transform the battery from a thermal state to a high-ergotropy state before the success probability vanishes. Our operations in charging are more efficient than those without measurements and do not invoke the initial coherence in both battery and chargers. Particularly, our finding features quantum measurement in shaping nonequilibrium systems.

Quantum thermal diode dominated by pure classical correlation via three triangular-coupled qubits.

A quantum thermal diode is designed based on three pairwise coupled qubits, two connected to a common reservoir and the other to an independent reservoir. It is found that the internal couplings between qubits can enhance heat currents. If the two identical qubits uniformly couple with the common reservoir, the crossing dissipation will occur, leading to the initial-state-dependent steady state, which can be decomposed into the mixture of two particular steady states: the heat-conducting state generating maximum heat current and the heat-resisting state not transporting heat. However, the rectification factor doesn't depend on the initial state. In particular, we find that neither quantum entanglement nor quantum discord is present in the steady state, but the pure classical correlation shows a remarkably consistent behavior as the heat rectification factor, which reveals the vital role of classical correlation in the system.

Mitigation of Quasiparticle Loss in Superconducting Qubits by Phonon Scattering

Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently uncorrelated in space and time. In superconducting qubits, this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high-energy phonons in the substrate. These phonons can break Cooper pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential mitigation technique is to place large volumes of normal or superconducting metal on the device, capable of reducing the phonon energy to below the superconducting gap of the qubits. To investigate the effectiveness of this method, we fabricate a quantum device with four nominally identical nanowire-based transmon qubits. On the device, half of the niobium-titanium-nitride ground plane is replaced with aluminum ($\mathrm{Al}$), which has a significantly lower superconducting gap. We deterministically inject high-energy phonons into the substrate by voltage biasing a galvanically isolated Josephson junction. In the presence of the small-gap material, we find a factor of 2--5 less degradation in the injection-dependent qubit lifetimes and observe that the undesired excited qubit state population is mitigated by a similar factor. We furthermore turn the $\mathrm{Al}$ normal with a magnetic field, finding no change in the phonon protection. This suggests that the efficacy of the protection in our device is not limited by the size of the superconducting gap in the $\mathrm{Al}$ ground plane. Our results provide a promising foundation for protecting superconducting-qubit processors against correlated errors from ionizing radiation.

DYNAMICS OF THE THREE-QUBITS TAVIS — CUMMINGS MODEL

In this article, we have studied the entanglement dynamics of three identical qubits (natural or artificial two-level atoms) resonantly interacting with the one mode of the thermal field of a microwave lossless resonator via one-photon transitions. An exact solution of the quantum time Schrodinger equation is found for the total wave function of the system for the initial separable and entangled states of qubits and the Fock initial state of the resonator. On the basis of this solution, an exact solution of the quantum Liouville equation for the total time-dependent density matrix of the system in the case of a thermal field of the resonator is constructed. The exact solution for the full density matrix is used to calculate the criterion of entanglement of pairs of qubits negativity. The resultsof numerical simulation of the time dependence of the negativity of pairs of qubits showed that with an increase in the intensity of the thermal resonator field, the degree of entanglement of pairs of qubits decreases. It is also shown that In the model under consideration, for any initial states of qubits and intensities of the thermal field of the resonator, the effect of sudden death of entanglement takes place. This behavior of the entanglement parameter in the model under consideration differs from that in the two-qubit model. For two-qubit model, the effect of the sudden death of entanglement takes place only for the initial entangled states of qubits and intense thermal fields of the resonator.

Inhibition of double excitation and strong quantum entanglement via engineered cavity interactions

We propose a method for realizing the inhibition of simultaneous excitation of two identical qubits in a single-mode cavity QED system, where two qubits are driven by a coherent field and coupled to a cavity with engineered qubit-cavity interactions. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we close many unwanted excitation pathways. This leads to an inhibition of the doubly excited state. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of nonexcitation of the doubly excited state is the bunching property of the cavity photons, which thus provides a possible measurement of this nonexcitation. We also present dynamical features of the inhibition of simultaneous excitation of two qubits. The proposal presented in this paper can be realized not only in traditional cavity QED, but also in noncavity topological photonics involving edge modes.

Quantum teleportation and thermal entanglement of two-scattering qubit state in graphene lattices

This paper presents an analytical investigation of the teleportation and the thermal entanglement of two identical qubits in the ground state of graphene lattices. The elastic interaction between the two qubits is considered as a scattering process. The modified Hamiltonian and density matrix of the system were presented. The teleportation and the entanglement of two-qubit system are analyzed through the study of the average fidelity of teleportation [Formula: see text], the concurrence [Formula: see text] and the entanglement of formation [Formula: see text]. The results depend on the temperature [Formula: see text] and also on the parameters of the system as the [Formula: see text] parameter of the interaction potential and [Formula: see text] band parameter. The study shows that the teleportation and the entanglement decrease when the temperature [Formula: see text] increases but they become all the more important as the parameters [Formula: see text] and [Formula: see text] increase.

Quantum thermodynamic methods to purify a qubit on a quantum processing unit

We report on a quantum thermodynamic method to purify a qubit on a quantum processing unit (QPU) equipped with (nearly) identical qubits. Our starting point is a three qubit design that emulates the well-known two qubit swap engine. Similar to standard fridges, the method would allow us to cool down a qubit at the expense of heating two other qubits. A minimal modification thereof leads to a more practical three qubit design that allows for enhanced refrigeration tasks, such as increasing the purity of one qubit at the expense of decreasing the purity of the other two. The method is based on the application of properly designed quantum circuits and can therefore be run on any gate model quantum computer. We implement it on a publicly available superconducting qubit based QPU and observe a purification capability down to 200 mK. We identify gate noise as the main obstacle toward practical application for quantum computing.

Quantum Reservoir Parameter Estimation via Fisher Information

In this study, we show that as a result of weak interaction of different information environments structured with a single probe qubit, these environments can perform binary classification of the information they contain. In this way, we refer to these environments as quantum information baths because they consist of sequences of identical qubits in certain pure quantum states. A micro-maser like master equation has been developed to clearly describe the system dynamics analytically and the quantum states of different information reservoirs. The model can also be treated as a quantum neuron, due to the single-qubit probe that makes a binary decision depending on the reservoir parameters in its steady state. The numerical results of the repeated interaction process based on the divisibility and additivity of the quantum dynamic maps are compared with the analytical results. Besides being a single quantum classifier, the model we present can also serve as a basic unit of a quantum neural network within the framework of the dissipative model of quantum computing.

Quantum Speed-Up in Collisional Battery Charging.

We present a collision model for the charging of a quantum battery by identical nonequilibrium qubit units. When the units are prepared in a mixture of energy eigenstates, the energy gain in the battery can be described by a classical random walk, where both average energy and variance grow linearly with time. Conversely, when the qubits contain quantum coherence, interference effects buildup in the battery and lead to a faster spreading of the energy distribution, reminiscent of a quantum random walk. This can be exploited for faster and more efficient charging of a battery initialized in the ground state. Specifically, we show that coherent protocols can yield higher charging power than any possible incoherent strategy, demonstrating a quantum speed-up at the level of a single battery. Finally, we characterize the amount of extractable work from the battery through the notion of ergotropy.

Entanglement Robustness via Spatial Deformation of Identical Particle Wave Functions.

We address the problem of entanglement protection against surrounding noise by a procedure suitably exploiting spatial indistinguishability of identical subsystems. To this purpose, we take two initially separated and entangled identical qubits interacting with two independent noisy environments. Three typical models of environments are considered: amplitude damping channel, phase damping channel and depolarizing channel. After the interaction, we deform the wave functions of the two qubits to make them spatially overlap before performing spatially localized operations and classical communication (sLOCC) and eventually computing the entanglement of the resulting state. This way, we show that spatial indistinguishability of identical qubits can be utilized within the sLOCC operational framework to partially recover the quantum correlations spoiled by the environment. A general behavior emerges: the higher the spatial indistinguishability achieved via deformation, the larger the amount of recovered entanglement.