Distant Quantum Dots Research Articles

Ferromagnetism in Ni quantum dots anchored graphdiyne

Achieving ferromagnetic ordering in two-dimensional carbon semiconductors like graphdiyne remains a challenge in spintronics. We synthesized Ni-doped graphdiyne (Ni-GDY) using an electrochemical method and found that adjusting the Ni atom concentration allows for a transition from paramagnetism to ferromagnetism, with a high Curie temperature of 175 K. Our density functional theory calculations revealed that the magnetic moment in Ni-GDY arises from Ni quantum dots. At low concentrations, the distant quantum dots result in paramagnetism, while at high concentrations, the formation of bound polarons and long-range exchange coupling through carbon p orbitals leads to ferromagnetism. This study clarifies the contradiction in magnetism observed in various transition metal-doped graphdiyne materials and highlights the potential applications of Ni-doped graphdiyne in electronic devices.

Generation of a Single-Cycle Acoustic Pulse: A Scalable Solution for Transport in Single-Electron Circuits

The synthesis of single-cycle, compressed optical and microwave pulses sparked novel areas of fundamental research. In the field of acoustics, however, such a generation has not been introduced yet. For numerous applications, the large spatial extent of surface acoustic waves (SAW) causes unwanted perturbations and limits the accuracy of physical manipulations. Particularly, this restriction applies to SAW-driven quantum experiments with single flying electrons, where extra modulation renders the exact position of the transported electron ambiguous and leads to undesired spin mixing. Here, we address this challenge by demonstrating single-shot chirp synthesis of a strongly compressed acoustic pulse. Employing this solitary SAW pulse to transport a single electron between distant quantum dots with an efficiency exceeding 99%, we show that chirp synthesis is competitive with regular transduction approaches. Performing a time-resolved investigation of the SAW-driven sending process, we outline the potential of the chirped SAW pulse to synchronize single-electron transport from many quantum-dot sources. By superimposing multiple pulses, we further point out the capability of chirp synthesis to generate arbitrary acoustic waveforms tailorable to a variety of (opto)nanomechanical applications. Our results shift the paradigm of compressed pulses to the field of acoustic phonons and pave the way for a SAW-driven platform of single-electron transport that is precise, synchronized, and scalable.

Collective cloaking of a cluster of electrostatically defined core–shell quantum dots in graphene

We study cloaking of a cluster of electrostatically defined core–shell quantum dots in graphene. Guided by the generalized multiparticle Mie theory, the Dirac electron scattering from a cluster of quantum dots is addressed. Indeed distant quantum dots may experience a sort of individual cloaking. But despite the multiple scattering of an incident electron from a set of adjacent quantum dots, collective cloaking may happen. Via a proper choice of the radii and bias voltages of shells, two most important scattering coefficients and hence the scattering efficiency of the cluster dramatically decrease. Energy-selective electron cloaks are realizable. More importantly, clusters simultaneously transparent to electrons of different energies, are achievable. Being quite sensitive to applied bias voltages, clusters of core–shell quantum dots may be used to develop switches with high on-off ratios.

Surpassing the classical limit in magic square game with distant quantum dots coupled to optical cavities

The emergence of quantum technologies is heating up the debate on quantum supremacy, usually focusing on the feasibility of looking good on paper algorithms in realistic settings, due to the vulnerability of quantum systems to myriad sources of noise. In this vein, an interesting example of quantum pseudo-telepathy games that quantum mechanical resources can theoretically outperform classical resources is the Magic Square game (MSG), in which two players play against a referee. Due to noise, however, the unit winning probability of the players can drop well below the classical limit. Here, we propose a timely and unprecedented experimental setup for quantum computation with quantum dots inside optical cavities, along with ancillary photons for realizing interactions between distant dots to implement the MSG. Considering various physical imperfections of our setup, we first show that the MSG can be implemented with the current technology, outperforming the classical resources under realistic conditions. Next, we show that our work gives rise to a new version of the game. That is, if the referee has information on the physical realization and strategy of the players, he can bias the game through filtered randomness, and increase his winning probability. We believe our work contributes to not only quantum game theory, but also quantum computing with quantum dots.

Design-flexible entanglement of two distant quantum dots bridged by dark whispering gallery modes.

We present a flexible design to realize the entanglement between two distant semiconductor quantum dots (QDs) embedded in separated photonic crystal nanobeam cavities. When bridged by a largely detuned microring cavity, photonic supermodes between two distant nanobeam cavities are formed via whispering gallery modes (WGMs). Due to the large detuning, WGMs in the microring exhibit almost no photonic excitation, showing the "dark WGMs." With the dyadic Green's functions of the nano-structure and the resolvent operators of the Hamiltonian, we numerically investigate the entanglement dynamics of two distant QDs. Furthermore, we prove that the entanglement can be tuned by adjusting the distances between the cavities. Such a scheme paves an efficient way for realizing a scalable quantum network in a solid-state system.

Excitation transfer between distant quantum dots in photonic crystal waveguide

Excitation transfer between distant quantum dots in photonic crystal waveguide

Sound-driven single-electron transfer in a circuit of coupled quantum rails

Surface acoustic waves (SAWs) strongly modulate the shallow electric potential in piezoelectric materials. In semiconductor heterostructures such as GaAs/AlGaAs, SAWs can thus be employed to transfer individual electrons between distant quantum dots. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. Here we present two essential building blocks of such a SAW-driven quantum circuit. First, we implement a directional coupler allowing to partition a flying electron arbitrarily into two paths of transportation. Second, we demonstrate a triggered single-electron source enabling synchronisation of the SAW-driven sending process. Exceeding a single-shot transfer efficiency of 99%, we show that a SAW-driven integrated circuit is feasible with single electrons on a large scale. Our results pave the way to perform quantum logic operations with flying electron qubits.

Entanglement of two distant quantum dots with the flip-flop interaction coupled to plasmonic waveguide

The entanglement of two distant quantum dots coupled to metallic waveguide has been investigated theoretically in the presence of the flip-flop interaction with the analytic solutions of eigenvalue equations of the system. High entanglement of two quantum dots could be achieved by adjusting the direct-coupling strength of two quantum dots, the coupling strength of quantum dots with surface plasmon along metallic waveguide, the group velocity of surface plasmon and detuning. The discussed system with the flip-flop interaction provides us with a rich way to realize the quantum device for quantum information processing, such as quantum communication and quantum computation.

Plasmon mediated entanglement dynamics of distant quantum dots**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274132 and 11550110180).

We investigate the time evolution of entanglement between two quantum dots in an engineered vacuum environment such that a metallic nanoring having a surface plasmon is placed near the quantum dots. Such engineering in environment results in oscillations in entanglement dynamics of the quantum dots systems. With proper adjustment of the separation between the quantum dots, entanglement decay can be stabilized and preserved for longer time than its decay without the surface plasmons interactions.

Non-classical correlations in two quantum dots coupled in a coherent resonator field under decoherence

An analytical description is obtained for two excitons; each exciton is in one of two distant quantum dots embedded in a nano-mechanical resonator which is initially prepared in a superposition of coherent states with intrinsic decoherence. We use a particular method based on the dressed states of the model Hamiltonian. The robustness of the generated non-classical correlations is investigated via the measurement-induced nonlocality and geometric quantum discord, compared with the log-negativity. The three measures present generated different correlations that depend on the initial coherence states and their intensities, and intrinsic decoherence. It is witnessed that the phenomena of sudden appearance and disappearance of entanglement are occurring and repeated at chosen intervals of time; they can disappear due to the intrinsic decoherence. The correlations were observed to attain highest robustness under initial coherent state, with decoherence parameter. Quantum correlation functions survive in the stationary states, and the amount of stationary correlations could be controlled by adjusting the values of the intrinsic decoherence, the initial state of the nanomechanical resonator and its coherence intensity.

Cavity-Mediated Coherent Coupling between Distant Quantum Dots.

Scalable architectures for quantum information technologies require one to selectively couple long-distance qubits while suppressing environmental noise and cross talk. In semiconductor materials, the coherent coupling of a single spin on a quantum dot to a cavity hosting fermionic modes offers a new solution to this technological challenge. Here, we demonstrate coherent coupling between two spatially separated quantum dots using an electronic cavity design that takes advantage of whispering-gallery modes in a two-dimensional electron gas. The cavity-mediated, long-distance coupling effectively minimizes undesirable direct cross talk between the dots and defines a scalable architecture for all-electronic semiconductor-based quantum information processing.

A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band

The implementation of fiber-based long-range quantum communication requires tunable sources of single photons at the telecom C-band. Stable and easy-to-implement wavelength-tunability of individual sources is crucial to (i) bring remote sources into resonance, (ii) define a wavelength standard, and (iii) ensure scalability to operate a quantum repeater. So far, the most promising sources for true, telecom single photons are semiconductor quantum dots, due to their ability to deterministically and reliably emit single and entangled photons. However, the required wavelength-tunability is hard to attain. Here, we show a stable wavelength-tunable quantum light source by integrating strain-released InAs quantum dots on piezoelectric substrates. We present triggered single-photon emission at 1.55 μm with a multi-photon emission probability as low as 0.097, as well as photon pair emission from the radiative biexciton–exciton cascade. We achieve a tuning range of 0.25 nm which will allow us to spectrally overlap remote quantum dots or tuning distant quantum dots into resonance with quantum memories. This opens up realistic avenues for the implementation of photonic quantum information processing applications at telecom wavelengths.

Charge reconfiguration in arrays of quantum dots

Semiconductor quantum dots are potential building blocks for scalable qubit architectures. Efficient control over the exchange interaction and the possibility of coherently manipulating electron states are essential ingredients towards this goal. We studied experimentally the shuttling of electrons trapped in serial quantum dot arrays isolated from the reservoirs. The isolation hereby enables a high degree of control over the tunnel couplings between the quantum dots, while electrons can be transferred through the array by gate voltage variations. Model calculations are compared with our experimental results for double, triple, and quadruple quantum dot arrays. We are able to identify all transitions observed in our experiments, including cotunneling transitions between distant quantum dots. The shuttling of individual electrons between quantum dots along chosen paths is demonstrated.

Phase-Tuned Entangled State Generation between Distant Spin Qubits.

Quantum entanglement between distant qubits is an important feature of quantum networks. Distribution of entanglement over long distances can be enabled through coherently interfacing qubit pairs via photonic channels. Here, we report the realization of optically generated quantum entanglement between electron spin qubits confined in two distant semiconductor quantum dots. The protocol relies on spin-photon entanglement in the trionic Λ system and quantum erasure of the Raman-photon path information. The measurement of a single Raman photon is used to project the spin qubits into a joint quantum state with an interferometrically stabilized and tunable relative phase. We report an average Bell-state fidelity for |ψ^{(+)}⟩ and |ψ^{(-)}⟩ states of 61.6±2.3% and a record-high entanglement generation rate of 7.3kHz between distant qubits.

Classical information transfer between distant quantum dots using individual electrons in fast moving quantum dots

AbstractOver the past two decades, lateral quantum dots have permitted a tremendous advancement in the manipulation of individual electrons. In order to have a complete toolbox for electronics at the single electron level, local manipulation in a quantum dot needs to be associated with the controlled transport of individual electrons. Here, we review results on the transfer of individual electrons and their spin degree of freedom between distant lateral quantum dots. The electron is transported in a surface acoustic wave‐generated moving quantum dot, with an efficiency of 92%. Furthermore, we will review recent results showing that classical spin information/magnetization can be partially transferred using this method. The fidelity was proven to be limited by the current sample design and implementation, and no fundamental limitation was met. This transfer capability opens new avenues in spin‐based quantum information processing and in the implementation of quantum optics experiments with flying electrons.

Strong coupling of a single electron in silicon to a microwave photon.

Silicon is vital to the computing industry because of the high quality of its native oxide and well-established doping technologies. Isotopic purification has enabled quantum coherence times on the order of seconds, thereby placing silicon at the forefront of efforts to create a solid-state quantum processor. We demonstrate strong coupling of a single electron in a silicon double quantum dot to the photonic field of a microwave cavity, as shown by the observation of vacuum Rabi splitting. Strong coupling of a quantum dot electron to a cavity photon would allow for long-range qubit coupling and the long-range entanglement of electrons in semiconductor quantum dots.

Steady-state entanglement between distant quantum dots in photonic crystal dimers

We show that two spatially separated semiconductor quantum dots under resonant and continuous-wave excitation can be strongly entangled in the steady-state, thanks to their radiative coupling by mutual interaction through the normal modes of a photonic crystal dimer. We employ a quantum master equation formalism to quantify the steady-state entanglement by calculating the system {\it negativity}. Calculations are specified to consider realistic semiconductor nanostructure parameters for the photonic crystal dimer-quantum dots coupled system, determined by a guided mode expansion solution of Maxwell equations. Negativity values of the order of 0.1 ($20\%$ of the maximum value) are shown for interdot distances that are larger than the resonant wavelength of the system. It is shown that the amount of entanglement is almost independent of the interdot distance, as long as the normal mode splitting of the photonic dimer is larger than their linewidths, which becomes the only requirement to achieve a local and individual qubit addressing. Considering inhomogeneously broadened quantum dots, we find that the steady-state entanglement is preserved as long as the detuning between the two quantum dot resonances is small when compared to their decay rates. The steady-state entanglement is shown to be robust against the effects of pure dephasing of the quantum dot transitions. We finally study the entanglement dynamics for a configuration in which one of the two quantum dots is initially excited and find that the transient negativity can be enhanced by more than a factor of two with respect to the steady-state value. These results are promising for practical applications of entangled states at short time scales.

Fast spin information transfer between distant quantum dots using individual electrons.

Transporting ensembles of electrons over long distances without losing their spin polarization is an important benchmark for spintronic devices. It usually requires injecting and probing spin-polarized electrons in conduction channels using ferromagnetic contacts or optical excitation. In parallel with this development, important efforts have been dedicated to achieving control of nanocircuits at the single-electron level. The detection and coherent manipulation of the spin of a single electron trapped in a quantum dot are now well established. Combined with the recently demonstrated control of the displacement of individual electrons between two distant quantum dots, these achievements allow the possibility of realizing spintronic protocols at the single-electron level. Here, we demonstrate that spin information carried by one or two electrons can be transferred between two quantum dots separated by a distance of 4 μm with a classical fidelity of 65%. We show that at present it is limited by spin flips occurring during the transfer procedure before and after electron displacement. Being able to encode and control information in the spin degree of freedom of a single electron while it is being transferred over distances of a few micrometres on nanosecond timescales will pave the way towards 'quantum spintronics' devices, which could be used to implement large-scale spin-based quantum information processing.

Effect of disorder on the radiative coupling between distant quantum dots embedded in a photonic crystal dimer

Quantifying the radiative coupling between distant quantum emitters can be relevant for applications in quantum information processing on chip. Here we focus on semiconductor quantum dots embedded in photonic crystal dimers formed by two nominally identical L3 cavities, and we show that the effective radiative coupling between them is relatively robust against non-perfect quantum dot positioning and also, to a smaller extent, to structural disorder in the photonic crystal. We show that the coupling between the quantum dots is enhanced at resonance with the dimer photonic modes and is proportional to the quality factors of these modes as long as the two cavities are strongly coupled. We find that, for the configuration where the L3 defects are aligned along the horizontal axis, the radiative coupling is almost two times larger than the values obtained in previous work.

Coupling Two Distant Double Quantum Dots with a Microwave Resonator.

We fabricated a hybrid device with two distant graphene double quantum dots (DQDs) and a microwave resonator. A nonlinear response is observed in the resonator reflection amplitude when the two DQDs are jointly tuned to the vicinity of the degeneracy points. This observation can be well fitted by the Tavis-Cummings (T-C) model which describes two two-level systems coupling with one photonic field. Furthermore, the correlation between the DC currents in the two DQDs is studied. A nonzero cross-current correlation is observed which has been theoretically predicted to be an important sign of nonlocal coupling between two distant systems. Our results explore T-C physics in electronic transport and also contribute to the study of nonlocal transport and future implementations of remote electronic entanglement.