Triple quantum dots (TQDs) are promising semiconductor spin qubits because of their all-electrical control via fast, tunable exchange interactions and immunity to global magnetic fluctuations. These qubits can experience strong transverse interaction with photons in the resonant exchange (RX) regime, when exchange is simultaneously active on both qubit axes. However, most theoretical work has been based on phenomenological Fermi-Hubbard models, which may not fully capture the complexity of the qubit spin-charge states in this regime. Here we investigate exchange in Si/SiGe and GaAs TQDs using full configuration interaction (FCI) calculations which better describe practical device operation. We show that high exchange operation in general, and the RX regime in particular, can differ significantly from simple models, presenting new challenges and opportunities for spin-photon coupling. We highlight the impact of device electrostatics and effective mass on exchange and identify a new operating point (XRX) where strong spin-photon coupling is most likely to occur in Si/SiGe TQDs. Based on our numerical results, we analyze the feasibility of a remote entanglement cavity iSWAP protocol and discuss design pathways for improving fidelity. Our analysis provides insight into the requirements for TQD spin-photon transduction and demonstrates more generally the necessity of accurate modeling of exchange in spin qubits.
We experimentally demonstrate that the Mollow triplet sidebands of a quantum dot strongly coupled to a cavity exhibit anomalous power induced broadening and enhanced emission when one sideband is tuned over the cavity frequency. We observe a nonlinear increase of the sideband linewidth with excitation power when the Rabi frequency exceeds the detuning between the quantum dot and the cavity, consistent with a recent theoretical model that accounts for acoustic phonon-induced processes between the exciton and the cavity. In addition, the sideband tuned to the cavity shows strong resonant emissio…
We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with sup…
Applying high power continuous-wave resonant s-shell excitation to a single self-assembled In- GaAs quantum dot, we demonstrate the generation of post-selected single-indistinguishable photons from the Mollow triplet sidebands. A sophisticated spatial filtering technique based on a double Michelson interferometer enabled us to separate the spectrally close lying individual Mollow components and perform almost background free two-photon interference measurements. Showing high consistency with the results of an independent determination of the emission coherence of 250 +/- 30 ps, our analysis re…
From our theoretical studies of resonant Raman transitions in two-electron quantum dots (artificial helium atoms) we show that in this system, the singlet-triplet Raman transitions are allowed (in polarized configuration) only in the presence of spin-orbit interactions. With an increase of the applied magnetic field this transition dominates over the singlet-singlet and triplet-triplet transitions. This intriguing effect can therefore be utilized to tune Raman transitions as well as the spin-orbit coupling in few-electron quantum dots.
Pauli spin blockade in quantum dot systems occurs when the charge transport is allowed only for some spin states, and it has been an efficient tool in spin-based qubit devices in semiconductors. We theoretically investigate a Pauli spin blockade in a triple quantum dot molecule consisting of three identical quantum dots in a semiconductor in the presence of an external magnetic field through the molecule. When the three-electron state is on resonance with two- or four-electron states, the Aharonov-Bohm oscillation and the Zeeman splitting lead to a periodic spin blockade effect. We focus on th…
The effect of an in-plane magnetic field on resonant tunneling in a symmetric GaAs/AlYGa1-YAs triple-barrier resonant tunneling TBRT structure is presented. The splitting of the resonant lines (and hence the coupling energy vs. magnetic induction) for the ground and the first excited resonant doublet shifts up in energy by increasing the magnetic induction. Each of branches in the resonant dispersion relations shows a parabolic behavior with the location of the cyclotron orbit center. It is expected that, at very high fields, the interaction with confined phonons be enhanced, due to phonon cou…
The ability to manipulate entanglement between multiple spatially-separated qubits is essential for quantum information processing. Although nitrogen-vacancy (NV) centers in diamond provide a promising qubit platform, developing scalable two-qubit gates remains a well-known challenge. To this end, magnon-mediated entanglement proposals have attracted attention due to their long-range spin-coherent propagation. Optimal device geometries and gate protocols of such schemes, however, have yet to be determined. Here we predict strong long-distance ($>μ$m) NV-NV coupling via magnon modes with cooper…
We report implementation of a resonantly driven singlet-triplet spin qubit in silicon. The qubit is defined by the two-electron anti-parallel spin states and universal quantum control is provided through a resonant drive of the exchange interaction at the qubit frequency. The qubit exhibits long $T_2^*$ exceeding 1 $μ$s that is limited by dephasing due to the $^{29}$Si nuclei rather than charge noise thanks to the symmetric operation and a large micro-magnet Zeeman field gradient. The randomized benchmarking shows 99.6 % single gate fidelity which is the highest reported for singlet-triplet qu…
The successes of superconducting quantum circuits at local manipulation of quantum information and photonics technology at long-distance transmission of the same have spurred interest in the development of quantum transducers for efficient, low-noise, and bidirectional frequency conversion of photons between the microwave and optical domains. We propose to realize such functionality through the coupling of electrical, piezoelectric, and optomechanical resonators. The coupling of the mechanical subsystems enables formation of a resonant mechanical supermode that provides a mechanically-mediated…
The resonant state expansion, a rigorous perturbation theory, recently developed in electrodynamics, is applied to non-relativistic quantum mechanical systems in one dimension. The method is used here for finding the resonant states in various potentials approximated by combinations of Dirac delta functions. The resonant state expansion is first verified for a triple quantum well system, showing convergence to the available analytic solution as the number of resonant states in the basis increases. The method is then applied to multiple quantum well and barrier structures, including finite peri…
Although individual spins in quantum dots have been studied extensively as qubits, their investigation under strong resonant driving in the scope of accessing Mollow physics is still an open question. Here, we have grown high quality positively charged quantum dots embedded in a planar microcavity that enable enhanced light-matter interactions. Under a strong magnetic field in the Voigt configuration, individual positively charged quantum dots provide a double-lambda level structure. Using a combination of above-band and resonant excitation, we observe the formation of Mollow triplets on all o…
We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe$_2$. Raman intensities are computed entirely from first principles by calculating finite differences of the dielectric susceptibility. In our analysis, we investigate the role of quantum interference effects and the electron-phonon coupling. With this method, we explain the experimentally observed intensity inversion of the $A^\prime_1$ vibrational modes in triple-layer MoTe2 with increasing laser photon energy. Finally, we show that a quantitative comparison with experiment…
We study the effect of electron-phonon interaction on current and zero-frequency shot noise in resonant tunneling through a series triple-quantum-dot coupling to a local phonon mode by means of a nonperturbative mapping technique along with the Green function formulation. By fixing the energy difference between the first two quantum dots to be equal to phonon frequency and sweeping the level of the third quantum dot, we find a largely enhanced current spectrum due to phonon effect, and in particular we predict current peaks corresponding to phonon-absorption and -emission assisted resonant tun…
Reminiscent of the bound character of a qubit's dynamics confined on the Bloch sphere, the observation of a Mollow triplet in the resonantly driven qubit fluorescence spectrum represents one of the founding signatures of Quantum Electrodynamics. Here we report on its observation in a hybrid spin-nanomechanical system, where a Nitro-gen Vacancy spin qubit is magnetically coupled to the vibrations of a Silicon Carbide nanowire. A resonant microwave field turns the originally parametric hybrid interac-tion into a resonant process, where acoustic phonons are now able to induce transitions between …
We have found experimentally that the shot noise in InAlAs-InGaAs-InAlAs Triple-Barrier Resonant-Tunneling Diodes (TBRTD) is reduced over the 2eI Poissonian value whenever their differential conductance is positive, and is enhanced over 2eI when the differential conductance is negative. This behavior, although qualitatively similar to that found in double-barrier diodes, differs from it in important details. In TBRTDs the noise reduction is considerably larger than predicted by a semi-classical model, and the enhancement does not correlate with the strength of the negative differential conduct…
A triple-quantum-dot system can be operated as either an exchange-only qubit or a resonant-exchange qubit. While it is generally believed that the decisive advantage of the resonant-exchange qubit is the suppression of charge noise because it is operated at a sweet spot, we show that the leakage is also an important factor. Through molecular-orbital-theoretic calculations, we show that when the system is operated in the exchange-only scheme, the leakage to states with double electron occupancy in quantum dots is severe when rotations around the axis 120$^\circ$ from $\hat{z}$ is performed. Whi…
We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubit…
Detailed properties of resonance fluorescence from a single quantum dot in a micropillar cavity are investigated, with particular focus on emission coherence in dependence on optical driving field power and detuning. Power-dependent series over a wide range could trace characteristic Mollow triplet spectra with large Rabi splittings of $|Ω| \leq 15$ GHz. In particular, the effect of dephasing in terms of systematic spectral broadening $\propto Ω^2$ of the Mollow sidebands is observed as a strong fingerprint of excitation-induced dephasing. Our results are in excellent agreement with prediction…
The electron transport through the triple-barrier resonant tunnelling diode (TBRTD) have been studied by the self-consistent numerical method for the Wigner-Poisson problem. The electron flow through the TBRTD can be controlled by the gate voltage applied to one of the potential well regions. For different gate voltage values we have determined the current-voltage characteristics, potential energy profiles, and electron density distribution. We have found the enhancement of the peak-to-valley ratio (up to $\sim$10), the appearance of the linear current versus bias voltage behaviour within the …
An enhancement in Brillouin light scattering of optical photons with magnons is demonstrated in magneto-optical whispering gallery mode resonators tuned to a triple resonance point. This occurs when both the input and output optical modes are resonant with those of the whispering gallery resonator, with a separation given by the ferromagnetic resonance (FMR) frequency. The identification and excitation of specific optical modes allows us to gain a clear understanding of the mode-matching conditions. A selection rule due to wavevector matching leads to an intrinsic single-sideband excitation. S…
Resonant excitation of solid state quantum emitters has the potential to deterministically excite a localized exciton while ensuring a maximally coherent emission. In this work, we demonstrate the coherent coupling of an exciton localized in a lithographically positioned, site-controlled semiconductor quantum dot to an external resonant laser field. For strong continuous-wave driving we observe the characteristic Mollow triplet and analyze the Rabi splitting and sideband widths as a function of driving strength and temperature. The sideband widths increase linearly with temperature and the squ…
Featuring spin-valley degree of freedom by a magnetic exchange field-induction to gain transport of charge carriers through a junction based on superconducting subgap tunneling can provide a new scenario for future electronics. Transmission of low-energy Dirac-like electron (hole) quasiparticles through a ferromagnet/superconductor (F/S) interface can be of noticeable importance due to strong spin-orbit coupling in the valence band of monolayer $MoS_2$ (ML-MDS). The magnetic exchange field (MEF) of a ferromagnetic section on top of ML-MDS may affect the electron (hole) excitations for spin-up …
We investigate a method for entangling two singlet-triplet qubits in adjacent double quantum dots via capacitive interactions. In contrast to prior work, here we focus on a regime with strong interactions between the qubits. The interplay of the interaction energy and simultaneous large detunings for both double dots gives rise to the double charge resonant regime, in which the unpolarized (1111) and fully polarized (0202) four-electron states in the absence of interqubit tunneling are near degeneracy, while being energetically well-separated from the partially polarized (0211 and 1102) states…
In this paper we report on a tuneable few electron lateral triple quantum dot design. The quantum dot potentials are arranged in series. The device is aimed at studies of triple quantum dot properties where knowing the exact number of electrons is important as well as quantum information applications involving electron spin qubits. We demonstrate tuning strategies for achieving required resonant conditions such as quadruple points where all three quantum dots are on resonance. We find that in such a device resonant conditions at specific configurations are accompanied by novel charge transfer …
We investigate transmission properties of triple barrier parabolic double quantum well structure using the non-equilibrium Green's function method. In particular, we examine the effect of system parameters on transmission coefficient. The properties of the electronic state are also studied as a function of the system parameters (such as the well and barrier widths) and electric field bias. We also tested energy electronic state with calculating the density of states. It has been found that the first resonant peaks shift towards the lower energy region as the middle barrier width increases. New…
A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values extracted from the wireless measurements agree with those determined from capacitance-voltage measurements, providing strong evidence that the sen…
Emission from a resonantly excited quantum emitter is a fascinating research topic within quantum optics and a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line below saturation of a quantum emitter which develops into a triplet at powers above saturation of the emitter. The spectral properties of the triplet strongly depends on pump power and detuning of the excitation laser. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures. We present a detailed photon-stati…
A complete numerical description of the charge and spin dynamics of a two-electron system confined in narrow nanowire quantum dots under oscillating electric field is presented in the context of recent electric dipole spin resonance experiments. We find that the spin-orbit coupling results in lifting the spin blockade by phonon mediated relaxation provided that the initially occupied state is close in energy to the ground state. This leads to suppression of the blockade from the triplet state with spins polarized parallel to the external magnetic field B. At higher B, after singlet-triplet gro…
We study the crucial role played by the solid-state environment in determining the photon emission characteristics of a driven quantum dot. For resonant driving, we predict a phonon-enhancement of the coherently emitted radiation field with increasing driving strength, in stark contrast to the conventional expectation of a rapidly decreasing fraction of coherent emission with stronger driving. This surprising behaviour results from thermalisation of the dot with respect to the phonon bath, and leads to a nonstandard regime of resonance fluorescence in which significant coherent scattering and …
We report on resonant optical spectroscopy of self-assembled InGaAs quantum dots in which the number of electrons can accurately be tuned to N=0,1,2 by an external gate voltage. Polarization, wave vector and magnetic field dependent measurements enable us to clearly distinguish between resonant Raman and resonant photoluminescence processes. The Raman spectra for N=1 and 2 electrons considerably differ from each other. In particular, for N=2, the quantum-dot He, the spectra exhibit both singlet and triplet transitions reflecting the elementary many-particle interaction. Also the resonant photo…
We operate a resonant exchange qubit in a highly symmetric triple-dot configuration using IQ-modulated RF pulses. At the resulting three-dimensional sweet spot the qubit splitting is an order of magnitude less sensitive to all relevant control voltages, compared to the conventional operating point, but we observe no significant improvement in the quality of Rabi oscillations. For weak driving this is consistent with Overhauser field fluctuations modulating the qubit splitting. For strong driving we infer that effective voltage noise modulates the coupling strength between RF drive and the qubi…
We propose a simple setup of three coupled quantum dots in the Coulomb blockade regime as a source for spatially separated currents of spin-entangled electrons. The entanglement originates from the singlet ground state of a quantum dot with an even number of electrons. To preserve the entanglement of the electron pair during its extraction to the drain leads, the electrons are transported through secondary dots. This prevents one-electron transport by energy mismatch, while joint transport is resonantly enhanced by conservation of the total two-electron energy.
We study electron transport through a triple quantum dot in ring configuration at finite bias. In particular, we analyze the influence of a gate voltage that detunes one of the dots, such that one branch of the interferometer becomes off-resonant. It turns out that then interference effects fade away, i.e., the current becomes independent of a penetrating flux and the detuning. The noise properties which are characterized by the full-counting statistics, however, are governed by bunching effects that may be tuned by the gate voltage. Analytical results for limiting cases support this picture. …
Creating a train of single photons and monitoring its propagation and interaction is challenging in most physical systems, as photons generally interact very weakly with other systems. However, when confining microwave frequency photons in a transmission line resonator, effective photon-photon interactions can be mediated by qubits embedded in the resonator. Here, we observe the phenomenon of photon blockade through second-order correlation function measurements. The experiments clearly demonstrate antibunching in a continuously pumped source of single microwave photons measured using microwav…
In this contribution, we investigated the spin coherence of high-mobility dense two-dimensional electron gases confined in multilayer systems. The dynamics of optically-induced spin polarization was experimentally studied employing the time-resolved Kerr rotation and resonant spin amplification techniques. For both the double and triple quantum wells doped beyond the metal-insulator transition, where the spin coherence is greatly suppressed, we found remarkably long spin lifetimes limited by the Dyakonov-Perel mechanism and spin hopping process between the donor sites as well as the spread of …
We study the two-time correlation function and the resonance fluorescence spectrum of a semiconductor quantum dot excited by a strong off-resonant laser pulse. The obtained analytical expressions exhibit a specific detuning-dependent damping of Rabi oscillations of the dressed quantum dot as well as a detuning-dependent width of Mollow-triplet lines. In the absence of pure dephasing, the central peak of the triplet is broadened, upon increasing detuning, but the blue and red side peaks are narrowed. We demonstrate that pure dephasing processes can invert these dependences. A crossover between …
We investigated the spin coherence of high-mobility two-dimensional electron gases confined in multilayer GaAs quantum wells. The dynamics of the spin polarization was optically studied using pump-probe techniques: time-resolved Kerr rotation and resonant spin amplification. For double and triple quantum wells doped beyond the metal-to-insulator transition, the spin-orbit interaction was tailored by the sample parameters of structural symmetry (Rashba constant), width and electron density (Dresselhaus linear and cubic constants) which allows us to attain long dephasing times in the nanoseconds…
Two-qubit quantum gates play an essential role in quantum computing, whose operation critically depends on the entanglement between two qubits. Resonantly driven controlled-NOT (CNOT) gates based on silicon double quantum dots (DQDs) are studied theoretically. The physical mechanisms for effective gate modulation of the exchange coupling between two qubits are elucidated. Scaling behaviors of the singlet-triplet energy split, gate-switching speed, and gate fidelity are investigated as a function of the quantum dot spacing and modulation gate voltage. It is shown that the entanglement strength …
A holy grail of photonics research is the realization of a laser that uses a single quantum emitter as the gain medium. Such a device would exhibit a plethora of new features, including lasing without a well-defined threshold and output intensity fluctuations that remain below the shot-noise limit. While single-atom lasers have been demonstrated, compact devices capable of continuous-wave operation require monolithic structures involving a solid-state quantum emitter. Here, we report the observation of steady-state laser amplification in Raman transitions between the lowest-energy entangled sp…
