We investigated quantum thermal machines powered by sequential nonselective generalized measurements, taking a double quantum dot with coherent interdot tunneling as a working substance. In this platform, the competition between detuning and tunneling hybridizes the localized states and modifies the energetic response of the cycle, allowing us to analyze measurement-driven thermodynamics beyond simple diagonal qubit models. We formulate a three-stroke cycle composed of thermalization with a single reservoir and two generalized measurement channels, and derive the corresponding internal-energy and entropy variations in order to identify the operational regimes of the device. Depending on the measurement parameters, the system can operate as a heat engine, accelerator, heater, or refrigerator. We show that the introduction of tunneling not only reshapes the boundaries between these modes, but also generates refrigeration configurations that are absent in the purely detuned model. In addition, the performance maps reveal that temperature, detuning, and tunneling amplitude jointly control the most favorable regions for work extraction and cooling. Our results demonstrate that coherent interdot coupling acts as an important resource for optimizing measurement-powered quantum thermal machines and highlight double quantum dots as a promising setting for experimentally relevant implementations of measurement-assisted thermodynamic devices.
For the solid state double-dot interferometer, the phase shifted interference pattern induced by the interplay of inter-dot Coulomb correlation and multiple reflections is analyzed by harmonic decomposition. Unexpected result is uncovered, and is discussed in connection with the which-path detection and electron loss.
Here we report the fabrication and quantum transport measurements of gates controlled parallel-coupled double quantum dot on both bilayer and single layer graphene. It is shown that the interdot coupling strength of the parallel double dots can be effectively tuned from weak to strong regime by both the in-plane plunger gates and back gate. All the relevant energy scales and parameters of the graphene parallel-coupled double dot can be extracted from the honeycomb charge stability diagrams revealed through the transport measurements.
Intriguing dichotomies in quantum measurement theory involving the role of the obersever, objective reality, collapse of wavefunctions and actualization of a measurement outcome are cast into a patholigical gedanken experiment involving a single electron in a double quantum dot system coupled via a weak link.
In this Letter, we present a physical scheme for implementing the discrete quantum Fourier transform in a coupled semiconductor double quantum dot system. The main controlled-R gate operation can be decomposed into many simple and feasible unitary transformations. The current scheme would be a useful step towards the realization of complex quantum algorithms in the quantum dot system.
We develop a scalable architecture for quantum computation using controllable electrons of double-dot molecules coupled to a microwave stripline resonator on a chip, which satisfies all Divincenzo criteria. We analyze the performance and stability of all required operations and emphasize that all techniques are feasible with current experimental technologies.
A crucial requirement for quantum information processing is the realization of multiple-qubit quantum gates. Here, we demonstrate an electron spin based all-electrical two-qubit gate consisting of single spin rotations and inter-dot spin exchange in a double quantum dot. A partially entangled output state is obtained by the application of the two-qubit gate to an initial, uncorrelated state. We find that the degree of entanglement is controllable by the exchange operation time. The approach represents a key step towards the realization of universal multiple qubit gates.
We propose a Bell measurement free scheme to implement a quantum repeater in GaAs/AlGa double qunatum dot systems.we prove the four pairs of double quantum dots compose an entanglement unit, given the the initial state is singlet states. Our shceme differs from the famous Duan-Lukin-Cirac-zoller(DLCZ) protocol in that Bell measurements are unneccessary for the entanglement swapping,which provides great advantages and conveniences in experimental implementaion. Our scheme significantly improve the success probability of quantum repeaters based on solid state quantum devices.
We propose a quantum computation architecture of double-dot molecules, where the qubit is encoded in the molecule two-electron spin states. By arranging the two dots inside each molecule perpendicular to the qubit scaling line, the interactions between neighboring qubits are largely simplified and the scaling to multi-qubit system becomes straightforward. As an Ising-model effective interaction can be expediently switched on and off between any two neighboring molecules by adjusting the potential offset between the two dots, universal two-qubit gates can be implemented without requiring time-d…
We propose a resonator-assisted entangling gate for spin qubits with high fidelity. Each spin qubit corresponds to two electrons in a nanowire double quantum dot, with the singlet and one of the triplets as the logical qubit states. The gate is effected by virtual charge dipole transitions. We include noise in our model to show feasibility of the scheme under current experimental conditions.
A single electron spin in a double quantum dot in a magnetic field is considered in terms of a four-level system. By describing the electron motion between the potential minima by spin-conserving tunneling and spin flip caused by a spin-orbit coupling, we inversely engineer faster-than-adiabatic state manipulation operations based on the geometry of four-dimensional (4D) rotations. In particular, we show how to transport a qubit among the quantum dots performing simultaneously a required spin rotation.
One of the key challenges for the implementation of scalable quantum information processing is the design of scalable architectures that support coherent interaction and entanglement generation between distant quantum systems. We propose a nanotube double quantum dot spin transducer that allows to achieve steady-state entanglement between nitrogen-vacancy center spins in diamond with spatial separations over micrometers. The distant spin entanglement further enables us to design a scalable architecture for solid-state quantum information processing based on a hybrid platform consisting of nitr…
We consider a nanomechanical resonator coupled to a double quantum dot. We demonstrate how the finite-frequency current noise spectrum through the double quantum dot can be used to distinguish classical and quantum behaviour in the nearby nano-electromechanical resonator. We also show how the full frequency current noise spectrum gives important information on the combined double quantum dot-resonator energy spectrum. Finally, we point out regimes where the quantum state of the resonator becomes squeezed, and also examine the cross-correlated electron-phonon current noise.
We investigate phonon-induced spin and charge relaxation mediated by spin-orbit and hyperfine interactions for a single electron confined within a double quantum dot. A simple toy model incorporating both direct decay to the ground state of the double dot and indirect decay via an intermediate excited state yields an electron spin relaxation rate that varies non-monotonically with the detuning between the dots. We confirm this model with experiments performed on a GaAs double dot, demonstrating that the relaxation rate exhibits the expected detuning dependence and can be electrically tuned ove…
Quantum gates that temporarily increase singlet-triplet splitting in order to swap electronic spins in coupled quantum dots, lead inevitably to a finite double-occupancy probability for both dots. By solving the time-dependent Schrödinger equation for a coupled dot model, we demonstrate that this does not necessarily lead to quantum computation errors. Instead, the coupled dot ground state evolves quasi-adiabatically for typical system parameters so that the double-occupancy probability at the completion of swapping is negligibly small. We introduce a measure of entanglement which explicitly t…
We propose a scheme to prepare arbitrary superpositions of quantum states in double quantum--dots irradiated by coherent microwave pulses. Solving the equations of motion for the dot density matrix, we find that dephasing rates for such superpositions can be quantitatively infered from additional electron current pulses that appear due to a controllable breakdown of coherent population trapping in the dots.
We study electron-phonon interaction induced decoherence between two-electron singlet and triplet states in a semiconductor double quantum dot using a spin-boson model. We investigate the onset and time evolution of this dephasing, and study its dependence on quantum dot parameters such as dot size and double dot separations, as well as the host materials (GaAs and Si). At the short time limit, electron-phonon interaction only causes an incomplete initial Gaussian decay of the off-diagonal density matrix element in the singlet-triplet Hilbert space. A complete long-time exponential decay due t…
We theoretically investigated the dephasing in an Aharonov-Bohm interferometer containing a lateral double quantum dot induced by coupling with a quantum dot charge sensor. We employed the interpolative 2nd-order perturbation theory to include the charge sensing Coulomb interaction. It is shown that the visibility of the Aharonov-Bohm oscillation of the linear conductance decreases monotonically as the sensing Coulomb interaction increases. In particular, for a weak sensing interaction regime, the visibility decreases parabolically, and it behaves linearly for a strong sensing interaction regi…
In Si quantum dots, valley degree of freedom, in particular the generally small valley splitting and the dot-dependent valley-orbit phase, adds complexities to the low-energy electron dynamics and the associated spin qubit manipulation. Here we propose a four-level model to extract tunnel coupling information for a Si double quantum dot (DQD). This scheme is based on a charge sensing measurement on the ground state as proposed in the widely used protocol for a GaAs double dot [DiCarlo et. al., PRL 92. 226801]. Our theory can help determine both intra- and inter-valley tunnel coupling with high…
We investigate the transport and coherence properties of a double quantum dot coupled to a single damped boson mode. Our numerically results reveal how the properties of the boson distribution can be steered by altering parameters of the electronic system such as the energy difference between the dots. Quadrature amplitude variances and the Wigner function are employed to illustrate how the state of the boson mode can be controlled by a stationary electron current through the dots.
We study the conditions for a double quantum dot system to work as a reliable electron spin entangler, and the efficiency of a beam splitter as a detector for the resulting entangled electron pairs. In particular, we focus on the relative strengths of the tunneling matrix elements, the applied bias and gate voltage, the necessity of time-dependent input/output barriers, and the consequence of considering wavepacket states for the electrons as they leave the double dot to enter the beam splitter. We show that a double quantum dot turnstile is, in principle, an efficient electron spin entangler …
We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield micro-second deph…
We apply the transitionless quantum driving method to control the electron spin of a two-electron double quantum dot with spin-orbit coupling by time-dependent electric fields. The $x$ and $y$ components of applied electric fields in each dot are designed to achieve fast adiabatic-like passage in the nanosecond timescale. To simplify the setup, we can further transform Hamiltonian by z axis to design an alternative speed-up adiabatic passage, without using the applied electric field in $y$ direction.
The three-electron configuration of gate-defined double quantum dots encodes a promising qubit for quantum information processing. I propose a two-qubit entangling gate using a pulse-gated manipulation procedure. The requirements for high-fidelity entangling operations are equivalent to the requirements for the pulse-gated single-qubit manipulations that have been successfully realized for Si QDs. This two-qubit gate completes the universal set of all-pulse-gated operations for the three-electron double-dot qubit and paves the way for a scalable setup to achieve quantum computation.
In the context of a semiconductor based implementation of a quantum computer the idea of a quantum storage bit is presented and a possible implementation using a double quantum dot structure is considered. A measurement scheme using a stimulated Raman adiabatic passage is discussed.
In this letter, we have considered an electron in a double quantum dot system interacting with a detector represented by a point contact. We present a dynamical model for the gradual decoherence of the density matrix due to the interaction with the detector. The interaction of the qubit (quantum system) on the quantum point contact (environment) leads to a discrete set of pointer states of the double quantum dot (apparatus). The necessary time for the qubit decoherence process (measurement time) has been calculated through this model. The existence of a minimum time for the quantum measurement…
Two main mechanisms dictate the tunneling process in a double quantum dot: overlap of excited wave functions, effectively described as a tunneling rate, and phonon-assisted tunneling. In this paper, we study different regimes of tunneling that arise from the competition between these two mechanisms in a double quantum dot molecule immersed in a unimodal optical cavity. We show how such regimes affect the mean number of excitations in each quantum dot and in the cavity, the spectroscopic resolution and emission peaks of the photoluminescence spectrum, and the second-order coherence function whi…
A quantum dot proposal for the implementation of topological quantum computation is presented. The coupling of the electron charge to an external magnetic field via the Aharonov-Bohm effect, combined with the control dynamics of a double dot, results in a two-qubit control phase gate. The physical mechanisms of the system are analysed in detail and the conditions for performing quantum computation resilient to control errors are outlined and found to be realisable with present technology.
We perform photon-assisted-tunneling (PAT) experiments on a GaAs double quantum dot device under high microwave excitation power. Photon-assisted absorption of up to 14 photons is observed, when electron temperature (>1K) are far above the lattice temperature. Signatures of Landau-Zener-Stückelberg (LZS) interference are found even in this non-equilibrium PAT spectrum. In addition, the charge state relaxation time T_1~8ns measured in this out of thermal equilibrium double quantum dot is in agreement with other previous reports.
We demonstrate the development of a double quantum dot with an integrated charge sensor fabricated in undoped GaAs/AlGaAs heterostructures using a double top-gated design. Based on the evaluation of the integrated charge sensor, the double quantum dot can be tuned to a few-electron region. Additionally, the inter-dot coupling of the double quantum dot can be tuned to a large extent according to the voltage on the middle gate. The quantum dot is shown to be tunable from a single dot to a well-isolated double dot. To assess the stability of such design, the potential fluctuation induced by 1/f n…
We propose a scheme for full Bell state measurement of spin qubits in a double quantum dot. Our scheme consists of Pauli spin-blockade measurements and biaxial electron-spin resonance. In order to eliminate the average of the Zeeman fields, the double quantum dot is designed so that the Landé g-factors of first and second dots satisfy g1=g2 with the use of g-factor engineering. Thus, we can swap one of the three spin-triplet states for the spin-singlet state without disturbing the other states. Our study shows that the sequential spin-to-charge conversions enable us to implement the full Bell …
Strong electron and spin correlations in a double-quantum-dot (DQD) can give rise to different quantum states. We observe a continuous transition from a Kondo state exhibiting a single-peak Kondo resonance to another exhibiting a double peak by increasing the inter-dot-coupling (t) in a parallel-coupled DQD. The transition into the double-peak state provides evidence for spin-entanglement between the excess-electron on each dot. Toward the transition, the peak splitting merges and becomes substantially smaller than t because of strong Coulomb effects. Our device tunability bodes well for futur…
We investigate the competition between pair entanglement of two spin qubits in double quantum dots attached to leads with various topologies and the separate entanglement of each spin with nearby electrodes. Universal behavior of entanglement is demonstrated in dependence on the mutual interactions between the spin qubits, the coupling to their environment, temperature and magnetic field. As a consequence of quantum phase transition an abrupt switch between fully entangled and unentangled states takes place when the dots are coupled in parallel.
