Distant Spins Research Articles

Critical metrology of minimally accessible anisotropic spin chains

We address quantum metrology in critical spin chains with anisotropy and Dzyaloshinskii–Moriya (DM) interaction, and show how local and quasi-local measurements may be exploited to characterize global properties of the systems. In particular, we evaluate the classical (magnetization) and quantum Fisher information of the relevant parameters for the density matrix of a single spin and that of a pair of spins ranging from nearest to sixth-nearest neighbors, to the limiting case of very distant spins. Our results allow us to elucidate the role of the different parameters and to individuate the optimal working regimes for the precise characterization of the system, also clarifying the effects of correlations on the estimation precision.

Entanglement generation between distant spins via quasilocal reservoir engineering

Entanglement generation between distant spins via quasilocal reservoir engineering

Spin–photon interface for a solid-state spin system via single trapped ion

An efficient spin–photon interface plays a significant role in building quantum network and distributed quantum computing with solid-state spin system, e.g., nitrogen-vacancy (NV) center in diamond. Solid-state spin system has the advantages of excellent portability and long coherence time, however, it suffers from a critical drawback of low efficiency of spontaneously emitted photons in the zero phonon line. This poses a severe challenge for the generation of an efficient interface between solid-state spin and photon. Here, we propose a method to establish a coherent interface between photon and solid-state spin system which is coupled to a trapped ion via a quantum transducer. Our method provides a new route to establish an efficient spin–photon interface allowing the generation of entanglement between distant spins.

Macroscopic sample shape effect on pulse electron double resonance (PELDOR) signal

Pulse electron double resonance (PELDOR), also called double electron–electron resonance (DEER), is a technique capable of measuring the strength of electron spin dipolar interactions, revealing spin–spin distance distributions in ordered and disordered solid materials. Previous work has shown that PELDOR signals acquire an out-of-phase component under conditions of high electron spin polarization, such as at low temperatures and high fields. In this paper, we show theoretically and experimentally that the size and sign of this effect depends on the macroscopic shape of the sample and its orientation in the external magnetic field. This effect is caused by dipolar interactions between distant spins and provides new insights into the fundamental physics of PELDOR.

Long-distance coupling of spin qubits via topological magnons

We consider two distant spin qubits in quantum dots, both coupled to a two-dimensional topological ferromagnet hosting chiral magnon edge states at the boundary. The chiral magnon is used to mediate entanglement between the spin qubits, realizing a fundamental building block of scalable quantum computing architectures: a long-distance two-qubit gate. Previous proposals for long-distance coupling with magnons involved off-resonant coupling, where the detuning of the spin-qubit frequency from the magnonic band edge provides protection against spontaneous relaxation. The topological magnon mode, on the other hand, lies in between two magnonic bands far away from any bulk magnon resonances, facilitating strong and highly tuneable coupling between the two spin qubits. Even though the coupling between the qubit and the chiral magnon is resonant for a wide range of qubit splittings, we find that the magnon-induced qubit relaxation is vastly suppressed if the coupling between the qubit and the ferromagnet is antiferromagnetic. A fast and high-fidelity long-distance coupling protocol is presented capable of achieving spin-qubit entanglement over micrometer distances with $1\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ gate speed and up to $99.9%$ fidelities. The resulting spin-qubit entanglement may be used as a probe for the long-sought detection of topological edge magnons.

Influence of errors on the transport of quantum information through distant quantum dot spin qubits

The ability to connect distant qubits plays a role in quantum computing and systems candidates for quantum computation must be able to interact their constituent qubits. We model the quantum dot spin qubits by a spin chain with nearest-neighbors interaction. Within this model, we interact distant qubits by consecutive SWAP gates. The SWAP gate exchanges the information of two different qubits and it is obtained by a time-dependent pulse. Also, we implement the CNOT gate, which is fundamental to universal quantum computation. These gates are applied in a system free from decoherence, which provides a fidelity with high accuracy. Furthermore, we analyze the situation where dephasing, amplitude-damping, and depolarizing types of errors occur in each site of the chain. We found that the order of the SWAP and CNOT gates is important and it can lead to a relevant difference in fidelity when the number of qubits is large.

Steady-state tunable entanglement thermal machine using quantum dots

We propose a solid state thermal machine based on quantum dots to generate steady-state entanglement between distant spins. Unlike previous approaches our system can be controlled by experimentally feasible steady state currents manipulated by dc voltages. By analyzing the Liouvillian eigenspectrum as a function of the control parameters, we show that our device operates over a large voltage region. As an extension, the proposed device also works as an entanglement thermal machine under a temperature gradient that can even give rise to entanglement at zero voltage bias. Finally, we highlight a post-selection scheme based on currently feasible non-demolition measurement techniques that can generate perfect Bell-pairs from the steady state output of our thermal machine.

Sign switching of superexchange mediated by a few electrons in a nonuniform magnetic field

Long-range interaction between distant spins is an important building block for the realization of a large quantum-dot network in which couplings between pairs of spins can be selectively addressed. Recent experiments on the coherent oscillation of logical states between remote spins facilitated by intermediate electron states were the first step for large-scale quantum information processing. Reaching this ultimate goal requires extensive studies on the superexchange interaction in different quantum-dot spatial arrangements and electron configurations. Here, we consider a linear triple quantum dot with two antiparallel spins in the outer dots forming the logical states while varying numbers of electrons in the middle dot form a mediator, which facilitates the superexchange interaction. We show that the superexchange is enhanced when the number of mediating electrons increases. In addition, we show that forming a four-electron triplet in the mediator dot further enhances the superexchange strength. Our work can be a guide to scale up the quantum-dot array with controllable and dense connectivity.

Atomic-Scale Quantum Sensing of Ensembles of Guest Molecules in a Metal-Organic Framework with Intrinsic Electron Spin Centers.

One of the exciting applications of electron-spin-based quantum sensing is the detection of distant nuclear spins of external molecular species. Here, we explore the application of a metal-organic framework (MOF) material as a host matrix for sensing spin centers. As a sensor, we employ inherent Cu2+ ions in the structure of a Zn-doped HKUST-1 framework. As a target molecular species, we use butane gas that exhibits no specific chemical reactivity toward the inner surface of HKUST-1 and is thus randomly distributed inside the MOF pore network. By employing a conventional double-resonance pulse sequence, we can effectively detect the coupling of the distant 1H nuclear spins of butane to the electron spin of the sensor and gain atomic-scale insight into their spatial distribution. Thus, our proof-of-the-concept experiment demonstrates that MOFs, the materials featuring extremely large surface area and great tunability, are perfectly suited as a key element for emerging magnetic quantum sensing solutions.

Strong long-range spin-spin coupling via a Kerr magnon interface

Strong long-range coupling between distant spins is crucial for spin-based quantum information processing. However, achieving such a strong spin-spin coupling remains challenging. Here we propose to realize a strong coupling between two distant spins via the Kerr effect of magnons in a yttrium-iron-garnet nanosphere. By applying a microwave field on this nanosphere, the Kerr effect of magnons can induce the magnon squeezing, so that the coupling between the spin and the squeezed magnons can be exponentially enhanced. This in turn allows the spin-magnon distance increased from nano- to micrometer scale. By considering the virtual excitation of the squeezd magnons in the dispersive regime, strong spin-spin coupling mediated by the squeezed magnons can be achieved, and a remote quantum-state transfer, as well as the nonlocal two-qubit iSWAP gate with high fidelity becomes implementable. Our approach offers a feasible scheme to perform quantum information processing among distant spins.

Programmable interactions and emergent geometry in an arrayof atom clouds.

Interactions govern the flow of information and the formation of correlations between constituents of many-body quantum systems, dictating phases of matter found in nature and forms of entanglement generated in the laboratory. Typical interactions decay with distance and thus produce a network of connectivity governed by geometry-such as the crystalline structure of a material or the trapping sites of atoms in a quantum simulator1,2. However, many envisioned applications in quantum simulation and computation require more complex coupling graphs including non-local interactions, which feature in models of information scrambling in black holes3-6 and mappings of hard optimization problems onto frustrated classical magnets7-11. Here we describe the realization of programmable non-local interactions in an array of atomic ensembles within an optical cavity, in which photons carry information between atomic spins12-19. By programming the distance dependence of the interactions, we access effective geometries for which the dimensionality, topology and metric are entirely distinct from the physical geometry of the array. As examples, we engineer an antiferromagnetic triangular ladder, a Möbius strip with sign-changing interactions and a treelike geometry inspired by concepts of quantum gravity5,20-22. The tree graph constitutes a toy model of holographic duality21,22, in which the quantum system lies on the boundary of a higher-dimensional geometry that emerges from measured correlations23. Our work provides broader prospects for simulating frustrated magnets and topological phases24, investigating quantum optimization paradigms10,11,25,26 and engineering entangled resource states for sensing and computation27,28.

Voltage-controlled Hubbard spin transistor

Transistors are key elements for enabling computational hardware in both classical and quantum domains. Here, we propose a voltage-gated spin transistor using itinerant electrons in the Hubbard model which acts at the level of single electron spins. Going beyond classical spintronics, it enables the controlling of the flow of quantum information between distant spin qubits. The transistor has two modes of operation, open and closed, which are realized by two different charge configurations in the gate of the transistor. In the closed mode, the spin information between source and drain is blocked while in the open mode we have free spin information exchange. The switching between the modes takes place within a fraction of the operation time which allows for several subsequent operations within the coherence time of the transistor. The system shows good resilience against several imperfections and opens up a practical application for quantum dot arrays.

Proposed rapid detection of nuclear spins with entanglement-enhanced sensors

Recently, there have been significant developments to detect nuclear spins with nitrogen vacancy (NV) centers in diamond. However, due to the nature of the short range dipole–dipole interaction, it takes a long time to detect distant nuclear spins with the NV centers. Here, we propose a rapid detection of nuclear spins with an entanglement between the NV centers. We show that the necessary time to detect the nuclear spins with the entanglement is several orders of magnitude shorter than that with separable NV centers. Our results pave the way for new applications in nanoscale nuclear magnetic resonance spectroscopy.

Efficient Entanglement of Spin Qubits Mediated by a Hot Mechanical Oscillator.

Localized electronic and nuclear spin qubits in the solid state constitute a promising platform for storage and manipulation of quantum information, even at room temperature. However, the development of scalable systems requires the ability to entangle distant spins, which remains a challenge today. We propose and analyze an efficient, heralded scheme that employs a parity measurement in a decoherence free subspace to enable fast and robust entanglement generation between distant spin qubits mediated by a hot mechanical oscillator. We find that high-fidelity entanglement at cryogenic and even ambient temperatures is feasible with realistic parameters and show that the entangled pair can be subsequently leveraged for deterministic controlled-NOT operations between nuclear spins. Our results open the door for novel quantum processing architectures for a wide variety of solid-state spin qubits.

Long-Distance Superexchange between Semiconductor Quantum-Dot Electron Spins.

Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin state of the chain, we observe oscillations between the distant end spins. We resolve the dynamics of both the end spins and the chain itself, and our measurements agree with simulations. Superexchange is a promising technique to create long-distance coupling between quantum-dot spin qubits.

Electron cascade for distant spin readout

The spin of a single electron in a semiconductor quantum dot provides a well-controlled and long-lived qubit implementation. The electron charge in turn allows control of the position of individual electrons in a quantum dot array, and enables charge sensors to probe the charge configuration. Here we show that the Coulomb repulsion allows an initial charge transition to induce subsequent charge transitions, inducing a cascade of electron hops, like toppling dominoes. A cascade can transmit information along a quantum dot array over a distance that extends by far the effect of the direct Coulomb repulsion. We demonstrate that a cascade of electrons can be combined with Pauli spin blockade to read out distant spins and show results with potential for high fidelity using a remote charge sensor in a quadruple quantum dot device. We implement and analyse several operating modes for cascades and analyse their scaling behaviour. We also discuss the application of cascade-based spin readout to densely-packed two-dimensional quantum dot arrays with charge sensors placed at the periphery. The high connectivity of such arrays greatly improves the capabilities of quantum dot systems for quantum computation and simulation.

Enhancing Spin-Phonon and Spin-Spin Interactions Using Linear Resources in a Hybrid Quantum System.

Hybrid spin-mechanical setups offer a versatile platform for quantum science and technology, but improving the spin-phonon as well as the spin-spin couplings of such systems remains a crucial challenge. Here, we propose and analyze an experimentally feasible and simple method for exponentially enhancing the spin-phonon and the phonon-mediated spin-spin interactions in a hybrid spin-mechanical setup, using only linear resources. Through modulating the spring constant of the mechanical cantilever with a time-dependent pump, we can acquire a tunable and nonlinear (two-phonon) drive to the mechanical mode, thus amplifying the mechanical zero-point fluctuations and directly enhancing the spin-phonon coupling. This method allows the spin-mechanical system to be driven from the weak-coupling regime to the strong-coupling regime, and even the ultrastrong coupling regime. In the dispersive regime, this method gives rise to a large enhancement of the phonon-mediated spin-spin interactions between distant solid-state spins, typically two orders of magnitude larger than that without modulation. As an example, we show that the proposed scheme can apply to generating entangled states of multiple spins with high fidelities even in the presence of large dissipations.

Entanglement between distant macroscopic mechanical and spin systems

Entanglement is an essential property of multipartite quantum systems, characterized by the inseparability of quantum states of objects regardless of their spatial separation. Generation of entanglement between increasingly macroscopic and disparate systems is an ongoing effort in quantum science, as it enables hybrid quantum networks, quantum-enhanced sensing and probing of the fundamental limits of quantum theory. The disparity of hybrid systems and the vulnerability of quantum correlations have thus far hampered the generation of macroscopic hybrid entanglement. Here, we generate an entangled state between the motion of a macroscopic mechanical oscillator and a collective atomic spin oscillator, as witnessed by an Einstein–Podolsky–Rosen variance below the separability limit, 0.83 ± 0.02 < 1. The mechanical oscillator is a millimetre-size dielectric membrane and the spin oscillator is an ensemble of 109 atoms in a magnetic field. Light propagating through the two spatially separated systems generates entanglement because the collective spin plays the role of an effective negative-mass reference frame and provides—under ideal circumstances—a back-action-free subspace; in the experiment, quantum back-action is suppressed by 4.6 dB. Einstein–Podolsky–Rosen entanglement between a millimetre-size mechanical membrane oscillator and a collective atomic spin oscillator formed by an ensemble of caesium atoms is achieved, although the two systems are spatially separated by one metre.

Electrical control of coherent spin rotation of a single-spin qubit

Nitrogen vacancy (NV) centers, optically active atomic defects in diamond, have attracted tremendous interest for quantum sensing, network, and computing applications due to their excellent quantum coherence and remarkable versatility in a real, ambient environment. One of the critical challenges to develop NV-based quantum operation platforms results from the difficulty in locally addressing the quantum spin states of individual NV spins in a scalable, energy-efficient manner. Here, we report electrical control of the coherent spin rotation rate of a single-spin qubit in NV-magnet based hybrid quantum systems. By utilizing electrically generated spin currents, we are able to achieve efficient tuning of magnetic damping and the amplitude of the dipole fields generated by a micrometer-sized resonant magnet, enabling electrical control of the Rabi oscillation frequency of NV spins. Our results highlight the potential of NV centers in designing functional hybrid solid-state systems for next-generation quantum-information technologies. The demonstrated coupling between the NV centers and the propagating spin waves harbored by a magnetic insulator further points to the possibility to establish macroscale entanglement between distant spin qubits.

Long-range interaction in an open boundary-driven Heisenberg spin lattice: A far-from-equilibrium transition to ballistic transport

We study an open Heisenberg XXZ spin chain with long-range Ising-type interaction, which is incoherently driven at its boundaries and therefore a far-from-equilibrium steady state current is induced. Quantum Monte Carlo techniques make a regime accessible, where the system size is large enough to examine the transition from a diffusive to a ballistic transport regime. We find that the chain lengths for this transition are increasing with decreasing range of the Ising-type interactions between distant spins, while the strength of the incoherent driving does not have a relevant effect on the transport transition. The transition can be explained by the suppression of ferromagnetic domains at the edges of the chain.