Electromagnetically induced squeezing of atomic spin

Taking into account the dephasing process in the realistic atomic ensemble, we theoretically study the generation of atomic spin squeezing via atomic coherence induced by the coupling and probe fields. Using the Heisenberg–Langevin approach, we find that the perfect spin squeezing in the X component can be obtained while the coupling and probe fields produce the maximum coherence between the ground state sublevels 1 and 2. Moreover, the degree of atomic spin squeezing in the X component can be strengthened with the increasing atomic density and/or Rabi frequency of the mixing field. The theoretical results provide a proof-of-principle demonstration of generating the atomic spin squeezing via quantum coherence in the realistic atomic ensemble which may find potential applications in quantum information processing and quantum networks.

We present a convenient and efficient scheme to generate arbitrarily multipartite continuous-variable entanglement via mechanical oscillator displacement induced by two strong input pump fields in the conventional single-cavity optomechanical system. It is shown that multipartite entanglement among the outputs of the two pump fields and any number of relatively weak probe fields can be realized and optimized when the two pump fields with suitable amplitude ratio and relative initial phase are, respectively, tuned to the red and blue mechanical sidebands of a single cavity mode and each probe field is red-detuned by the mechanical frequency with respect to a different neighboring cavity mode. This method can, in principle, be extended to flexibly and conveniently generate arbitrarily multiple nondegenerate bright entangled fields by using only coherent laser fields, and may find promising applications in realistic quantum communication and networks.

Pretty good state transfer on Cayley graphs over dihedral groups

We study relations between bosonic quadrature squeezing and atomic spin squeezing, and find that the latter reduces to the former in the limit of a large number of atoms for even and odd states. We demonstrate this reduction by treating even and odd spin coherent states, for which analytical solutions are readily obtained, and prove that even spin coherent states always exhibit spin squeezing, whereas odd spin coherent states do not, analogous to the squeezing characteristic of even and odd bosonic coherent states. Finally, we examine the squeezing transfer between photons and atoms via the Dicke Hamiltonian, where a perfect transfer of squeezing is demonstrated in the limit of a large number of atoms.

Single‐photon sources are essential for testing fundamental physics and for the development of quantum technologies. In this work, a single‐photon source is investigated, based on a two‐photon Jaynes–Cummings system, where the resonator works as the quantum emitter rather than the two‐level system. This role reversal provides certain advantages, such as robustness against losses from the two‐level system (e.g., dephasing), as it remains in its ground state throughout the entire dynamics. This provides higher efficiency, purity, and indistinguishability compared to sources based on the usual Jaynes–Cummings model under the same parameter conditions in both models. Another advantage of this system is the possibility of direct conversion of a coherent excitation pulse with one photon on average to a single‐photon pulse with efficiency, purity, and indistinguishability above . Since the entire excitation pulse is consumed in the generation of a single photon, the system also minimizes energy waste. The potential for implementing the two‐photon JC model across different platforms expands the possibilities for controlled single‐photon generation in applications in quantum information processing and computation.

Quantum information has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.

Efficient verification of the functioning of quantum devices is a key to the development of quantum technologies, but is a daunting task as the system size increases. Here we propose a simple and general framework for verifying unitary transformations that can be applied to both individual quantum gates and gate sets, including quantum circuits. This framework enables efficient verification of many important unitary transformations, including but not limited to all bipartite unitaries, Clifford unitaries, generalized controlled-$Z$ gates, generalized controlled-NOT gates, the controlled-SWAP gate, and permutation transformations. For all these unitaries, the sample complexity increases at most linearly with the system size and is often independent of the system size. Moreover, little overhead is incurred even if one can only prepare Pauli eigenstates and perform local measurements. Our approach is applicable in many scenarios in which randomized benchmarking (RB) does not apply and is thus instrumental to quantum computation and many other applications in quantum information processing.

Recently, there has been much interest in using Group IV elements from the periodic table to fabricate and study XV color centers in diamond, where X=Si, Ge, Sn, or Pb and V is a carbon vacancy. These Group IV color centers have a number of interesting spin and optical properties, which could potentially make them better candidates than NV− centers for important applications in quantum computing and quantum information processing. Unfortunately, the very same ion implantation process that is required to create these XV color centers in diamond necessarily also produces many carbon vacancies (VC), which can form complexes with these color centers (VC−XV) that can dramatically affect the properties of the isolated XV color centers. The main focus of this work is to use density-functional theory to study the thermodynamics and kinetics of the formation of these VC−XV complexes and to suggest experimental ways to impede this process such as varying the Fermi level of the host diamond material through chemical doping or applying an external electrical bias. We also include a discussion of how the simple presence of many VC can negatively impact the spin coherence times (T2) of Group IV color centers through the presence of acoustic phonons.

Molecular nanomagnets have attracted the attention of the scientific community since they represent model systems to study many quantum phenomena, such as quantum entanglement and decoherence. They are also interesting for their envisaged technological applications, including magnetic refrigeration, high-density information storage and quantum information processing. Driven by these motivations, the present Thesis focuses on the study of magnetic properties and spin dynamics of different classes of molecular nanomagnets, with the purpose to understand their quantum behavior and their potential applications in future technologies. Magnetic frustration is at the origin of many exotic phenomena in matter. Here frustration-induced properties are studied and analyzed for the clusters Ni7, Fe7, Fe6 and Mn6, together with their effects on phonon-induced relaxation dynamics. Indeed, the comprehension of relaxation mechanisms is crucial in order to address the implementation of these systems in the fields of quantum information processing or information storage. Molecular nanomagnets are also promising materials for very-low-temperature magnetic refrigeration due to a potentially enhanced magnetocaloric effect. By explicitly considering Carnot refrigeration cycles, the theoretical recipe to design the best molecules for cryogenic magnetic refrigeration is identified. Another important class of molecular nanomagnets is that of antiferromagnetic rings. Of these, Cr7Ni is of great importance for applications in quantum information processing. The local spin density distribution in Cr7Ni is here obtained from 53Cr-NMR and confirmed by theoretical calculations. The origin of magnetic anisotropy of these rings grafted on surfaces is investigated using XMCD and theoretical calculations. Antiferromagnetic “purple” rings are completely characterized by the comparison of our calculations with inelastic neutron scattering and EPR data. The characterization of purple rings represents a first step in the description of a new family of “purple-green” entangled dimers, which represent model systems to study spin entanglement and its application in the field of quantum computation.

We propose a multifunction phase-shifting manipulator with low noise at a single-photon level,by using a threelevel atomic scheme.This three-level system interacts with a strong pumping field and a weak probe field with a large detuning.Due to this large detuning,two lower states can be coherently prepared prior to the injection of the pump and probe fields.In our configuration,the duration of the pumping field is much longer than that of the probe field. By solving the Heisenberg-Langevin equations of our system under the steady state approximation,we calculate the linear susceptibility of the system and examine the quantum noise properties of the probe field in detail.We show that this scheme,which rests on the process of two-wave mixing with initial atomic coherence,exhibits many interesting properties that neither typical electromagnetically induced transparency (EIT) schemes nor active Raman gain (ARG) schemes possess.Although both EIT-and ARG-based schemes have been widely investigated in atomic medium,the direct generalizations of these schemes to the single/few photon limit prove to be more problematic.The low fidelity due to the significant probe-field attenuation in EIT medium and the large quantum noise due to the amplification of the probe field in an active Raman gain medium are the main obstacles that prohibit a high-fidelity,low-noise phase shifter from being realized in the single/few photon limit.Physically,this scheme can be viewed as a hybrid scheme in which two processes of different physical principles are allowed to interfere with each other to achieve many desired functionalities. For instance,it can be used as a lossless two-photon-broadband phase-shifter with suitable system parameters.It can also be used as an attenuator/amplifier and a total transparency with a zero phase shift.In particular,we show that by locking the pump field intensity and the two-photon detuning simultaneously a flat constant π-phase shift can be realized with unit probe fidelity in a broad probe field frequency range.Applying the quantum regression theorem,we calculate the noise spectrum of the outgoing probe field as a large phase shift is achieved,and show that this two-photon-insensitive π-phase shift may significantly reduce the quantum noise fluctuations associated with a Raman gain process,and have a lot of potential applications for quantum information processing and optical telecommunication.The realization of this broadband π-phase-shift with significantly reduced quantum noise fluctuations makes this scheme attractive for the realization of low-noise phase-gate/polarization-gate at single-photon level.

In this dissertation, we present a detailed theoretical analysis of the spin dynam- ics in small antiferromagnetic systems in view of macroscopic quantum phenom- ena, possible applications in quantum information processing, and transport of the magnetization. Ferromagnetic and antiferromagnetic systems with a size on the nanometer scale such as nanoparticles or magnetic molecular clusters, show intriguing quantum e®ects which are in stark contrast to the behavior of a macroscopic magnetic moment. In recent years, the interest in small magnetic systems has been renewed due to the discovery of possible future technological applications such as data storage or quantum information processing. Fer- romagnetic molecular magnetic systems with a large net spin show incoherent tunneling of the magnetization on a long timescale. This quantum phenomenon is, by now, well established both experimentally and theoretically. Small an- tiferromagnetic systems have so far attracted less attention although quantum e®ects are even more pronounced than in their ferromagnetic counterparts. The main reason for this is that the predicted quantum phenomena such as coherent quantum tunneling of the N¶eel vector are not easily accessible in experiments. On a theoretical level, the description of an antiferromagnetic system is chal- lenging because of the pronounced quantum °uctuations of the spins. Several antiferromagnetic molecular ring molecules have been synthesized to date. The ferric wheels are the most prominent examples. These systems are promising candidates for macroscopic quantum coherence in the form of coherent N¶eel vector tunneling. Although the tunneling rate can be determined from the measurement of thermodynamic properties, a thorough understanding requires theoretical analysis and experimental observation of the spin dynamics. We calculate spin correlation functions using spin coherent state path integrals and ¯nd analytical expressions for the correlation functions of both the N¶eel vector and the total spin. Our results are in good agreement with numerical exact diagonalization for the small systems that are accessible numerically. From the correlation functions, we deduce that the observation of N¶eel vec- tor tunneling requires an experimental probe that couples to a single spin of the antiferromagnetic system only. Both nuclear magnetic resonance and elec- tron spin resonance on doped rings meet this criterion. Nuclear spins coupled only to single electron spins are ideal candidates for local probes because the nuclear spin susceptibiliy exhibits signatures of the coherent electron spin dy- namics. Alternatively, by doping of the ring molecule, an antiferromagnetic system emerges that has uncompensated sublattice spins. The resulting tracer spin would allow one to detect N¶eel vector tunneling with electron spin reso- nance or magnetic susceptibility measurements. Small antiferromagnetic systems with a ¯nite net spin are interesting in view of quantum information processing. Single electron spins are among the most promising candidates for qubits in a solid state system. However, quantum computing is also possible with a wide range of antiferromagnetic clusters which form an e®ective two-state system in the low energy sector. The main advantage of a qubit formed by a spin cluster is that initialization, quantum gate operation, error correction, and readout are possible with techniques applicable to single spins, while the requirements on local control are relaxed. Spin cluster qubits are very insensitive to the details of intracluster exchange interactions and spin placement. Quantum computing is only one of the exciting perspectives in the emerging ¯eld of spintronics in which the spin and charge degrees of freedom of an elec- tron are treated on an equal footing. We analyze transport of magnetization in insulating systems described by a spin Hamiltonian in which the magnetization current is not accompanied by a charge current. The magnetization current through a quasi one-dimensional magnetic wire of ¯nite length suspended be- tween two bulk magnets is determined by the spin conductance which remains ¯nite in the ballistic limit due to contact resistance. Magnetization currents produce an electric ¯eld and hence can be measured directly. For magnetiza- tion transport in an external electric ¯eld, phenomena analogous to the Hall e®ect emerge.

We elucidate the geometry of quantum adiabatic evolution. By minimizing the deviation from adiabaticity we find a Riemannian metric tensor underlying adiabatic evolution. Equipped with this tensor, we identify a unified geometric description of quantum adiabatic evolution and quantum phase transitions, which generalizes previous treatments to allow for degeneracy. The same structure is relevant for applications in quantum information processing, including adiabatic and holonomic quantum computing, where geodesics over the manifold of control parameters correspond to paths which minimize errors. We illustrate this geometric structure with examples, for which we explicitly find adiabatic geodesics. By solving the geodesic equations in the vicinity of a quantum critical point, we identify universal characteristics of optimal adiabatic passage through a quantum phase transition. In particular, we show that in the vicinity of a critical point describing a second order quantum phase transition, the geodesic exhibits power-law scaling with an exponent given by twice the inverse of the product of the spatial and scaling dimensions.

Motivated by the progress on shortcuts to adiabaticity, we propose three schemes for speeding up (fractional) stimulated Raman adiabatic passage, and achieving rapid and non-adiabatic creation and transfer of maximal coherence in a triple-quantum-dot system. These different but relevant protocols, designed from counter-diabatic driving, dress-state method, and resonant technique, require their own pumping fields, applied gate voltages and varying tunneling couplings between two spatially separated dots. Such fast and reliable shortcuts not only allow for feasibly experimental realization in solid-state architectures but also may have potential applications in quantum information processing and quantum control.

In this serie of lectures we present some of the basic issues in semiconductor spintronics at an introductory level. We start with a short overview of low dimensional semiconductor structures and their applications. We then move on to discuss the notions of spin generation, injection, relaxation and detection in spintronics, with an emphasis in the application of diluted magnetic semiconductors for spin generation and injection. Finally, we discuss the manipulation of spin in semiconductor nanostructures like quantum dots, and its applications in quantum computation.

We present a scheme to significantly enhance multipartite entanglement with an atom-assisted single-cavity optomechanical system. By embedding an ensemble of Λ-type three-level atoms into the cavity, the stationary tripartite entanglement among two longitudinal cavity modes and a mirror oscillation mode can be greatly enhanced due to quantum coherence induced between the two lower atomic states when one (another) cavity mode is driven at the red (blue) mechanical sideband, and both cavity modes are blue-detuned by the mechanical frequency to the respective atomic resonant transitions; moreover, strong light–light–atom–mirror quadripartite entanglement can also be obtained. The present hybrid system can serve as a promising platform for realizing various quantum protocols, and may find potential applications in quantum information processing and quantum networks.