Efficient verification of quantum gates with local operations

Pretty good state transfer on Cayley graphs over dihedral groups

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.

Shortcut to adiabaticity in various quantum systems has attracted much attention with the wide applications in quantum information processing and quantum control. In this paper, we concentrate on stimulated Raman shortcut-to-adiabatic passage in quantum three-level systems. To implement counter-diabatic driving but without additional coupling, we first reduce the quantum three-level systems to effective two-level problems at large intermediate-level detuning, or on resonance, apply counter-diabatic driving along with the unitary transformation, and eventually modify the pump and Stokes pulses for achieving fast and high-fidelity population transfer. The required laser intensity and stability against parameter variation are further discussed, to demonstrate the advantage of shortcuts to adiabaticity.

We present a special quantum effect named electromagnetically induced squeezing of atomic spin in the conventional Λ-type atomic system driven by a strong pump field and a relatively weak probe field in an atomic ensemble. Nearly perfect atomic spin squeezing in the y component can be achieved with fully on resonance at high pump-field intensity, large optical depth of the atomic ensemble, and a low coherence decay rate between the two lower levels. This method would greatly facilitate the generation of atomic spin squeezing by using only coherent laser fields, and may find potential applications in realistic quantum metrology, quantum computation, and quantum information processing.

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.

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.

Fast characterization of pulsed spontaneous parametric down conversion (SPDC) sources is important for applications in quantum information processing and communications. We propose a simple method to perform this task, which only requires measuring the counts on the two output channels and the coincidences between them, as well as modeling the filter used to reduce the source bandwidth. The proposed method is experimentally tested and used for a complete evaluation of SPDC sources (pair emission probability, total losses, and fidelity) of various bandwidths. This method can find applications in the setting up of SPDC sources and in the continuous verification of the quality of quantum communication links.

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.

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.

The recently discovered negatively charged tin-vacancy centre in diamond is a promising candidate for applications in quantum information processing (QIP). We here present a detailed spectroscopic study encompassing single photon emission and polarisation properties, the temperature dependence of emission spectra as well as a detailed analysis of the phonon sideband and Debye–Waller factor. Using photoluminescence excitation spectroscopy we probe an energetically higher lying excited state and prove fully lifetime limited linewidths of single emitters at cryogenic temperatures. For these emitters we also investigate the stability of the charge state under resonant excitation. These results provide a detailed insight into the spectroscopic properties of the SnV− centre and lay the foundation for further studies regarding its suitability in QIP.

We introduce the concept and proof-of-principle design of a new type of high-speed photonic integrated analyzers for continuous-variable quantum states. These analyzers aim to address both unconditionally entangled states and noise-squeezed states. Unlike current squeezed state analyzers and entangled state analyzers, our devices do not rely on optical phase-locked loops. Instead, they use a phase-diversity architecture where the local oscillator operates as a free-running laser. This method greatly simplifies system design and allows for photonic integrated circuit realizations that meet the strict size, weight, and power requirements of space-based quantum communication systems. We explain how the integrated analyzers for unconditionally entangled states and noise-squeezed states work, outline their methods for characterizing entanglement and squeezing in the digital domain, and provide a tolerance analysis of the essential photonic components. Numerical modeling shows that our design achieves over 10dB parametric gain with sub-10mW pump powers. It also allows for squeezing levels that exceed the 8dB threshold needed for fault-tolerant continuous-variable quantum computing. This work offers a route to scalable, practical photonic analyzers for quantum communication and information processing applications.

Hyperentanglement has significant applications in quantum information processing. Here we present an efficient hyperentanglement concentration protocol (hyper-ECP) for partially hyperentangled Bell states simultaneously entangled in polarization, spatial-mode and time-bin degrees of freedom (DOFs) with the parameter-splitting method, where the parameters of the partially hyperentangled Bell states are known to the remote parties. In this hyper-ECP, only one remote party is required to perform some local operations on the three DOFs of a photon, only the linear optical elements are considered, and the success probability can achieve the maximal value. Our hyper-ECP can be easily generalized to concentrate the N-photon partially hyperentangled Greenberger–Horne–Zeilinger states with known parameters, where the multiple DOFs have largely improved the channel capacity of long-distance quantum communication. All of these make our hyper-ECP more practical and useful in high-capacity long-distance quantum communication.

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.

The bandwidth and versatility of optical devices have revolutionized information technology systems and communication networks. Precise and arbitrary control of an optical field that preserves optical coherence is an important requisite for many proposed photonic technologies. For quantum information applications, a device that allows storage and on-demand retrieval of arbitrary quantum states of light would form an ideal quantum optical memory. Recently, significant progress has been made in implementing atomic quantum memories using electromagnetically induced transparency, photon echo spectroscopy, off-resonance Raman spectroscopy and other atom-light interaction processes. Single-photon and bright-optical-field storage with quantum states have both been successfully demonstrated. Here we present a coherent optical memory based on photon echoes induced through controlled reversible inhomogeneous broadening. Our scheme allows storage of multiple pulses of light within a chosen frequency bandwidth, and stored pulses can be recalled in arbitrary order with any chosen delay between each recalled pulse. Furthermore, pulses can be time-compressed, time-stretched or split into multiple smaller pulses and recalled in several pieces at chosen times. Although our experimental results are so far limited to classical light pulses, our technique should enable the construction of an optical random-access memory for time-bin quantum information, and have potential applications in quantum information processing.

In this study, we demonstrate the coherent control of plexcitonic modes in (i) off-resonant and (ii) resonant coupled plasmon-emitter systems through the optical Stark effect (OSE). By leveraging the Stark shift in degenerate energy eigenstates of two- or three-level quantum emitters (QEs) embedded in plasmonic nanocavities, we demonstrate tunable photoluminescence, coherent control of plexcitonic states, and Stark-induced transparency. Our findings highlight the potential of Stark tuning to mitigate decoherence and enable on-demand control of spontaneous photon emission in the visible regime. These results pave the way for developing scalable and actively tunable photonic integrated circuits for quantum computing and information processing applications.