One of the main challenges in designing large scale quantum processors is connecting separated qubits. In this thesis, we explore new opportunities that mesoscopic many-body systems provide for creating quantum correlation between isolated quantum systems. In particular, we study entangling two non-interacting qubits through an intermediate mesoscopic system consisting of identical two-level systems. Two uncoupled qubits can be entangled either by projectively measuring a joint property of them or by creating an indirect interaction between them. The focus of this thesis is on procedures that are based on joint measurement on the qubits. \nWe propose a new method for entangling two non-interacting qubits by measuring their parity indirectly through an intermediate mesoscopic system. Indirect joint measurement scheme benefits from coherent magnification of the target qubits’ state in the collective state of the mesoscopic system; such that a low-resolution measurement on the mesoscopic system suffices to prepare post-selected entanglement on the target qubits. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits’ state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a new design element that can be integrated into quantum processor architectures and quantum measurement devices. \nWe quantify the resources required for implementing the indirect joint measurement technique when the intermediate mesoscopic system consists of spin-1/2 particles with internal dipolar coupling. A mesoscopic spin system consisting of two non-interacting halves, each coupled to one of the target qubits is proved to provide a helpful geometry that allows implementing the coherent magnification process with experimentally available control tools. We show that the requirements on the amplified state of the target qubits and the mesoscopic spin system perfectly maps to the specifications of micro-macro entanglement between each target qubit and its nearby half of the mesoscopic spin system. In the light of this equivalence, the effects of experimental imperfections are explored; in particular, bipartite entanglement between the target qubits is shown to be robust to imperfect preparation of the mesoscopic spin system. Our analysis provides a new approach for using an intermediate spin system for connecting separated qubits. It also opens a new path in exploring entanglement between microscopic and mesoscopic spin systems.
One of the main challenges in designing large scale quantum processors is connecting separated qubits. In this thesis, we explore new opportunities that mesoscopic many-body systems provide for creating quantum correlation between isolated quantum systems. In particular, we study entangling two non-interacting qubits through an intermediate mesoscopic system consisting of identical two-level systems. Two uncoupled qubits can be entangled either by projectively measuring a joint property of them or by creating an indirect interaction between them. The focus of this thesis is on procedures that are based on joint measurement on the qubits. \nWe propose a new method for entangling two non-interacting qubits by measuring their parity indirectly through an intermediate mesoscopic system. Indirect joint measurement scheme benefits from coherent magnification of the target qubits’ state in the collective state of the mesoscopic system; such that a low-resolution measurement on the mesoscopic system suffices to prepare post-selected entanglement on the target qubits. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits’ state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a new design element that can be integrated into quantum processor architectures and quantum measurement devices. \nWe quantify the resources required for implementing the indirect joint measurement technique when the intermediate mesoscopic system consists of spin-1/2 particles with internal dipolar coupling. A mesoscopic spin system consisting of two non-interacting halves, each coupled to one of the target qubits is proved to provide a helpful geometry that allows implementing the coherent magnification process with experimentally available control tools. We show that the requirements on the amplified state of the target qubits and the mesoscopic spin system perfectly maps to the specifications of micro-macro entanglement between each target qubit and its nearby half of the mesoscopic spin system. In the light of this equivalence, the effects of experimental imperfections are explored; in particular, bipartite entanglement between the target qubits is shown to be robust to imperfect preparation of the mesoscopic spin system. Our analysis provides a new approach for using an intermediate spin system for connecting separated qubits. It also opens a new path in exploring entanglement between microscopic and mesoscopic spin systems.
We quantify the resources required for entangling two uncoupled spin qubits through an intermediate mesoscopic spin system (MSS) by indirect joint measurement. Indirect joint measurement benefits from coherent magnification of the target qubits' state in the collective magnetization of the MSS, such that a low-resolution collective measurement on the MSS suffices to prepare postselected entanglement on the target qubits. A MSS consisting of two noninteracting halves, each coupled to one of the target qubits, is identified as a geometry that allows implementing the magnification process with experimentally available control tools. It is proved that the requirements on the amplified state of the target qubits and the MSS perfectly map to the specifications of micro-macro entanglement between each target qubit and its nearby half of the MSS. In the light of this equivalence, the effects of experimental imperfections are explored; in particular, bipartite entanglement between the target qubits is shown to be robust to imperfect preparation of the MSS. Our study provides an approach for using an intermediate spin system for connecting separated qubits. It also opens another path in exploring entanglement between microscopic and mesoscopic spin systems.
The amplification of quantum information carried by a single quantum excitation is a recurring challenge across diverse quantum platforms. The coupling between a single qubit and a mesoscopic ensemble of spins, for example, can be leveraged to realize non-destructive detection of the qubit state. However, realizing robust couplings between such systems is experimentally challenging and typically requires programmable quantum gates or native long-range interactions. Here, we introduce a platform that couples a single qubit, encoded in the ground-to-Rydberg transition of a control atom, to a Rydberg-dressed target ensemble of ground-state atoms trapped in an optical lattice. We show that the state of the control qubit can be coherently mapped onto the ensemble via a qubit-controlled collective phase shift. By Rydberg-dressing the ensemble, the controlled phase shift per target atom becomes independent of the number of target atoms, making the protocol intrinsically insensitive to atom-number fluctuations and atom loss, which are the dominant experimental imperfections in our system. Exploiting the collective response of up to eight target spins, we demonstrate the efficacy of the scheme by realizing non-destructive detection of a single Rydberg excitation with a state-assignment fidelity of F=99.81−1.47+0.17%. Our approach demonstrates the key ingredients for high-fidelity transfer of quantum information from a single qubit to a mesoscopic ensemble, opening a route to non-destructive mid-circuit readout of Rydberg states and to efficient interfaces between single qubits and photonic modes.
We propose a method for entangling two noninteracting qubits by measuring their parity indirectly through an intermediate mesoscopic system. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits' state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a design element that can be integrated into quantum processor architectures and quantum measurement devices.
The generation of entanglement in atomic systems plays a central topic in the fields of quantum information storage and processing. Moreover, a special category of entangled states of multi-atom ensembles, spin squeezed states, have been proven to lead to considerable improvement in the sensitivity of precision measurements compared to systems involving uncorrelated atoms. A treatment of entanglement in open systems is, however, incomplete without a precise description of the process of decoherence which necessarily accompanies it. The theory of entanglement and decoherence are the two main topics of this thesis. Methods are described for the generation of strong correlations in large atomic ensembles using either cavity quantum electrodynamics or measurement outcome conditioned quantum dynamics. Moreover, the description of loss of entanglement resulting from the coupling to a noise reservoir (electromagnetic vacuum) is explored. A spin squeezing parameter is used throughout this thesis as both a measure of entanglement strength and as an indication of the sensitivity improvement above the so-called standard quantum limit (sensitivity obtained with uncorrelated particles) in metrology. The first scheme considered consists of a single mode cavity field interacting with a collection of atoms for which spin squeezing is produced in both resonant and off-resonant regimes. In the resonant case, transfer of squeezing from a field state to the atoms is analyzed, while in the off-resonant regime squeezing is produced via an effective nonlinear interaction (one-axis twisting Hamiltonian). A second, more experimentally realistic case, is one involving the interaction of free space atoms with laser pulses; a projective measurement of a source field originating from atomic fluctuations provides a means of preparing atomic collective states such as spin squeezed and Schrodinger cat states. A new unravelling is proposed, that employs the detection of photon number in a single output channel and is capable of producing mesoscopic Schrodinger cat states in a single step. As a first step to understanding the role of cooperative decoherence, of importance in the case of dense pencil-shaped atomic ensembles, the collective spin decoherence of a two multilevel atom system is derived. This calculation is also relevant to entanglement loss for two qubits manipulated using with reading/writing pulses. Finally, a scheme is proposed in which arbitrarily strong entanglement can be produced in a four-wave mixing setup, where the preparation of the atoms in a dark state limits the decoherence to negligible values.
This thesis discusses various schemes and protocols for quantum information processing in mesoscopic systems with particular focus on using the spin of a particle as the bearer of information. The first chapter introduce various aspects of the field of quantum information used in this thesis such as qubits, entanglement, its quantification, quantum logic gates and entanglement swapping. In this chapter concepts such as AKLT states, decoherence and adiabatic elimination are introduced as they will be relevant in the thesis. In chapter 2 we introduce the Quantum Dots as the solid state system that will primarily be used as the hardware for the development of Quantum Information Processing (QIP). The different properties of quantum dots depending on their size are discussed. The exchange interaction between tunnel coupled quantum dots and the background of quantum computation in quantum dots is described. The principal sources of decoherence and the measurement techniques for spin qubits are presented. In chapter 3, carbon nanowires filled with N@C60 dimers are studied to analyse the entanglement between nuclear spins. The dimer is modelled as a two coupled nuclear spin- electron spin pair with a Heisenberg interaction. The entanglement have been studied depending on the temperature and the intensity of an external magnetic field. Witnessing the entanglement, and particularly bound entanglement are discussed. In chapter 4, the way to extract a singlet from a quantum dot is explored. The system that we model will be consisting of a triple dot and analyse the best way to get the singlet out, with each electron in a separate dot. The chief motivation is to create a singlet between separate dots in a time-scale much faster than that given by spinspin exchange interactions. In chapter 5, quantum logic gates in a triple dot system has been studied. Such gates have been widely studied in double and single quantum dots. Motivated by the advent of experimental set ups of triple dots, we have studied the natural quantum gates that came out of a triple dot system. There are still two spin quantum bits in the three dots and there is an empty intervening dot, which imparts the scheme some advantages, as well as a substantial difference from the class of schemes studied so far. In chapter 6, we model a large square dot. As we describe in chapter 2, the properties of the large dots make them behave with some interesting properties such as hosting Wigner molecules of electrons inside. We explore the application of these structures for quantum information processing. We show here how to get singlet/triplet measurement, entanglement swapping, and how to prepare a 1D AKLT state, using the square dot as a construction block of the system. Finally in chapter 7 conclusions and further work. Here we indicate the further work that could be done with the knowledge present in this thesis and motivated by future advances in the technology.
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.