Quantum computing promises exponential speedup for particular computational tasks, such as integer factoring, which bears importance for encryption technology, and quantum simulation, which holds vast scientific potential. Comparing with the traditional circuit-based quantum computers, one-way quantum computers are experimentally appealing because it requires only local measurements on an entangled resource called a cluster state. In this thesis, I first review the fundamental theory of one-way quantum computing and its experimental implementation using the quantum optical frequency comb. After that, I present two novel schemes for generating ultra-scalable cluster states with rich structures that can be used for universal quantum computing.
Quantum computing promises exponential speedup for particular computational tasks, such as integer factoring, which bears importance for encryption technology, and quantum simulation, which holds vast scientific potential. Comparing with the traditional circuit-based quantum computers, one-way quantum computers are experimentally appealing because it requires only local measurements on an entangled resource called a cluster state. In this thesis, I first review the fundamental theory of one-way quantum computing and its experimental implementation using the quantum optical frequency comb. After that, I present two novel schemes for generating ultra-scalable cluster states with rich structures that can be used for universal quantum computing.
One-way quantum computing allows any quantum algorithm to be implemented easily using just measurements. The difficult part is creating the universal resource, a cluster state, on which the measurements are made. We propose a scalable method that uses a single, multimode optical parametric oscillator (OPO). The method is very efficient and generates a continuous-variable cluster state, universal for quantum computation, with quantum information encoded in the quadratures of the optical frequency comb of the OPO.
In the one-way model of quantum computing, quantum algorithms are implemented using only measurements on an entangled initial state. Much of the hard work is done upfront when creating this universal resource, known as a cluster state, on which the measurements are made. Here we detail a new proposal for a scalable method of creating cluster states using only a single multimode optical parametric oscillator (OPO). The method generates a continuous-variable cluster state that is universal for quantum computation and encoded in the quadratures of the optical frequency comb of the OPO. This work expands on the presentation in Menicucci, Flammia and Pfister (2008 Phys. Rev. Lett. 101 , 130501).
We propose and fully analyze the simplest technique to date (to our knowledge) for generating light-based universal quantum computing resources, namely, 2D, 3D, and n -hypercubic cluster states in general. The technique uses two standard optical components: first, a single optical parametric oscillator pumped below threshold by a monochromatic field, which generates Einstein–Podolsky–Rosen entangled states, a.k.a. two-mode squeezed states, over the quantum optical frequency comb; second, phase modulation at frequencies that are multiples of the comb spacing (via RF or optical means). The compactness of this technique paves the way to implementing quantum computing on chip using quantum nanophotonics.
Ever since its discovery, quantum mechanics has remained an intensely active field, and its real-world applications continue to unfold rapidly. In 1982, Richard Feynman proposed a new type of computer operating directly under quantum mechanics laws: – the quantum computer \cite{Feynman1982}. Compared with the classical computer, whose information is encoded in “bits”, the quantum computer, whose information is encoded in “quantum bits”, or “qubits”, will be able to perform calculations exponentially faster for such problems as factoring large integers into primes and simulating complicated quantum systems. Due to their extremely powerful calculation speeds and abilities, quantum computers have been the long-pursued dreams for both experimentalists and theorists in many research groups, government agencies, industrial companies, etc., and the fast-paced developments in their architecture and speed continue to make them more and more attractive. There are two principal models of quantum computing: the circuit model and the measurement-based model. The circuit model is similar to a traditional computer where there are inputs, gates and outputs. The measurement-based model is different, as it is crucially based on the cluster state, a type of highly entangled quantum state. In this new model, quantum computing begins with an initial cluster state and then carries out calculations by physical measurements of the cluster state itself along with feedforward. Thus, the cluster state serves as the material and resource for the entire set of calculations, and it is an extremely important part of measurement-based quantum computing. This thesis will discuss an experimental and theoretical work that holds the world record for the largest entangled cluster state ever created whose 60 qumodes (optical versions of qubits) are all available simultaneously. Moreover, the entangled state we created is not random, and it is a cluster state which meets the specific requirements for implementing quantum computing. In the race to build a practical quantum computer, the ability to create such a large cluster state is paramount. Also, our creative optical method to generate massive entanglement advances many other methods due to its high efficiency, super-compactness and large scalability. The entanglement proceeds from interfering multiple EPR entangled pairs, which are generated from the down-converting process of a nonlinear crystal in an optical parametric oscillator, into a very long dual-rail wire cluster state. Moreover, many copies of the same state can easily be obtained by merely adjusting the frequencies of the pump lasers. These cluster states serve as building blocks of the universal quantum computer, and also are, in their own right, important resources for studying and exploring quantum mechanics in large systems.
One-way quantum computing is experimentally appealing because it requires only local measurements on an entangled resource called a cluster state. Record-size, but nonuniversal, continuous-variable cluster states were recently demonstrated separately in the time and frequency domains. We propose to combine these approaches into a scalable architecture in which a single optical parametric oscillator and simple interferometer entangle up to ($3\ifmmode\times\else\texttimes\fi{}{10}^{3}$ frequencies) $\ifmmode\times\else\texttimes\fi{}$ (unlimited number of temporal modes) into a computationally universal continuous-variable cluster state. We introduce a generalized measurement protocol to enable improved computational performance on this entanglement resource.
Large and complex optical quantum states are a key resource for fundamental science and applications such as quantum communications, information processing, and metrology. In this context, cluster states are a particularly important class because they enable the realization of universal quantum computers by means of the so-called `one-way' scheme, where processing operations are performed through measurements on the state. While two-level (i.e. qubit) cluster states have been realized thus far, further boosting this computational resource by increasing the number of particles comes at the price of significantly reduced coherence time and detection rates, as well as increased sensitivity to noise. In contrast, the realization of d-level (with d > 2) cluster states offers the possibility to increase quantum resources without changing the number of particles, enables the implementation of efficient computational protocols, as well as coincides with a reduction in the noise sensitivity of the states. Here, we experimentally realize, characterize, and perform one-way processing operations on three-level, four-partite cluster states formed by two photons in the time and frequency domain. We make use of a unique approach based on integrated photonic chips and optical fiber communications components, enabling scalable, deterministic new functionalities.