Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn ͓2001, Nature ͑London͒ 409, 46͔ explicitly demonstrates that efficient scalable quantum computing with single photons, linear optical elements, and projective measurements is possible. Subsequently, several improvements on this protocol have started to bridge the gap between theoretical scalability and practical implementation. The original theory and its improvements are reviewed, and a few examples of experimental two-qubit gates are given. The use of realistic components, the errors they induce in the computation, and how these errors can be corrected is discussed.
We show how to construct a near deterministic CNOT using several single photons sources, linear optics, photon number resolving quantum non-demolition detectors and feed-forward. This gate does not require the use of massively entangled states common to other implementations and is very efficient on resources with only one ancilla photon required. The key element of this gate are non-demolition detectors that use a weak cross-Kerr nonlinearity effect to conditionally generate a phase shift on a coherent probe, if a photon is present in the signal mode. These potential phase shifts can then be measured using highly efficient homodyne detection.PACS numbers: 03.67. Lx, 03.67.Mn, In the past few years we have seen the emergence of single photon optics with polarisation states as a realistic path for achieving universal quantum computation. This started with the pioneering work of Knill, Laflamme and Milburn [KLM][1] who showed that with only single photon sources and detectors and linear elements such as beam-splitters, a near deterministic CNOT gate could be created, through with the use of significant but polynomial resources. With this architecture for the CNOT gate and trivial single qubit rotations a universal set of gates is hence possible and a route forward for creating large devices can be seen. Since this original work there has been significant progress both theoretically [2,3,4,5,6] and experimentally [7,8,9], with a number of CNOT gates actually demonstrated.Much of the theoretical effort has focused on determining more efficient ways to perform the controlled logic. The standard model for linear logic uses only[1]:• Single photon sources, • Linear optical elements including feed-forward, • Photon number resolving single photon detectors, and it has been shown by Knill[4] that the maximum probability for achieving the CNOT gate is 3/4. While these upper bounds are not thought to be tight, with the best success probabilities for the CNOT gate being 2/27[10], it does indicate that near deterministic gates are not possible using only the above resources and strategy. These gates can be made efficient using the "standard" optical teleportation tricks which require the use of massively entangled resources. Are there other natural ways to increase the efficient of these gate operations? Franson et al. [2] showed that if you can increase your allowed physical resources to include maximally entangled two photon states, then the CNOT gate can have its probability of success boosted to 1/4, though this is still far below the 3/4 maximum. Alternatively it is possible to use single photons for the cluster state method of one way quantum computation [5,6]. This can dramatically decrease the number of single photons sources required to perform a CNOT gate (from up to 10000 for KLM logic to 45 for the cluster approaches). The overhead here in single photon sources is large (but polynomial and hence still efficient in a sense). Can we however build near deterministic (or deterministic) linear optics gates with a low ...
We describe a quantum repeater protocol for long-distance quantum communication. In this scheme, entanglement is created between qubits at intermediate stations of the channel by using a weak dispersive light-matter interaction and distributing the outgoing bright coherent light pulses among the stations. Noisy entangled pairs of electronic spin are then prepared with high success probability via homodyne detection and postselection. The local gates for entanglement purification and swapping are deterministic and measurement-free, based upon the same coherent-light resources and weak interactions as for the initial entanglement distribution. Finally, the entanglement is stored in a nuclear-spin-based quantum memory. With our system, qubit-communication rates approaching 100 Hz over 1280 km with fidelities near 99% are possible for reasonable local gate errors.PACS numbers: 03.67. Hk, 03.67.Mn, 42.50.Pq In a quantum repeater, long-distance entanglement is created by distributing entangled states over sufficiently short segments of a channel such that the noisy entangled states in each segment can be purified and connected via entanglement swapping [1,2]. The resulting entanglement between the qubits at distant stations can then be used, for example, to teleport quantum information [3] or transmit secret classical information [4]. Existing approaches to quantum repeaters generate entanglement using postselection with single-photon detection [5,6,7]. In these schemes, high-fidelity entanglement is created and the subsequent entanglement purification is needed primarily to compensate the degrading effect of connecting the imperfect entangled pairs via swapping. However, due to their rather low success probabilities in the initial entanglement distribution, these protocols feature very low communication rates.More efficient schemes, compatible with existing classical optical communication networks, would involve bright multi-photon signals. In this Letter, we propose such a scheme that operates in a regime of modest initial fidelities, but creates entangled states at high speed. The high rate in the generation of entangled pairs is mainly due to the near-unit efficiencies for the homodyne detection of bright signals, as opposed to the low efficiencies of single-photon detectors. In our scheme, the resulting entangled pairs will be discrete atomic qubit states, but the probe system we use is a bright light pulse described and measured via a continuous phase observable; hence, our quantum repeater is "hybrid" not only because it employs matter signals and light probes (as in other schemes), but more distinctly, by utilizing both discrete and continuous quantum variables.In general, in order to realize universal quantum computation or, more relevant to us here, long-distance quantum communication, a nonlinear element is needed for the implementation. Optically, this nonlinear element may be introduced in at least two possible ways. The first method uses only linear transformations, but a measurement-induced nonlinear...
During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen-vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 10(7) such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.
We obtain sufficient conditions for the efficient simulation of a continuous variable quantum algorithm or process on a classical computer. The resulting theorem is an extension of the Gottesman-Knill theorem to continuous variable quantum information. For a collection of harmonic oscillators, any quantum process that begins with unentangled Gaussian states, performs only transformations generated by Hamiltonians that are quadratic in the canonical operators, and involves only measurements of canonical operators (including finite losses) and suitable operations conditioned on these measurements can be simulated efficiently on a classical computer.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.