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Functional quantum circuit combines photon gates

Shigeki Takeuchi

Published Jan 1, 2009• 0 citations
Publisher

Abstract

An optical-quantum-circuit entanglement filter can pass a pair of pho-tons with the desired polarization correlations. Photons are an important information-conveyance medium for enhanced computers and communications based on quantum mechanics. Filtering multiple photon states is a key capabili-ty in quantum-information science and technology, where one-quantum binary-digit (one-qubit) filters, or polarizers, have found wide application. Filtering based on the correlation or en-tanglement of photons requires multi-qubit filters with qubit-qubit interactions. We demonstrate an optical-entanglement filter that passes a pair of photons if they have the desired polarization correlations.1 The entangling capability of the filter was verified, distinguishing it from classical ones. Because our entanglement filter acts on photonic qubits, it is promising for quantum tech-

DOI
10.1117/2.1200908.1752

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An optical-quantum-circuit entanglement filter can pass a pair of pho-tons with the desired polarization correlations. Photons are an important information-conveyance medium for enhanced computers and communications based on quantum mechanics. Filtering multiple photon states is a key capabili-ty in quantum-information science and technology, where one-quantum binary-digit (one-qubit) filters, or polarizers, have found wide application. Filtering based on the correlation or en-tanglement of photons requires multi-qubit filters with qubit-qubit interactions. We demonstrate an optical-entanglement filter that passes a pair of photons if they have the desired polarization correlations.1 The entangling capability of the filter was verified, distinguishing it from classical ones. Because our entanglement filter acts on photonic qubits, it is promising for quantum tech-
This paper discusses high-performance quantum optical circuits for processing polarization entanglement between correlated photon pairs. The primary application of this technology is secure communication links with quantum encryption that rely on the inherent properties of laser light and physical-layer processes in optical components. It can be used for generating identical pairs of encryption keys in quantum key distribution applications, for a variety of encryption protocols ranging from single-photon to multiple-photon, keyed communication in quantum noise, but we see a potential spectrum extending further to include multi-access systems. Success in these applications can be achieved by introducing additional degrees-of-freedom in each single-particle quantum state. We see hyper-entanglement as a very promising approach to do this. The concept is visualized by having multiple entangled photons with specifically assigned frequencies in the 100 GHz ITU grid and arriving simultaneously from multiple sources. Processing of such signals requires hyperspectral optical circuits capable of responding to non-classical features of quantum states. The function of these circuits is to direct the arriving signals along different processing paths to single photon detectors, which can be realized by a combination of different technologies, such as highly selective Lyot filters, dense wavelength division multiplexing, and others. Processing capabilities of these quantum circuits, while seemingly straight-forward in theory, still present a great implementation challenge. Practical operating conditions and characteristics of optical components must be taken into consideration to address the underlying design problems and make this technology feasible. This constitutes the main focus of our paper.
Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2,3]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [4]), measurement (eg. [5]) and other quantum technologies [6]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturization and scalability [7]. Here we report high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [8]. We have demonstrated controlled manipulation of up to four photons on-chip, including highfidelity single qubit operations, using a lithographically patterned resistive phase shifter [9]. We have used this architecture to implement a small-scale compiled version of Shor's quantum factoring algorithm [10], demonstrated heralded generation of tunable four photon entangled states from a six photon input [11], a reconfigurable two-qubit circuit [12], and combined waveguide photonic circuits with superconducting single photon detectors [13]. We describe complex quantum interference behavior in multi-mode interference devices with up to eight inputs and outputs [14], and quantum walks of correlated particles in arrays of coupled waveguides [15]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [16,17] and diamond [18,19] and nonlinear [20,21] photon sources.
We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T=(20+/-9)%.
In the emerging field of quantum information technology the two basic subfields are quantum communication and quantum computation. Photonic qubits are considered as most promising information carriers for this new technology due to the immense advantage of suffering negligible decoherence. Additionally, the very small photon-photon interactions can be replaced by inducing effective nonlinearities via measurements which allow for the implementation of crucial two-qubit gate operations. Although the spontaneous parametric down-conversion gives access to the generation of highly entangled few-photon states, such as four-qubit cluster states which allow to demonstrate the new concept of the one-way quantum computer, its applicability is highly limited due to the poor scaling of the simultaneous emission of more than one-entangled photon pair. Therefore of particular interest is the reversible mapping of qubits from photon states to atomic states. This might allow the implementation of photonic quantum repeaters for long-distance quantum communication or the generation of arbitrary multi-photon states as required for linear-optics quantum computing. Thus for the realization of such a quantum network several approaches for achieving the required quantum control between matter and photons have been studied during the past few years. Recent experiments demonstrating the generation of narrow-bandwidth single photons using a room-temperature ensemble of 87Rb atoms and electromagnetically induced transparency should emphasize the progress towards such a quantum network.