Linear Optical Quantum Computing Research Articles

Superconducting Resonators as Beam Splitters for Linear-Optics Quantum Computation

We propose and analyze a technique for producing a beam-splitting quantum gate between two modes of a ring-resonator superconducting cavity. The cavity has two integrated superconducting quantum interference devices (SQUIDs) that are modulated by applying an external magnetic field. The gate is accomplished by applying a radio frequency pulse to one of the SQUIDs at the difference of the two mode frequencies. Departures from perfect beam splitting only arise from corrections to the rotating wave approximation; an exact calculation gives a fidelity of >0.9992. Our construction completes the toolkit for linear-optics quantum computing in circuit quantum electrodynamics.

Hong–Ou–Mandel Dip Using Degenerate Photon Pairs from a Single Periodically Poled Lithium Niobate Waveguide with Integrated Mode Demultiplexer

We experimentally observed a Hong–Ou–Mandle dip with photon pairs generated in a periodically poled reverse-proton-exchange lithium niobate waveguide with an integrated mode demultiplexer at a wavelength of 1.5 µm. The visibility of the dip in the experiment was 80% without subtraction of any noise terms at a peak pump power of 4.4 mW. The technology developed in the experiment can find various applications in the research field of linear optics quantum computation in fiber or quantum optical coherence tomography with near infrared photon pairs.

Preparation of narrow-band photons for atomic-based quantum memory with a type-I phase matched periodical poled KTP crystal

Transferring a quantum state between a photon and a quantum memory is the key point for realizing a long-distance quantum communication, and is also a basic ingredient of linear optical quantum computation. In an atomic-based network, the efficient coupling between a photon and an atomic system is a prerequisite for realizing the transfer of information between them, which requires that the photon should have a comparable bandwidth with the natural bandwidth of an atom. Therefore, generating a narrow-band photon has become a very important topic in the quantum information field. One simple and efficient way is cavity-enhanced spontaneously parametric down-conversion. In this paper, we will review and introduce a series of experiments done in our group for realizing this goal. We believe these works are very useful for the research in this direction.

Linear optical quantum computation with imperfect entangled photon-pair sources and inefficient non–photon-number-resolving detectors

We propose a scheme for efficient cluster state quantum computation by using imperfect polarization-entangled photon-pair sources, linear optical elements and inefficient non-photon-number-resolving detectors. The efficiency threshold for loss tolerance in our scheme requires the product of source and detector efficiencies should be >1/2 - the best known figure. This figure applies to uncorrelated loss. We further find that the loss threshold is unaffected by correlated loss in the photon pair source. Our approach sheds new light on efficient linear optical quantum computation with imperfect experimental conditions.

Scheme for implementing linear optical quantum iSWAP gate with conventional photon detectors

A simple scheme is proposed to implement a two-qubit linear optical quantum iSWAP gate that is a universal gate in quantum computation and quantum information processing. By the interference effect of the polarized photons, a quantum iSWAP gate can be achieved with a low success probability (η4/32, with η being the quantum efficiency of photon detectors). The scheme is based only on simple linear optical elements, a pair of two-photon polarization entangled states, and conventional photon detectors that only distinguish the vacuum and nonvacuum Fock number states, which greatly decreases the experimental difficulty of implementing linear optical quantum computation.

Erratum: Spin-resolved quantum-dot resonance fluorescence

Two experiments observe the so-called ‘Mollow triplet’ in the emission spectrum of a quantum dot—originating from resonantly driving a dot transition—and demonstrate the potential of these systems to act as single-photon sources and as a readout modality for electron-spin states. Confined spins in self-assembled semiconductor quantum dots promise to serve both as probes for studying mesoscopic physics in the solid state and as stationary qubits for quantum-information science1,2,3,4,5,6,7. Moreover, the excitations of self-assembled quantum dots can interact with near-infrared photons, providing an interface between stationary and ‘flying’ qubits. Here, we report the observation of spin-selective photon emission from a resonantly driven quantum-dot transition. The Mollow triplet8 in the scattered photon spectrum—the hallmark of resonance fluorescence when an optical transition is driven resonantly—is presented as a natural way to spectrally isolate the photons of interest from the original driving field. We also demonstrate that the relative frequencies of the two spin-tagged photon states can be tuned independent of an applied magnetic field through the spin-selective dynamic Stark effect, induced by the same driving laser. This demonstration should be a step towards the realization of challenging tasks such as electron-spin readout, heralded single-photon generation for linear-optics quantum computing and spin–photon entanglement.

Experimental quantum teleportation and multiphoton entanglement via interfering narrowband photon sources

In this paper, we report a realization of synchronization-free quantum teleportation and narrowband three-photon entanglement through interfering narrowband photon sources. Since both the single-photon and the entangled photon pair utilized are completely autonomous, it removes the requirement of high-demanding synchronization techniques in long-distance quantum communication with pulsed spontaneous parametric down-conversion sources. The frequency linewidth of the three-photon entanglement realized is on the order of several MHz, which matches the requirement of atomic ensemble based quantum memories. Such a narrowband multiphoton source will have applications in some advanced quantum communication protocols and linear optical quantum computation.

Proposal for Pulsed On-Demand Sources of Photonic Cluster State Strings

We present a method to convert certain single photon sources into devices capable of emitting large strings of photonic cluster state in a controlled and pulsed "on-demand" manner. Such sources would greatly reduce the resources required to achieve linear optical quantum computation. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques, and shows that the photonic machine gun can be fired for arbitrarily long times. Using realistic parameters for current quantum dot sources, we conclude high entangled-photon emission rates are achievable, with Pauli-error rates per photon of less than 0.2%. For quantum dot sources, the method has the added advantage of alleviating the problematic issues of obtaining identical photons from independent, nonidentical quantum dots, and of exciton dephasing.

Linear optical generation of multipartite entanglement with conventional photon detectors

A simple experimental scheme is proposed to generate a three-photon entangled $W$ state and a genuine four-photon entangled state $|{\ensuremath{\chi}}^{00}⟩$ that has many interesting entanglement properties and possible applications in quantum information processing. These multipartite entangled states can be generated with a certain success probability. We show that even when the quantum efficiency of the detector is $\ensuremath{\eta}=0.75$, the success probability can reach $2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ for $W$-state production and $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for $|{\ensuremath{\chi}}^{00}⟩$-state production. The scheme is based only on simple linear optical elements and conventional photon detectors, which greatly decrease the experimental difficulty of implementing linear optical quantum computation.

Spin-resolved quantum-dot resonance fluorescence

In the quest for physically realizable quantum information science (QIS) primitives, self-assembled quantum dots (QDs) serve a dual role as sources of photonic (flying) qubits and traps for electron spin; the prototypical stationary qubit. Here we demonstrate the first observation of spin-selective, near background-free and transform-limited photon emission from a resonantly driven QD transition. The hallmark of resonance fluorescence, i.e. the Mollow triplet in the scattered photon spectrum when an optical transition is driven resonantly, is presented as a natural way to spectrally isolate the photons of interest from the original driving field. We go on to demonstrate that the relative frequencies of the two spin-tagged photon states are tuned independent of an applied magnetic field via the spin-selective dynamic Stark effect induced by the very same driving laser. This demonstration enables the realization of challenging QIS proposals such as heralded single photon generation for linear optics quantum computing, spin-photon entanglement, and dipolar interaction mediated quantum logic gates. From a spectroscopy perspective, the spin-selective dynamic Stark effect tunes the QD spin-state splitting in the ground and excited states independently, thus enabling previously inaccessible regimes for controlled probing of mesoscopic spin systems.

Single-photon sources–an introduction

This review surveys the physical principles and recent developments in manufacturing single-photon sources. Special emphasis is placed on important potential applications such as linear optical quantum computing (LOQC), quantum key distribution (QKD) and quantum metrology that drive the development of these sources of single photons. We discuss the quantum-mechanical properties of light prepared in a quantum state of definite photon number and compare it with coherent light that shows a Poissonian distribution of photon numbers. We examine how the single-photon fidelity directly influences the ability to transmit secure quantum bits over a predefined distance. The theoretical description of modified spontaneous decay, the main principle behind single-photon generation, provides the background for many experimental implementations such as those using microresonators or pillar microcavities. The main alternative way to generate single photons using postselection of entangled photon pairs from parametric down-conversion, will be discussed. We concentrate on describing the underlying physical principles and we will point out limitations and open problems associated with single-photon production.

Adaptive Measurements in the Optical Quantum Information Laboratory

Adaptive techniques make practical many quantum measurements that would otherwise be beyond current laboratory capabilities. For example, they allow discrimination of nonorthogonal states with a probability of error equal to the Helstrom bound, measurement of the phase of a quantum oscillator with accuracy approaching (or in some cases attaining) the Heisenberg limit (HL), and estimation of phase in interferometry with a variance scaling at the HL, using only single qubit measurement and control. Each of these examples has close links with quantum information, in particular, experimental optical quantum information: the first is a basic quantum communication protocol; the second has potential application in linear optical quantum computing; the third uses an adaptive protocol inspired by the quantum phase estimation algorithm. We discuss each of these examples and their implementation in the laboratory, but concentrate upon the last, which was published most recently [Higgins <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Nature</i> , vol. 450, p. 393, 2007].

Design of high brightness entangled source based on periodically poled KTiOPO4 crystal

Entangled states are key resource for quantum information processing. In linear optics quantum computing，the most widely used entanglement resource is polarization entangled photon pairs generated via type-Ⅱ spontaneous parametric down-conversion. However，the brightness is not sufficient to perform quantum communication experiment between ground and satellite. In this paper we have developed a high-brightness entangled resource based on periodic polarization and quasi-phase marched periodically poled KTiOPO4 crystal. The number of generated entangled pairs is about 1.6×104s-1mW-1 and the visibility is 27∶1. Our source is with about one order of magnitude brighter than previous sources based on β-barium borate crystal. This leads to an immediate possibility for applications in the field of quantum key distribution，quantum teleportation and quantum computation. Particularly it would form a solid basis for future global quantum network.

Experimental Characterization of a Telecommunications-Band Quantum Controlled- not Gate

The quantum controlled-not gate is an example of the maximally entangling gate, which is a broad class of operations that are necessary for scalable linear optics quantum computation. Here, we characterize a telecommunications-wavelength (1550 nm) quantum controlled- not gate, and for the first time, experimentally bound its process fidelity by measuring its operation in two complementary polarization bases. The gate's final process fidelity F is given by 91% ? F ? 95%.

Generalized parity measurements

Measurements play an important role in quantum computing (QC), by either providing the nonlinearity required for two-qubit gates (linear optics QC), or by implementing a quantum algorithm using single-qubit measurements on a highly entangled initial state (cluster state QC). Parity measurements can be used as building blocks for preparing arbitrary stabilizer states, and, together with 1-qubit gates are universal for quantum computing. Here we generalize parity gates by using a higher dimensional (qudit) ancilla. This enables us to go beyond the stabilizer/graph state formalism and prepare other types of multi-particle entangled states. The generalized parity module introduced here can prepare in one-shot, heralded by the outcome of the ancilla, a large class of entangled states, including GHZ_n, W_n, Dicke states D_{n,k}, and, more generally, certain sums of Dicke states, like G_n states used in secret sharing. For W_n states it provides an exponential gain compared to linear optics based methods.

Demonstration of a scheme for the generation of “event-ready” entangled photon pairs from a single-photon source

We present a feasible and efficient scheme, and its proof-of-principle demonstration, of creating entangled photon pairs in an event-ready way using only simple linear optical elements and single photons. The quality of entangled photon pair produced in our experiment is confirmed by a strict violation of Bell's inequality. This scheme and the associated experimental techniques present an important step toward linear optics quantum computation.

Quantum cloning of a pair of orthogonally polarized photons with linear optics

A linear optical probabilistic scheme for the optimal cloning of a pair of orthogonally polarized photons is devised, based on single- and two-photon interferences. It consists in a partial symmetrization device realized with a modified unbalanced Mach-Zehnder interferometer, followed by two balanced beam splitters where the Hong-Ou-Mandel photon bunching occurs. This scheme has the advantage that it enables quantum cloning without the need for stimulated amplification in a nonlinear medium. It can also be modified so to make an optical two-qubit partial SWAP gate, thereby providing a potentially useful tool to linear optics quantum computing.

Demonstration of a Quantum Controlled-NOT Gate in the Telecommunications Band

We present the first quantum controlled-not (cnot) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 microm telecommunications band. Using this free-space cnot gate, all four Bell states are produced and fully characterized by performing quantum-state tomography, demonstrating the gate's unambiguous entangling capability and high fidelity. Telecom-band operation makes this cnot gate particularly suitable for quantum-information-processing tasks that are at the interface of quantum communication and linear optical quantum computing.

How Good Must Single Photon Sources and Detectors Be for Efficient Linear Optical Quantum Computation?

We present a scheme for linear optical quantum computation that is highly robust to imperfect single photon sources and inefficient detectors. In particular we show that if the product of the detector efficiency with the source efficiency is greater than 2/3, then efficient linear optical quantum computation is possible. This high threshold is achieved within the cluster state paradigm for quantum computation.

Loss-tolerant operations in parity-code linear optics quantum computing

A heavy focus for optical quantum computing is the introduction of error correction, and the minimization of resource requirements. We detail a complete encoding and manipulation scheme designed for circuit-model linear optics quantum computing, incorporating scalable operations and loss-tolerant architecture. We also calculate loss thresholds for the gate systems described, with an efficiency of $83%$ required for the single-qubit operations, and $90%$ for the two-qubit controlled-NOT gate.