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We analyze how the performance of a quantum-repeater network depends on the protocol employed to distribute entanglement, and we find that the choice of repeater-to-repeater link protocol has a profound impact on entanglement-distribution rate as a function of hardware parameters. We develop numerical simulations of quantum networks using different protocols, where the repeater hardware is modeled in terms of key performance parameters, such as photon generation rate and collection efficiency. These parameters are motivated by recent experimental demonstrations in quantum dots, trapped ions, and nitrogen-vacancy centers in diamond. We find that a quantum-dot repeater with the newest protocol ('MidpointSource') delivers the highest entanglement-distribution rate for typical cases where there is low probability of establishing entanglement per transmission, and in some cases the rate is orders of magnitude higher than other schemes. Our simulation tools can be used to evaluate communication protocols as part of designing a large-scale quantum network.

Ultracold atoms can be used to sympathetically cool a membrane with a mass ten billion times larger than that of the atoms. Sympathetic cooling with ultracold atoms 1 and atomic ions 2 enables ultralow temperatures in systems where direct laser or evaporative cooling is not possible. It has so far been limited to the cooling of other microscopic particles, with masses up to 90 times larger than that of the coolant atom 3 . Here, we use ultracold atoms to sympathetically cool the vibrations of a Si 3 N 4 nanomembrane 4 , 5 , the mass of which exceeds that of the atomic ensemble by a factor of 10 10 . The coupling of atomic and membrane vibrations is mediated by laser light over a macroscopic distance 6 , 7 and is enhanced by placing the membrane in an optical cavity 8 . We observe cooling of the membrane vibrations from room temperature to 650 ± 230 mK, exploiting the large atom–membrane cooperativity 9 of our hybrid optomechanical system 10 , 11 . With technical improvements, our scheme could provide ground-state cooling and quantum control of low-frequency oscillators such as nanomembranes or levitated nanoparticles 12 , 13 , in a regime where purely optomechanical techniques cannot reach the ground state 8 , 9 .

Transducing non-classical states of light from one wavelength to another is required for integrating disparate quantum systems that take advantage of telecommunications-band photons for optical-fibre transmission of quantum information and near-visible, stationary systems for manipulation and storage. In addition, transducing a single-photon source at 1.3 µm to visible wavelengths would be integral to linear optical quantum computation because of near-infrared detection challenges. Recently, transduction at single-photon power levels has been accomplished through frequency upconversion, but it has yet to be demonstrated for a true single-photon source. Here, we transduce triggered single photons from a semiconductor quantum dot at 1.3 µm to 710 nm with 21% (75%) total detection (internal conversion) efficiency. We demonstrate that the upconverted signal maintains the quantum character of the original light, yielding a second-order intensity correlation, g (2) ( τ ), that shows that the optical field is composed of single photons with g (2) (0) = 0.165 < 0.5. Colour conversion of single photons may allow the advantages of quantum systems operating at different wavelengths to be simultaneously utilized. Researchers demonstrate the colour conversion of triggered single photons from a semiconductor quantum dot between 1.3 µm to 710 nm. The up-converted signal maintains the quantum character of the original light.

Quantum computation features known examples of hardware acceleration for certain problems, but is challenging to realize because of its susceptibility to small errors from noise or imperfect control. The principles of fault tolerance may enable computational acceleration with imperfect hardware, but they place strict requirements on the character and correlation of errors 1 . For many qubit technologies 2 – 21 , some challenges to achieving fault tolerance can be traced to correlated errors arising from the need to control qubits by injecting microwave energy matching qubit resonances. Here we demonstrate an alternative approach to quantum computation that uses energy-degenerate encoded qubit states controlled by nearest-neighbour contact interactions that partially swap the spin states of electrons with those of their neighbours. Calibrated sequences of such partial swaps, implemented using only voltage pulses, allow universal quantum control while bypassing microwave-associated correlated error sources 1 , 22 – 28 . We use an array of six 28 Si/SiGe quantum dots, built using a platform that is capable of extending in two dimensions following processes used in conventional microelectronics 29 . We quantify the operational fidelity of universal control of two encoded qubits using interleaved randomized benchmarking 30 , finding a fidelity of 96.3% ± 0.7% for encoded controlled NOT operations and 99.3% ± 0.5% for encoded SWAP. The quantum coherence offered by enriched silicon 5 – 9 , 16 , 18 , 20 , 22 , 27 , 29 , 31 – 37 , the all-electrical and low-crosstalk-control of partial swap operations 1 , 22 – 28 and the configurable insensitivity of our encoding to certain error sources 28 , 33 , 34 , 38 all combine to offer a strong pathway towards scalable fault tolerance and computational advantage. In this alternative approach to quantum computation, the all-electrical operation of two qubits, each encoded in three physical solid-state spin qubits, realizes swap-based universal quantum logic in an extensible physical architecture.

Quantum computation requires qubits that satisfy often-conflicting criteria, which include long-lasting coherence and scalable control1. One approach to creating a suitable qubit is to operate in an encoded subspace of several physical qubits. Although such encoded qubits may be particularly susceptible to leakage out of their computational subspace, they can be insensitive to certain noise processes2,3 and can also allow logical control with a single type of entangling interaction4 while maintaining favourable features of the underlying physical system. Here we demonstrate high-fidelity operation of an exchange-only qubit encoded in a subsystem of three coupled electron spins5 confined in gated, isotopically enhanced silicon quantum dots6. This encoding requires neither high-frequency electric nor magnetic fields for control, and instead relies exclusively on the exchange interaction4,5, which is highly local and can be modulated with a large on–off ratio using only fast voltage pulses. It is also compatible with very low and gradient-free magnetic field environments, which simplifies integration with superconducting materials. We developed and employed a modified blind randomized benchmarking protocol that determines both computational and leakage errors7,8, and found that unitary operations have an average total error of 0.35%, with half of that, 0.17%, coming from leakage driven by interactions with substrate nuclear spins. The combination of this proven performance with complete control via gate voltages makes the exchange-only qubit especially attractive for use in many-qubit systems.

Optoelectronic devices that provide non-classical light states on demand have a broad range of applications in quantum information science 1 , including quantum‐key‐distribution systems 2 , quantum lithography 3 and quantum computing 4 . Single-photon sources 5 , 6 in particular have been demonstrated to outperform key distribution based on attenuated classical laser pulses 7 . Implementations based on individual molecules 8 , nitrogen vacancy centres 9 or dopant atoms 10 are rather inefficient owing to low emission rates, rapid saturation and the lack of mature cavity technology. Promising single-photon-source designs combine high-quality microcavities 11 with quantum dots as active emitters 12 . So far, the highest measured single-photon rates are ∼ 200 kHz using etched micropillars 13 , 14 . Here, we demonstrate a quantum-dot-based single-photon source with a measured single-photon emission rate of 4.0 MHz (31 MHz into the first lens, with an extraction efficiency of 38%) due to the suppression of exciton dark states. Furthermore, our microcavity design provides mechanical stability, and voltage-controlled tuning of the emitter/mode resonance and of the polarization state.

An off day for computing Reset your perceptions for a foray into the quantum world. Counterfactual computation has been proposed as a logical consequence of quantum mechanics. Using appropriate algorithms, the theory goes, it should be possible to infer the outcome of a quantum computation without actually running the computer. Hosten et al . now report experimental confirmation that this does indeed happen. Their all-optical quantum computer was prepared in a superposition of interacting with and not interacting with an algorithm, and they obtained information about the result even when the photon did not interact with the algorithm. Surprisingly, the counterfactual approach worked better than randomly guessing the solution. It should be possible to use a similar approach in other systems, including the trapped ions popular in quantum computing architecture. The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run 1 . Relying on similar arguments to interaction-free measurements 2 (or quantum interrogation 3 ), counterfactual computation is accomplished by putting the computer in a superposition of ‘running’ and ‘not running’ states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach 4 . It was believed that the overall probability of such counterfactual inference is intrinsically limited 1 , 5 , so that it could not perform better on average than random guesses. However, using a novel ‘chained’ version of the quantum Zeno effect 6 , we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence.

By using a one-dimensional optical lattice to control and confine the location of cold 87 Rb atoms, researchers have created a distributed Bragg reflector that enables optical parametric oscillation solely from atoms.

This dissertation describes several quantum optics experiments that rely on the coupling between an atomic-like system and the confined optical modes of a cavity as described by cavity quantum electrodynamics (QED). The novelty of these experiments is that they are performed in the solid-state and as such are extremely interesting for applications of quantum information. These results have been obtained through a collaborative effort between the research groups of D. Bouwmeester in Physics, P. M. Petroff in Materials and ECE, L. A. Coldren in Materials and ECE, and E. L. Hu in ECE. The first set of these experiments explores this coupling in photonic crystal defect cavities. The first of these experiments shows how very few quantum dots can act as a sufficient gain medium to generate extremely low-threshold lasing. This surprising result, which arises due to the non-atomic-like nature of the quantum dots, is verified by a measurement of the photon statistical transition of the cavity mode. This is done for a series of such devices to elucidate the differences between macroscopic lasers and nanolasers. Next, a short experiment is discussed which uses the adsorption of material inside the cryostat to spectrally tune the resonance of a photonic crystal cavity. This section of the dissertation concludes with an experiment demonstrating an all-optical scheme to precisely determine the spacial location of a single quantum dot. Then, using this location, a high-quality photonic crystal cavity is fabricated, and strong coupling between the quantum and cavity is realized. The second set of experiments employs a novel, electrically-gated, oxide-apertured micropillar cavity to demonstrate a bright source of optically-generated single photons as well as electrically-generated single photons. Furthermore, the intra-cavity electric field generated by the gating of the structures enabled the demonstration of cavity QED with charged quantum dots, which has important ramifications for solid-state quantum information schemes that use the spin of an electron (or hole) for manipulation. Finally, the intra-cavity field is used to achieve spectral resonance between a quantum dot and a cavity mode by the Stark effect. This effect, in combination with a slightly elliptical micropillar, is used to demonstrate a polarization-switchable single photon source.