arXiv subjects

Ronald Hanson

Publications and source records attributed to Ronald Hanson.

53 records · Page 3Linked to original sources

arXiv2017

Multiplexed entanglement generation over quantum networks using multi-qubit nodes

Quantum networks distributed over distances greater than a few kilometers will be limited by the time required for information to propagate between nodes. We analyze protocols that are able to circumvent this bottleneck by employing multi-qubit nodes and multiplexing. For each protocol, we investigate the key network parameters that determine its performance. We model achievable entangling rates based on the anticipated near-term performance of nitrogen-vacancy centres and other promising network platforms. This analysis allows us to compare the potential of the proposed multiplexed protocols in different regimes. Moreover, by identifying the gains that may be achieved by improving particular network parameters, our analysis suggests the most promising avenues for research and development of prototype quantum networks.

quant-ph↗

arXiv2017

Entanglement Distillation between Solid-State Quantum Network Nodes

The potential impact of future quantum networks hinges on high-quality quantum entanglement shared between network nodes. Unavoidable real-world imperfections necessitate means to improve remote entanglement by local quantum operations. Here we realize entanglement distillation on a quantum network primitive of distant electron-nuclear two-qubit nodes. We demonstrate the heralded generation of two copies of a remote entangled state through single-photon-mediated entangling of the electrons and robust storage in the nuclear spins. After applying local two-qubit gates, single-shot measurements herald the distillation of an entangled state with increased fidelity that is available for further use. In addition, this distillation protocol significantly speeds up entanglement generation compared to previous two-photon-mediated schemes. The key combination of generating, storing and processing entangled states demonstrated here opens the door to exploring and utilizing multi-particle entanglement on an extended quantum network.

quant-ph↗

arXiv2016

Repeated quantum error correction on a continuously encoded qubit by real-time feedback

Reliable quantum information processing in the face of errors is a major fundamental and technological challenge. Quantum error correction protects quantum states by encoding a logical quantum bit (qubit) in multiple physical qubits. To be compatible with universal fault-tolerant computations, it is essential that the states remain encoded at all times and that errors are actively corrected. Here we demonstrate such active error correction on a continuously protected qubit using a diamond quantum processor. We encode a logical qubit in three long-lived nuclear spins, repeatedly detect phase errors by non-destructive measurements using an ancilla electron spin, and apply corrections on the encoded state by real-time feedback. The actively error-corrected qubit is robust against errors and multiple rounds of error correction prevent errors from accumulating. Moreover, by correcting correlated phase errors naturally induced by the environment, we demonstrate that encoded quantum superposition states are preserved beyond the dephasing time of the best physical qubit used in the encoding. These results establish a powerful platform for the fundamental investigation of error correction under different types of noise and mark an important step towards fault-tolerant quantum information processing.

quant-ph↗

arXiv2016

Quantum Zeno subspaces by repeated multi-spin projections

Repeated observations inhibit the coherent evolution of quantum states through the quantum Zeno effect. In multi-qubit systems this effect provides new opportunities to control complex quantum states. Here, we experimentally demonstrate that repeatedly projecting joint observables of multiple spins creates coherent quantum Zeno subspaces and simultaneously suppresses dephasing caused by the environment. We encode up to two logical qubits in these subspaces and show that the enhancement of the dephasing time with increasing number of projections follows a scaling law that is independent of the number of spins involved. These results provide new insights into the interplay between frequent multi-spin measurements and non-Markovian noise and pave the way for tailoring the dynamics of multi-qubit systems through repeated projections.

quant-ph↗

arXiv2016

Robust quantum-network memory using decoherence-protected subspaces of nuclear spins

The realization of a network of quantum registers is an outstanding challenge in quantum science and technology. We experimentally investigate a network node that consists of a single nitrogen-vacancy (NV) center electronic spin hyperfine-coupled to nearby nuclear spins. We demonstrate individual control and readout of five nuclear spin qubits within one node. We then characterize the storage of quantum superpositions in individual nuclear spins under repeated application of a probabilistic optical inter-node entangling protocol. We find that the storage fidelity is limited by dephasing during the electronic spin reset after failed attempts. By encoding quantum states into a decoherence-protected subspace of two nuclear spins we show that quantum coherence can be maintained for over 1000 repetitions of the remote entangling protocol. These results and insights pave the way towards remote entanglement purification and the realisation of a quantum repeater using NV center quantum network nodes.

quant-ph↗

arXiv2015

Optimized quantum sensing with a single electron spin using real-time adaptive measurements

Quantum sensors based on single solid-state spins promise a unique combination of sensitivity and spatial resolution. The key challenge in sensing is to achieve minimum estimation uncertainty within a given time and with a high dynamic range. Adaptive strategies have been proposed to achieve optimal performance but their implementation in solid-state systems has been hindered by the demanding experimental requirements. Here we realize adaptive d.c. sensing by combining single-shot readout of an electron spin in diamond with fast feedback. By adapting the spin readout basis in real time based on previous outcomes we demonstrate a sensitivity in Ramsey interferometry surpassing the standard measurement limit. Furthermore, we find by simulations and experiments that adaptive protocols offer a distinctive advantage over the best-known non-adaptive protocols when overhead and limited estimation time are taken into account. Using an optimized adaptive protocol we achieve a magnetic field sensitivity of $6.1 \pm 1.7$ nT *Hz$^{-1/2}$ over a wide range of 1.78 mT. These results open up a new class of experiments for solid-state sensors in which real-time knowledge of the measurement history is exploited to obtain optimal performance.

quant-ph↗

arXiv2014

Unconditional quantum teleportation between distant solid-state qubits

Realizing robust quantum information transfer between long-lived qubit registers is a key challenge for quantum information science and technology. Here we demonstrate unconditional teleportation of arbitrary quantum states between diamond spin qubits separated by 3 meters. We prepare the teleporter through photon-mediated heralded entanglement between two distant electron spins and subsequently encode the source qubit in a single nuclear spin. By realizing a fully deterministic Bell-state measurement combined with real-time feed-forward we achieve teleportation in each attempt while obtaining an average state fidelity exceeding the classical limit. These results establish diamond spin qubits as a prime candidate for the realization of quantum networks for quantum communication and network-based quantum computing.

quant-ph↗

arXiv2013

Top-down fabrication of plasmonic nanostructures for deterministic coupling to single quantum emitters

Metal nanostructures can be used to harvest and guide the emission of single photon emitters on-chip via surface plasmon polaritons. In order to develop and characterize photonic devices based on emitter-plasmon hybrid structures a deterministic and scalable fabrication method for such structures is desirable. Here we demonstrate deterministic and scalable top-down fabrication of metal wires onto preselected nitrogen vacancy centers in nanodiamonds using clean room nano-fabrication methods. We observe a life-time reduction of the emitter emission that is consistent with earlier proof-of-principle experiments that used non-deterministic fabrication methods. This result indicates that top-down fabrication is a promising technique for processing future devices featuring single photon emitters and plasmonic nanostructures.

cond-mat.mes-hall↗

arXiv2013

High-fidelity projective readout of a solid-state spin quantum register

Initialization and readout of coupled quantum systems are essential ingredients for the implementation of quantum algorithms. If the state of a multi-qubit register can be read out in a single shot, this enables further key resources such as quantum error correction and deterministic quantum teleportation, as well as direct investigation of quantum correlations (entanglement). While spins in solids are attractive candidates for scalable quantum information processing, thus far single-shot detection has only been achieved for isolated qubits. Here, we demonstrate preparation and measurement of a multi-spin quantum register by implementing resonant optical excitation techniques originally developed in atomic physics. We achieve high-fidelity readout of the electronic spin associated with a single nitrogen-vacancy (NV) centre in diamond at low temperature, and exploit this readout to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we can distinguish the state of the nuclear spins in a single shot by mapping it onto and subsequently measuring the electronic spin. Finally, we show compatibility with qubit control by demonstrating initialization, coherent manipulation, and single-shot readout in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. These results pave the way for the first test of Bell's inequalities on solid-state spins and the implementation of measurement-based quantum information protocols.

quant-ph↗

arXiv2012

Demonstration of entanglement-by-measurement of solid state qubits

Projective measurements are a powerful tool for manipulating quantum states. In particular, a set of qubits can be entangled by measurement of a joint property such as qubit parity. These joint measurements do not require a direct interaction between qubits and therefore provide a unique resource for quantum information processing with well-isolated qubits. Numerous schemes for entanglement-by-measurement of solid-state qubits have been proposed, but the demanding experimental requirements have so far hindered implementations. Here we realize a two-qubit parity measurement on nuclear spins in diamond by exploiting the electron spin of a nitrogen-vacancy center as readout ancilla. The measurement enables us to project the initially uncorrelated nuclear spins into maximally entangled states. By combining this entanglement with high-fidelity single-shot readout we demonstrate the first violation of Bells inequality with solid-state spins. These results open the door to a new class of experiments in which projective measurements are used to create, protect and manipulate entanglement between solid-state qubits.

quant-ph↗

arXiv2012

Opening up the Quantum Three-Box Problem with Undetectable Measurements

One of the most striking features of quantum mechanics is the profound effect exerted by measurements alone. Sophisticated quantum control is now available in several experimental systems, exposing discrepancies between quantum and classical mechanics whenever measurement induces disturbance of the interrogated system. In practice, such discrepancies may frequently be explained as the back-action required by quantum mechanics adding quantum noise to a classical signal. Here we implement the 'three-box' quantum game of Aharonov and Vaidman in which quantum measurements add no detectable noise to a classical signal, by utilising state-of-the-art control and measurement of the nitrogen vacancy centre in diamond. Quantum and classical mechanics then make contradictory predictions for the same experimental procedure, however classical observers cannot invoke measurement-induced disturbance to explain this discrepancy. We quantify the residual disturbance of our measurements and obtain data that rule out any classical model by > 7.8 standard deviations, allowing us for the first time to exclude the property of macroscopic state-definiteness from our system. Our experiment is then equivalent to a Kochen-Spekker test of quantum non-contextuality that successfully addresses the measurement detectability loophole.

quant-ph↗

arXiv2012

Two-photon quantum interference from separate nitrogen vacancy centers in diamond

We report on the observation of quantum interference of the emission from two separate nitrogen vacancy (NV) centers in diamond. Taking advantage of optically induced spin polarization in combination with polarization filtering, we isolate a single transition within the zero-phonon line of the non-resonantly excited NV centers. The time-resolved two-photon interference contrast of this filtered emission reaches 66%. Furthermore, we observe quantum interference from dissimilar NV centers tuned into resonance through the dc Stark effect. These results pave the way towards measurement-based entanglement between remote NV centers and the realization of quantum networks with solid-state spins.

quant-ph↗

arXiv2010

Spin dynamics in the optical cycle of single nitrogen-vacancy centres in diamond

We investigate spin-dependent decay and intersystem crossing in the optical cycle of single negatively-charged nitrogen-vacancy (NV) centres in diamond. We use spin control and pulsed optical excitation to extract both the spin-resolved lifetimes of the excited states and the degree of optically-induced spin polarization. By optically exciting the centre with a series of picosecond pulses, we determine the spin-flip probabilities per optical cycle, as well as the spin-dependent probability for intersystem crossing. This information, together with the indepedently measured decay rate of singlet population provides a full description of spin dynamics in the optical cycle of NV centres. The temperature dependence of the singlet population decay rate provides information on the number of singlet states involved in the optical cycle.

quant-ph↗

arXiv2010

Control and coherence of the optical transition of single defect centers in diamond

We demonstrate coherent control of the optical transition of single Nitrogen-Vacancy defect centers in diamond. On applying short resonant laser pulses, we observe optical Rabi oscillations with a half-period as short as 1 nanosecond, an order of magnitude shorter than the spontaneous emission time. By studying the decay of Rabi oscillations, we find that the decoherence is dominated by laser-induced spectral jumps. By using a low-power probe pulse as a detuning sensor and applying post-selection, we demonstrate that spectral diffusion can be overcome in this system to generate coherent photons.

quant-ph↗

arXiv2008

Quenching Spin Decoherence in Diamond through Spin Bath Polarization

We experimentally demonstrate that the decoherence of a spin by a spin bath can be completely eliminated by fully polarizing the spin bath. We use electron paramagnetic resonance at 240 gigahertz and 8 Tesla to study the spin coherence time $T_2$ of nitrogen-vacancy centers and nitrogen impurities in diamond from room temperature down to 1.3 K. A sharp increase of $T_2$ is observed below the Zeeman energy (11.5 K). The data are well described by a suppression of the flip-flop induced spin bath fluctuations due to thermal spin polarization. $T_2$ saturates at $\sim 250 μs$ below 2 K, where the spin bath polarization is 99.4 %.

quant-ph↗

arXiv2007

Fabrication and Characterization of Two-Dimensional Photonic Crystal Microcavities in Nanocrystalline Diamond

Diamond-based photonic devices offer exceptional opportunity to study cavity QED at room temperature. Here we report fabrication and optical characterization of high quality photonic crystal (PC) microcavities based on nanocrystalline diamond. Fundamental modes near the emission wavelength of negatively charged nitrogen-vacancy (N-V) centers (637 nm) with quality factors (Qs) as high as 585 were observed. Three-dimensional Finite-Difference Time-Domain (FDTD) simulations were carried out and had excellent agreement with experimental results in the values of the mode frequencies. Polarization measurements of the modes were characterized; their anomalous behavior provides important insights to scattering loss in these structures.

cond-mat.mtrl-sci↗

arXiv2007

Universal set of quantum gates for double-dot spin qubits with fixed interdot coupling

We propose a set of universal gate operations for the singlet-triplet qubit realized by two electron spins in a double quantum dot, in the presence of a fixed inhomogeneous magnetic field. All gate operations are achieved by switching the potential offset between the two dots with an electrical bias, and do not require time-dependent control of the tunnel coupling between the dots. We analyze the two-electron dynamics and calculate the effective qubit rotation angle as a function of the applied electric bias. We present explicit gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT gate sequence, completing the universal gate set.

cond-mat.mes-hall↗