arXiv subjects

M. A. Broome

Publications and source records attributed to M. A. Broome.

13 recordsLinked to original sources

arXiv2019

Experimental distribution of entanglement with separable carriers

The key requirement for quantum networking is the distribution of entanglement between nodes. Surprisingly, entanglement can be generated across a network without direct transfer - or communication - of entanglement. In contrast to information gain, which cannot exceed the communicated information, the entanglement gain is bounded by the communicated quantum discord, a more general measure of quantum correlation that includes but is not limited to entanglement. Here, we experimentally entangle two communicating parties sharing three initially separable photonic qubits by exchange of a carrier photon that is unentangled with either party at all times. We show that distributing entanglement with separable carriers is resilient to noise and in some cases becomes the only way of distributing entanglement through noisy environments.

quant-ph↗

arXiv2019

Benchmarking high fidelity single-shot readout of semiconductor qubits

Determination of qubit initialisation and measurement fidelity is important for the overall performance of a quantum computer. However, the method by which it is calculated in semiconductor qubits varies between experiments. In this paper we present a full theoretical analysis of electronic single-shot readout and describe critical parameters to achieve high fidelity readout. In particular, we derive a model for energy selective state readout based on a charge detector response and examine how to optimise the fidelity by choosing correct experimental parameters. Although we focus on single electron spin readout, the theory presented can be applied to other electronic readout techniques in semiconductors that use a reservoir.

quant-ph↗

arXiv2018

Two-Electron Spin Correlations in Precision Placed Donors in Silicon

Substitutional donor atoms in silicon are promising qubits for quantum computation with extremely long relaxation and dephasing times demonstrated. One of the critical challenges of scaling these systems is determining inter-donor distances to achieve controllable wavefunction overlap while at the same time performing high fidelity spin readout on each qubit. Here we achieve such a device by means of scanning tunnelling microscopy lithography. We measure anti-correlated spin states between two donor-based spin qubits in silicon separated by 16${\pm}1$ nm. By utilizing an asymmetric system with two phosphorus donors at one qubit site and one on the other (2P-1P), we demonstrate that the exchange interaction can be turned on and off via electrical control of two in-plane phosphorus doped detuning gates. We determine the tunnel coupling between the 2P-1P system to be 200 MHz and provide a roadmap for the observation of two-electron coherent exchange oscillations.

cond-mat.mes-hall↗

arXiv2018

Singlet-triplet minus mixing and relaxation lifetimes in a double donor dot

We measure singlet-triplet mixing in a precision fabricated double donor dot comprising of 2 and 1 phosphorus atoms separated by $16{\pm}1$ nm. We identify singlet and triplet-minus states by performing sequential independent spin readout of the two electron system and probe its dependence on magnetic field strength. The relaxation of singlet and triplet states are measured to be $12.4{\pm}1.0$ s and $22.1{\pm}1.0$ s respectively at $B_z{=}2.5$ T.

cond-mat.mes-hall↗

arXiv2018

High Fidelity Single-Shot Singlet-Triplet Readout of Precision Placed Donors in Silicon

In this work we perform direct single-shot readout of the singlet-triplet states in exchange coupled electrons confined to precision placed donor atoms in silicon. Our method takes advantage of the large energy splitting given by the Pauli-spin blockaded (2,0) triplet states, from which we can achieve a single-shot readout fidelity of 98.4$\pm$0.2%. We measure the triplet-minus relaxation time to be of the order 3s at 2.5T and observe its predicted decrease as a function of magnetic field, reaching 0.5s at 1T.

cond-mat.mes-hall↗

arXiv2017

BosonSampling with single-photon Fock states from a bright solid-state source

A BosonSampling device is a quantum machine expected to perform tasks intractable for a classical computer, yet requiring minimal non-classical resources as compared to full-scale quantum computers. Photonic implementations to date employed sources based on inefficient processes that only simulate heralded single-photon statistics when strongly reducing emission probabilities. BosonSampling with only single-photon input has thus never been realised. Here, we report on a BosonSampling device operated with a bright solid-state source of single-photon Fock states with high photon-number purity: the emission from an efficient and deterministic quantum dot-micropillar system is demultiplexed into three partially-indistinguishable single-photons, with a single-photon purity $1{-}g^{(2)}(0)$ of $0.990{\pm}0.001$, interfering in a linear optics network. Our demultiplexed source is between one and two orders-of-magnitude more efficient than current heralded multi-photon sources based on spontaneous parametric downconversion, allowing us to complete the BosonSampling experiment faster than previous equivalent implementations.

quant-ph↗

arXiv2016

Mapping the Chemical Potential Landscape of a Triple Quantum Dot

We investigate the non-equilibrium charge dynamics of a triple quantum dot and demonstrate how electron transport through these systems can give rise to non-trivial tunnelling paths. Using a real-time charge sensing method we establish tunnelling pathways taken by particular electrons under well-defined electrostatic configurations. We show how these measurements map to the chemical potentials for different charge states across the system. We use a modified Hubbard Hamiltonian to describe the system dynamics and show that it reproduces all experimental observations.

cond-mat.mes-hall↗

arXiv2016

Scalable performance in solid-state single-photon sources

The desiderata for an ideal photon source are high brightness, high single-photon purity, and high indistinguishability. Defining brightness at the first collection lens, these properties have been simultaneously demonstrated with solid-state sources, however absolute source efficiencies remain close to the 1% level, and indistinguishability only demonstrated for photons emitted consecutively on the few nanosecond scale. Here we employ deterministic quantum dot-micropillar devices to demonstrate solid-state single-photon sources with scalable performance. In one device, an absolute brightness at the output of a single-mode fibre of 14% and purities of 97.1-99.0% are demonstrated. When non-resontantly excited, it emits a long stream of photons that exhibit indistinguishability up to 70%---above the classical limit of 50%---even after 33 consecutively emitted photons, a 400 ns separation between them. Resonant excitation in other devices results in near-optimal indistinguishability values: 96% at short timescales, remaining at 88% in timescales as large as 463 ns, after 39 emitted photons. The performance attained by our devices brings solid-state sources into a regime suitable for scalable implementations.

quant-ph↗

arXiv2015

Observation of Entanglement-Dependent Two-Particle Holonomic Phase

Holonomic phases---geometric and topological---have long been an intriguing aspect of physics. They are ubiquitous, ranging from observations in particle physics to applications in fault tolerant quantum computing. However, their exploration in particles sharing genuine quantum correlations lack in observations. Here we experimentally demonstrate the holonomic phase of two entangled-photons evolving locally, which nevertheless gives rise to an entanglement-dependent phase. We observe its transition from geometric to topological as the entanglement between the particles is tuned from zero to maximal, and find this phase to behave more resilient to evolution changes with increasing entanglement. Furthermore, we theoretically show that holonomic phases can directly quantify the amount of quantum correlations between the two particles. Our results open up a new avenue for observations of holonomic phenomena in multi-particle entangled quantum systems.

quant-ph↗

arXiv2015

Engineering integrated photonics for heralded quantum gates

Scaling up linear-optics quantum computing will require multi-photon gates which are compact, phase-stable, exhibit excellent quantum interference, and have success heralded by the detection of ancillary photons. We investigate implementation of the optimal known gate design which meets these requirements: the Knill controlled-Z gate, implemented in integrated laser-written waveguide arrays. We show that device performance is more sensitive to the small deviations in the coupler reflectivity, arising due to the tolerance values of the fabrication method, than phase variations in the circuit. The mode fidelity was also shown to be less sensitive to reflectivity and phase errors than process fidelity. Our best device achieves a fidelity of 0.931+/-0.001 with the ideal 4x4 unitary circuit and a process fidelity of 0.680+/-0.005 with the ideal computational-basis process.

quant-ph↗

arXiv2011

Two-photon quantum walks in an elliptical direct-write waveguide array

Integrated optics provides an ideal test bed for the emulation of quantum systems via continuous-time quantum walks. Here we study the evolution of two-photon states in an elliptic array of waveguides. We characterise the photonic chip via coherent-light tomography and use the results to predict distinct differences between temporally indistinguishable and distinguishable two-photon inputs which we then compare with experimental observations. Our work highlights the feasibility for emulation of coherent quantum phenomena in three-dimensional waveguide structures.

quant-ph↗

arXiv2010

Efficient measurement of quantum dynamics via compressive sensing

The resources required to characterise the dynamics of engineered quantum systems-such as quantum computers and quantum sensors-grow exponentially with system size. Here we adapt techniques from compressive sensing to exponentially reduce the experimental configurations required for quantum process tomography. Our method is applicable to dynamical processes that are known to be nearly-sparse in a certain basis and it can be implemented using only single-body preparations and measurements. We perform efficient, high-fidelity estimation of process matrices on an experiment attempting to implement a photonic two-qubit logic-gate. The data base is obtained under various decoherence strengths. We find that our technique is both accurate and noise robust, thus removing a key roadblock to the development and scaling of quantum technologies.

quant-ph↗

arXiv2010

Discrete single-photon quantum walks with tunable decoherence

Quantum walks have a host of applications, ranging from quantum computing to the simulation of biological systems. We present an intrinsically stable, deterministic implementation of discrete quantum walks with single photons in space. The number of optical elements required scales linearly with the number of steps. We measure walks with up to 6 steps and explore the quantum-to-classical transition by introducing tunable decoherence. Finally, we also investigate the effect of absorbing boundaries and show that decoherence significantly affects the probability of absorption.

quant-ph↗