38 research outputs found
Adaptive homodyne phase discrimination and qubit measurement
Fast and accurate measurement is a highly desirable, if not vital, feature of
quantum computing architectures. In this work we investigate the usefulness of
adaptive measurements in improving the speed and accuracy of qubit measurement.
We examine a particular class of quantum computing architectures, ones based on
qubits coupled to well controlled harmonic oscillator modes (reminiscent of
cavity-QED), where adaptive schemes for measurement are particularly
appropriate. In such architectures, qubit measurement is equivalent to phase
discrimination for a mode of the electromagnetic field, and we examine adaptive
techniques for doing this. In the final section we present a concrete example
of applying adaptive measurement to the particularly well-developed circuit-QED
architecture.Comment: 9 pages, 8 figures. Published versio
Multidimensional cluster states using a single spin-photon interface coupled strongly to an intrinsic nuclear register
Photonic cluster states are a powerful resource for measurement-based quantum
computing and loss-tolerant quantum communication. Proposals to generate
multi-dimensional lattice cluster states have identified coupled spin-photon
interfaces, spin-ancilla systems, and optical feedback mechanisms as potential
schemes. Following these, we propose the generation of multi-dimensional
lattice cluster states using a single, efficient spin-photon interface coupled
strongly to a nuclear register. Our scheme makes use of the contact hyperfine
interaction to enable universal quantum gates between the interface spin and a
local nuclear register and funnels the resulting entanglement to photons via
the spin-photon interface. Among several quantum emitters, we identify the
silicon-29 vacancy centre in diamond, coupled to a nanophotonic structure, as
possessing the right combination of optical quality and spin coherence for this
scheme. We show numerically that using this system a 2x5-sized cluster state
with a lower-bound fidelity of 0.5 and repetition rate of 65 kHz is achievable
under currently realised experimental performances and with feasible technical
overhead. Realistic gate improvements put 100-photon cluster states within
experimental reach
All-optical formation of coherent dark states of silicon-vacancy spins in diamond
Spin impurities in diamond can be versatile tools for a wide range of
solid-state-based quantum technologies, but finding spin impurities which offer
sufficient quality in both photonic and spin properties remains a challenge for
this pursuit. The silicon-vacancy center has recently attracted a lot of
interest due to its spin-accessible optical transitions and the quality of its
optical spectrum. Complementing these properties, spin coherence is essential
for the suitability of this center as a spin-photon quantum interface. Here, we
report all-optical generation of coherent superpositions of spin states in the
ground state of a negatively charged silicon-vacancy center using coherent
population trapping. Our measurements reveal a characteristic spin coherence
time, T2*, exceeding 250 nanoseconds at 4 K. We further investigate the role of
phonon-mediated coupling between orbital states as a source of irreversible
decoherence. Our results indicate the feasibility of all-optical coherent
control of silicon-vacancy spins using ultrafast laser pulses.Comment: Additional data and analysis is available for download in PDF format
at the publications section of http://www.amop.phy.cam.ac.uk/amop-m
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Optical signatures of silicon-vacancy spins in diamond
Colour centres in diamond have emerged as versatile tools for solid-state quantum technologies ranging from quantum information to metrology, where the nitrogen-vacancy centre is the most studied to date. Recently, this toolbox has expanded to include novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with highly desirable photonic properties. The challenge for utilizing this centre is to realize the hitherto elusive optical access to its electronic spin. Here we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. Our measurements reveal a spin-state purity approaching unity in the excited state, highlighting the potential of the centre as an efficient spin-photon quantum interface
Charge-carrier complexes in monolayer semiconductors
The photoluminescence (PL) spectra of monolayer (1L) semiconductors feature
peaks ascribed to different charge-carrier complexes. We perform diffusion
quantum Monte Carlo simulations of the binding energies of these complexes and
examine their response to electric and magnetic fields. We focus on quintons
(charged biexcitons), since they are the largest free charge-carrier complexes
in transition-metal dichalcogenides (TMDs). We examine the accuracy of the
Rytova-Keldysh interaction potential between charges by comparing the binding
energies of charge-carrier complexes in 1L-TMDs with results obtained using
interaction potentials. Magnetic fieldsT change the
binding energies (BEs) by meV,T, in agreement with experiments,
with the BE variations of different complexes being very similar. Our results
will help identify charge complexes in the PL spectra of 1L-semiconductor
Ultrafast optical control of entanglement between two quantum dot spins
The interaction between two quantum bits enables entanglement, the
two-particle correlations that are at the heart of quantum information science.
In semiconductor quantum dots much work has focused on demonstrating single
spin qubit control using optical techniques. However, optical control of
entanglement of two spin qubits remains a major challenge for scaling from a
single qubit to a full-fledged quantum information platform. Here, we combine
advances in vertically-stacked quantum dots with ultrafast laser techniques to
achieve optical control of the entangled state of two electron spins. Each
electron is in a separate InAs quantum dot, and the spins interact through
tunneling, where the tunneling rate determines how rapidly entangling
operations can be performed. The two-qubit gate speeds achieved here are over
an order of magnitude faster than in other systems. These results demonstrate
the viability and advantages of optically controlled quantum dot spins for
multi-qubit systems.Comment: 24 pages, 5 figure
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Coherent control of the silicon-vacancy spin in diamond
Spin impurities in diamond have emerged as a promising building block in a wide range of solid-state-based quantum technologies. The negatively charged silicon-vacancy centre combines the advantages of its high-quality photonic properties with a ground-state electronic spin, which can be read out optically. However, for this spin to be operational as a quantum bit, full quantum control is essential. Here we report the measurement of optically detected magnetic resonance and the demonstration of coherent control of a single silicon-vacancy centre spin with a microwave field. Using Ramsey interferometry, we directly measure a spin coherence time, T2*, of 115±9 ns at 3.6 K. The temperature dependence of coherence times indicates that dephasing and decay of the spin arise from single-phonon-mediated excitation between orbital branches of the ground state. Our results enable the silicon-vacancy centre spin to become a controllable resource to establish spin-photon quantum interfaces.We acknowledge financial support by the University of Cambridge, the ERC Grant PHOENICS, FP7 Marie Curie Initial Training Networks S3NANO and SPIN NANO, and the NQIT programme. This research has been partially funded by the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant agreement no. 611143 (DIADEMS). B.P. thanks Wolfson College (Cambridge) for support through a Research Fellowship
Tunable Indistinguishable Photons From Remote Quantum Dots
Single semiconductor quantum dots have been widely studied within devices
that can apply an electric field. In the most common system, the low energy
offset between the InGaAs quantum dot and the surrounding GaAs material limits
the magnitude of field that can be applied to tens of kVcm^-1, before carriers
tunnel out of the dot. The Stark shift experienced by the emission line is
typically 1 meV. We report that by embedding the quantum dots in a quantum well
heterostructure the vertical field that can be applied is increased by over an
order of magnitude whilst preserving the narrow linewidths, high internal
quantum efficiencies and familiar emission spectra. Individual dots can then be
continuously tuned to the same energy allowing for two-photon interference
between remote, independent, quantum dots