A sequence of single photons is emitted on demand from a single three-level atom strongly coupled to a high-finesse optical cavity. The photons are generated by an adiabatically driven stimulated Raman transition between two atomic ground states, with the vacuum field of the cavity stimulating one branch of the transition, and laser pulses deterministically driving the other branch. This process is unitary and therefore intrinsically reversible, which is essential for quantum communication and networking, and the photons should be appropriate for all-optical quantum information processing.

Reply on H. J. Kimble's comment on "Deterministic single-photon source for distributed quantum networking"

Silicon is evolving as a versatile photonic platform with multiple functionalities that can be seamlessly integrated. The tool box is rich starting from the ability to guide and amplify multiple wavelength sources at GHz bandwidths, to optomechanical MEMS. The strong confinement of light in ultra small structures also enables the generation of strong optical forces. We have recently shown that nonlinear optical forces can enable controllable manipulation of photonic structures. These advances should enable future micro-optomechanical systems (MOMS) with novel and distinct functionalities. A research area that recently has emerged is nonlinear optics using silicon photonics. Since the birth of nonlinear optics, researchers have continually focused on developing efficient nonlinear optical devices that require low optical powers. The strong light confinement in silicon waveguides results in a high effective nonlinearity ad enables fine tuning of waveguide dispersion which is essential for phase matching of parametric nonlinear optical processes such as four-wave-mixing (FWM) We demonstrated FWM-based frequency conversion in waveguides using as little as 1 mW of pump power in a ring-resonator geometry, and ∼100 mW of pump power over bandwidths exceeding 800 nm in a straight-waveguide device. In addition, by using the concept of time-space duality we have shown the temporally stretching and compressing of optical waveforms which allows for seamless transformation between the GHz and THz regimes.

Nonlinear silicon photonics

We performed an loophole-free test of Bells inequalities. The probability that local realism is compatible with our results is less than 5.9×10-9.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-020168.pdf

A Strong Loophole-Free Test of Local Realism

Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ions, atoms and atomic ensembles, which are then used to connect remote long-term memory nodes in distributed quantum networks. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks.

Quantum entanglement between an optical photon and a solid-state spin qubit

We review our work on ultrafast laser sources and frequency generation in high-index doped silica (Hydex) integrated structures.

CMOS compatible nonlinear optics based on Hydex

The exploitation of quantum mechanics has the potential to solve important challenges and
introduce novel technologies enabling, among others, powerful computation (which can solve
certain problems exponentially faster than classical computers), perfectly secure communications,
as well as sensing and imaging with precision and resolutions not achievable by classical means.
In order to make use of the unique properties of quantum physics, experimental platforms
that provide access to, and allow the control of quantum states, need to be developed. In
particular, quantum systems composed of multiple parties are required, where each party is in a
coherent superposition of states, and where multiples of such parties are in a coherent
superposition with each other (e.g. being entangled). Additionally, in order to make use of such
entangled quantum systems, it is required that each individual party can be coherently controlled
and measured. The usability and available quantum resources of the entangled system is related
to its size, which is proportional to the number of entangled parties, as well as the dimensionality
of the superposition each party is in. For this reason, there is a imminent demand to achieve,
investigate, and improve the generation and control of large quantum systems, comprised of as
many entangled parties as possible, where each party is in an ideally high-dimensional
superposition.
Many quantum platforms are currently subject to extensive research, such as trapped ions,
superconducting circuits, defect centers in solid-state crystals, mechanical oscillators, and optical
quantum states (i.e. photons). While all platforms provide distinct advantages (as well as
challenges), optical photon states are of particular interest, because they can interact with other
quantum systems, and can be transmitted over long distances while preserving their quantum
coherence without the need for extensive shielding. A large variety of sources for optical
quantum states has been demonstrated, however most such implementations suffer from high
complexity, ultimately limiting their scalability. Integrated photonics has recently become a
leading platform for the compact, cost-efficient, and stable generation and processing of optical
quantum states. However, to date, integrated quantum sources have only been able to generate
photon pairs, which operated on single channels, and single polarization modes. Furthermore, in
the few realizations of on-chip entanglement, they were limited to two entangled parties, where
each party was in a two-dimensional superposition. These limitations of current integrated photon sources arise in large part from the concepts currently exploited to generate and control these
states. In particular, path-entangled sources are the most commonly used, but increasing the
quantum state complexity requires a significant, and very challenging, increase in device
complexity. There is therefore an urgent need to develop new concepts that can achieve the onchip
generation of large and complex photon states without increasing source complexity, while
still enabling coherent quantum state control and detection.
In this work, we aim to address these limitations by making use of multiple frequency and
temporal modes within a single spatial waveguide mode (therefore readily compatible with
standard optical fibers) of an on-chip microring resonator. This approach is in contrast to more
commonly used techniques relying on path entanglement, which exploits multiple spatial modes
and generates entanglement by means of complex waveguide structures. Working within a single
spatial mode, we achieve the first demonstration of pure heralded single photons and entangled
photon pairs, which are emitted on multiple frequency modes from a single source. We then
demonstrate the first generation of multi-photon entangled quantum states on a photonic chip.
Finally, by making use of the spectral multi-mode nature of optical frequency combs, we achieve
the generation of photon pairs in a coherent superposition of multiple frequency modes,
representing the first generation of high-dimensional entangled states on a photonic chip.
Our work represents a significant step forward in the generation of complex quantum
states on a photonic chip and in a single spatial mode compatible with standard optical fibers. In
addition, we demonstrated coherent quantum state control and manipulation for all generated
states by means of standard telecommunications components, which could in the future be also
integrated on single, monolithic chip. Our results indicate that the exploitation of integrated
optical frequency comb sources for the generation of optical quantum states can provide a
scalable and practical platform for quantum technologies.

Optical quantum state generation with integrated frequency comb sources.

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Optical quantum state generation with integrated frequency comb sources.