Alberto Politi

Silica-on-Silicon Waveguide Quantum Circuits

Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization, and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8 ± 0.5%; a controlled-NOT gate with an average logical basis fidelity of 94.3 ± 0.2%; and a path-entangled state of two photons with fidelity of >92%. These results show that it is possible to directly “write” sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, including information processing, communication, metrology, and lithography, as well as the fundamental science of quantum optics.

Quantum Walks of Correlated Photons

A Correlated Quantum Walk Random walks are powerful tools for modeling statistical events. The analogous quantum walk involves particles tunneling between available sites. Peruzzo et al. (p. 1500; see the Perspective by Hillery) now report on the quantum walk of a correlated pair of photons propagating through a coupled waveguide array. The output pattern resulting from the injection of two correlated photons possess quantum features, indicating that the photons retain their correlations as they walk randomly through the waveguide array, allowing scale-up and parallel searches over many possible paths. Pairs of correlated photons retain their quantum-mechanical correlations as they propagate through a waveguide maze. Quantum walks of correlated particles offer the possibility of studying large-scale quantum interference; simulating biological, chemical, and physical systems; and providing a route to universal quantum computation. We have demonstrated quantum walks of two identical photons in an array of 21 continuously evanescently coupled waveguides in a SiOxNy chip. We observed quantum correlations, violating a classical limit by 76 standard deviations, and found that the correlations depended critically on the input state of the quantum walk. These results present a powerful approach to achieving quantum walks with correlated particles to encode information in an exponentially larger state space.

Shor's Quantum Factoring Algorithm on a Photonic Chip

A quantum algorithm to factor numbers is implemented on an optical chip. Shor’s quantum factoring algorithm finds the prime factors of a large number exponentially faster than any other known method, a task that lies at the heart of modern information security, particularly on the Internet. This algorithm requires a quantum computer, a device that harnesses the massive parellism afforded by quantum superposition and entanglement of quantum bits (or qubits). We report the demonstration of a compiled version of Shor’s algorithm on an integrated waveguide silica-on-silicon chip that guides four single-photon qubits through the computation to factor 15.

Manipulation of multiphoton entanglement in waveguide quantum circuits

Precise control of single-photon states and multiphoton entanglement is demonstrated on-chip. Two- and four-photon entangled states have now been generated in a waveguide circuit and their interference tuned. These results open up adaptive and reconfigurable photonic quantum circuits not just for single photons, but for all quantum states of light.

Laser written waveguide photonic quantum circuits

We report photonic quantum circuits created using an ultrafast laser processing technique that is rapid, requires no lithographic mask and can be used to create three-dimensional networks of waveguide devices. We have characterized directional couplers--the key functional elements of photonic quantum circuits--and found that they perform as well as lithographically produced waveguide devices. We further demonstrate high-performance interferometers and an important multi-photon quantum interference phenomenon for the first time in integrated optics. This direct-write approach will enable the rapid development of sophisticated quantum optical circuits and their scaling into three-dimensions.

Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit

Researchers demonstrate a reconfigurable integrated quantum photonic circuit. The device comprises a two-qubit entangling gate, several Hadamard-like gates and eight variable phase shifters. The set-up is used to generate entangled states, violate a Bell-type inequality with a continuum of partially entangled states and demonstrate the generation of arbitrary one-qubit mixed states.

Polytype control of spin qubits in silicon carbide.

Crystal defects can confine isolated electronic spins and are promising candidates for solid-state quantum information. Alongside research focusing on nitrogen-vacancy centres in diamond, an alternative strategy seeks to identify new spin systems with an expanded set of technological capabilities, a materials-driven approach that could ultimately lead to ‘designer’ spins with tailored properties. Here we show that the 4H, 6H and 3C polytypes of SiC all host coherent and optically addressable defect spin states, including states in all three with room-temperature quantum coherence. The prevalence of this spin coherence shows that crystal polymorphism can be a degree of freedom for engineering spin qubits. Long spin coherence times allow us to use double electron–electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Together with the distinct optical and spin transition energies of such inequivalent states, these interactions provide a route to dipole-coupled networks of separately addressable spins.

Multimode quantum interference of photons in multiport integrated devices

Photonics is a leading approach in realizing future quantum technologies and recently, optical waveguide circuits on silicon chips have demonstrated high levels of miniaturization and performance. Multimode interference (MMI) devices promise a straightforward implementation of compact and robust multiport circuits. Here, we show quantum interference in a 2×2 MMI coupler with visibility of V=95.6±0.9%. We further demonstrate the operation of a 4×4 port MMI device with photon pairs, which exhibits complex quantum interference behaviour. We have developed a new technique to fully characterize such multiport devices, which removes the need for phase-sensitive measurements and may find applications for a wide range of photonic devices. Our results show that MMI devices can operate in the quantum regime with high fidelity and promise substantial simplification and concatenation of photonic quantum circuits.

Direct evidence of light confinement and emission enhancement in active silicon-on-insulator slot waveguides

The authors experimentally demonstrate strong light confinement and enhancement of emission at 1.54μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO2 with Er3+ doped Si nanoclusters. Angle-resolved attenuated total reflectance is used to excite the slab guided modes, giving a direct evidence of the strong confinement of the electric field in the low-index active material for the fundamental transverse-magnetic mode. By measuring the guided photoluminescence from the cleaved-edge of the sample, the authors observe a more than fivefold enhancement of emission for the transverse-magnetic mode over the transverse-electric one. These results show that Si-based slot waveguides could be important as starting templates for the realization of Si-compatible active optical devices.

Observing fermionic statistics with photons in arbitrary processes

Quantum mechanics defines two classes of particles-bosons and fermions-whose exchange statistics fundamentally dictate quantum dynamics. Here we develop a scheme that uses entanglement to directly observe the correlated detection statistics of any number of fermions in any physical process. This approach relies on sending each of the entangled particles through identical copies of the process and by controlling a single phase parameter in the entangled state, the correlated detection statistics can be continuously tuned between bosonic and fermionic statistics. We implement this scheme via two entangled photons shared across the polarisation modes of a single photonic chip to directly mimic the fermion, boson and intermediate behaviour of two-particles undergoing a continuous time quantum walk. The ability to simulate fermions with photons is likely to have applications for verifying boson scattering and for observing particle correlations in analogue simulation using any physical platform that can prepare the entangled state prescribed here.