Session M37: Focus Session: Quantum Optics and Photonic Architectures

Demonstration of Backward-wave Parametric Down-Conversion

Ultrabright sources of temporally long and spectrally narrow photons are essential for efficient light-matter interaction at the single-photon level. To achieve high brightness, parametric down-conversion of the backward-wave type was proposed for single-mode generation of long biphotons [1]. In this talk I will describe the demonstration of backward-wave parametric down-conversion in a nonlinear crystal, of which the time-energy entanglement was characterized by the Franson interference. I will also discuss the possibility of realizing a miniature ultrabright biphoton source.\\[4pt] [1] C.-S. Chuu and S. E. Harris, Phys. Rev. A 83, 061803(R) (2011)   

Nanomechanical single-qubit gates and iSWAP gate of single-electron spins in a carbon nanotube

A universal gate set for quantum computation can be built with one-qubit and iSWAP gates. We theoretically investigate mechanically-induced single-electron spin resonance in a quantum dot [1] and a phonon mediated iSWAP gate of two separate single electron spins in two quantum dots on a suspended carbon nanotube which is driven by an external electric field. The intrinsic spin-phonon coupling between the spin and the mechanical mode is induced by the spin-orbit coupling. Arbitrary-angle rotations about arbitrary axes of the single electron spin can be achieved by varying the frequency and the strength of the external electric driving field. If two single-electron spins in two quantum dots couple to the same vibrational mode simultaneously, the two spins are indirectly coupled via phonon exchange. Both electron spin resonance and the iSWAP gate can be turned off by suppressing the spin-phonon coupling by electrostatically shifting the electron wave function on the nanotube. Combining iSWAP and single spin gates, maximally entangled states of two spins can be generated in a single step. [1] H. Wang and G. Burkard, Phys. Rev. B 90, 035415 (2014).   

Nanophotonic photon echo memory based on rare-earth-doped crystals

Rare earth ions (REIs) are promising candidates for implementing solid-state quantum memories and quantum repeater devices. Their high spectral stability and long coherence times make REIs a good choice for integration in an on-chip quantum nano-photonic platform. We report the coupling of the 883 nm transition of Neodymium (Nd) to a Yttrium orthosilicate (YSO) photonic crystal nano-beam resonator, achieving Purcell enhanced spontaneous emission by 21 times and increased optical absorption. Photon echoes were observed in nano-beams of different doping concentrations, yielding optical coherence times T$_{2}$ up to 80 $\mu $s that are comparable to unprocessed bulk samples. This indicates the remarkable coherence properties of Nd are preserved during nanofabrication, therefore opening the possibility of efficient on-chip optical quantum memories. The nano-resonator with mode volume of $1.6(\lambda /n)^{3}$ was fabricated using focused ion beam, and a quality factor of 3200 was measured. Purcell enhanced absorption of 80{\%} by an ensemble of $\sim$ 1 $\times$ 10$^{6}$ ions in the resonator was measured, which fulfills the cavity impedance matching condition that is necessary to achieve quantum storage of photons with unity efficiency.   

Photons in synthetic gauge fields

Electronic transport is localized in low-dimensional disordered media. The addition of gauge fields to disordered media leads to fundamental changes in the transport properties. We implement a synthetic gauge field for photons using silicon-on-insulator technology. By determining the distribution of transport properties, we confirm that waves are localized in the bulk and localization is suppressed in edge states. Furthermore, we measure corresponding topological invariants and investigate the chiral gauge anomaly in the context of Chern-Simon theory. Our system provides a new platform for investigating the transport properties of photons in the presence of synthetic gauge fields.   

Harnessing gauge fields for maximally entangled state generation

We study the generation of entanglement between two species of bosons living on a ring lattice, where each group of particles can be described by a d-dimensional Hilbert space (qudit). Gauge fields are exploited to create an entangled state between the pair of qudits. Maximally entangled eigenstates are found for well-defined values of the Aharonov-Bohm phase, which are zero-energy eigenstates of both the kinetic and interacting parts of the Bose-Hubbard Hamiltonian, making them quite exceptional. We propose a protocol to reach the maximally entangled state (MES) by starting from an initially prepared ground state. Also, an indirect method to detect the MES by measuring the current of the particles is proposed.   

What does it mean for half of an empty cavity to be full?

It is well known that the vacuum state of a quantum field is spatially entangled. This is true both in free and confined spaces, for example in an optical cavity. The obvious consequence of this, however, is surprising and intuitively challenging. Namely, that in some sense half of an empty box is full. Formally this is clear, but what does this physically mean in terms of, say, measurements that can actually be made? In this contribution I will discuss a new and simple perspective that answers this question precisely and physically concretes the phenomenon. In so doing I will also propose a simple experimental setup for the verification of, and indeed the efficient harvesting of, vacuum entanglement. Preprint: http://arxiv.org/abs/1409.4203   

Protocols for a quantum network based on single photons

Two protocols for interfacing single optical photons with individual qubits are theoretically discussed. The first is a protocol which allows one to interface a single optical photon with a superconducting qubit. It makes use of a a spin ensemble, where the individual emitters possess both an optical and a magnetic dipole transition. Reversible frequency conversion is realized by combining optical photon storage, for instance by means of EIT, with the controlled switching on and off the coupling of the magnetic dipole transition with a microwave cavity, which in turn couples to a superconducting qubit. We test various strategies and compare their efficiencies in terms of robustness and transfer time. The second protocol aims at achieving perfect absorption of a photon by a single trapped atom, or solid-state emitter, by means of optimal control theory. We make use of the Krotov algorithm for the purpose of identifying pulses driving the atom, that maximize the efficiency and fidelity of absorption in the setup of [Reiser et al., Nature {\bf 508}, 237 (2014)]. These protocols contribute to the development of a toolbox for quantum networks using hybrid platforms.   

QNIX: A Linear Optical Architecture for Quantum Computing

There is currently a great deal of effort to develop a large-scale quantum computer, and one of the most promising platforms to do so is integrated linear optics. We present a proposal for a dynamical scheme for an integrated linear optics implementation of a one-way quantum computer. We go beyond the purely theoretical work and address practical issues in order to create a physically realistic design. We describe every step of cluster state construction and processing, showing the outstanding issues left to be addressed and our contributions to the different stages of the dynamical process. These include optimised interferometers for the generation of GHZ states, a universal and scalable architecture which requires entangled sources of no more than 3 photons with no active feed-forward, and loss-tolerant and fault-tolerant strategies specifically tailored to our proposed architecture. Our work demonstrates that building a linear optical quantum computer need be less challenging than previously thought, and brings large-scale switch-free linear optical architectures for quantum computing much closer to experimental realisation.   

Reshaping quantum wave packets through time-dependent absorption

The problem of control and reliable manipulation of quantum states finds importance in many areas of physics including quantum metrology and matter-wave interferometry. Here we propose a new approach to reshaping the spatial wave function of a quantum particle, e.g., an atom, by passing the latter through a time-dependent absorbing barrier. Experimentally, such a barrier can be realized by means of a sharply focused laser beam or a light sheet, with the radiation frequency chosen to make the passing atom undetectable, for instance, by ionizing the atom or changing its internal state. In particular, we show how the proposed method can be used to shift, squeeze, or split spatially-localized quantum wave packets.   

Correlated random walks induced by dynamical wavefunction collapse

Wavefunction collapse models modify Schr\"odinger's equation so that it describes the collapse of a superposition of macroscopically distinguishable states as a genuine physical process [PRA {\bf 42}, 78 (1990)]. This provides a basis for the resolution of the quantum measurement problem. An additional generic consequence of the collapse mechanism is that it causes particles to exhibit a tiny random diffusive motion. Furthermore, the diffusions of two sufficiently nearby particles are positively correlated --- it is more likely that the particles diffuse in the same direction than would happen if they behaved independently [PRA {\bf 89}, 032713 (2014)]. The use of this effect is proposed as an experimental test of wave function collapse models in which pairs of nanoparticles are simultaneously released from nearby traps and allowed a brief period of free fall. The random displacements of the particles are then measured. The experiment must be carried out at sufficiently low temperature and pressure for the collapse effects to dominate over the ambient environmental noise. It is argued that these constraints can be satisfied by current technologies for a large class of viable wavefunction collapse models.