P. Milman

Entanglement-enabled delayed choice experiment

Delaying Quantum Choice Photons can display wavelike or particle-like behavior, depending on the experimental technique used to measure them. Understanding this duality lies at the heart of quantum mechanics. In two reports, Peruzzo et al. (p. 634) and Kaiser et al. (p. 637; see the Perspective on both papers by Lloyd) perform an entangled version of John Wheelers delayed-choice gedanken experiment, in which the choice of detection can be changed after a photon passes through a double-slit to avoid the measurement process affecting the state of the photon. The original proposal allowed the wave and particle nature of light to be interchanged after the light had entered the interferometer. By contrast in this study, entanglement allowed the wave and particle nature to be interchanged after the light was detected and revealed the quantum nature of the photon, for example, it displays wave- and particle-like behavior simultaneously. Quantum entanglement is used to probe the nature of the photon. Wave-particle complementarity is one of the most intriguing features of quantum physics. To emphasize this measurement apparatus–dependent nature, experiments have been performed in which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after a photon has already entered the device. A recent extension suggested using a quantum beam splitter at the interferometer’s output; we achieve this using pairs of polarization-entangled photons. One photon is tested in the interferometer and is detected, whereas the other allows us to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore, this experiment allows us to continuously morph the tested photon’s behavior from wavelike to particle-like, which illustrates the inadequacy of a naive wave or particle description of light.

Direct bell states generation on a III-V semiconductor chip at room temperature

Summary form only given. In these last years, a great deal of effort has been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four-wave mixing in Silicon waveguides have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 775 nm), impinging on the surface of the waveguide with an incidence angle θ, generates two counterpropagating orthogonally polarized beams (around 1550 nm). The waveguide core is surrounded by distributed Bragg reflectors to enhance the pump field. We demonstrate the direct emission of polarization entangled photons by pumping the device with two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of 0.8 to the Bell state ( Hν +eiφ vx ) is obtained. A theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement.These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.

Topological phase for spin-orbit transformations on a laser beam

We investigate the topological phase associated with the SO(3) representation in terms of maximally entangled states. An experimental demonstration of this topological phase is provided for polarization and spatial mode transformations of a laser beam.

Topological phase for entangled two-qubit states.

Entangled states play a crucial role in quantum physics, ranging from fundamental aspects to quantum information processing. We show here that entangled two-qubit states can also be used to characterize unambiguously the subtlety of the SO(3) rotation group topology. The well known two distinct families of path in this group are put in one-to-one correspondence with cyclic evolutions of these entangled states, resulting in a pi phase difference. We propose a simple quantum optics interference experiment to demonstrate this topological phase shift.

Quantum teleportation in the spin-orbit variables of photon pairs

We propose a polarization to orbital angular momentum teleportation scheme using entangled photon pairs generated by spontaneous parametric down-conversion. By making a joint detection of the polarization and angular momentum parity of a single photon, we are able to detect all the Bell states and perform, in principle, perfect teleportation from a discrete to a continuous system using minimal resources. The proposed protocol implementation demands experimental resources that are currently available in quantum optics laboratories.

Topologically decoherence-protected qubits with trapped ions

The authors introduced a new kind of Hamiltonian describing an infinite range coupling between particles placed on a square array with a large number of symmetries and describe its spectral properties. The authors also shows in particular that this Hamiltonian can be naturally realized using ions confined in a surface trap and that the induced protection is more effective than the one predicted for a short range Hamiltonian.

Parity-dependent State Engineering and Tomography in the ultrastrong coupling regime

Reaching the strong coupling regime of light-matter interaction has led to an impressive development in fundamental quantum physics and applications to quantum information processing. Latests advances in different quantum technologies, like superconducting circuits or semiconductor quantum wells, show that the ultrastrong coupling regime (USC) can also be achieved, where novel physical phenomena and potential computational benefits have been predicted. Nevertheless, the lack of effective decoupling mechanism in this regime has so far hindered control and measurement processes. Here, we propose a method based on parity symmetry conservation that allows for the generation and reconstruction of arbitrary states in the ultrastrong coupling regime of light-matter interactions. Our protocol requires minimal external resources by making use of the coupling between the USC system and an ancillary two-level quantum system.

Bell-type inequalities for cold heteronuclear molecules.

We introduce Bell-type inequalities allowing for nonlocality and entanglement tests with two cold heteronuclear molecules. The proposed inequalities are based on correlations between each molecule spatial orientation, an observable which can be experimentally measured with present day technology. Orientation measurements are performed on each subsystem at different times. These times play the role of the polarizer angles in Bell tests realized with photons. We discuss the experimental implementations of the proposed tests, which could also be adapted to other high dimensional quantum angular momenta systems.

Continuous-Variable Instantaneous Quantum Computing is Hard to Sample

Instantaneous quantum computing is a subuniversal quantum complexity class, whose circuits have proven to be hard to simulate classically in the discrete-variable realm. We extend this proof to the continuous-variable (CV) domain by using squeezed states and homodyne detection, and by exploring the properties of postselected circuits. In order to treat postselection in CVs, we consider finitely resolved homodyne detectors, corresponding to a realistic scheme based on discrete probability distributions of the measurement outcomes. The unavoidable errors stemming from the use of finitely squeezed states are suppressed through a qubit-into-oscillator Gottesman-Kitaev-Preskill encoding of quantum information, which was previously shown to enable fault-tolerant CV quantum computation. Finally, we show that, in order to render postselected computational classes in CVs meaningful, a logarithmic scaling of the squeezing parameter with the circuit size is necessary, translating into a polynomial scaling of the input energy.

Superradiant phase transition in the ultrastrong-coupling regime of the two-photon Dicke model

The controllability of current quantum technologies allows to implement spin-boson models where two-photon couplings are the dominating terms of light-matter interaction. In this case, when the coupling strength becomes comparable with the characteristic frequencies, a spectral collapse can take place, i.e. the discrete system spectrum can collapse into a continuous band. Here, we analyze the thermodynamic limit of the two-photon Dicke model, which describes the interaction of an ensemble of qubits with a single bosonic mode. We find that there exists a parameter regime where two-photon interactions induce a superradiant phase transition, before the spectral collapse occurs. Furthermore, we extend the mean-field analysis by considering second-order quantum fluctuations terms, in order to analyze the low-energy spectrum and compare the critical behavior with the one-photon case.