M. E. Goggin

Towards quantum chemistry on a quantum computer

Exact first-principles calculations of molecular properties are currently intractable because their computational cost grows exponentially with both the number of atoms and basis set size. A solution is to move to a radically different model of computing by building a quantum computer, which is a device that uses quantum systems themselves to store and process data. Here we report the application of the latest photonic quantum computer technology to calculate properties of the smallest molecular system: the hydrogen molecule in a minimal basis. We calculate the complete energy spectrum to 20 bits of precision and discuss how the technique can be expanded to solve large-scale chemical problems that lie beyond the reach of modern supercomputers. These results represent an early practical step toward a powerful tool with a broad range of quantum-chemical applications.

Remote state preparation: arbitrary remote control of photon polarization.

We experimentally demonstrate the first remote state preparation of arbitrary single-qubit states, encoded in the polarization of photons generated by spontaneous parametric down-conversion. Utilizing degenerate and nondegenerate wavelength entangled sources, we remotely prepare arbitrary states at two wavelengths. Further, we derive theoretical bounds on the states that may be remotely prepared for given two-qubit resources.

Optimizing type-I polarization-entangled photons

Optical quantum information processing needs ultra-bright sources of entangled photons, especially from synchronizable femtosecond lasers and low-cost cw-diode lasers. Decoherence due to timing information and spatial mode-dependent phase has traditionally limited the brightness of such sources. We report on a variety of methods to optimize type-I polarization-entangled sources--the combined use of different compensation techniques to engineer high-fidelity pulsed and cw-diode laser-pumped sources, as well as the first production of polarization-entanglement directly from the highly nonlinear biaxial crystal BiB(3)O(6) (BiBO). Using spatial compensation, we show more than a 400-fold improvement in the phase flatness, which otherwise limits efficient collection of entangled photons from BiBO, and report the highest fidelity to date (99%) of any ultrafast polarization-entanglement source. Our numerical code, available on our website, can design optimal compensation crystals and simulate entanglement from a variety of type-I phasematched nonlinear crystals.

Estimation of a quantum interaction parameter using weak measurements: Theory and experiment

We investigate the estimation of a small interaction parameter from the outcomes of weak quantum measurements implemented by the interaction. The relation of weak values and sensitivity is explained and the different contributions of postselected results are identified using experimental data. The results show how weak values can be used to control the distribution of input state sensitivity between different postselected outcomes.

Remote state preparation: arbitrary remote control of photon polarizations for quantum communication

By using a partial polarizer to apply a generalized polarization measurement to one photon of a polarization entangled pair, we remotely prepare single photons in arbitrary polarization qubits. Specifically, we are able to produce a range of states of any desired degree of mixedness or purity, over (and within) the entire Poincare sphere, with a typical fidelity exceeding 99.5%. Moreover, by using non-degenerate entangled pairs as a resource, we can prepare states in multiple wavelengths. Finally, we discuss the states remotely preparable given a particular two-qubit resource state.

Complementarity in variable strength quantum non-demolition measurements

Using a linear optic quantum gate we perform a variable strength quantum non-demolition measurement, to elucidate the role of which-path knowledge in a complementarity experiment. Specifically, we demonstrate that the entanglement created by the measurement interaction prevents an exhaustive description in terms of complementary wave-like and particle-like behaviour of a single photon in an interferometer.

The Conversion Revolution: Down, Up and Sideways

Continued progress in quantum communication and optical approaches to quantum computation, requires the development of enabling technologies for photonic quantum information processing [1]. Such resources include high efficiency single photon detectors, high-fidelity single-photon sources, high brightness sources of entanglement, and multiple microsecond optical quantum memories. Here we discuss our current efforts to improve this quantum information “toolbox”. Section II describes the realization of an extremely bright, high-fidelity source of entangled photon pairs, created via the process of parametric downconversion, as well as one possible method for storing such quantum bits. Section III presents one application for entanglement: the realization of the quantum communication protocol of remote state preparation, a “sideways” conversion of the state of one photon contingent on particular measurements made on the other. Finally, Section IV describes an inverse nonlinear process which enables efficient and coherent detection of infrared photons by first up-converting them into visible frequencies.

Polarization Independent Photon Storage with Variable Time Delay

We discuss a high-efficiency photon storage system that is independent of photons polarization. This scheme may be critical for enhancing the efficiency of existing quantum computers and quantum storage devices for qubits.

Continuous- and Discrete-time Quantum Walks with Non-classical Two-Photon Inputs

We present two-photon continuous- and discrete-time quantum walks, respectively implemented in an integrated coupled waveguide array and a polarization-based beam-displacer network. We observe distinctly non-classical signatures, constituting progress towards quantum simulation capabilities.

Quantum Chemistry on a Quantum Computer: First Steps and Prospects

We use a photonic quantum computer to simulate the hydrogen molecule. This is the first experimental demonstration of efficient quantum chemistry, which promises to be a powerful new tool in biology, chemistry, and materials science.