6 research outputs found
Experimental Superposition of Orders of Quantum Gates
In a quantum computer, creating superpositions of quantum bits (qubits) in
different states can lead to a speed-up over classical computers [1], but
quantum mechanics also allows for the superposition of quantum circuits [2]. In
fact, it has recently been theoretically predicted that superimposing quantum
circuits, each with a different gate order, could provide quantum computers
with an even further computational advantage [3-5]. Here, we experimentally
demonstrate this enhancement by applying two quantum gates in a superposition
of both possible orders to determine whether the two gates commute or
anti-commute. We are able to make this determination with only a single use (or
query) of each gate, while all quantum circuits with a fixed order of gates
would require at least two uses of one of the gates [3]. Remarkably, when the
problem is scaled to N gates, creating a superposition of quantum circuits is
likely to provide an exponential advantage over classical algorithms, and a
linear advantage over quantum algorithms with fixed gate order [4]. The new
resource that we exploit in our experiment can be interpreted as a
"superposition of causal orders". We demonstrate such a superposition could
allow some quantum algorithms to be implemented with an efficiency that is
unlikely to be achieved on a quantum computer with a fixed gate order.Comment: 10 pages, 7 figures, 2 table
Probing the fate of Mn complexes in Nafion: a combined multifrequency EPR and XAS study
Multifrequency electron paramagnetic resonance (EPR, 9.4 and 263 GHz) and X-ray absorption spectroscopy (XAS) were employed to study structural and electrochemical changes of selected Mn complexes in contact with dried Nafion films upon electro-oxidation and after long-term illumination. It was found that when in contact with Nafion the Mn-Me₃TACN complexes are reduced into Mn(II) complexes with an octahedral geometry (Me₃TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane). The reduction process involves an intermediate product in which Mn has been reduced from the initial +III or +IV state of the precursor Mn complex but is still coordinated to the TACN ligand. Electro-oxidation yields a MnOₓ mineral with a birnessite structure, in which the Mn(III) or Mn(IV) ions exhibit very strong magnetic coupling. Long-term illumination of the oxide-containing Nafion film while it is exposed to an aqueous electrolyte partially decomposes the mineral and forms a Mn(II) species with octahedral coordination
Experimental superposition of orders of quantum gates
Quantum computers achieve a speed-up by placing quantum bits (qubits) in superpositions of different states. However, it has recently been appreciated that quantum mechanics also allows one to ‘superimpose different operations’. Furthermore, it has been shown that using a qubit to coherently control the gate order allows one to accomplish a task—determining if two gates commute or anti-commute—with fewer gate uses than any known quantum algorithm. Here we experimentally demonstrate this advantage, in a photonic context, using a second qubit to control the order in which two gates are applied to a first qubit. We create the required superposition of gate orders by using additional degrees of freedom of the photons encoding our qubits. The new resource we exploit can be interpreted as a superposition of causal orders, and could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer
High-harmonic generation enhancement with graphene heterostructures
We investigate high-harmonic generation in graphene heterostructures
consisting of metallic nanoribbons separated from a graphene sheet by either a
few-nanometer layer of aluminum oxide or an atomic monolayer of hexagonal boron
nitride. The nanoribbons amplify the near-field at the graphene layer relative
to the externally applied pumping, thus allowing us to observe third- and
fifth-harmonic generation in the carbon monolayer at modest pump powers in the
mid-infrared. We study the dependence of the nonlinear signals on the ribbon
width and spacer thickness, as well as pump power and polarization, and
demonstrate enhancement factors relative to bare graphene reaching 1600 and
4100 for third- and fifth-harmonic generation, respectively. Our work supports
the use of graphene heterostructures to selectively enhance specific nonlinear
processes of interest, an essential capability for the design of nanoscale
nonlinear devices
Giant enhancement of third-harmonic generation in graphene-metal heterostructures
Nonlinear nanophotonics leverages engineered nanostructures to funnel light
into small volumes and intensify nonlinear optical processes with spectral and
spatial control. Due to its intrinsically large and electrically tunable
nonlinear optical response, graphene is an especially promising nanomaterial
for nonlinear optoelectronic applications. Here we report on exceptionally
strong optical nonlinearities in graphene-insulator-metal heterostructures,
demonstrating an enhancement by three orders of magnitude in the third-harmonic
signal compared to bare graphene. Furthermore, by increasing the graphene Fermi
energy through an external gate voltage, we find that graphene plasmons mediate
the optical nonlinearity and modify the third-harmonic signal. Our findings
show that graphene-insulator-metal is a promising heterostructure for
optically-controlled and electrically-tunable nano-optoelectronic components
Tuning single-photon sources for telecom multi-photon experiments
Multi-photon state generation is of great interest for near-future quantum simulation and quantum computation experiments. To-date spontaneous parametric down-conversion is still the most promising process, even though two major impediments still exist: accidental photon noise (caused by the probabilistic non-linear process) and imperfect single-photon purity (arising from spectral entanglement between the photon pairs). In this work, we overcome both of these difficulties by (1) exploiting a passive temporal multiplexing scheme and (2) carefully optimizing the spectral properties of the down-converted photons using periodically-poled KTP crystals. We construct two down-conversion sources in the telecom wavelength regime, finding spectral purities of > 91%, while maintaining high four-photon count rates. We use single-photon grating spectrometers together with superconducting nanowire single-photon detectors to perform a detailed characterization of our multi-photon source. Our methods provide practical solutions to produce high-quality multi-photon states, which are in demand for many quantum photonics applications