18 research outputs found
Direct generation of three-photon entanglement using cascaded downconversion
High quality entangled photon sources are a key requirement for many promising quantum optical technologies. However, the production of multi-photon entangled states with good fidelity is challenging. Current sources of multi-photon entanglement require the use of post-selection, which limits their usefulness for some applications. It has been an open challenge to create a source capable of directly producing three-photon entanglement. An important step in this direction was achieved with the demonstration of photon triplets produced by a new process called cascaded downconversion, but these previous measurements were not sufficient to show whether these photons were in an entangled state and only had detection rates of five triplets per hour. In this thesis, we show the first demonstration of a direct source of three-photon entanglement. Our source is based on cascaded downconversion, and we verify that it produces genuine tripartite entanglement in two degrees of freedom: energy-time and polarization.
The energy-time entanglement is similar to a three-particle generalization of an Einstein-Podolski-Rosen state; the three photons are created simultaneously, yet the sum of their energies is well defined, which is an indication of energy-time entanglement. To prove it, we use time-bandwidth inequalities which check for genuine tripartite entanglement. Our measurements show that the state violates the inequalities with what constitute, to the best of our knowledge, the strongest violation of time-bandwidth inequalities in a tripartite continuous-variable system to date.
We create polarization entanglement by modifying our experimental setup so that two downconversion processes producing orthogonally polarized triplets interfere to create Greenberger-Horne-Zeilinger states. By using highly efficient superconducting nanowire single photon detectors, we improve the detected triplet rate by 2 orders of magnitude to 660 triplets per hour. We characterize the state using quantum state tomography, and find a fidelity of 86\% with the ideal state, beating the previous best value for a three-photon entangled state fidelity measured by tomography. We also use the state to perform two tests of local realism. We violate the Mermin and Svetlichny inequalities by 10 and 5 standard deviations respectively, the latter being the strongest violation to date. Finally, we show that, unlike previous sources of tree-photon entanglement, our source can be used as a source of heralded Bell pairs. We demonstrate this by measuring a CHSH inequality with the heralded Bell pairs, and by reconstructing their state using quantum state tomography.4 month
Realization of novel entangled photon sources using periodically poled materials
This thesis deals with the production of entangled photons using spontaneous parametric down-conversion (SPDC). We start with a short overview of some important theoretical concepts. First we provide a brief reminder of the theory of entanglement. We then discuss how the state of quantum systems can be determined using quantum state tomography. We also explain SPDC, the physical process which we use to produce entangled photons. Finally, we give an overview of the methods which have been used to produce entangled photons in the past, both for two- and three-photon entanglement.
The first experiment is the design of an efficient source of entangled photon pairs based on a polarizing Sagnac interferometer configuration. With this configuration, we can use quasi-phasematched materials which allow for higher efficiencies than standard bulk nonlinear materials. The source is pumped by a low-power continuous-wave laser diode, and produces degenerate photon pairs at 809nm. It has a spectral brightness of 87,500 pairs/(s mW nm), and the fidelity of the produced quantum states with a Bell state is 98.9%. The source is used for experiments in quantum key distribution, cluster state quantum computing, remote state preparation, state discrimination, and entanglement-enhanced classical communication.
The second experiment discussed in this thesis is the generation of photon triplets using cascaded SPDC. In this experiment, a primary SPDC source is pumped with a low-power, continuous-wave laser diode, producing photon pairs. Single photons from these pairs serve as the pump for a second down-conversion, resulting in photon triplets. This is the first demonstration of the direct production of photon triplets, and the first observation of SPDC at the single photon level. This method could potentially be used to produce entangled photon triplets without post-selection, and as a source of triggered Bell pairs
Amplification of cascaded downconversion by reusing photons with a switchable cavity
The ability to efficiently produce and manipulate nonclassical states of
light is a critical requirement for the development of quantum optical
technologies. In recent years, experiments have demonstrated that cascaded
spontaneous parametric down-conversion is a promising approach to implement
photon precertification, providing a way to overcome photon transmission losses
for quantum communication, as well as to directly produce entangled
three-photon states and heralded Bell pairs. However, the low efficiency of
this process has so far limited its applicability beyond basic experiments.
Here, we propose a scheme to amplify triplet production rates by using a fast
switch and a delay loop to reuse photons that fail to convert on the first pass
through the cascade's second nonlinear crystal. We construct a theoretical
model to predict amplification rates and verify them in an experimental
implementation. Our proof-of-concept device increases the rate of detected
photon triplets as predicted, demonstrating that the method has the potential
to dramatically improve the usefulness of cascaded down-conversion for
device-independent quantum communication and entangled state generation.Comment: 6 pages, 5 figure
Observation of genuine three-photon interference
Multiparticle quantum interference is critical for our understanding and
exploitation of quantum information, and for fundamental tests of quantum
mechanics. A remarkable example of multi-partite correlations is exhibited by
the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles
are correlated while no pairwise correlation is found. The manifestation of
these strong correlations in an interferometric setting has been studied
theoretically since 1990 but no three-photon GHZ interferometer has been
realized experimentally. Here we demonstrate three-photon interference that
does not originate from two-photon or single photon interference. We observe
phase-dependent variation of three-photon coincidences with 90.5 \pm 5.0 %
visibility in a generalized Franson interferometer using energy-time entangled
photon triplets. The demonstration of these strong correlations in an
interferometric setting provides new avenues for multiphoton interferometry,
fundamental tests of quantum mechanics and quantum information applications in
higher dimensions.Comment: 7 pages, 7 figure
An Ultra-Low Noise Telecom Wavelength Free Running Single Photon Detector Using Negative Feedback Avalanche Diode
It is challenging to implement genuine free running single photon detectors
for the 1550 nm wavelength range with simultaneously high detection efficiency
(DE), low dark noise, and good time resolution. We report a novel read out
system for the signals from a negative feedback avalanche diode (NFAD) which
allows useful operation of these devices at a temperature of 193 K and results
in very low dark counts (~100 CPS), good time jitter (~30 ps), and good DE
(~10%). We characterized two NFADs with a time correlation method using photons
generated from weak coherent pulses (WCP) and photon pairs produced by
spontaneous parametric down conversion (SPDC). The inferred detector
efficiencies for both types of photon sources agree with each other. The best
noise equivalent power of the device is estimated to be 8.1 x 10^(-18) W
Hz^(-1/2), more than 10 times better than typical InP/InGaAs SPADs show in free
running mode. The afterpulsing probability was found to be less than 0.1% per
ns at the optimized operating point. In addition, we studied the performance of
an entanglement-based quantum key distribution (QKD) using these detectors and
develop a model for the quantum bit error rate (QBER) that incorporates the
afterpulsing coefficients. We verified experimentally that using these NFADs it
is feasible to implement QKD over 400 km of telecom fibre. Our NFAD photon
detector system is very simple, and is well suited for single-photon
applications where ultra-low noise and free-running operation is required, and
some afterpulsing can be tolerated.Comment: 28 pages, 16 figures, and 1 tabl
Time-resolved double-slit experiment with entangled photons
The double-slit experiment strikingly demonstrates the wave-particle duality
of quantum objects. In this famous experiment, particles pass one-by-one
through a pair of slits and are detected on a distant screen. A distinct
wave-like pattern emerges after many discrete particle impacts as if each
particle is passing through both slits and interfering with itself. While the
direct event-by-event buildup of this interference pattern has been observed
for massive particles such as electrons, neutrons, atoms and molecules, it has
not yet been measured for massless particles like photons. Here we present a
temporally- and spatially-resolved measurement of the double-slit interference
pattern using single photons. We send single photons through a birefringent
double-slit apparatus and use a linear array of single-photon detectors to
observe the developing interference pattern. The analysis of the buildup allows
us to compare quantum mechanics and the corpuscular model, which aims to
explain the mystery of single-particle interference. Finally, we send one
photon from an entangled pair through our double-slit setup and show the
dependence of the resulting interference pattern on the twin photon's measured
state. Our results provide new insight into the dynamics of the buildup process
in the double-slit experiment, and can be used as a valuable resource in
quantum information applications
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