Non-classical light will be used in a variety of quantum-enhanced measurements such as imaging and metrology, and quantum measurements and quantum networking. One example is the discrete single or entangled photon source, a light source that provides at most one photon or one entangled photon pair at a time. One of the brightest of these sources is engineered from epitaxial single semiconductor quantum dots in optical cavities. I will discuss how these sources are made and characterized — particularly their nonclassical characteristics.
Semiconductor quantum dots are excellent emitters of single photons. Often, the same mode is used to resonantly excite a QD and to collect the emitted single-photons, requiring cross polarization to separate out scattered laser light. This reduces the source brightness to ≤50%, and potentially eliminates their use in some quantum applications. We demonstrate a resonant-excitation approach to creating single photons that is free of any filtering whatsoever. This integrated device allows us to resonantly excite single quantum-dot states in several cavities in the plane of the device using connected cavity-waveguides, while the cavity-enhanced single-photon fluorescence is directed vertically (off-chip) in a Gaussian mode.
We investigate the charging dynamics in epitaxially grown InAs quantum dots (QDs) under resonant excitation with and without additional low-power above-band excitation. Time-resolved resonance fluorescence from a charged exciton (trion) transition is recorded as the above-band excitation is modulated on and off. The fluorescence intensity varies as the QD changes from charged to neutral and back due to the influence of the above-band excitation. We fit the transients of the decay of the time-resolved resonance fluorescence after the above-band excitation turns off with a model that represents the neutralization process. The time dependence of the transients indicate that Auger recombination of resonantly excited trions is largely responsible for neutralization of the charged state when the above-band excitation is off. Additionally, a stretched exponential component of the transient of the fluorescence decay indicates the QD is supplied with charges via carrier migration through a stochastic collection of weakly-binding sites, resulting in sub-diffusion-like dynamics.
We characterize an efficient and nearly-noiseless parametric frequency upconverter. The ultra-low noise regime is reached by the wide spectral separation between the input and pump frequencies and the low pump frequency relative to the input photons. The background of only ≈100 photons per hour is demonstrated. We demonstrate phase preservation in a frequency upconversion process at the single-photon level. We summarize our efforts to measure this ultra-low noise level, and discuss both single-photon avalanche photodiode measurements and a photon-counting transition edge sensor (TES) measurements. To reach the required accuracy, we supplemented our TES with a dark count reduction algorithm. The preservation of the coherence was demonstrated by simultaneously upconverting the input of each arm of a Mach-Zehnder interferometer through high interference fringe contrast. We observe fringe visibilities of ≥0.97 with faint coherent input.
Single semiconductor quantum dots, due to their discrete energy structure, form single photon and twin photon sources that are characterized by a well-defined frequency of the emitted photons and inherently sub-Poissonian statistics. The single photons are generated through a recombination of an electron-hole pair formed by an electron from the conduction band and a hole from the valence band. When excited to the biexciton state quantum dots can provide pairs of photons emitted in a cascade. It has been shown that this biexciton-exciton cascade can deliver entangled pairs of photons. To achieve a deterministic generation of photon pairs from a quantum dot system one requires exciting it using a two-photon resonant excitation of the biexciton. Particularly, an efficient and coherent excitation of the biexciton requires the elimination of the single exciton probability amplitude in the excitation pulse and reaching the lowest possible degree of dephasing caused by the laser excitation. These two conditions impose contradictory demands on the excitation pulse-length and its intensity. We addressed this problem from a point of view that does not include interaction of the quantum dot with the semiconductor environment. We found an optimized operation regime for the system under consideration and provide guidelines on how to extend this study to other similar systems. In particular, our study shows that an optimal excitation process requires a trade-off between the biexciton binding energy and the excitation laser pulse length.
We experimentally realize a solid-state spin-photon transistor using a quantum dot strongly coupled to a photonic crystal cavity. We are able to control the light polarization through manipulation of the quantum dot spin states. The spinphoton transistor is crucial for realizing a quantum logic gate or generating hybrid entanglement between a quantum dot spin and a photon. Our results represent an important step towards semiconductor based quantum logic devices and onchip quantum networks.
KEYWORDS: Quantum dots, Photonic crystals, Molecular photonics, Picosecond phenomena, Molecules, Polaritons, Energy transfer, Chemical species, Molecular energy transfer, Control systems
Vacuum Rabi oscillation is a damped oscillation in which energy can transfer between an atomic excitation and a photon when an atom is strongly coupled to a photonic cavity. This process is challenging to be coherently controlled due to the fact that interaction between the atom and the electromagnetic resonator needs to be modulated in a quick manner compared to vacuum Rabi frequency. This control has been achieved at microwave frequencies, but has remained challenging to be implemented in the optical domain. Here we demonstrated coherent control of energy transfer in a semiconductor quantum dot strongly coupled to a photonic crystal molecule by manipulating the vacuum Rabi oscillation of the system. Instead of using a single photonic crystal cavity, we utilized a photonic crystal molecule consisting two coupled photonic crystal defect cavities to obtain both strong quantum dot-cavity coupling and cavityenhanced AC stark shift. In our system the AC stark shift modulates the coupling interaction between the quantum dot and the cavity by shifting the quantum dot resonance, on timescales (picosecond) shorter than the vacuum Rabi period. We demonstrated the ability to transfer excitation between a quantum dot and cavity, and performed coherent control of light-matter states. Our results provides an ultra-fast approach for probing and controlling light-matter interactions in an integrated nanophotonic device, and could pave the way for gigahertz rate synthesis of arbitrary quantum states of light at optical frequencies.
Generating strong interactions between single quanta of light and matter is central to quantum information science, and a key component of quantum computers and long-distance quantum networks. In quantum information processing, these interactions are required to create elementary logic operations (quantum gates) between stationary matter quantum bits (qubits) and photonic qubits that can be transmitted over long distances. Efficient quantum gates between photonic and matter qubits are a crucial enabler for a broad range of applications that include robust quantum networks, nondestructive quantum measurements, and strong photon-photon interactions. So far these qubit-photon gates have been achieved using single atoms and at microwave frequencies in circuit QED systems. Their implementation with solidstate quantum emitters, however, has remained a difficult challenge. We demonstrate that the qubit state of a photon can be controlled by a single solid-state qubit composed of a quantum dot (QD) strongly coupled to an optical nanocavity. We show that the QD acts as a coherently controllable qubit system that conditionally flips the polarization of a photon reflected from the cavity mode on picosecond timescales. This operation implements a controlled NOT (cNOT) logic gate between the QD and the incident photon, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and quantum computations. Our results represent an important step towards an all solid-state implementation of quantum networks and quantum computers, and provide a versatile approach for controlling and probing interactions between a photon and a single quantum emitter on ultra-fast timescales.
Low power optical nonlinearities are a crucial requirement for data routing and next generation all-optical processing.
The majority of nonlinear optical devices to date exploit weak nonlinearities from a large ensemble of atomic systems,
resulting in both high power dissipation and a large device footprint. Quantum dots (QDs) coupled to photonic crystals
can provide significant reduction in both device size and power dissipation. The interaction between these two systems
creates extremely strong light-matter interaction owing to the tight optical confinement of photonic crystals and large oscillator strengths of QDs. Such interactions enable optical nonlinearities near the single photon level. In this work we investigate the nonlinear properties of QDs coupled to photonic crystals. We demonstrate large optical Stark shift with only 10 photons. We then propose and demonstrate a novel photonic circuit that can route light on a chip with extremely low optical powers.
We demonstrate interference between discrete photons emitted by two different semiconductor quantum dots
and quantify their degree of indistinguishability. The quantum dot emission energies are tuned into resonance by
straining the samples. Upon interference on a beamsplitter, the photons are shown to be 18.1% indistinguishable,
resulting in a coincidence detection rate below the classical limit. Post-selecting only those detections occurring
within a short time of each other increases the measured indistinguishability to 47%. The photons are partially
distinguishable due to dephasing of the exciton states, and post-selection is also affected by the detector response
time.
We demonstrated lasing in localized optical resonances of deterministic aperiodic structures
with pseudo-random morphologies. The localized lasing modes in the Rudin-Shapiro arrays of air
nanoholes in GaAs membranes occur at reproducible spatial locations and their frequencies are
only slightly affected by the structural fluctuations in different samples. Numerical study on the
resonances of the passive systems and optical imaging of lasing modes enabled us to interpret the
observed lasing behavior in terms of distinctive localized resonances in the two-dimensional Rudin-
Shapiro structures. The deterministic aperiodic media with controllable structural and optical
properties provide a novel platform, alternative to random lasers and different from photonic crystals
lasers, for the engineering of multi-frequency coherent light sources suitable for technological
integration.
The discrete states resulting from three-dimensional confinement in semiconductor quantum dots retain much of
the character of their bulk-band origins, for example their angular momentum and effective mass. In addition they
have many features of discrete atomic-like single particle states. Strong optical field interactions and reasonable
dephasing rates make this system attractive for basic quantum optics experiments, as well as applications in
quantum information sciences. However, semiconductor quantum dots have large inhomogeneous state broadens
due to variations in size and shape. In addition, epitaxial semiconductor quantum dots, one of the classes in
common use, rarely have ideal symmetry. Here we show how an optical technique can be used to fine-tune the
transition energies of semiconductor quantum dots states and if desired restore targeted symmetry elements.
This approach can be applied to establish degeneracies in biexciton-exciton decays to form discrete entangled
photon pairs or to establish indistinguishability between different quantum dots.
We model a new THz laser device structure based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. An ensemble of QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We simulate the rate equations governing lasing and discuss a variety of processes affecting lasing including nonradiative recombination and the ground state decay rate.
Single-photon sources rarely emit two or more photons in the same
pulse, compared to a Poisson-distributed source of the same
intensity, and have numerous applications in quantum information
science. The quality of such a source is evaluated based on three
criteria: high efficiency, small multi-photon probability, and
quantum indistinguishability. We have demonstrated a single-photon
source based on a quantum dot in a micropost microcavity that
exhibits a large Purcell factor together with a small multi-photon
probability. For a quantum dot on resonance with the cavity, the
spontaneous emission rate has been increased by a factor of five,
while the probability to emit two or more photons in the same
pulse has been reduced to 2% compared to a Poisson-distributed
source of the same intensity. The indistinguishability of emitted
single photons from one of our devices has been tested through a
Hong-Ou-Mandel-type two-photon interference experiment;
consecutive photons emitted from such a source have been largely
indistinguishable, with a mean wave-packet overlap as large as
0.81. We have also designed and demonstrated two-dimensional
photonic crystal GaAs cavities containing InAs quantum dots that
exhibit much higher quality factors together with much smaller
mode volumes than microposts, and therefore present an ideal
platform for construction of single photon sources of even higher
quality.
Quantum cryptography is a method to exchange secret messages with unconditional security over a potentially hostile environment using single photons. Previous implementations of quantum cryptography have relied on highly attenuated laser light to approximate single photo states. Such sources are vulnerable to eavesdropping attacks based on photon splitting. Here we present an experimental demonstration of quantum cryptography using a single photon source based on Indium Arsenide quantum dots. We achieve a communication rate of 25kbits/s. This source allows secure communication over a quantum channel with up to 28dB of channel loss, as opposed to only 23dB for an attenuated laser.
A high efficiency, triggered single photon source with applications to quantum communications is discussed. The sources is formed from an InAs-based quantum dot located in the center of a micropost cavity formed from GaAs, with top and bottom GaAs/AlAs distributive Bragg reflector pairs, and lateral processing. When pumped above band into the semiconductor host, correlation measurements show a reduction in the two-photon probability to 0.14, compared to unity for a Poisson source. The external efficiency of this structure is 0.24.
A single-photon device based on a semiconductor quantum dot
embedded in an optical microcavity is described. The spontaneous
emission lifetime, multi-photon suppression, and spectral linewidth
are measured. It is then shown that consecutively emitted photons possess a large degree of quantum-mechanical indistinguishability, with a mean wave-packet overlap as large as 0.8. This demonstration is accomplished through a Hong-Ou-Mandel-type two-photon interference experiment.
In experiments conducted nearly 20 years ago, the spontaneous emission from single atoms was modified using electromagnetic cavities. In a condensed matter analogy to a single atom, we demonstrate that the spontaneous emission from an isolated InAs quantum dot can be modified as well. The single quantum dot spontaneous emission is coupled with high efficiency to a single, polarization-degenerate cavity mode using a compact, semiconductor resonator structure. The quantum dot is embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%. It is believed that this structure will be useful in triggered photons sources for quantum cryptography.
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