This thesis presents the experimental demonstration of an atomic source of narrowband nonclassical states of light. Employing four-wave mixing in hot atomic Rubidium vapour, the optical states produced are naturally compatible with atomic transitions and may be thus employed in atom-based quantum communication protocols. We first demonstrate the production of two-mode intensity-squeezed light and analyze the correlations between the two produced modes. Using homodyne detection in each mode, we verify the production of two-mode quadrature-squeezed light, achieving a reduction in quadrature variance of 3 dB below the standard quantum limit. Employing conditional detection on one of the channels, we then demonstrate the generation of single-photon Fock states as well as controllable superpositions of vacuum and 1-photon states. We fully characterize the produced light by means of optical homodyne tomography and maximum likelihood estimation. The narrowband nature of the produced light yields a resolvable temporal wave-function, and we develop a method to infer this wave function from the continuous photocurrent provided by the homodyne detector. The nature of the atomic process opens the door to a new direction of research: generation of arbitrary superpositions of collective atomic states. We perform the first proof-of-principle experiment towards this new field and discuss a proposal for extending this work to obtain full control over the collective-atomic Hilbert space.
This thesis presents the experimental demonstration of an atomic source of narrowband nonclassical states of light. Employing four-wave mixing in hot atomic Rubidium vapour, the optical states produced are naturally compatible with atomic transitions and may be thus employed in atom-based quantum communication protocols. We first demonstrate the production of two-mode intensity-squeezed light and analyze the correlations between the two produced modes. Using homodyne detection in each mode, we verify the production of two-mode quadrature-squeezed light, achieving a reduction in quadrature variance of 3 dB below the standard quantum limit. Employing conditional detection on one of the channels, we then demonstrate the generation of single-photon Fock states as well as controllable superpositions of vacuum and 1-photon states. We fully characterize the produced light by means of optical homodyne tomography and maximum likelihood estimation. The narrowband nature of the produced light yields a resolvable temporal wave-function, and we develop a method to infer this wave function from the continuous photocurrent provided by the homodyne detector. The nature of the atomic process opens the door to a new direction of research: generation of arbitrary superpositions of collective atomic states. We perform the first proof-of-principle experiment towards this new field and discuss a proposal for extending this work to obtain full control over the collective-atomic Hilbert space.
This thesis presents a study of the four-wave mixing process in hot Rubidium vapor and its applicability to generating arbitrary quantum states, both in the optical domain and as atomic collective spin excitations. The four-wave mixing process involves a double Raman scattering event, where conservation of energy and momentum entangle these Raman-scattered photons. Heralding upon detection of one of these photons in a given mode projects the other photon onto a single photon Fock state in a well-defined mode, which may serve as a narrowband quantum light source that is compatible with atom-based quantum communication protocols. We integrate the existing technique of conditional measurements into our detection scheme, which allows us to engineer more complex quantum states. Experimentally we demonstrate the production of arbitrary superpositions of |0> and |1> in the optical Fock basis. We also present a theoretical model of this process which incorporates experimental complications such as losses, higher photon-number states, and a non-trivial temporal mode. This scheme can be extended to generate arbitrary quantum states with increasing photon numbers, at the expense of repetition rate. The atomic nature of this four-wave mixing process has an added benefit in that it allows access to excitations of the collective atomic ensemble. The optical results demonstrate the creation of transient collective spin excitations, and we present a proposal to extend this work to generate longer lived arbitrary collective atomic states.
The spatial character of the noise on a light field affects its usefulness for imaging. Multi-spatial-mode (MSM) squeezed light has noise below the quantum noise limit in multiple spatial modes, and can be used for super resolution imaging. The generation of such light has long been an experimental goal within the field of quantum optics. \n \nThis work introduces the theory behind the generation of squeezed light, and its measurement using a homodyne detector. A four-wave mixing process in a rubidium 85 vapour is used to experimentally generate squeezed light. The properties of this squeezed light are investigated, through the use of homodyne detection with a bichromatic LO. \n \nThis thesis further investigates how the squeezed quadrature changes from amplitude to phase over a range of 40 MHz. The MSM nature of a squeezed light field is directly investigated. The field is shown to contain at least 75 squeezed spatial modes in the frequency domain, each squeezed at a level of up to -2.5 dB. This thesis develops techniques to measure the spatial character of noise on a light field in the time domain. These are promising techniques for the analysis of the MSM nature of a squeezed light in the time domain.
In this thesis we describe a series of calculations
and experiments that use atomic coherence for the generation of
two-mode squeezed states of the electromagnetic field. Atomic
coherence makes it possible to eliminate the problems of absorption
losses and spontaneous emission that have limited the amount of
squeezing obtained with atomic systems. Effects such as
electromagnetically induced transparency allow for efficient
generation of atomic coherence and are used as the basis for the
generation of squeezed states. We analyze the
use of a double-A configuration in order to combine a four-wave
mixing process and atomic coherence in a single system. The use of
this configuration makes it possible to eliminate absorption and
enhance the four-wave mixing process responsible for the generation
of a two-mode squeezed state. We implemented a
new detection technique, which we devised, based on the use of a
bichromatic local oscillator in a balanced heterodyne detection
scheme for the characterization of two-mode squeezed states. This
new detection technique makes it possible to characterize squeezed
states independently of the frequency separation between the modes.
In order to obtain the required phase-coherent
lasers, we designed and implemented a laser system that consists of
three stabilized external-cavity diode lasers. We obtained a system
with a residual phase noise of less than 0.04 rad². We studied
electromagnetically induced transparency in the Dl line of ⁸⁷Rb and
obtained a reduction in absorption of 92%. Experimentally, we
implemented the double-A configuration in the Dl line of ⁸⁷Rb in a
vapor cell. We were able to verify the presence of correlations
between the fields generated from the four-wave mixing process and
produced more than 10 decibels of phase-dependent noise modulation.
Finally, we propose a new quantum
communication scheme that takes advantage of the properties of a
two-mode squeezed state. This new scheme offers the possibility of
securely transmitting either a cryptographic key or a deterministic
message. We analyze the security of the scheme and find that it can
be verified with the use of the quantum correlations present in a
two-mode squeezed state.
Light is used across a broad range of applications from imaging to communications. These fields have advanced sufficiently that measurements are now limited by the fundamental behaviour of light, such as fluctuations inherent to its quantum nature. This is dictated by quantum mechanics, most noticeably Heisenberg's uncertainty principle, having been adapted to describe light and form quantum optics. \nAs it is bound by the uncertainty principle, measuring an observable of light can be made more accurate by adding uncertainty to conjugate observables. This improved accuracy is then gauged against the quantum noise limit (QNL) of the measured observable. Such measurements are then squeezed where the noise of the measurement is below this QNL. In this thesis, we examine the squeezing of light via four wave mixing (4WM) in a rubidium 85 (Rb85) atomic vapour. This includes the generation and detection of squeezed light in its temporal and spatial degrees of freedom. We consider various methods of detection, such as homodyne detection to measure field quadratures, then intensity difference after a waveguide to observe preservation of local correlations, and finally local intensity difference squeezing with a CCD camera. \nThe three experiments each present different results. The broadband squeezing of light yielded variable squeezing across a bandwidth of 60MHz, displaying the multi-temporal mode nature of the process. Photodetection measurements and intensity difference of entangled light after being passed through a waveguide displayed local correlations between a range of corresponding regions within the beam profile, with squeezing of \\(\\sim\\)-1dB. CCD photodetection and intensity difference of this light showed squeezing of approximately -0.75dB in a single set of spatial Fourier frequencies encircling the central spatial frequency, 0mm\\(^{-1}\\), also referred to as DC, corresponding to 8 distinct squeezed spatial modes within the beams. \nThe use of a waveguide in transmitting local multi-spatial mode (MSM) correlations presents interesting possibilities in guiding said correlations for use in quantum communications and encryption protocols, with a vastly improved bit rate, if not also transmission of quantum images. This supplements the use of a CCD camera in measuring local MSM correlations in these beams which, in yielding positive results, can be utilized as a means of improved imaging beyond the QNL. \nGiven this exhaustive computational investigation, the results have conclusively shown MSM squeezing in signal beams generated from 4WM in a Rb85 vapour. Using the results from Chapter 6, this squeezing is distributed across 8 distinct spatial modes, with vertical spatial frequencies of ±4.96mm\\(^{-1}\\), ±2.48mm\\(^{-1}\\) and 0mm\\(^{-1}\\) and horizontal frequencies ±0.60mm\\(^{-1}\\), ±0.30mm\\(^{-1}\\) and 0mm\\(^{-1}\\). \nThese frequencies correspond to real space vertical sizes of 0.20mm and 0.40mm and horizontal sizes of 1.66mm and 3.33mm. These can be seen to be MSM squeezed regions, given that the size of the beams in the results were larger than this. The average height of the probe was 0.70mm, while the conjugate was 0.66mm while the average lengths were 6.17mm and 5.95mm. As these size exceed the length scales derived from the spatial frequencies, these frequencies thus show MSM squeezing.
Quantum correlations and entanglement shared among multiple quantum modes are important for both fundamental science and the future development of quantum technologies. This development will also require an efficient quantum interface between multimode quantum light sources and atomic ensembles, which makes it necessary to implement multimode quantum light sources that match the atomic transitions. Here, we report on such a source that provides a method for generating quantum correlated beams that can be extended to a large number of modes by using multiple four-wave mixing (FWM) processes in hot rubidium vapor. Experimentally, we show that two cascaded FWM processes produce strong quantum correlations between three bright beams but not between any two of them. In addition, the intensity-difference squeezing is enhanced with the cascaded system to -7.0±0.1 dB from the -5.5±0.1/-4.5±0.1 dB squeezing obtained with only one FWM process. One of the main advantages of our system is that as the number of quantum modes increases, so does the total degree of quantum correlations. The proposed method is also immune to phase instabilities due to its phase insensitive nature, can easily be extended to multiple modes, and has potential applications in the production of multiple quantum correlated images.
Quantum information systems require an interface between photons as a physical system to transport information and atoms which are better suited to store and process it. In this thesis the construction of a novel pulsed, bright and narrow-band light source is presented. It consists of an optical parametric oscillator featuring a periodically poled KTP crystal and a mechanical chopper and produces ?s-pulses of squeezed vacuum. The generated states are resonant to the Rubidium-DI-transition, are almost transform-limited and show more than 3 dB of squeezing. The physical effect of electromagnetically induced transparency (EIT) in hot rubidium vapor can be used to generate ultra-low group velocities and allows for a reversible adiabatic conversion of optical quantum states into collective spin excitations which can be used to stop pulses of light. I report about recent progress in slowing down a pulse of squeezed vacuum by one third of its width while still preserving 0.36 dB of squeezing in the process. I analyze electromagnetically induced transparency and light storage in an ensemble of atoms with multiple excited levels (multi-A configuration) which are coupled to one of the ground states by quantized signal fields and to the other one via classical control fields. A basis transformation of atomic and optical states which reduces the analysis of the system to that of electromagnetically induced transparency in a regular three-level configuration is presented. We demonstrate the existence of dark state polaritons and propose a protocol to transfer quantum information from one optical mode to another by adiabatic control of the control fields and present a proof-of principle experiment to adiabatically transfer light between modes of different frequencies in Rb-87 vapor. To that end a digital phase lock circuit has been designed that allows to electronically phase lock an external-cavity diode laser to a reference laser over a 7 GHz frequency range with sub-Hertz precision.