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Quantum light memory in atomic ensemble, further development of the theory and its results

Hassan Kaatuzian, Ali Rostami, Ahmad Ajdarzadeh Oskouei

Published Feb 9, 2004• 0 citations
OpenAlex

Abstract

We extend the theory of quantum light memory in atomic ensemble of Lambda type atoms with considering lower levels coherency decay rate and one and two-photon detunings from resonances in low intensity and adiabatic passage limit. We obtain that with considering these parameters, group velocity of probe (information) pulse and its amplitude does not tend to zero by turning off the control field, therefore there is no complete storage of light. Also, we obtain that there will be a considerable decay of probe pulse and stored information. We calculate the minimum velocity and decay rate for probe (information) pulse. In addition, we obtain that in the Off-resonance case there will be a considerable distortion of the output light pulse that causes in loss of the stored information, then we present limitations for detunings and therefore for bandwidths of practical lasers also limitations for maximum storage time to have negligible distortion of stored information. We finally present the numerical calculations and compare them with analytical results.

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We extend the theory of quantum light memory in atomic ensemble of Lambda type atoms with considering lower levels coherence decay rate and one and two-photon detunings from resonances in low intensity and adiabatic passage limit. We obtain that with considering these parameters, that there will be a considerable decay of probe pulse and stored information; also, we obtain that the group velocity of probe (light) pulse and its amplitude does not tend to zero by turning off the control field. We propose a method to keep the probe pulse in small values in turn off time of control field and to reduce the loss of the stored probe pulse. In addition, we obtain that in the Off-resonance case there will be a considerable distortion of the output light pulse that causes in loss of the stored information, then we present limitations for detunings and therefore for bandwidths of practical lasers also limitations for maximum storage time to have negligible distortion of stored information. We finally present the numerical calculations and compare them with analytical results.

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T. Yu. Golubeva, Yu. M. Golubev, O. S. Mishina +3 more

We study the coherent storage and retrieval of a very short multimode light pulse in an atomic ensemble. We consider a quantum memory process based on the conversion of a signal pulse into a long-lived spin coherence via light matter interaction in an on-resonant $\ensuremath{\Lambda}$ -type system. In order to study the writing and reading processes we analytically solve the partial differential equations describing the evolution of the field and of the atomic coherence in time as well as in space. We show how to optimize the process for writing as well as for reading. If the medium length is fixed, for each length, there is an optimal value of the pulse duration. We discuss the information capacity of this memory scheme and we estimate the number of transverse modes that can be stored as a quantum hologram.

arXiv:1012.5016•Physical Review A•May 9, 2011•31 citations
We extend the theory of quantum light memory in atomic ensemble of Lambda type atoms with considering lower levels coherency decay rate and one and two-photon detunings from resonances in low intensity and adiabatic passage limit. We obtain that with considering these parameters, group velocity of probe (information) pulse and its amplitude does not tend to zero by turning off the control field, therefore there is no complete storage of light. Also, we obtain that there will be a considerable decay of probe pulse and stored information. We calculate the minimum velocity and decay rate for probe (information) pulse. In addition, we obtain that in the Off-resonance case there will be a considerable distortion of the output light pulse that causes in loss of the stored information, then we present limitations for detunings and therefore for bandwidths of practical lasers also limitations for maximum storage time to have negligible distortion of stored information. We finally present the numerical calculations and compare them with analytical results.
We extend the theory of quantum light memory in atomic ensemble of Lambda type atoms with considering lower levels coherence decay rate and one and two-photon detunings from resonances in low intensity and adiabatic passage limit. We obtain that with considering these parameters, that there will be a considerable decay of probe pulse and stored information; also, we obtain that the group velocity of probe (light) pulse and its amplitude does not tend to zero by turning off the control field. We propose a method to keep the probe pulse in small values in turn off time of control field and to reduce the loss of the stored probe pulse. In addition, we obtain that in the Off-resonance case there will be a considerable distortion of the output light pulse that causes in loss of the stored information, then we present limitations for detunings and therefore for bandwidths of practical lasers also limitations for maximum storage time to have negligible distortion of stored information. We finally present the numerical calculations and compare them with analytical results.
We extend the theory to describe the quantum light memory in type atoms with considering (lower levels coherency decay rate) and detuning for the probe and the control fields. We obtain that with considering these parameters, group velocity of the probe pulse does not tend to zero by turning off the control field. We show that there are considerable decay for the probe pulse and the stored information. Also, it is inferred that the light field does not tend to zero when the control field is turned off. In addition, we obtain that in the off-resonance case there is considerable distortion of the output light pulse (with fast oscillations) that destroys the stored information. Then we present the limitations on detuning (bandwidths) of the probe and the control fields to achieve negligible distortion. We finally present the numerical calculations and compare them with obtained analytical results.
We show that there is no need for adiabatic passage in the storage and retrieval of information in the optically thick vapor of Lambda-type atoms. This information can be mapped into and retrieved out of long-lived atomic coherence with nearly perfect efficiency by strong writing and reading pulses with steep rising and falling edges. We elucidate similarities and differences between the ``adiabatic'' and ``instant'' light storage techniques, and conclude that for any switching time, an almost perfect information storage is possible if the group velocity of the signal pulse is much less than the speed of light in the vacuum c and the bandwidth of the signal pulse is much less then the width of the two-photon resonance. The maximum loss of the information appears in the case of instantaneous switching of the writing and reading fields compared with adiabatic switching, and is determined by the ratio of the initial group velocity of the signal pulse in the medium and speed of light in the vacuum c, which can be very small. Quantum restrictions to the storage efficiency are also discussed.
Ideal photonic quantum memories can store arbitrary pulses of light with unit efficiency. This requires operating in the adiabatic regime, where pulses have a duration much longer than the bandwidth of the memory. In the nonadiabatic regime of short pulses, memories are therefore imperfect, and information is always lost. We theoretically investigate the bandwidth limitations for setups based on individual atoms, or ensembles thereof, confined inside optical cavities. We identify an effective strategy for optimizing the efficiencies of the storage and retrieval process regardless of the duration or the shape of the pulses. Our protocol is derived almost completely analytically and attains efficiencies that are comparable to those obtained by numerical optimization. Furthermore, our results provide an improved understanding of the performance of quantum memories in several regimes. When considering pulses defined on an infinite time interval, the shapes can be divided into two categories, depending on their asymptotic behaviors. If the intensity of the pulse increases with time slower than or as an exponential function then the storage efficiency is only limited by the pulse width. For pulses defined on a finite interval, on the other hand, the efficiency is determined by the shape at the beginning of the storage or, correspondingly, at the end of the retrieval process.
The emergence of quantum physics from the page to the lab and the world at large is an exciting development of recent years. The prospects of absolutely secure communication and efficient simulation of physical systems have spurred great human effort into understanding these possibilities and turning them into realities. Photons are the most easily manipulated quantum particles and are a promising candidate for implementing these technologies. Limitations of photons include the difficulty of keeping objects that move at the speed of light, and producing strong interactions between particles that do not normally interact. The work presented in this thesis is motivated by the possibility of overcoming these limitations. The ability to faithfully store and reproduce a quantum state is essential for many quantum information technologies. Quantum memories for light have been developed over the last two decades to provide this ability. The group at the Australian National University developed the gradient echo memory (GEM): A quantum state of light can be controllably stored and released from an atomic ensemble by the use of additional optical fields and magnetic field gradients. This scheme was previously shown to preserve the quantum characteristics of the light. We used the GEM scheme with a cold rubidium ensemble to create the first optical memory that simultaneously beat the no-cloning limit, a benchmark for many of the technologies relying on quantum memories, and the loss rate for a delay line composed of optical fibre. We also created an analogue to a pulsed optical resonator using GEM with a warm rubidium vapour. This was done by replacing the circulating optical field of a resonator with light stored in the memory, and replacing the coupling of light into and out of that circulating mode with storage and recall from the memory. The bandwidth and repetition rate of this resonator were rapidly tunable as they were controlled by external optical and magnetic fields. We worked on implementing GEM with strings of thousands of atoms strongly coupled to the evanescent field of an optical nanofibre. This raised new possibilities for creating a true random access memory that would allow a more flexible use of the multi-mode capacity of GEM. We developed the theory for a novel type of stationary light in the gradient echo memory. Our stationary light scheme relies on the destructive interference of counter-propagating optical fields throughout the memory. The optical intensity scales with optical depth, as with other forms of stationary light. However, as the destructive interference could be set up over a much greater distance, more of the optical depth is available for generating stationary light. Finally, we studied how a control-phase gate for single-photon optical states could be implemented using a nonlinear interaction with stationary light. The stationary light generated by one state modulates the phase of another state stored in the memory. The second state modifies the stationary light, also producing a back-action on the first state and generating the required cross-phase shift.
We study the coherent storage and retrieval of a very short multimode light pulse in an atomic ensemble. We consider a quantum memory process based on the conversion of a signal pulse into a long-lived spin coherence via light matter interaction in an on-resonant $\ensuremath{\Lambda}$ -type system. In order to study the writing and reading processes we analytically solve the partial differential equations describing the evolution of the field and of the atomic coherence in time as well as in space. We show how to optimize the process for writing as well as for reading. If the medium length is fixed, for each length, there is an optimal value of the pulse duration. We discuss the information capacity of this memory scheme and we estimate the number of transverse modes that can be stored as a quantum hologram.