Squeezed light and application toward Quantum technology. Qtalk, QSB Yeditepe, Turkey
Hello, I am Saesun Kim, I am speaking with you from the University of Oklahoma Working at Albert Marino’s quantum optics lab at the center for quantum research and technology. Today, it is a great pleasure for me to share with you a perspective of quantum light, and How the quantum light play role in this quantum technology. Quantum technology is a newly emerging field that relies on quantum mechanical principles such as entanglement, superposition, and coherence. It is becoming the foundation for the next industrial revolution.
The quantum technologies that will emerge from this revolution Are quantum computers, and quantum sensors. quantum communication, Quantum computers will provide an exponential increase in computation power for specific problems in machine learning, drug development, battery design, and financial analysis. Quantumsensors can enhance the sensitivity of devices that are used in self-driving cars, medical devices, and satellites. Finally, as sensors and devices are connecting and exchanging data with other systems over the internet, Securing the internet of things has become a critical problem to address in order to protect the privacy of individuals. To this end, quantum key distribution can provide a secure network that is fundamentally secure against possible attacks from an eavesdropper. and also you have seen in the previous talk, the quantum network will connect the computers and sensors so we can have interaction between quantum devices to other quantum devices So we can have interaction between quantum device to other quantum devices.
To take advantage of quantum technology, it is crucial to generate and control the quantum resources quantum resources that can contain the quantum properties Those properties can be contained in either Light, matter, and other system For example, first picture is light resource that we generated in our lab. We can access the entanglement by taking picture with camera The second picture is the atomic resource from our collaborator Dr. Schwartzman. They are cooling down the atoms into the BEC level which can conatin the quantum property. The third picture is the IBM’s five Qubit processor. This is done by quantum rsource the superconducting Transmon using microwave resonator There are many quantum resources Amaong many quantum resources, Light have their unique property.
it allow us to sense, allow us to control, and allow us to communicate. People make quantum resource with atom and other matter, but they have to use light to measure the property People want to capture and control the particle, they need to use light to control the system by tweezers. People can generate qubit and circuit, they need light to communicate and teleport the state. Electromagnetic wave which include visible light, UV light, microwave and others, are foundation and platform for other quantum resources.
Here, I will share our approach to enhance the fundamental quantum resources, light And show how this play role in quantum technology. Before I talk about the quantum mechanical behavior of the light I want to spend some time in undergraduate level of quantum mechanics. Some people already know, and if you didn’t know, I am so sorry for the spoiler.
Light behave both wave and particle at the same time. This means that Particle exist in a single place, we know both momentum and position at the same time. Waves on the other hand, does not exist at the single point. It rather spread out on the space. So we cannot assigne the position of the wave.
In 1905, Einstein suggest insane idea that light is particle Which is called photon . He showed Energy of Photon is proportional to momentum which is defined by its frequency. So he conclude that light is both wave and particle. Since light is wave, now we have interesting property As I mentioned that we do not know the exact position of the wave, If we can concentrate the wave. we can get more accuracy of the position Now we know with more precision where the photon is, but we do not have enough time to measure the frequency, So we are less sure about the frequency. Einstein showed that frequency is related to momentum, So if we know more about the position, we loss information about the momentum Similarly, If we starch the wave, now I have more time to measure the frequency, So I know the frequency with more accuracy, but I unsure about the position of the light So there is some relationship between uncertainty of position and momentum.
Heisenberg discovered this uncertainty relationship, and he found the fundamental limit of how much we can know Einstein received noble prize for understanding the photoelectric effect Heisenberg also got Nobel prize for creation of quantum mechanics. This uncertainty is not the practical limit of our measurement, Even though we spend billions of dollar, we cannot do better. It is the fundamental limit of the universe. One thing to note is that Uncertainty of the frequency and position is not quantum effect, it is just furrier transformation, However, Heinsberg was able to connect with physical relationship of position and momentum uncertainty as a quantum meahncal effect. Now, we are ready to see how we describe light in quantum mechanics. For example, if we want to represent the object in in classical mechanics, We will use something called phase space diagram where x axis represent the position information, And y axis represent momentum information.
So if duck has positon of 4 and velocity of45, we can represent it on the plot with single point However, in quantum mechanics, because of the uncertainty principle, we do not have well defined position and momentum, So we cannot represent as point, Instead there is always some uncertainty associate with the point as shown in black region Similarly, if we want to describe the quantum mechanical behavior of the light in the phase space diagram, We have to represent the uncertainty region instead of the point And one more change is that x axis is now representing the real part of electric field and y axis is now imaginary part of the electric field Those quantity are related to the amplitude and phase. If we can do the time evolution of the field, it is more clear how they are related to amplitude and phase. If we can measure this state of light with really fast detector, We will see electric something like this. And it is clear how the real and imaginary part is related to the amplitude and phase. Imaginary part is oscillating, and it is correspond to the amplitude. And the real part here represtn the phase information.
Here you can see that red dot represent the classical description of the electric field, As you can see there is well defined amplitude and phase. However, in quantum mechanical description, there is noise coupled into the system, so we cannot define the amplitude and phase. So This is the quantum mechanical description of the classical light. This is the fundamental limit of noise in quantum mechanics, This is called Quantum noise Limit. Not every light can reach that limit, but this is the limit that good laser source could reach.
Since laser light is collection of photons, We can count them one by one Let’s say if we can measure and count the number of the photon arriving to the detector. The probability distribution we end of getting is the poissonian distribution. And the Possionain distribution occurs when the event is random and indepent. Like if you are counting how many people enter the convience store, The event will be more likely random and independent, so the number of the coustumer will follow the possionain distribution/ This is because that the light is emitted from the source randomly and independently l Like pouring a bucket of shots, and measuign the number of the shots on the detector.
So This Quantum noise limit, is from the particle nature of the light, Because of the particle nature of the light, this is also referred as shot noise limit. So what we have describes, is quantum mechanical description of the classical light. Then the question is that can we show the quantum nature of the light? The answer is yes, if we can squeezed the light, we can show the quantum nature of the light.
Obviously, It is not the physical squeezing of the light, we are squeezing the light in phase space. If we can squeezed the quantum noise limit so that uncertaintly relationship still hold while we can reduce the noise in one axes below the QNL What this mean is that if I make a measurement. Such as amplitude of the light, I will see less amplitude noise from the detector, but large undercetainty in phase. Now let’s re-think about the what it means by the particle nature.
if the photon is indepednetly and randomly emitted, it reveil the particle nature of light, Then, if the photon is not indepently emitted, more ordering, then it shows the quantum nature of the light. For example, if two photon are emitted together dependent to each other, This has quantum property we already know, it called entangled light which we are familiar from the previous talk. So in order to create the quantum light, we need medium that can generate the two photons at the same time. There are many way to generate the squeezed light, but the most popular way to generate it is using the crystal and atomic process. The first squeezing is done by the sodium system, and it has extended to rubidium and cesium to generate large squeezing.
However, The best squeezing is reported by the PPKTP crystal using parametric down conversion. The previous talk introduce the generation and application very well. In our lab, I am also trying build the squeezed light using crystal, but mainly we are generating squeezed state of light using atomic system. Atomic system is nice to work with it because it does not require any fabrication and locking the cavity. Additionally, it has great potential to interacting the quantum light with atomic system which I will talk about it later.
So if we send strong light field into the atom, It will start interacting wit the light. We can find the process that can generate ethe two photon at the same time. Since the generation are correlated, The intensity of the two beam must be correlated. Which means that intensity fluectuation is also correlated. Because of the correlation, if we subtract two noise together, We can see the noise reduction below the quantum noise limit.
In our lab, we used the process called four-wave mixing process in rubidium vapor to gernate the squeezed stet of light. In this process, we send light in to rubidium vapor cell. Here we send pump beam into the cell, Two pump photon is abospred while the two photn probe and conjagatea are emitted at the same time.
Due to their simultaneous generation, when we measure the intensity difference of the two beams, we can find that the intensity noise is reduced below the shot-noise limit. In our lab, we can generate 9 dB of noise reduction, which correposnde to 90% of the noise is reduced from the quantum noise limit. And also those light have nice property such as entanglement. So squeezed light have reduced noise property, Due to their property, it has been first proposed to use enhance the optical devices. This was proposed by Caves in 1981, He proposed that squeezed state of light can be used and mix wit the interferomenter to enhance the sensitivity.
This has been achieved and now people in LIGO, they are integrating the squeezed light to measure the gravitational wave of the far away objects Recently Ligo measure the merging black hole from 1.3 billion light-years away from us. Two black hole as they are merging each other, they create massive gravitional wave, make a ripples in space-time And this interferometers are sensitive enough to measure the those exicitng evenest. Therefore Quantum light is becoming a important part of this exciting journey of studying cosmo. In our lab, we are working with the squeezed state of light, And we are trying to extend the applicability and interactiibility of our system. We are using three approachs First we use quantum resource to enhance other system.
For example, we are integrating squeezed light with fiber and plasomic sensor to improve the sensitivity of the current sensors. Second, we are tyring to connect two types of quantum resources atom and light to make hybrid quantum resource. That can be beneficial for extrme sensitive sensors and control of the system.
And finally, we are distrubting the light into multiple system, so the quantum light can be more applicable in quantum technology. First thing we have enhanced is the plasmonic sensor. This is acually done by my colleague javad and ashok, they have shown the enhancement sensor called plasmonic sensor. Plasmoic sensor is high sensitive sensor which has wide range of application in chemical detection, liquid, water sensing, bioimaging and medical application.
This sensor is relies on resource plasmon, which is the light interactiving with surface of the silver structure. Because of the unique pattern on the surface of the silver layer, electrons are collectively excited and coupled with the light, and this interaction can transmit the light to the other side of the surface. Which is called extraordinary optical transmission, because ordinarly light will reflect on silver layer like mirror. But when the plasmon interact wit the light, they allow light to go though the surface. The useful property of the plasmoic sensor is that this transmission is depend on property the environment. For example, if I change the air density, this will introduce the shift of the resonances of plasmonic interaction, and we can see that our transmission change as the resonance shift.
Sensitivity , how small resonance shift we can detect, is given by how well we can detect the change in transmission, which is limited by the noise of the measurement So if we send the classical light from laser into plasmonic sensor, This is what you will see on measurmenet. This plot is from the experimental setup that environment is modulated with modulation frequency at 199KHz. On the left side, you can see the signal from the noise, but on the right hand side, you can notice that there is no signal, because the modulation signal is smaller than the left side of the plot, so we cannot distinguish the signal from the noise. Since this is limited by the noise of the measurement, we can use squeezed light to make it better. Squeezed light has reduced noise property below the shot noise limit, so if we use squeezed light, As you can see the noise is lower, and signal is in the same level, so it is now easy to distinguish the signal from the noise. This is how our lab can enhance the sensitivity of the sensor using the quantum state of the light.
We are now working on other type of sensor, which is fiber sensor. Physics is very similar. As the light propergate through the optical fiber, Light is interacting with the phonon from the vibration mode in the material. Light will scatter because the interaction of environment so by looking at the transmission of the light, We can sense the change of environment near the fiber. And fiber can sense the change of environment such as heat, vibration, stress and sound. That is why this sensors are used in commercial domain such as building control, leak detection, and highway monitoring.
Since they are using the light, I am currently working on enhancing the sensor using squeezed light. I hope to share more once our result are in public. I have shown some of the way to enhance the system. However, some of the most sensitive sensors are based on atomic system Because they are quite sensitive to the external field.
Amazingly, some of the atomic sensors, like atomic clocks are now commercially avaible, and this can enhance the synchronization of the satellite and GPS signal. Beside the sensor, people also use atom to do quantum computing by Rydberg atom, And store the quantum information to use it for quantum memory So atomic ensembles is another powerful quantum resource. Therefore, there are significant interest connecting light resource and atomic resource to take advantage of the benefits of both system In order to do that, we need the strong interaction between atom and light In general, if the light is not on resonant, there will be no interaction between them , So we need to first generate the atomic resonant squeezed state of light, And then it is important to match linewidth of the laser to interact with the atomic system. Unfortunately, the efficient generation of narrow-band resonant squeezed has proven to be a challenge. there are two different ways to generate atomic resonant squeezed state of light. Using the nonlinear crystal with cavity, process called optical parametic oscillator, people can generate narrowband squeezed light.
There is a challenge to generate atomic resonant light, because crystal must be pumped by the UV light which does not give good efficienty Although there are many difficulty, it has been shown to generate rubidium and cesium resonant squeezed state of light. On the other hand, it is more natural to generate atomic squeezed light by atomic system. Because atom uses the atomic processeses to generate the squeezed light which already provide narrowband and near resonant light. However, it is difficult to generate the resonant squeezed light due to the strong absorption near resonance.
To avoid the absorption at the resonant transition, What we did previously was taking advantage of different isotope of rubidium Isotoype is like twin sisters, rubidium85 has twin sister named rubidium87. They have very similar energy level structure. Since their energy levels are very close each other, we could operate our generation in different regime and optimize to generate resonant squeezed light. For more details, please check my papers.
So, we have shown that we can generate the large resonant squeezed light with 87. However, this approach only works for the certain transition of Rubidium 87 and we do not have much frequency tunability. And also other atoms does not isotope like rubidium does, Therefore in order to generate other atomic resonant squeezed state of light and also have tunability. we need alternate approach to solve the problem.
Here we provide the solution to generate the atomic resonant squeezed light by shifting the energy level. The idea is simple, if we can move the absorption away, we do not need to worry about the being a resonant. There are three common way we can shift the energy level. Manetic field, optical fiend, and electric field. If we apply Magnetic field to atomic system, called zeeman shift, energy level will start to split into many different way, And it is too messy for us to use it. Another possibility is using a strong optical field, called AC stark shift, However, additional light will couple with our process, and it will make it difficult to generate the squeezed light.
The last option is using the electric field, called DC start shift. We can do it by applying high electric field with two metal plate. In this case Electorn from the atom will be attracted to one direction, and the nucluis will be attractive to the other direction, And effectively it will shift the electron orbit, and this will shift the energy level sturctue. Nice thing about the DC start shift is that whole energy level will shift together, and it does not affect our four-wave mixing process. Howevfer ,there is one big problem It requires very strong electric field.
To shift the amount for what we need, we need to apply 25KV in 2mm spacing of the plate. Which corresponds to 12 MV/m Yes, the problem of the research is that I have to work with extremely high voltage. Since we are working with low current, it is not too dangerous, But I have touched 10KV twice, it was very interesting experience. So don’t try at home, Here is the my experinetal demonstration of how we can apply high electric filed to the source. We assemble the vacuum chamber, inject the rubidium, we apply high electric field.
Very simple, but there is One big problem, it is the arcing. If the surface of the plate or any inperfection inside of the chamber will create the dense electric field, and this will introduce electrical arcing. If you know what arcing is, that it is high intesnti electromagnetic pulse. So basically every time I fail, I send EMP to entire our lab where the all the expensive electronics are. I broke laser twice, fried two photodiode, two power suppliers, and list goes on. Anyway, we were able to demonstrate the physics, that’s what matters right? Here is the result.
This plot is the transmission plot, where x axis represent the laser detuning from rubidium atom. And the y acis represent the transmission. The green trace is like a reference trace, each peaks represent the energy level from rubidium85 and 87. Once we have FWM process, You can see two beam are generated.
Red and black. we can see the gain is around 1GHZ away from the one particular transition of the rubium. This is where we get the best squeezing. By applying DC electric field, we show that we can shift this process toward the resonance by 250MHZ . We also measure intensity difference squeezing, Current system we start with 4dB of squeezing and we preserve 4dB after dressing the system with DC electric field. Therefore, we succefully demonstrate that we can shift the frequency of the light with external electric field.
We are also working with BEC group, and currently trying to generate sodium resonant squeezed light. I am really excited to know that kind of physics we will see when I interact the quantum light with atomic quantum state. We have shown by interaction between squeezed light and matter, that squeezed light can enhance the system, we can generate the light which can interact with atom. The last part of the talk is about distribution the squeezing.
In order to connect with multiple quantum system, it is important to distribute the information we have to the multiple party. Elise give great overview of the quantum network, it is possible that we can store our amplitude and phase information as a bit of information And code into the system. Especially, our system can have entanglement, The ability to distribute the entaglement into multiple party allow us to create the genuine multi-partite entanglement And this will extend our applicability of squeezed light to other quantum technology such as Distribution quantum sensor, Quantum teleportation quantum key distribution, and one-way quantum computing One of the way to distribute the resource is by distributing in the space , And Our first approach to distribute the quantum resource is by quantum imaging Which is done by my colleague Ashok and Gaurav. The four-wave miinxg have to conserve energy and momentum, this leads to the quantum correlation in temperal and spatial domain so They take a picture of the two entangled light by camera, And show that there is minimum size of the correlation area known as the coherence area And the coherence area can used as pixel of the image.
And as you can see the information in the pixels are correlated to each other, Uisng the pixel, they show there is entanglement between two image. Now, Our group is working on how the distribution of correlations can be controlled by modifying the configuration of the four-wave mixing. So they are trying to make different shape of area are correlated with other pixel This gives an extra degree of freedom to encode quantum information and interface with other quantum systems.
So the first menthod show how we can spread the correlation in the space, Our second approach is to distribute the entanglement in multiple beam. This is done by repeat the four-wave mxing process, and making a network using the process. For example, here each box represent four-wave mixing process. Which Generate entagled light Then this two light will be entangled each other, and by passing it to second stage process, So we can distribute the entanglement to four different beam By using this compact method, we have theotically show that we can genretate four, six and eight partite entanglement, And we are now designing the experiment to verify that we can actually generate the genuine multipartite entanglement. The ability to generate the multi-partite entanglement will open the door for applications in quantum communication, and one-way quantum computation so Our quantum resource squeezed light can enhance the current system, interact with other quantum resources, And distribute in space and multiple party. All the work I present is done by our group.
I want to make sure the all the credit goes to the right people. Alebrto Marino is our advisor, who made everything possible. enahcnement of the plasmonic sensor is by Ashok, Javad, Umang Quantum imaging is by Gaurav, Siva and ahosk. And Kit is starting to work with the resonant light project.
As we already know that, it is important to create quantum computer, and quantum sensors. However, it is also important to make a network of the quantum hardwares. For example, you and me, we’re talking over zoom on your computer. No matter how fast your computer is, if we do not have network, functionality of the computer is limited. For quantum computer, it requires a way to exchange the quantum information and communicate each other, but the classical channel will destroy the quantum information. Therefore, the ability to interact with other system is very important.
What we do mainly is sensing for now, but interacting the quantum resource to plasomon, phonon, and otheratomic system, Also spread the quantum resource in space and to the multiple party, we believe that we are slowly improving the ability to interact with other quantum resource and provide the foundation for the quantum networking. I am really excited to witness the convergence of difference quantum resources and prepare the next industrial revolution based on quantum technology using squeezed state of light Thank you for your attention.