00:00:00 Could you tell me what is so fascinating about quantum mechanics?
00:00:08 There are two aspects I find most fascinating about quantum mechanics.
00:00:16 The first aspect is that... this is a difficult question actually, I have to think about it a little bit.
00:00:28 What is so fascinating about quantum mechanics is that until now no one truly understands quantum mechanics, especially this interesting concept called quantum superposition.
00:00:42 What quantum superposition leads to is this concept called quantum entanglement.
00:00:49 Even though it has been over a hundred years since we have come up with quantum mechanics, we still don't understand why there is quantum entanglement.
00:00:55 So the more you don't understand, the more interesting it becomes.
00:01:04 And can you explain entanglement, what it is, what it means?
00:01:09 The basic principles of quantum entanglement can be understood like this:
00:01:12 Let's say we have two dice, like the ones we normally use for gambling.
00:01:20 It doesn't matter how far apart the dice are, one could be in China and the other in the Netherlands.
00:01:27 So if you roll your dice and then call to ask me, "Jian Wei, do you know what my dice says?"
00:01:35 I just have to look at the dice in my hand and can tell you what your dice says.
00:01:40 It's to say that no matter how great the distance between the dice is, according to Einstein there is some strange interaction between these two far away locations.
00:01:49 When the result appears here, the far away dice has the same result.
00:01:55 This is a typical result we get when we experiment with quantum entanglement.
00:02:05 What I like about your work is that you somehow are able to put the theory into practice.
00:02:14 Can you elaborate on that?
00:02:17 There are two types of people study quantum mechanics.
00:02:24 The first type goes, "This is what quantum entanglement is; we don't have to understand why it exists."
00:02:30 They go straight onto testing physics phenomena within the physical system.
00:02:36 And this has resulted in various useful applications such as
00:02:40 semiconductor lasers, nuclear magnetic resonance, giant magnetoresistance, and even energy generation.
00:02:48 Because these type experiments on condensed state and quantum phenomena but do not ask why entanglement exists, they have already greatly changed our lives.
00:03:00 The other type, my and colleagues in the U.S. and Europe, including the Netherlands, also have had great results when last year
00:03:10 we conducted experiments on the existence of quantum entanglement.
00:03:15 During this process we gradually developed a technique to actively control one or more particles.
00:03:28 Once you can actively control particles, a new field emerges
00:03:33 called quantum information science.
00:03:35 Quantum information science has some practical applications.
00:03:39 The first is quantum communications.
00:03:43 Quantum communication can theoretically provide us with an unfailingly safe communication method.
00:03:51 This could transform cyber security
00:03:57 This is very important to our daily lives and of our desires to keep our secrets secure.
00:04:07 If I tangle up the tiny particles that make up the quantum world, they will gain a great computational ability that we can use for quantum computation.
00:04:26 For example, if we combine all of the computational power on Earth we can perhaps count to 2^80 a year.
00:04:39 But if we can harness only a hundred of those small, entangled particles, it may hold a counting power a million times more powerful than all of Earth's computational power combined.
00:04:54 That's to say if there were a million Earths in this universe, all these Earths combined would have the same computational power as what we could achieve with 100 tiny particles.
00:05:08 When you think about it this way, quantum communications can bring about a great advancement in computational power.
00:05:14 The third aspect is that we can use quantum mechanics for getting close measurements, called quantum measurements.
00:05:21 Quantum measurement could greatly advance GPS navigation and make it more precise.
00:05:33 We could also use the same techniques to advance biomedical detection technology as well.
00:05:43 Now you can see that this very fundamental, physics question can be used for many practical real life applications.
00:05:58 When I visualise a quantum computer, what does it look like?
00:06:12 People often ask me this question.
00:06:16 If we try to picture a regular computing device, we would picture a everyday computer with computer chips, like a PC.
00:06:26 However if we go back 200 years, people in China would have used an abacus made out of bamboo to do calculations.
00:06:38 If someone back then tried to imagine what a computer should look like, they would imagine something made out of wood, or bamboo, or perhaps marble.
00:06:50 So I think that currently it is hard to imagine what a quantum computer would look like.
00:06:57 I can tell you though, that I once had a dream about a quantum computer.
00:07:03 That quantum computer was not like a typical computer, but was a pile of material that was neither solid nor liquid, and was constantly shifting and emitting a blue light.
00:07:22 It is hard to imagine what an actual quantum computer from the future would look like.
00:07:27 Currently many scientists around the world are taking different approaches to try constructing a quantum computer
00:07:35 Some people are using photon, others are using atoms, and some are even using superconducting qubits.
00:07:49 So in the future there might be many different types of quantum computers made out of different martials that can perform different functions.
00:08:00 So it is very difficult to say what a quantum computer is going to look like.
00:08:05 When I look at your work in quantum satellites, what big step is that?
00:08:16 What happened last week, can you elaborate on that?
00:08:19 We are ecstatic that we were able to successfully launch the first ever quantum science satellite.
00:08:29 After the launch we went through several days of testing, and so far the satellite has been functioning stably.
00:08:36 There were several reasons we wanted to launch this quantum science satellite.
00:08:42 The first was that if we use fibre optics for quantum communication on the ground, because quantum signals cannot replicate it would be difficult for signals to get very far.
00:08:55 One of our colleagues in Switzerland, Nicolas Gisin estimated that if we have a thousand kilometre of fibre optics
00:09:08 and even if we use the very best technology available to make this quantum communications device
00:09:17 we can only send a single byte(?) of information every 300 years .
00:09:22 So this is not very useful for long distance quantum communication.
00:09:29 But what is interesting is that if we have a quantum satellite which can send signals through the air, according to our theories and what we have tried to build, we can send several hundred kb(?) of information over a thousand kilometres, every second.
00:09:47 This is enough to transmit important quantum information tasks securely.
00:09:54 This also means after launching into the atmosphere, we are able to do experiments that were not to do on the ground.
00:10:05 Also from a scientist’s perspective, this is an important contribution to fundamental research.
00:10:12 Once we are able to send send quantum light particles through the atmosphere we have the ability to investigate whether quantum entanglement exists and
00:10:25 what kind of changes will be happen to the quantum state under the influence of relativity and gravity.
00:10:35 One likely possibility is that we will move towards testing quantum mechanics in space.
00:10:41 And once we are in space, we we can conduct further investigations on quantum nonlocality on a larger scale.
00:10:49 According an American colleague Tony Leggett, there is one last experiment in quantum physics nonlocality we have not yet performed
00:11:01 To do the final test of quantum nonlocality, we have to observe quantum entanglement separated over a distance of 300 thousand km, when there will be only residual free will.
00:11:20 So we will have the ability to conduct that final test of quantum nonlocality.
00:11:27 Everyone really hopes that we can unify quantum and gravity theories into a theory of everything
00:11:42 There are many models for this, but which one of these is compatible with the natural world?
00:11:48 When we transport quantum photon hundreds of thousands of kilometres away, we can test many different quantum models.
00:12:02 Let's see if we can shift the boundaries of physics exploration.
00:12:08 It's mainly because of these two reasons: practical and fundamental research.
00:12:17 The theory of everything; could you explain a little bit more what that is?
00:12:23 Us physicists all have all have this ultimate dream.
00:12:29 Currently we know of four fundaments forces of nature within the natural world.
00:12:35 We have known of gravity the longest, and then we have electromagnetism, strong nuclear force, and weak nuclear force.
00:12:46 We already have a standard model in which we have unified electromagnetism, strong nuclear force, and weak nuclear force.
00:12:58 So what we dream of now, which Einstein had started to do, is to combine all four forces into one unified theory.
00:13:10 But in this process of unifying has had many difficulties, and fitting gravity into this is a great challenge.
00:13:17 Scientists have developed many different theories about this, such as quantum gravity.
00:13:23 Quantum gravity tries to combine the theories of relativity and gravity.
00:13:30 If we can combine these theories, according to our current understanding we will have created a theory of everything, a theory that we can use to explain every natural phenomena.
00:13:42 But there are many candidate theories, so which one is the one truly reflective of our natural world?
00:13:52 You've got this theory of everything. The steps you have been taking in the past year, when you look at the theory of everything, how big of a step have you made?
00:14:07 In order to do these kind of experiments the first important step is to move the laboratory to space.
00:14:22 We have officially only started making satellites in 2007 and before that we have done related experiments on the ground for a while.
00:14:30 The earliest we have done these kinds of experiments is in 2003.
00:14:33 It is through many years of effort we could move our experiments to the atmosphere.
00:14:39 On the way we have developed and refined techniques.
00:14:44 The parameters of our satellite is based on the calculations done by an American colleague Paul Grant(?).
00:14:52 With our techniques we can now make a highly precise telescope that can see a license plate on one of Jupiter's moons.
00:15:03 This is because our spacial resolution is very high.
00:15:11 We also have to be able to accurately detect of low-level light.
00:15:19 Now lets say we are on the Moon, and you light a match to smoke a cigarette.
00:15:25 When you strike the match you produce light, and we need to be able to perceive that light on Earth.
00:15:31 When we have the refined technology to do so, if we are successful in our experiments this time
00:15:36 we hope for our next step that we can work with China's lunar landing programme and experiment with distanced quantum entanglement of 300 thousand kilometres apart.
00:15:51 Currently we have completed some experiments on the ground
00:15:56 so if the upcoming tests are successful, we should be able to achieve hundreds of thousands of kilometres distanced quantum entanglement.
00:16:04 After these tasks have been completed, I think we can then conduct the primary investigations into the accuracy of proposed quantum models I have mentioned earlier.
00:16:18 So we have three steps, and this is just the first.
00:16:26 When you say three steps, what are the three steps?
00:16:34 The first step is hopefully to conduct important tests in the air, test if it feasible to do so.
00:16:47 After testing, we will be able to realise new quantum ground
00:16:52 that can be used for secure quantum communication.
00:16:56 The second step is for a 1200 kilometre quantum entanglement distribution
00:17:02 to discover if quantum engagement can still occur with a separation of thousands of kilometres.
00:17:12 With the foundations from these two experiments, we can go on with the second step, which is to have a satellite between the Moon and Earth
00:17:25 at the Lagrangian point, from where we will distribute two points of quantum entanglement.
00:17:30 This way we achieve free willed quantum nonlocality from a great collective distance of 300 thousand kilometres.
00:17:43 According to our colleague Tony Leggett, if after we conduct this experiment we will prove the nonlocality of quantum mechanics.
00:17:50 The theory of quantum nonlocality will be confirmed and the debate will end.
00:17:57 With these foundations we will then use the techniques we have developed to transmit light over a great distance
00:18:05 to investigate quantum gravity models.
00:18:11 These are the three steps
00:18:18 Entanglement communication, how does that work, what do you mean by that?
00:18:25 We have two kinds of experiments with quantum key distribution
00:18:33 The first type is using a single photon, and not using quantum entanglement
00:18:41 This occurs between a satellite and the ground.
00:18:44 There will be a weak light source from the satellite which will shoot several hundred individual photons to the ground every second.
00:18:54 On the ground there is a ordinary antenna that will receive the signal
00:18:58 and can conduct classical communication with the satellite.
00:19:04 After a couple exchanges we come up with a indecipherable key together.
00:19:10 Here we have used two fundamental principles.
00:19:14 The first is called quantum indivisibility.
00:19:18 Let's say we are drinking a glass of water and I split the glass of water into two half glasses of water
00:19:25 And we split the half glasses of water, which are now a quarter full.
00:19:29 After slowly dividing we now have single water molecules
00:19:41 but we can’t have half molecules or quarter molecules.
00:19:46 This is the same with light.
00:19:47 If you take a 25 W light bulb and magnify it, you can see that the light is composed of individual particles.
00:19:55 We call these particles photons, a single quantum.
00:20:00 If someone tries to eavesdrop, he cannot divide the photon in half
00:20:07 so either the eavesdropper steals away the photon or it will not be stolen and the photon reach the ground.
00:20:12 The photons that reach the ground are the ones that were not stolen by the eavesdropper.
00:20:18 Using this method you can say if you have been eavesdropped on, you won't receive a signal and you can give up on those signals.
00:20:24 This is its first principle.
00:20:27 The other principle is quantum superposition
00:20:34 Quantum superposition guarantees the quantum no cloning theorem.
00:20:43 When a photon is being transmitted, if you take a picture of it with a camera and you want to measure and make a copy of it
00:20:53 this influences the photon’s state of being so that when you receive the signal on the ground you will know that the signal has been observed by someone else.
00:21:02 You can then reject the signal
00:21:04 Because quantums have two characteristics
00:21:06 the first being indivisibility, which says it is individual and whole
00:21:10 the second being that it cannot be copied
00:21:15 we can guarantee the security of the quantum key.
00:21:21 This is the first type of quantum key distribution, which has nothing to do with quantum entanglement.
00:21:23 Once we can apply quantum entanglement to quantum key distribution
00:21:29 we can have our satellite be our source of quantum entanglement during our experiments.
00:21:35 It will produce several tens of millions of entangled photons every second and
00:21:43 they will be distributed to two points on the surface of the earth that are approximately 1200 km apart.
00:21:50 If you detect a photon on one point it is guaranteed we will detect an entangled photon on the other point.
00:21:57 If you don't detect anything at that point than the other point will also come up empty.
00:22:02 If there is one here, there is one there.
00:22:04 This kind of situation is called a state of quantum entanglement.
00:22:08 With this connection, you can achieve quantum key distribution.
00:22:15 You can either use single photons or quantum entanglement to achieve quantum communication.
00:22:25 Why is your team, your lab, so ahead of other labs in the world?
00:22:36 I think this is because of three reasons.
00:22:40 First of all I was lucky to be in Austria to join Zeilinger's team in 1996 when they were first starting out.
00:22:51 I was a PhD student at the time, and was very lucky to be a part of the first wave of people working on quantum communications.
00:23:01 By chance I was one of the founding members of the team that grappled with the first theories and techniques of quantum communications.
00:23:15 Later in China, we started to work with quantum communications and we needed the latest technology to get a satellite launched.
00:23:23 Our team is composed of those who have picked up the latest lab skills from all around the world
00:23:32 For example, I myself was first in Austria then in Germany, so we have quantum storage technology and quantum entanglement control technology.
00:23:43 We have students who went to Stanford and brought back the best single photon detection technique back to China.
00:23:51 We also have students who went to Cambridge and brought back the best quantum light source techniques.
00:23:57 At the same time I have colleagues who brought back from Canada and Japan the latest theories on quantum key distribution.
00:24:06 Because we are a relatively large team whose skills complement each other,
00:24:16 we are able to do things that my supervisor back in Austria, who had a smaller team, could not do.
00:24:23 This an critical reason why we have come to the forefront of fundamental research in our field.
00:24:29 The third aspect is our project is supported by the Chinese Academy of Sciences.
00:24:34 If we, for example, need very precise adaptive optics
00:24:43 China has already has been supporting this technology as well as space technology for several years.
00:24:48 China had dozens of labs in Shanghai, Beijing, Chengdu, and others just in the field of quantum communications.
00:25:01 They have joined our group and we now have a new and very complementary group with collective knowledge of many of the latest technologies.
00:25:11 Also because the Chinese economy has been good the past few year
00:25:17 we have been able to ask for a quite substantial amount of funding and thus advance quickly.
00:25:26 So it's almost China's pride
00:25:27 Yeah
00:25:29 In what way? In what way is China proud of this project?
00:25:35 In China we are trying to understand approach a satellite launch from a different angle.
00:25:45 Before when we came to science, we've always been following and imitating
00:25:54 but very rarely were in at the international forefront.
00:25:58 Eventually we want China to be running with the rest, and perhaps in some areas lead the way.
00:26:08 From a certain perspective, when it comes to quantum communication field our satellite has put us in the front of the international footrace.
00:26:17 So when naming our satellite, after consulting with Chinese colleagues, we decided to name our Satellite Micius.
00:26:26 Micius was an ancient Chinese philosopher, educator and scientist.
00:26:35 According to literature he is the first in the world to suggest that light travels in a straight line.
00:26:45 This was something that was not suggested in the West until much later.
00:26:53 Micius when it came to the study of light, of particles, but others also thought of particle theory too later (Micius was the first to theorise of particles, but it was others after him that thought of particle theory???)
00:27:08 By naming the satellite Micius, we wanted to explain that as long as we have the opportunity to challenge ourselves
00:27:16 us Chinese people can make meaningful contributions to science.
00:27:21 We also have another reason, which is that throughout this project we benefited from a flow of international collaboration.
00:27:27 Therefore, after we finish our first round of scientific missions we are opening our project to scientists from all around the world.
00:27:37 We welcome everyone to work with us.
00:27:40 We are working toward international collaboration, firstly with Austria's scientific society
00:27:46 to achieve quantum communication between the satellite and Beijing, and between the satellite and Vienna.
00:27:54 From there we can achieve quantum communication between Vienna and Beijing
00:28:01 We also currently have agreements for collaboration with our German and Italian colleagues.
00:28:07 We want, with everybody’s help, work towards advancement in quantum communication science and quantum technology applications.
00:28:22 When you look at your dream this was a big step you made by sending out the satellite. What are the steps, what are you striving?
00:28:34 If this satellite is a success, we have two further steps to consider.
00:28:42 This satellite does have the latest technology
00:28:50 If you want to guarantee that the satellite is able to successfully complete its missions
00:28:55 the quality and stability of its engineering is very important.
00:29:02 Currently this satellite can only work at night
00:29:05 because the photon signals are too weak compared to sunlight.
00:29:12 The satellite is only functional at night when there is no sunlight and it is easier to do experiments.
00:29:20 So the next step is to develop a technology that can make sure we can conduct experiments during daytime too.
00:29:25 Then we can perform experiments both day and night.
00:29:32 We need this ability in order to do the experiment I mentioned before with the distance of 300 thousand kilometres.
00:29:41 Once we have the technology to do experiments throughout the day we want to use it for secure quantum communication
00:29:56 and also link our experiments with the moon landing programme
00:30:05 , towards big scale quantum nonlocality validation.
00:30:11 China's moon project could you explain a little bit what this project is? So I can understand what it means?
00:30:18 China's moon landing plan is also called the Chang'e moon landing plan
00:30:25 and has already been going on for several years.
00:30:28 The first Chang’e, I don't remember when it launched
00:30:33 but currently they have a long term plan to send people on the moon in the near future.
00:30:41 When it comes to China's space exploration there are some ambitious plans.
00:30:50 When it comes to the details, because I am not directly involved
00:30:56 I am not sure about the time scale.
00:31:00 However they have started to seriously consider sending a satellite to the moon.
00:31:08 We have some equipment on the moon that is working because of Chang'e.
00:31:17 Chang'e is goddess from a famous Chinese fairy tale who drank an elixir of immortality
00:31:29 and flew to the moon; this is why the programme is called Chang'e.
00:31:38 Chang'e is the first Chinese satellite to orbit the earth.
00:31:42 It later delivered equipment to the moon
00:31:52 The next step is for China to perhaps send a satellite to the far side of the moon to bring back a sample to Earth.
00:32:01 After this mission is complete I estimate that within 10, 15 years China could be able to send people to the moon.
00:32:17 Looking at your work here in China is it how do the Chinese authorities look at your work? It is very important to be able to conquer space and time?
00:32:37 I think there are many sides to this.
00:32:42 I think everyone is very excited about this.
00:32:45 Actually this quantum science satellite was launched in the twelfth...China has something called a 5-year plan.
00:32:55 There is a first 5-year plan, a second... and now we are past the 12th and currently in the 13th 5-year plan
00:33:03 The quantum science satellite is one of the several satellites that were a part of the 12th 5-year plan.
00:33:08 Despite this being the first quantum science satellite, this is actually the Chinese Academy of Sciences' third science satellite.
00:33:17 We previously had a dark matter detection satellite
00:33:22 and another, the Shijian-10, to work with microgravity and the life sciences.
00:33:30 The third is our quantum satellite.
00:33:34 The reason why people were exited about our quantum satellite was because
00:33:36 firstly, this satellite was the first of its kind.
00:33:39 So despite it being the third, it was also the first.
00:33:43 Besides that, this satellite incorporated techniques that we did not possess before.
00:33:53 And because were not able to get what we needed from Europe or the US
00:34:01 this satellite was manufactured with solely Chinese technology.
00:34:07 With this in mind, we are very happy and satisfied with our technological progress.
00:34:16 This satellite to our country is to certain degree a symbol
00:34:29 that as time progresses, we are slowly able to do something meaningful.
00:34:41 We are not only making purely technological progress but we are also contributing to fundamental research.
00:34:51 We think that China should be able to contribute to global scientific advancement.
00:34:59 However, we also often hear other sentiments.
00:35:05 What are they?
00:35:08 There are several different ones.
00:35:09 The first is because quantum entanglement is a very difficult to understand concept.
00:35:16 Before this kind of concept is universally accepted, those who are not familiar with quantum mechanics
00:35:24 may have doubts or suspicions of whether quantum entanglement is real.
00:35:31 However within the scientific community there is a general acceptance of quantum entanglement.
00:35:39 We frequently hear criticisms online of quantum mechanics and the theory of relativity.
00:35:44 However, I think with the advancement of science this kind of criticism should slowly dissipate.
00:35:49 People also are anxious for our technology to become usable.
00:35:59 Of course we think they should be of use.
00:36:01 In the near future, we should be able to use quantum key distribution to secure quantum communication between Beijing and Vienna, for example.
00:36:09 But when this would this technology be available to the public so everyone can benefit from this?
00:36:15 I think we need to be patient and wait until we have refined the technology.
00:36:21 Some people might think that after the satellite launch, we have solved all the problems.
00:36:29 This is something that we often need to explain, that we need to wait, we need to have patience
00:36:34 We have to wait for technology to improve.
00:36:38 What kind of other applications can you imagine that are ahead for us with quantum mechanics and all the developments that are going on?
00:36:48 The first thing, as I mentioned earlier, is to increase the security of our communication and our cyber security.
00:37:02 The second is to use the developed technology to can see an image from very far away.
00:37:08 We can use quantum imaging to from explore earth's resources (?).
00:37:23 So far it seems these are the two main applications.
00:37:28 In the a long term there are some interesting plans
00:37:31 to use the quantum entanglement distribution in space and have two clocks on Earth synchronise.
00:37:41 I'm talking about quantum clock synchronisation.
00:37:42 We only have theorised about this.
00:37:45 Mikhail Lukin from Harvard have recently published some interesting articles
00:37:52 in which he proposes that we can use free space quantum engagement distribution to achieve two clock synchronisation on Earth.
00:38:02 Everyone knows that this clock synchronisation would be very useful when it comes to GPS navigation.
00:38:09 So aside from communication there will be also real life uses in imaging and time synchronisation.
00:38:18 Hopefully an answer to the theory of everything.
00:38:22 Yes but it's a long-term project that will probably take another twenty years
00:38:31 Maybe you can say that in Chinese?
00:38:32 That is of course our most ambitious thought.
00:38:40 If us Chinese scientists are able to investigate and contribute to the theory of everything
00:38:49 it would something to be proud of.
00:38:53 This would, according to our colleague Tony Leggett,
00:38:58 take 30 years, or perhaps 100 years.
00:39:02 However we are more inclined to believe that it won't take that long, it wont take a 100 years.
00:39:10 I very much believe that within my lifetime I will we able to witness the ultimate proof of the theory of quantum nonlocality;
00:39:18 perhaps in 15 years or perhaps in 20, but at least much shorter than 100 hundreds.
00:39:27 It's magic right? There’s something magical about it
00:39:31 And when you look at your personal dream, how do you envision the dreams you still have?
00:39:44 My dream is not the about the application of quantum technologies.
00:39:52 My dream is to do fundamental research and to work on and contribute to the theory of everything.
00:40:12 A great wish of mine is to discover through experimentation with quantum entanglement
00:40:22 why quantum entanglement exists.
00:40:24 When I was learning about quantum physics
00:40:28 at first i had some difficulties(?)
00:40:29 Afterwards I learned about Einstein’s' work on quantum entanglement
00:40:35 deep in my heart, even though there is innumerable proof that Einstein's thoughts were wrong and even though
00:40:42 experiments with quantum mechanics again and again disprove Einstein’s theory on the principle of locality,
00:40:49 deep in my heart I want to do experiments that could perhaps go on to prove that on a higher level
00:40:56 we come back to a variation of Einstein's principle of locality theory.
00:41:04 So in my heart I am still a believer in the principle of locality and I hope to do something in this field.
00:41:10 Yet I don’t know if I can do discover something new within my lifetime.
00:41:14 Those are my wishes; I think that us scientists spend so much money
00:41:21 in hopes that the technologies we make are of some use and so we push it towards to it being used.
00:41:27 I hope that these technologies can be put to good use in the future
00:41:33 like in secure communication, or in imaging, or in clock synchronisation.
00:41:44 Then, to the general public their investment in scientists
00:41:49 is not in vain and they will give us even more money to let us spend on fundamental research.
00:42:02 You were talking about quantum entanglement. I know I asked you before, but could you explain again what is quantum entanglement?
00:42:10 Quantum entanglement is actually is an embodiment of the theory of quantum superposition multi-particle system.
00:42:22 What does that mean?
00:42:24 I'll give an example: to simplify this lets assume there are only two states.
00:42:31 There is something we call spin, and it can be in a state of either up or down
00:42:35 If we have two particles, according to quantum mechanics, a particles in superposition has a spin
00:42:43 state of either up or down.
00:42:45 This can occur simultaneously.
00:42:47 So if there are two particles, it will exists in a strange state of being.
00:42:52 Mathematically speaking, I will say it will be spin up up plus spin down down.
00:42:59 So if there are two particles, one in Shanghai and the other in the Netherlands,
00:43:07 and at this moment if we measure the particles, we discover that
00:43:12 if you find that one particle has a upward spin, then the other particle will also have an upward spin.
00:43:21 If you find that one particle has a downward spin, then the other particle will also have a downward spin.
00:43:28 It seems that everyone thinks this is strange, but you can repeat this experiment many times
00:43:33 and you can send many pairs of entangled particles to different places
00:43:36 but every time the result, if the particles are in an entangled state,
00:43:39 will be that either both particles have an upward spin, or both have a downward spin.
00:43:43 Einstein called this strange phenomenon "spooky action at a distance."
00:43:52 This is what we mean when we talk about quantum entanglement.
00:44:01 And when you look around now, can you see quantum mechanics working, are there machines, are there topics that somehow prove quantum mechanics?
00:44:22 Up until now we have proven quantum mechanics is correct through many experiments.
00:44:29 We have concrete examples like superconducting phenomena, nuclear energy,
00:44:37 nuclear magnetic resonance, giant magnetoresistance and other
00:44:47 and other examples from various scientific fields that prove quantum mechanics works very well.
00:44:53 That it is correct.
00:44:55 When it comes to detailed experiments into quantum mechanics
00:44:58 ,what we call fundamental research into quantum mechanics,
00:45:03 we often use quantum entanglement as an example.
00:45:09 After quantum Schrödinger and Einstein proposed entanglement in 1935,
00:45:18 people started doing experiments with it in the 70’s
00:45:25 The earliest experiments were conducted by Clauser and his colleagues at the University of California, Berkeley.
00:45:35 Later it was France's Alain Aspect
00:45:38 who also did experiments in this field.
00:45:41 Successively they proved the existence of quantum entanglement.
00:45:44 But some people say there are gaps within these experiments.
00:45:48 What I want to prove is "spooky action at a distance"
00:45:53 because then you guarantee that during the experiments the condition of locality is met.
00:46:01 The locality loophole was shut down in the 90's Weihs'(?) team in Innsbruck.
00:46:13 Everyone then suggested the the detection loophole.
00:46:17 Well, last year the detection loophole was closed by Dutch scientists
00:46:25 who also simultaneously closed the locality loophole.
00:46:28 Thus we are left with the final loophole, the freedom of choice loophole which is also called the collapse locality loophole.
00:46:38 We hope to close this loophole within the next 10, 20 years.
00:46:45 All the experiments until now already proved quantum mechanics works
00:46:50 but quantum mechanics at long distance, and this final loophole, can they also be solved?
00:46:56 Well we don't know that now.
00:46:58 In the future there are two possibilities
00:47:00 I think it is a great possibility is that we we go on prove quantum mechanics is correct
00:47:04 and we close all the loopholes.
00:47:06 But there is also a small possibility that we discover a new field of physics.
00:47:12 Then it will get very exciting for us.
00:47:20 Both can be exciting.
00:47:21 Right
00:47:24 What kind of other theories that can be found when you fantasize about that?
00:47:28 I think throughout this process, when we are talking about the validation of quantum mechanics nonlocality
00:47:35 it is still very fundamental, still the foundations of of quantum mechanics.
00:47:42 The next step would be what I mentioned before:
00:47:45 we can test quantum gravity models.
00:47:50 This would be what we would do after we finish our second mission.
00:47:56 Afterwards, theoretical physicists are very smart
00:48:03 and they will propose all sorts of models.
00:48:06 But each model the propose will be consistent with the natural world
00:48:09 and while some scientists will say that's good enough
00:48:11 this is when we need to do experiments.
00:48:14 We wish that when we finish proving quantum mechanics nonlocality
00:48:20 we will come back and to experimental research work in quantum gravity related theories and models.
00:48:29 And quantum gravity, could you explain that? What is quantum gravity?
00:48:37 I am not really qualified to discuss quantum gravity because this theory
00:48:43 is something (theoretical physicists like) Hawking and other scientists have suggested.
00:48:48 To an experimental physicists like me I just know that that they took quantum theory
00:48:54 and gravity theory and combined them.
00:49:00 After they were combined they proposed several further theories
00:49:04 but which of them is the one congruent with our natural universe?
00:49:09 This needs to be proved with experiments.
00:49:11 When we do the experiments do the predictions from the theories align with our experimental results?
00:49:17 The ones that match up are the ones that are more likely to approximate natural law.
00:49:24 Because I am not an expert in quantum gravity, I can't elaborate on or criticise the models
00:49:37 Thank you