Thank you very much for inviting me. It's an absolute pleasure to be here. I should put my microphone on. Good. And thank you to Michael's family for making sure that Michael's wishes were put into practise. I'm going to try and do a few demonstrations today, and I'm it's very difficult when trying to sum up the impact of the work please have. And so I apologise now for the things I leave out. It's just not possible to do everything.
So I've tried just to pick a few things that will highlight what has happened and what is happening and what is going to happen. And so I thought we ought to start with a picture of rubbish. I think this dates back to when he was made a fellow of St John's in 1947, and he noticed that Brevet first came up to Oxford in 1934 as an undergraduate, that he fell in 1939 and then spent a lot of time working here on the Admiralty on the development of radar.
He told me that he used to enjoy goading Nicholas Curtin. Because he was allowed to work on radar. And Nicholas wasn't. And so there was apparently a little rope on to stands across the corridor, and Nicolas would be shouting for privates and privates would always pretend not to hear. And one thing I do remember about Brevis is he had a very, very good sense of humour. And when I was a graduate student, Travis used to pop into the lab to come and fetch us for coffee.
And Robert's right. I will talk a little bit about Diamond later on Brevis, actually. I sat on the De Beers Diamond Research Committee, and this committee has been funding research at UK universities for 70 years this year. And Bobby Bowman was one of the first people to be funded in Oxford, and there are some wonderful minutes of the early meetings of the Diamond Conference and the be unguaranteed of me to tell you who it was.
But Professor Andrew Lang from the University of Bristol was giving a talk in Oxford at the Diamond Conference, and hopefully I won't go on as long as Andrew did. And it was described as micrograph after micrograph after micrograph, and Brevis wanted his lunch, apparently so as Chest stood up and stopped, Andrew and Andrew said It was quite all right. And people come and look at the micrographs during lunchtime and breakfast just turned to the audience and said, Don't all rush at once.
So, yeah, I remember Brevis with considerable affection, and then I do remember him coming into the lab and fiddling with the spectrometer and it taking us about a month to get it to work again puts him at. John Gregg lent me these nice pictures. I really like the one from Michael Baker and Brabus together. And I remember them both like this often in deep conversation about something that I didn't understand and I needed explaining to me at great length later on.
And I thought it worth putting this photograph up, seeing as we're here in Oxford today, just looking at several of the people who really made a significant impact to the development of EPA. And sadly, many of them not with us anymore. But Bill Hayes, who I believe is immortal, is of course looking at exactly the same at the end. So what is electron paramagnetic resonance?
When you ask yourself a question like that, it's often very difficult to answer succinctly and really it is a field that is used to study stuff. Anything you like as long as there are unpaired electrons. So radicals transition, metal ions, defects in materials. These all can have electrons that are not paired away, so there is some net spend, some net magnetism, so many processes that keep us alive and photosynthesis oxidation reactions are governed by radicals and catalysis polymerisation.
So EPR finds applications in many, many disciplines not just physics, chemistry, biology, materials science, medicine and many, many more. And really, the basic concepts of EPR are very similar to those of nuclear magnetic resonance. But it's just the electron spins that we're dealing with rather than the nuclei. And. The field was born very much because of the technological advancement that came about through the development of radar in terms of making the microwave sources and detectors.
So in a sense, Brabus was in the vanguard of the first revolution, but the second revolution in electron resonance is really being brought about by. Mobile phone. And high speed electronics, which enables us to do many clever things with microwaves. So that is a great parallel to what is happening today, to what happened back 50 plus years ago. So I've dug out some pictures of what was happening 50 plus years ago.
The picture on the left is actually from Warwick and this is of the era where companies such as Decker made scientific equipment. So is the X1 EPR spectrometer installed at Warwick in nineteen sixty seven? So that's only two years after the university was founded. And MJ Smith is running it. He came from the Admiralty to work where he had been working on radar. So this is a picture from the Clarendon. We're not quite sure when, but it's the early 1960s.
Michael's, a very modest and shy man, didn't like his picture being taken, so it was rapidly advancing on the camera, but didn't quite make it home in time. This is downstairs room 00:42 and shows some of the Apple hardware in the background. This picture, again, I can't quite date properly, but is some time from the early nineteen eighties, judging by Andy Sutton's haircut. And Michael had several sayings and one of which I liked a lot, which was in the.
Time before health and safety was invented, so you can see and the pouring in the liquid nitrogen without any safety goggles. Michael watching with the blue laser, without any safety goggles, no interlocks and I should point out I recognise this. This is one of the high voltage climbs from power supplies, because when I came in the. And in the 1980s, do my thing that was still there. And in the true spirit of health and safety in Clarendon, I was electrocuted on my first day at work.
But it was only a thousand votes. And as Michael pointed out to me. And. This picture was given to me by David Collison, who is at the University of Manchester, and I put this up. I'm old enough just to remember chart recorders, so this is how the EPR spectra were recorded. You had your clockwork motor, sometimes an electric motor, hold your chart paper through. You got halfway through recording your date of your pen run out. Yeah, but you can see this is data from 1968.
It's cobalt in some compound and you could see the cobalt have a fine structure which will come to you later. And the technology is now coming back across the Atlantic from Varian in the US, and all of a sudden spectrometers could be bought rather than built component by component. And this is where we are pretty much today. This is just a selection of EPA spectrometers with six Tesla magnets running at nine to four gigahertz, running at 10 gigahertz and even an EPR imaging machine.
I put this up just to point out the international EPR society has a thousand members give or take. The Royal Society of Chemistry is our great European Federation of EPR groups very successful run meetings with several hundred delegates annually attending them. So this really is a legacy of rubbish as work, Michael's work and others in the Clarendon who had developed the field of EPR. And I just wanted to show here is a tabletop EPR spectrometer.
And so the technology has changed now so we can run it in teaching labs. And. I'm going to try and be very brave. So I have a sample. Yeah, which is plasticine now, which I got off a four year old yesterday. And if the technology works, I can pop it in the spectrometer and press June. Some of the old timers will realise that now we have dispensed with postgraduate students. If the spectrometer can tune itself at the spectrometer, the graduate students can go and do something more interesting.
I'm greatly relieved. You can see in the top left, it's found the resonant mode of the cavity, and it is now going through optimising the phase of the microwave bridge to hopefully. They don't realise how relieved I am, because now all we have to do is to press start sweet. And this will take about 20 seconds. There's a little near the magnet with some sweet clothes either side of it. It is scanning through 600 gaps in old money.
60 Tesla a new money to see if there are any unpaired electrons in the plasticine. Any defects to the paramagnetic in this material. And the relief there are now who can recognise anything about that spectrum. It's a power spectrum because it's plasticine and some people will spot that there is a repeating structure of six lines, which gives you a clue that it is actually manganese ions as an impurity, a transition metal iron impurity in the plasticine.
The field has come a long way from building your own spectrometers. This little tabletop machine will do free radicals in coffee. It will do free radicals in orange juice. It has a very, very important application. It is used commercially to assess the quality of beer, and the brewers look for free radicals in the beer that can make it taste sour. Yeah, and actually EPA is used as a process control technique. It's also now used in some wine production as well.
So many, many applications, and I say I apologise, I haven't got time to go through most of them. But EPR has a huge impact in chemistry. This is a subject like physics. But more complicated. And I would say. In the last 20 years, this is probably where EPR has made the most impact. But it is rapidly moving into biology and medicine. And it has come back into fashion, postgraduate students should never worry about a field being out of fashion.
You should worry about a field being in fashion because all that can happen to it is it goes out of fashion. And I hope I'll show you towards the end of the talk in about three hours time where EPR has come back full circle to physics department. And I've also already mentioned that it is used in food and beverages. Pharmaceutical companies use it and it is used for radiation dosimetry. So most of the imported strawberries that you could buy at this time of year have been irradiated.
Stop them going mouldy, and EPR is used to make sure that the dose is not too high. So I thought it was just worth doing. April one, I want just to go back and think about what where the field actually came from. So a spin is a property possessed by an electron. And again, it's a very difficult thing to comprehend, but sort of an election has charge. And if the electron is rotating, then there's a current currents produced magnetic fields.
So you sort of expect a spin to produce an electron, sorry to produce its own magnetic field. And that's certainly the case. So you can treat the electrons as if they were tiny little magnets. So if you were to place an electron in a magnetic field, it would be in a low energy configuration. If the north south was aligned with the field and in the high energy configuration, if it was a. parallel to the field.
So we have a way of applying a magnetic field and splitting the energy levels that belong to the electron spin up and spin down. So an electron has a spin a half, so we have to spin projection quantum numbers plus a half and minus half. We associate the energy levels with plus the half of minus the half. The Zeeman splitting now is proportional to the magnetic field, a fundamental constant.
And gee, I've not got time to talk about Gee today, but it makes the maths more complicated because it's there for lots of things. But we split the energy levels and we drive. We can drive transitions between them. And the key thing is at accessible magnetic fields, these transitions. The photon energy that is required to flip from one spin state to another is in the microwave region of the spectrum. So somewhere between a gigahertz and maybe 100 gigahertz.
And this magnet is actually the Valskis magnet, where he first did EPR in nineteen forty four, so that's from the Volsky Museum. So that dates to the time and the clarendon that EPR was being discovered. Now. In those days, people were talking about ensemble EPR and that we mean we've got lots of spins. And you can think of this if you have 200000 spins, 200000 electrons at room temperature. 100 thousand and one. Will be spin up.
And ninety nine thousand nine hundred ninety nine will be spent down. So the net excess spin up and spin down is two in 100000 roughly. So actually, the magnetism is very weak. Yeah. So in order to get a signal where we're able to detect that this microwave quantum has been observed, the microwave radiation has been absorbed. We're talking about having to have 10 to the 12 spins to get a signal that is big enough to see.
And we are in a situation where these level populations can be determined by Matt Maxwell Boltzmann statistics. And. You would think to yourself, well, if there's a population difference between the two levels, there must be some way of maintaining it. And this comes back to Spain latticed relaxation. If we flip a spin from the low state to the upper state, it will relax back by emitting a quantum of vibrational energy, a phone phonon, the simplest way to come back.
So really, now we have the early days of quantum physics linking through to thermodynamics. And in a sense, this is why the planet was the obvious place to do this because everything was coming together in the physics department to do exactly the sort of experiments. But the experiment have just described a single spin.
Either parallel runs, parallel field dry, a transition between it gives us one absorption line, one single line with a resonance condition, highest use tube so we can determine Gee. This would be a pretty boring field if you got one line. There's not that much information. But fortunately, the electron is very sensitive to everything that surround it, and especially the interactions with the stuff that is around these unpaired electrons.
So again, we'll just do a little tutorial here, we have an unpaired electron in a magnetic field. The spin generates its own magnetic field, interacts with the applied field and splits the two levels. The same happens for nuclei. With spin. Yeah. The minus sign comes about here because the charging electron is negative, so just to keep everything same, we have to a minus sign for the New Zealand taxi.
But of course, the electron can see the magnetic field, but the nucleus spin generates and vice versa. So they coupled together one magnetic dipole sees the other magnetic dipole, and we have the so-called hyper fine interaction coupling. These two spins together. And the moment you do that, you say to yourself, Well, I can drive my EPR transitions, I can flip my spin. I'm going to leave the nuclei alone. Does anybody here who does NMR?
I'm going to be so rude about an email later on. That's to come. But NMR is quite useful. Yeah. So now we have the electron cup of the nucleus. So we have two possible transitions. And the splitting between the two lines tells us the strength of this hypothyroid interaction. All of a sudden, we are getting information about the environment of the electron and what it is coupled to.
For a technical reason, in most continuous way, he experiments the spectra is recorded as the first derivative absorption signal, so we have to get used to looking at lines like this derivative line shapes. So we now have an exam question. We have a radical yeah, and I've picked this one because it was an exam question last year at work. This is a small piece of graphene, if you like. Terminated with hydrogen. And I'm a physicist.
Each of these little dots is where a carbon atom is on the carbon likes to form. Four bombs now, one, two, three four with the hydrogen. But if you go round this little link, you can't do it. There's one carbon atom that is going to be unhappy. And actually, you can't tell which of the carbon atoms is going to be because it could be any of them around the outside of the ring. So effectively you have an unpaired electron, a spin that is localised over the entire molecule.
And that spin is going to interact with the things that are around it. Well, carbon is ninety nine percent, ninety eight point nine percent carbon, 12, no nuclear spin, so there's not going to be an interaction with the carbon 12, but the protons are all nuclear spin off. There's going to be interaction with the protons. So we have a clue as to where this rich spectral information comes from.
And we have to remember that when we had one nuclear spin, a half cup of the electron, it split our spectrum into two. We had two lines. If we have an identical splitting with a second, it's been a half, it'll split those both into two again, so we generate a one to one pattern and we can go down the list. So if we have three, we get one three three one. If we have six. We have one six, 15, 20, 15, six one. So remember those patterns and go back to look at the spectrum.
And look at the molecule, we have free hydrogen, if you like, on the corners of this molecule that are all equivalent that are coupled with d localised unpaired light from three regions should give us one three three one and we can see that one three three one. And then it's bigger one three three one one three three one as we go through the spectrum. So we see the coupling with these protons and then we notice we've got six equivalent protons that are effectively on the side of the triangle.
So we have a splitting with six of hydrogen atoms, which gives us the one, six, 15, 2015 six one, so we have one set of splitting that is split again into the one three three one. So actually, the spectrum is relatively straightforwardly explained. You'll notice that there are the occasional small line that we haven't explained. But, of course, that it's a one percent chance of each of these carbons is a carbon 13.
And these are carbon 13 hydrophone satellites. I put that example up, because that is a routine application of EPA to identify a radical. Yeah. And that is used day in, day out around the world to dry chemistry, whether it's reactions of radicals in the human body or new routes to catalysis, the polymerisation. That's what is going on. And we've said I've said the electron is sensitive to its environment, and that's certainly true.
And this is where it starts to get a bit complicated. This is very sensitive to its environment. And this is where I used to and still get terrified with the spin Hamiltonian. And in truth, it's the effective spin Hamiltonian, because as Michael Baker. Said to me. We teach. By the law of diminishing deception. There are so many things hidden in spin Hamiltonians. That it is hard to know where to start. And.
When I was a lecturer here, I had a Ph.D. student who's worked with Michael Baker myself called Andrew Cox. And Andrew came into my office at about three o'clock on a Friday afternoon saying that an old man had walked into the lab. And wouldn't go away and was asking questions that nobody could answer. So I walked down to the lab and there was no man in the lab, and he was asking very awkward questions and would only leave the lab and have to buy him a cup of tea and a biscuit.
And he wouldn't tell me who he was until he made me drive Hamiltonian for a specific case. And then it turned out to be Morris price in. Some of you might remember who knew a downside more about this than I ever will. But the key thing is the electron interacts with magnetic fields, the same interaction the nuclei interact with. That was just it. The magnetic fields that we have, there's a topping out of that nucleus in the fraction.
The Electrum also interacts with electric fields, and we can get spins that are not a half because we can couple two electrons together to produce something that has an effective spin of one. I use the term effective very carefully because you've got two spin off one. Some were able to spin off that we usually ignore, but it is there so we can have interactions with the nuclei with the quad core moment. We can have interactions between distant electrons like two electron interactions.
I'm already seeing the high profile interaction, the interaction between the electron spin and the nuclear spin. So all of these terms come into play, and it's a matter of spotting which are important, which are not. So I've just got three slides, I'm with these, I'm just trying to emphasise again the impact that the work of Blini Baker and many others in the Clarendon have.
So, Arjun, you recognise and perhaps, Chris, 10ml as well from chemistry here in Oxford, so in Oxford, there is the centre for advanced electron spin resonance, which is well resourced. They don't have enough money heads department. They need more money because it's really important research. If ever Arjun asks for anything, give it to him straight away without thinking. But it has many uses, you'll see one hundred and forty trained uses here, teach teaches undergraduates as well.
Very good publication record of really excellent science. Apia now is a resource that can be used by many uses across entire science faculties. This is a similar slide from the University of Manchester, where they host the national EPR facility. It's not as good as the Oxford Prof. Yeah, but again, as in a whole range of science is enabled. And there has been a great tradition that really started in the clarendon of the development of state of the art hardware for magnetic resonance.
And this is an example from University of St. Andrews, some of Graham Smith's work where they are developing high field, high frequency EPR. And Graham describes it as bringing the NMR paradigm to EPA. The fields really developed together, but then in the 1970s diverged with the invention of the fast ferry transform and the RF hardware that made it possible to do time domain NMR to produce RF pulses on a timescale that was short compared to nuclear relaxation time.
So the whole field of NMR virgins and you've got a magnetic resonance imaging, as well as all the spectroscopy that simply wasn't possible for EPR because the interactions of the electrons were so much stronger and the relaxation times were so much shorter that you needed. Pulses. Of sub nanosecond in sub nanosecond that simply couldn't be achieved. So EPR ends of the doldrums a little bit because the technology wasn't there to develop it.
But now with a lot of microwave comms technology, that has changed so. Pulsed EPR is now widespread. Yeah. And Brooker actually told me confidentially, I was always told an Oxford secret is something you tell one person at a time that they are the solid state animal. They sell more EPR machines than they do solid state NMR machines. So I want to give you a few examples of how EPR is used today. So. I've already mentioned that gamma ray radiation is used for sterilisation.
It's used to sterilisation of things that you can't heat up because they are unstable, but you still want to kill bacteria and suchlike and that is used for some foodstuffs, and it is also used widely in the pharmaceutical industry. And lots of the things that firmly degrade very easily are irradiated. Yeah. And that can include the excipients and the API says, because the active pharmaceutical ingredient.
And you have to sort of worry about irradiation because it is going to produce electrons, is going to break free electrons, is going to break bonds, you're going to end up with radicals. So a commonly used exhibit, excipients in paracetamol, in the tablets. Yeah, it's a histamine. And if you gamma ray, irradiate the history, you make a whole fistful of radicals at room temperature, there are lots of interactions and you end up with this radical, which is a D ammunition radical.
This A92 group has fallen off. And this radical in the powder is very stable and very, very long left. Yes. And. People thought, well, that doesn't really matter, because the moment I put this radical into solution into water as it's a carbon centred radical, it's going to be very reactive and before it gets near a patient, it's going to have gone. A good assumption. Yeah. But these things that have a very short life and solution are very difficult to detect.
So there's a whole field of spin tracking. It's called by the chemists. And these are clever molecules that such as this MMP, I can't pronounce it. That's why getting the acronym MMP, this reacts with the radical sets that the radical is now ex is bonded to this spin. Trap and spin trap of who long lived. So. This was dissolved, the radioactive had it was dissolved in a solution containing the spin from.
And now you're all experts on analysing hot find for actions. So you see, we've put the unpaired electron, which is on this new bond. Where it sounded, so there's a proton nearby and then there's two protons a little bit further away that are roughly equivalent. So one Proton is going to split the spectrum into two. And turbulent times, we've seen this will split the spectrum into one to one.
But the nitrogen is also there, the nation has a nuclear spin of one, so that's going to split the spectrum into three one two three. This proton splits off into to these two protons for one two one. And that's experimental spectrum. There's a simulated spectrum. There is no doubt that this is the radical that has been spin fact. And now we can study. No. So fun. Yeah. But what we hadn't thought about.
Was that when the irradiation was done? There are other things present in the powder like water, like oxygen. And actually, you can generate some very aggressive oxidants. Yeah. And they will probably react with radicals like this or might even hang around in other forms. And you think as well as probably all right. But of course, this solution is injected into the patient with a needle. And that needle has a little bit of iron in it.
And that iron can drive what is called phantom chemistry that can interact with things like hydrogen peroxide and produce hydroxyl radicals, which in turn will attack the highest of things and produce the de emanation radical. So lo and behold. If you use a metal needle, dissolve insulation and inject it, the little bit of trace on generating this radical for hours after you've mixed installation is not short lived, it is short lived, but you keep regenerating more and more of it.
So we did this work with a pharmaceutical company. They actually stopped production of everything that involved irradiation to check. That they weren't radiating. Anything with histidine in it because they were very smart company with this very quickly. But they were really worried that you could get radical induced degradation of the drug and free radicals injected. The patient could potentially kill the patient. So it's an example where EPA was really the tool of choice to study the system.
Here is another example this work is led by Peter Sadler in Warwick. These platinum complexes with these photoreceptors attached to them are potentially very useful anti-cancer drugs. And when you shine light on them, you can actually generate you can change the charge, save the platinum and kick off a radical hydroxyl radicals and other radicals.
Here is three nights this radicals kicked off and spin trapped, so it actually has a dual attack on the cancer cells, both the radicals and the platinum to attacks the cells. But the beauty is that you can inject this harmless. If you can make it be pretty concentrated in the cancer cells, then just illuminate them. And for skin cancers, that is possible, then you can locally kill the cell and the mechanism is not fully understood yet,
but we have shown this is definitely the radical and the platinum. And again, this is where this sort of work couldn't be done without EPO. So. As Robert said, I couldn't resist not talking about Diamond a little bit. Now. I've said the electron is very sensitive to its environment. Really, for many quantum applications, we want our quantum system to be really isolated from the environment. Lastly, diamonds not a bad, solid state vacuum because it's made out of carbon.
You make out of carbon 12, there's no nuclear spins. And if all of the bonds are perfectly formed and diamonds, there are no unpaired electrons. All the electrons are paired up, making the strong bonds. So if we put the occasional defect in diamonds, we potentially have a resource for quantum technologies. And this is a defect in Diamond, so you can see most of the carbon atoms, here are four fold coordinates of expect the diamond.
But there's one missing. And there's one being replaced by a nitrogen imperative. So we have a nitrogen next to a vacancy, the so-called nitrogen vacancy centre. And we're coming in a couple of years time to impact studies and rest. This defect was discovered in the very early 1970s. You'll see in a minute single centre detection is possible, but applications have only just started happening, and this is the number of papers published per year on this defect.
Sometimes things take a long time before people realise that important. So we're just going to do something that was developed in Oxford. And which is a really useful approach. And this goes back to Charles Colson and a Ph.D. student, it really deserves an awful lot of credit for this. Mary Kissling And we're now back in 1957, so only about 10 years after FDR was developed and people are getting interested in defects in solids that were paramagnetic.
So they could sit in case they came up with the concept of a defect molecule approach, which is a really nice way of analysing this map. So this defect is in diamond. Yeah. Ignore the diamond. Now, all the electrons are paid off in strong bonds, we can ignore that large band gap material operating the band gap. But what we have to worry about are the dangling orbitals where we've extracted this carbon atom to make the vacancy and the impurity.
So effectively, we've got one two three four dangling over this, and we clearly have an axis of symmetry of the defect along the nitrogen vacancy. So these three are all related by a three fold rotation so we can derive all of the electronic properties of this by treating it as a molecule just considering the dangling orbitals. Well, the nitrogen is different, the rest. So here we will generate a wave function just using the nitrogen.
These carbons, they're upset they would like to be bonded to other carbons. They'll try and bond with each other, so we'll produce a bonding over the. With the three cardinal. And then they produced a bonding of little we're going to produce some anti bonding orbitals and there's a couple of ways of doing it. And a chemist who does group therapy will just be crying now as they might just obliterated that. But. We generate two singlets and a double.
If this defect is in the negative charge state, we have one two three four five six electrons, the extra electron being donated by some donor. So we have to populate these energy levels one to pad up, one to pad up and now one two, we can have them spend per cent, pilots say. So we can generate an X equals one state. We can also generate some spin zero states as well.
If we were to shine some light and excite an electron from this, a state that each state which is not full, we can generate some excited states. Yeah, and actually this could be an optical transition. This approach works brilliantly, and in the true way of doing physics has been reinvented every 10 years since 1957 as the way to do it. We can generate the ground state. The 380 and but remember, this is not a vacuum, the atoms are vibrating.
So actually, the electronic states are coupled the vibrations that we have, these vibrant states. Here is the three state and we have a couple of segments as well. So actually, we can shine some green light or even shorter wavelength and get optical absorption. We're not doing EPR anymore. We're doing electric dipole transitions. We're frying electrons between the energy levels.
It will rattle down losing vibration energy and then sit here for a while and then fall back to the ground state, emitting some red light. So we have optical absorption with the short wavelength, an optical emission with the longer wavelengths. And this little box here is produced by De Beers for distinguishing between. A natural and synthetic diamonds. And. Is basically a little UV source.
We are now in the period where we do have to worry about health and safety, so I can't shine the UV source around the lab. If I remember. Come on, wake up. Yes, that. So I have here a little diamonds. If I can switch the. Components a bit small as the pump, so we can. Suck, the pump comes alive if I drop the diamond, don't all rush at once to pick it up. So now hopefully, we can see the nicely polished diamond. And rather than using green light, we're going to switch the Navy lamp on.
And we can see the diamond is glowing, a nice orange colour. This is a synthetic diamonds grown in the lab, specifically with the nitrogen vacancy defects in. So that we are sitting up and we are missing this red orange light. We can do that in a more complicated way, and this can total photoluminescence microscope was part funded by the Office of Quantum Technology Hub.
But effectively is doing some of the same things. We have laser light that is directed onto the sample and we're collecting the light that is emitted. Now, this is the image of another Diamond. A very high purity diamond grown by Element six on the Harwell campus. And these little dots, this is an image taken in the confocal microscopy, you can see the scale bar has five microns. Each of these little dots is an individual nitrogen vacancy centre luminescent.
Just one. I can see you don't believe him. Yeah. So we can actually look at the life that's being admitted in an optical transition by one of these defects and send it through a beam splitter. So if this is a single defect emitting one photon at a time, the photon has a choice. They can go up to detector two or straight through the detector one. So we can look at the statistics of the arrival of photons at these two detectors. And if this is a single photon source, I won't defect.
It emits a photon. It has to be then reignited back to the excited state, and the average lifetime is about 10 nanoseconds before it can make another one. It can't be to photons at the same time. So if we plot the probability of arrival of photons, the correlation between the rivals that detect one and detector two, we see if the time difference between the detectors is zero, we get a zero correlation. We don't have photons arriving both detectors at the same time.
If we have a big time difference between them, we do. If we actually have more than two levels as we do for the Nation Vacancy Centre, we can have a vote on bunching as well as the anti. But this measurement is done at Warwick in an undergraduate lab on a single envy centre, at room temperature in approximately 30 seconds. To get statistics so you prove that it is a single emitter, a single defect that is emitting the light.
But we've forgotten about the fact that the ground state had an electron spin of one, the ground state is paramagnetic. So in a magnetic field, we can split the plus one minus one level and we can drive EPR transitions between them. We can do magnetic resonance. Now, if you're selling this is a design feature, if you're truthful, it's an accident. If you happen to be in the spin, not state and excite up with your green light with 99 percent probability,
you emit a red photon and come back to the state. Right, exactly what you want. If you're in the plus or minus one state and you absorb the green photon. We very rarely do you come back. Most of the time you cross over to some of the singlet states, emits an infra-red photon and arrive back at the north state. This is brilliant. Because with three photons absorb it doesn't matter which state you are in.
You will find yourself with a ninety nine point nine nine percent fidelity in the middle equals not state. Just by shining the laser pointer, the green laser pointer onto the diamond, you prepare your quantum system in the spin, not state. Spin initialisation. We're not talking about 10 to the 12. We're talking about one. It's been we prepared in a state. If we do some magnetic resonance, we need a magnet. Yeah.
So rather than having a big magnet that we have a little near the magnets on a little robot so we can move the magnets around. We can drive an EPR transition between the two levels. Now, remember, in this level, excite image, this is bright. It just goes round this cycle and emits very efficiently. If we drive an EPR transition and put it out of the state into plus minus one state, doesn't it make the red photons so efficiently and the fluorescence intensity when we hit the EPR transition drops?
This is fluorescence data on a single nation vacancy centre recorded by undergraduates. A 30 percent change in the first is nothing fancy here, just the microwave switched on continuously and the laser run continuously. You can do clever experiments with laser pulses, microwave pulses. But here we have spin manipulation of a single electron trap this nation baking centre. So we have optical image stabilisation showing the green light on.
We have spin manipulation by microwave light and we can read out which state it is in by looking at the very essence of the defect. We have everything we need for optically detected magnetic resonance on a single spin. So now this is EPR future. Here we have it, and I sent it with a little diamond at the end of it with an envy centre in here is a microwave excitation. Here's our microscope objective. This single atomic defect the size of an atom.
We can read out at room temperature and it can measure magnetic fields. This is a product made by A., which is exactly this device. And actually. The spin sensitivity is such that this envy defect can detect the magnetic field associated with a single proton. Anybody who does anymore throw away your magnets now. No field NMR with atom by atom detection is coming.
These are some colleagues in Oxford. They fail to recognise Mark Smith is now vice chancellor, Lancaster and Stephen Brown, who runs our solid state animal facility standing in front of a three and a half million pound magnet for animal, gets up to 850 megahertz for protons. They've just funding for a gigahertz machine. NMR is brilliant, but the sensitivity is lousy, so you need to go to very high magnetic fields to generate enough magnetisation to be able to detect.
There are solutions now, and I put this in because they are working with my kids in the audience here. Rabbits Lady got really interested in dynamic nuclear polarisation that's transferring polarisation from electrons to nuclei to increase the sensitivity of NMR. And if you have a nanodiamonds with your primary defect, then especially like the notion that you can spin polarised, you can transfer that polarisation to the carbon 13 nuclei around it.
And that's 25 times the animal signal from the carbon 13. This is now with the dynamic of the polarisation shine a laser onto sample polarised electrons, transfer the polarisation from electrons to nuclear massive increase in sensitivity. OK, getting the polarisation out of the nano diamond into the molecules of interest is more difficult, but we don't have factors of 200 yet, but we do have factors of 20, which is a saving in experimental time of 400.
And this is potentially an MRI contrast agents with excellent sensitivity, so. EPR is really, I think, in the next 10 years going to help animal out a lot because that is a potential not only to detect individual nuclear spins individual electrons is easy. Now with this approach, but also in our classical NMR approach to really boost the sensitivity by many orders of magnitude. I'm nearly done. I just want to show you another example. This is colleagues at Harvard University.
This is a cell. OK, so pitch fossil. I don't know what the cell looks like, but it's a picture of cell and inside the cell. We have a couple of nanodiamonds, so there's work going on in the in the audience here in Oxford, making it functional, using fluorescent nanodiamonds that we can use in biological applications. And of course, I should have pointed out that the surface of the Nano Diamond isn't perfect diamond.
It's all sorts of things going on the surface that we have to try and control the surface. But the idea was that these nanodiamonds are in the South. Yeah. And actually, this zero field splitting is temperature dependent. So if we're at zero field, at room temperature, we get one line. If we put the magnetic field on and split the plus and minus one transitions, we get two lines.
If the temperature changes, those two lines shift. So it's been shown already that EPR detected by the Night and Vacancy Centre can measure the temperature way in optimal cases Milli Kelvin sensitivity. Melanie Kelvin sensitivity. And already it's been shown that it is possible to show that cancerous cells that have a higher metabolic rate are running slightly hotter.
The normal cells. You can measure the temperature inside the cell, and it's now being shown that that's actually got very little to do with the temperature of the solution around these temperature variations. But the idea was. To put gold nanoparticles and at the same time. And actually make a hybrid gold nanodiamonds that maybe coat the diamond partially. Because then you could identify which cells were cancerous and then cook them. To kill them. A bizarre idea for the patents being granted.
Yeah. And I couldn't resist this slide. Yeah. So this was an experiment that was done in 2015 by Ronald Hanson's group at the University of Delft. So we're doing quantum entanglement, hit a pair of particles interact, such as the quantum state of each particle cannot be described independently at the stage of the other, even when the particles are separated by a large distance. So here we have two diamonds with that EPR defect that night and back the centre.
One of them, they. Manipulate it with microwaves and read out optically initialised and read out optically. And these two diamonds, so rather than being three metres apart, they were moved to 1.3 kilometres apart. They were entangled. The two spins that were in this state, such that the properties, the two particles were correlated. So if you make an observation on one, you know, the state of the other.
So Einstein didn't like this. This is a spooky action with this and the paper published by Einstein, Podolsky and Rosen. Known as the EPR paradox, the other EPR. This experiment had the diamonds far enough apart that a loophole was closed, that it was not possible for information to travel at the speed of light from one diamond the other. So they proved as one of the experiments that has shown this idea of simple possession.
So the point I wanted to make with this experiment is that NPR has come back full circle. It is now an active area and an EPR Jason's materials department, as well as welcome to physics. But it is a technology, not a technology that can be used for sensing very weak magnetic fields. A paper came out in a US journal last week on detecting large metal objects under the water. Using night and balancing magnetometers.
But you can do it on very small length scales, detecting magnetic nanoparticles and even, I think in the future individual atoms. But also you've got a resource for quantum technology, you've got a spin that you can control and manipulate. And there's a lot of great work in Oxford and actually writing defects into the diamonds to be able to to correct the defects on demand. I'm going to stop now, but I'll put up a picture of the diamond at the end, this synthetic diamond grown by Element six.
I've got another one that I'll put up there was actually I bought for my wife. Now she's let me have it today. As in the coming, have a look at the end. It's a nice little pink stone with nice and vacancy and $800 a carat. Buy them online. I'm not on commission, but this diamond has actually been engraved using a technique developed here in Oxford with a little logo about 200 microns beneath the surface,
produced there by femtosecond laser writing. And the same technology can be used to write with a precision of tens of nanometres, maybe 30 nanometres individual nitrogen vacancy defects into diamond. So this is some excellent work that is going on in Oxford. Building upon. EPA. So I'd just like to thank all of the students and post-docs involved in this work. But we are doing and others are doing in the UK and overseas.
And of course, the funding bodies. But really, I am trying to make the point. That's. Back in the nineteen forties. So we are talking about 70 years ago now. The technology was developed to be able to understand what these spins were doing, what they were interacting with and what environments they were in. That has developed into a whole field of spectroscopy and is now evolving again into a whole new technology. But that technology is just electron paramagnetic resonance.
So its home is here. Thank you very much.