06/07/2019

A New View On Quantum Weirdness

11:44 minutes

The “spooky physics” of the quantum world has long been marked by two key ideas: the idea of superposition, meaning that a quantum particle can exist in multiple states simultaneously, and the idea of randomness, meaning that it’s impossible to predict when certain quantum transitions will take place.

Writing in the journal Nature, Zlatko Minev and colleagues report that they may be able to make the quantum behavior slightly less mysterious. By using a sensitive form of continuous monitoring, they’ve been able to identify signs that a quantum leap is imminent in an artificial atom. The timing of the leap is still completely random—the researchers can’t predict when it will happen—but they do get a warning flag of an upcoming jump a few microseconds before it occurs. And if they are able to spot an upcoming leap, they can apply the correct stimulus to prevent that jump from occuring in the first place.

Minev joins Ira to talk about the finding, and what new directions it might open up in quantum research.


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Segment Guests

Zlatko Minev

Zlatko Minev is a research scientist at IBM Quantum Computing in Yorktown Heights, New York.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’ll be talking about new advances in cancer drugs and what your cats do when you’re not there. But first, the spooky physics of the quantum world has long been marked by two key ideas. There’s the idea of superposition, meaning that a quantum particle can exist in multiple states simultaneously, and the idea of randomness, meaning that it’s impossible to predict where certain quantum transitions will take place. 

But now researchers say that they may be able to make the quantum behavior slightly less mysterious. By using a sensitive form of continuous monitoring, they have been able to identify signs that a quantum leap is imminent in an artificial atom. The timing of the leap is still completely random. The researchers cannot predict when it will happen. But they do get a warning flag of an upcoming jump a few microseconds before it occurs. 

Let me introduce my guest who will explain it. Zlatko Minev is a research scientist at IBM Quantum Computing in Yorktown Heights, New York. He did his work as part of his post-doctoral dissertation at Yale published this week in the journal Nature. Welcome to Science Friday, Dr. Minev. 

ZLATKO MINEV: Ira, it’s a pleasure to be on Science Friday. 

IRA FLATOW: Well, it’s our pleasure to have you. Thank you. Set the scene for us. What is a quantum leap? 

ZLATKO MINEV: That’s a great question. Well, that goes back to the very founding of quantum physics and Bohr’s ideas that in quantum physics, unlike in classical physics, the energy of an atom can only take discrete levels. 

For instance, it can only be measured to ever be 0, 1, 2, or 3. And it can never be, so to speak, in the middle. Well, then the question is, how does the atom transition from one discrete energy to another without ever tracking a flight in between the two? And Bohr’s idea to answer this was a quantum jump. 

IRA FLATOW: And this was a big debate in physics, right, between bore and Einstein and Schrodinger? 

ZLATKO MINEV: You’re absolutely right, yes. Bohr had this idea in 1913. And it was then thought that these jumps occur in an abrupt instantaneous fashion. Now, Schrodinger, on the other hand, another chief architect of quantum physics, had quite the opposite to say about it. He actually quite almost polemically opposed the whole idea of quantum jumps all together. 

He wrote an article called “Are There Quantum Jumps?” In which he wrote, “if all this damn quantum jumping were really here to stay, I should be sorry I ever got involved with quantum physics.” We can maybe understand some of Schrodinger’s uneasiness about these discrete, abrupt transitions in that nothing within the jurisdiction of Schrodinger’s equation– which governs basically quantum physics– ever jumps or is abrupt or instantaneous. 

IRA FLATOW: Let’s talk about what you did. You developed a way to monitor the behavior of an artificial atom very closely. Why did you do that? How did you do that? And what did you see? 

ZLATKO MINEV: Exactly. And maybe to lead into that, I should explain that these quantum jumps were not observed for about 70 years after Bohr proposed them. And Bohr and Schrodinger and others went back and forth. And in about 1986, there was three simultaneous experiments in atomic physics which observed quantum jumps for the first time. And in all those experiments, we indeed observed these discrete random transitions between the energy levels of these atoms. 

And so in a sense, the story has since been that these quantum jumps do indeed exist and occur by this manner. Now, in our experiment, we go back to those original observations and take basically a metaphorical lens to the experiment, zooming in on the very fine timescale where the time of that quantum jump is said to occur, where we see that abrupt transition. Until now, in a sense, this has been far out of reach of experimental atomic physics. Because it hinges on the ability to resolve every single photon that’s being emitted by the atom. 

And in our experiment, with this measurement scheme I proposed, what we’re able to do is to actually look at the time of the last piece of information– the atom amidst the last photon, the last flash of light– and zoom into the various vast dynamics of where the transition is said to occur at this abrupt transition. What we observe is that on this fine timescale, the jumps are actually not discrete, but continuous, coherent. 

And in the sense that the jump has a flight and mid-flight, it’s in a superposition of having jumped and having not jumped. A Schrodinger cat-like state of the atom didn’t jump and did jump between the ground state and the excited state. Not only that, but surprisingly, every single time a quantum jump occurs between the ground state and excited state in these systems, the glide between the ground and excited state is always the same. It’s predictable. 

IRA FLATOW: So are you able to know then when it’s going to make the leap? Is there a warning, a flag that comes up comes up says, uh oh, here comes a leap? 

ZLATKO MINEV: You’re absolutely right, exactly. It’s subtle in the sense that we can’t say Sunday at 2 PM there’s going to be a quantum jump. But Sunday at 1:59 PM, right before a quantum jump does occur, there are certain telltale signals that you can detect that there is a process underway, a transition that’s about to turn into a quantum jump. And so in that sense, you can always get this advance warning of a quantum jump occurring and always catch it mid-flight before it does occur and is deterministically prevented from occurring. 

IRA FLATOW: How do you do that? How do you prevent it from making the leap? [LAUGHTER] 

ZLATKO MINEV: Well, the trick consists in two parts. The first part is to be able to catch the quantum jump, to detect this advance warning signal. To make a kind of loose analogy, you could say that quantum jumps of an atom are somewhat analogous to the eruption of a volcano. They’re completely unpredictable in the long-term. No one exactly knows when that will occur. Nonetheless, with the correct monitoring, with just the right efficiency and so forth, we can with certainty detect this warning of an imminent disaster and act on it before it has occurred. 

Now, in the case of the atoms we work with, it turns out that there is a very particular signal that you can pick out from the measurement. So we observe the atom continuously. When the atom is in the ground state before it has taken its jump, we see a series of flashes of light. We detect light that is being scattered by the atom. 

And the way we observe the atom is by shining light on it. And if a lot of light is being scattered and we see a lot of flashes, then we know with certainty it’s in the ground state. On the other hand, if no light is being scattered, there are no flashes, then we know the atom is with certainty in the excited state. Now, if we can look at every single one of those flashes and zoom into the last flash and we find a lull in the flashes– we find a flash followed by a lull in the flashes of a very particular duration that is predicted ahead of time– then we can pause the evolution of the jumps of the atom, or if you want, act. 

And this is the telltale signal that the quantum jump is mid-flight. And in fact, here we can see that it’s in the superposition state. So conditioned on measuring the signal by looking at the atom in real time, we can then apply an intervention– a microwave pulse– that tries to reverse the evolution of the atom from the ground state to the excited state right back down to where it started. 

IRA FLATOW: So you push it back down. Is that like preventing the death of Schrodinger’s cat, in this case? [LAUGHTER] 

ZLATKO MINEV: In a way. I mean, of course, I should clarify that there were no felines that were harmed or involved in the actual experiment. And here we focus on the excitations of an atom. But if you want, we can, in a sense, anticipate the jump of the cat from, say, the ground to the excited state and reverse it by applying a force of just the right amplitude and direction. And if you get either the amplitude or direction wrong, then this experiment completely fails. 

IRA FLATOW: Oh, yeah. Bye bye, cat. In the minute or so I have left, can you explain, what use is this? I mean, besides pure research, in the realm of pure research, what can you learn from preventing the quantum leap? 

ZLATKO MINEV: I think the main message to begin with is that the experiment tells us there’s more to the story of quantum physics. And in particular, there’s more to learn about randomness and predictability. That there are subtle effects that allow you to do things which you might have thought impossible before. 

And now, this first demonstration, I think, also shows us that perhaps it could be possible to explore these effects and use these kind of interventions, these detections of early errors, in the control of quantum systems. For instance, in the early detection of error syndromes in, say, quantum error correction. In the quantum computer, if a quantum jump occurs, that can lead to an error in the calculation that one is performing. 

Now, if one can, in a sense, potentially anticipate that error before it does occur, then you can prevent it from propagating in your calculation and corrupting what you’re trying to do. I think it also has very interesting applications to quantum sensing, and so forth. But this is still a very open area of exploration. 

IRA FLATOW: Well, it’s fascinating. I don’t think we’ve ever talked about preventing a quantum leap. And we wish you great luck. And congratulations on your success, Zlatko. Zlatko Minev, a research scientist at the IBM Quantum Computing in Yorktown Heights, New York. And he did his work as part of his doctoral dissertation at Yale.

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