Imagine that your fourteen-year-old daughter, Alice, has been arguing with you day and night for the past week about going out with friends to party. Every night, you tell her no, and every night, you hear her groaning and complaining in her room. One night as you are washing the dishes, you notice that the house is quiet – too quiet. You run up to Alice’s room and fling open the door, catching Alice just as she is climbing out the window to freedom. Determining if a teenager is about to sneak out of the house may be difficult, but even more unpredictable than a disgruntled teenager is a quantum system, where artificial atoms can jump between different levels with complete randomness. However, recent research from the Devoret Lab at Yale has shown that there may be a way to predict, catch, and even reverse such a jump, even if the atom is partway through its daring escape.
While it is obvious why you care about catching escaping teenage delinquents, understanding tiny quantum systems can have even greater reward. One of the hallmarks of quantum mechanics is the absolute randomness present within a quantum system. Such limits, derived by physicists like Heisenberg and Schrodinger, show that even if you knew how every single particle in the universe moved, you would still be unable to predict what happens in certain quantum systems. This uncertainty can create problems for quantum computing, as it makes it especially different to prevent errors in the system from accumulating. To attack this problem, the Devoret Lab created a quantum system that they could finely control and test out a new way of predicting errors.
Designing a quantum system that can be carefully probed and measured is not easy, but Zlatko Minev, an applied physics graduate student at Yale, has plenty of experience. His previous work was in a quantum system of superconducting circuits that is often referred to as an artificial atom, because like an atom, the system has different energy levels that it can jump between with complete randomness. These energy levels are similar to the states that the teenage daughter can occupy – in her bedroom, on her phone, or going out the window. At any time, it is possible for the daughter to make a leap away from talking on the phone and bolt for the window. For the artificial atom, it can jump between a ground state to either an excited state or a dark state.
Unlike a regular atom, an artificial atom can be very carefully controlled using microwave signals and measured using highly sensitive photon detectors. Both these properties were crucial for this experiment’s success: the microwave control allows the experimenters to freeze the status of the atom at different points to look more closely at it, while the sensitive photon detectors allow for the detection of the tiniest signals.
When the quantum system is turned on, it must always be jumping to one of its excited states. Every time it makes the jump between the excited state and the ground state, the system emits a photon that can then be measured with high precision by the experimenters. However, the dark state is isolated and cannot be directly measured in the same way. Instead, the experimenters can deduce that the atom is now jumping back and forth from the dark state to the ground by looking for an absence of signal in the excited state transition. This is analogous to the daughter talking on her telephone: every time she speaks, her parents can hear her and know that she is in her room, while a prolonged silence would indicate that she has escaped.
Minev’s experiment hinges on a prediction made by Quantum Jump Theory, which governs the random probability of making hops from the ground to the excited state to making hops from the ground to the dark state. Although this theory predicts that it is impossible to directly predict when the atom would suddenly start jumping to the dark states, it does provide a loophole to peer into this bizarre world. The experimenters can determine that the atom is about to make the transition if they observe a warning signal. This warning signal comes from a prolonged gap in the signal from the excited state, like how it may take time for the daughter to hang up the phone before she is able to leap out the window.
This warning signal may seem like a simple concept, but it is incredibly hard to measure. To begin, the normal photon signal from the excited state is not continuous, meaning that there are natural breaks in seeing photons. To properly predict a transition to the dark state, the experimenters need to tell the difference between a natural lull in and a warning signal. In addition, both natural lulls and warning signals require incredible precision to measure, equivalent to not missing a single signal within one hundred thousand. Such a measurement can perhaps only be done accurately in a superconducting system, and not in a natural atom system.
Although building a superconducting system is difficult, the payoff is great: Minev’s team was able to predict the atom jumps before they happened. Using the microwave control signals, they were able to freeze the system to observe exactly how far the atom had jumped at several different time steps, and by reversing the control signals, they can even force the atom to jump backwards halfway through a jump. These measurements not only show the validity of the quantum jump theory, they also provide new insights into the nature of randomness.
In the future, the group hopes to use this powerful warning signal to understand other quantum behaviors and to correct for errors before they even happen. Sitting on a stable island amidst a sea of chaos, there is hope yet ahead for a fully implementable quantum computer.