2<\/a><\/p>\n 3<\/a><\/p>\n from the mid 1980s by John Clarke, Michel Devoret, and John Martinis (all, at the time, at the University of California, Berkeley) that convincingly demonstrated that quantum tunneling and energy-level quantization can occur in a millimeter-scale electronic circuit. The experiments are noteworthy less for their results\u2014it would have been far more surprising if the circuits didn\u2019t obey the predictions of quantum mechanics\u2014than for their ramifications. The laureates showed that the macroscopic quantum world could be brought under experimental control. And their work laid the foundations for the superconducting qubits that are at the cutting edge of quantum computing research today.<\/p>\n Posing the question<\/p>\n In one sense, \u201cWhy don\u2019t we see quantum effects in the macroscopic world?\u201d is easy to answer: Planck\u2019s constant \u210f defines a physical scale that, compared with most of what we encounter in our everyday experience, is small. Beginning students of quantum mechanics are often amused to find that they can calculate the probability of some classically absurd thing\u2014walking through a wall, for example, or part of your left earlobe spontaneously appearing on Jupiter\u2014and that that number is not identically zero. But it might as well be. The time it would take a human body to tunnel through a wall, multiplied by the energy barrier it would have to overcome to do so, is so large relative to \u210f that the tunneling probability has a gargantuan negative exponent, and the event would never happen. (For some pandemic-era musings on other unphysical calculations, gargantuan negative exponents, and the meaning of \u201cnever,\u201d see the 2020 PT column \u201cA pea, the Sun, and a million monkeys <\/a>.\u201d)<\/p>\n In another sense, \u201cWhy don\u2019t we see quantum effects in the macroscopic world?\u201d evokes a different easy answer: We do. The flow of persistent currents in superconductors is a quantum phenomenon. So is the photoelectric effect. So are the existence of crystals with well-defined facets and chemicals with well-defined colors. So is the mere existence of solid matter. The echoes of quantum mechanics in our everyday experience are not sparse. But in each case, the entities behaving quantum mechanically are atoms or subatomic particles, not macroscopic collective variables like the position of a bowling ball or a person. Microscopic quantum effects make themselves known at the macroscopic level, but a macroscopic system showing its own tunneling or energy-level quantization would be an entirely different thing.<\/p>\n In yet a third sense, the question becomes significantly more subtle. The time-dependent Schr\u00f6dinger equation states that systems\u2019 wavefunctions evolve deterministically, and it makes no allowance for the probabilistic collapse of those wavefunctions during measurements. It would seem like any system set in motion would accumulate many superpositions of macroscopic states, of the type that Erwin Schr\u00f6dinger highlighted with his eponymous cat paradox, that are never observed in the real world.<\/p>\n In a 1991 Physics Today feature article, \u201cDecoherence and the transition from quantum to classical <\/a>,\u201d Wojciech Zurek made the case that those superpositions are not observed because dissipation and decoherence conspire to destroy them. No real-world system is perfectly isolated from its environment, and all the minute couplings and exchanges of energy break down the coherence between widely separated parts of a wavepacket. In effect, they transform the spookily quantum \u201cThe cat is simultaneously alive and dead\u201d into the familiarly classical \u201cThe cat is either alive or dead, but we don\u2019t know which.\u201d And because large systems have more channels for interacting with their surroundings than small systems do, their superpositions disappear far more quickly. Regardless of how completely that argument explains the nonexistence of dead-and-alive cats, dissipation certainly makes it harder to observe pure quantum behavior in macroscopic systems.<\/p>\n So the question becomes, Can one create a macroscopic apparatus that exhibits behavior described by a collective coordinate, with energy and time scales that are not large relative to \u210f, and that is also sufficiently decoupled from its environment that its quantum states don\u2019t decohere? And the answer, as of the early 1980s, was \u201cMaybe.\u201d<\/p>\n Designing the experiment<\/p>\n The key to observing quantum behavior in a macroscopic coordinate was that the coordinate could be something other than the physical position of a particle: Tunneling through a classically forbidden barrier doesn\u2019t have to involve literally walking through a wall. (More recently, researchers have started to harness the quantum behavior of position coordinates in mesoscopic and macroscopic mechanical resonators. For some examples from PT\u2019s archive, see the 2025 Back Scatter \u201cA macroscopic qubit <\/a>,\u201d the 2023 news story \u201cMacroscopic mechanical oscillator is herded into a Schr\u00f6dinger cat state <\/a>,\u201d the 2015 news story \u201cA quantum squeezed state of a mechanical resonator has been realized <\/a>,\u201d the 2010 news brief \u201cQuantum properties in the mechanical world <\/a>,\u201d and references therein.)<\/p>\n To see what such a quantum macroscopic variable could look like, consider the circuit in figure 1(a): The state of the inductor\u2013capacitor combination is characterized by the charge on the capacitor, which sloshes back and forth like a mass on a spring. The harmonic-oscillator potential, shown in figure 4<\/a><\/p>\n Figure 1.<\/p>\n In an inductor\u2013capacitor circuit (a), charge bounces between the plates of the capacitor like a mass on a spring. The harmonic-oscillator potential (b) gives rise to a series of discrete energy levels. But because the levels are all equally spaced, observing their quantization would be difficult.<\/p>\n (Figure by Freddie Pagani.)<\/p>\n View larger<\/a><\/p>\n But how could you tell? You could try to observe the energy-level quantization by spectroscopically exciting transitions among the energy levels. But the levels are all equally spaced, and the frequency of transitions between them is equal to the circuit\u2019s classical resonant frequency, so there\u2019s no clear way to distinguish a quantum resonance from a classical one. Furthermore, there\u2019s no option to observe quantum tunneling, because with only one well in the energy potential, the system has nowhere to tunnel to.<\/p>\n Both those problems are solved with the switch from an inductor\u2013capacitor circuit to a Josephson junction: two overlapping strips of superconducting material, as shown in figure The state of the Josephson junction is characterized by the superconducting phase difference across the interface. That sounds like an exotic quantum mechanical quantity, but you can think of it as roughly analogous to the charge in the inductor\u2013capacitor circuit: Both are macroscopic parameters that describe the collective state of all the charge carriers in the system. The phase difference is defined modulo 2\u03c0, and it follows a sine-wave potential rather than a parabolic one. The result, as shown in figure If a Josephson-junction circuit is prepared in a low-lying state in one well of the sine-wave potential, classical physics would dictate that, barring any energy input into the system, it would stay there forever. But quantum mechanics predicts that the system has some probability of turning up in a different energy well: Despite lacking the energy to climb over the barrier, it can tunnel through it. And that tunneling probability can be made significant, even in a circuit that\u2019s not too small: In the one the laureates used, the interface between the superconductors was 10 \u00b5m by 10 \u00b5m. In a circuit of that size, tunneling through the energy barrier would involve the concerted motion of billions of Cooper pairs. Mathematically, it makes sense to describe their state as a single collective variable. But would that variable obey the Schr\u00f6dinger equation, or would decoherence degrade or ruin its quantum behavior?<\/p>\n Figure 2.<\/p>\n A Josephson junction (a)\u2014two strips of superconductor separated by a thin nonsuperconducting interface\u2014provided the ideal testing ground for macroscopic quantum effects. Its energy potential (b) is a sine wave, rather than a parabola, so its states are unequally spaced, and the system can tunnel from one energy well into another.<\/p>\n (Panel (a) adapted from J. M. Martinis, M. H. Devoret, J. Clarke, \u201cQuantum Josephson junction circuits and the dawn of artificial atoms,\u201d Nat. Phys. 16, 234, 2020; panel (b) by Freddie Pagani.)<\/p>\n View larger<\/a><\/p>\n Clarke, Devoret, and Martinis weren\u2019t the first to appreciate that a Josephson junction could be an ideal testing ground for macroscopic quantum effects.<\/p>\n 5<\/a><\/p>\n Nor were they the first to attempt the experiment.<\/p>\n 6<\/a><\/p>\n What set their work apart was the care with which they made their measurements\u2014and, consequently, the clarity of their results.<\/p>\n They started by thoroughly characterizing the circuit in the classical regime to pin down the parameters of the sine-wave potential\u2014complete with error bars\u2014and therefore the tunneling probability that they could expect under any given conditions. Because cooling to absolute zero is impossible, there was always some lingering probability that the circuit could get enough of an energy kick from the environment to hurdle over the barrier rather than tunnel through it. They needed to understand the likelihood of the first possibility to demonstrate the existence of the second.<\/p>\n For the test itself, the laureates biased the Josephson junction with a small current, which transformed the level sine wave of figure Starting at 1 K and cooling the system to progressively lower temperatures, the laureates measured how readily the voltage drop appeared. In the upper part of the temperature range, there was still plenty of thermal energy for the system to surmount the energy barrier classically. But as the temperature fell, the classical probability diminished. If the voltage drop kept appearing, it would have to be due to quantum tunneling.<\/p>\n Figure In another series of experiments, the laureates used microwaves to excite the Josephson-junction circuit from the lowest metastable energy level to a higher one. Rather than varying the microwave frequency to home in on the quantum resonance, they varied the bias current, which changed the tilt and shifted the energy-level spacings. When it was in resonance with the microwaves, the circuit was excited to a higher energy level, which had less of an energy barrier to tunnel through, so the researchers observed the excitation as an enhanced escape rate from the metastable well. And the resonances always appeared where quantum mechanics said they would.<\/p>\n But qubits?<\/p>\n The energy-quantized Josephson junction wasn\u2019t yet a qubit. For one thing, in 1985, the word \u201cqubit\u201d didn\u2019t even exist\u2014and it wouldn\u2019t be coined by Benjamin Schumacher until a decade later, after Peter Shor discovered that a hypothetical quantum computer could find the prime factors of a large number faster than a classical computer could. The advent of Shor\u2019s algorithm helped launch the study of quantum information from a niche intellectual pursuit into something with potential real-world applications. (For more on the algorithm and its genesis, see the annotated version of David Zierler\u2019s interview with Shor <\/a> published in PT in April 2025.)<\/p>\n For another thing, the Josephson junction still had more quantum properties to reveal. The laureates had demonstrated tunneling and energy-level quantization. But a useful qubit also needs the ability to be prepared in a superposition of states, which can be manipulated in conjunction with other qubits to create complex entangled states.<\/p>\n Figure 3.<\/p>\n Applying a bias current to a Josephson junction transforms the flat sine-wave potential from figure 2 into a tilted one (a). The system can then prove its quantum nature by tunneling out of the metastable energy well. The plot in (b), from one of the laureates\u2019 landmark papers, shows one clear demonstration of the effect. The horizontal coordinate T is the system\u2019s real temperature, and the vertical coordinate Tesc is the temperature that would yield the observed escape rate if all the escapes happened classically. At higher temperatures the two are equal, but at lower temperatures they diverge: evidence of macroscopic quantum tunneling (MQT).<\/p>\n (Panel (a) by Freddie Pagani; panel (b) from ref. 3.)<\/p>\n View larger<\/a><\/p>\n There are several ways to create superposable states out of a superconducting Josephson-junction-based circuit. Physics Today has covered superconducting qubits at several stages of their development: To read about them in more detail, see the November 2005 feature article \u201cSuperconducting circuits and quantum information <\/a>,\u201d by J. Q. You and Franco Nori; the 2002 news story \u201cTwo realization schemes raise hopes for superconducting quantum bits <\/a>\u201d; and the 2009 news story \u201cSuperconducting qubit systems come of age <\/a>.\u201d<\/p>\n Perhaps the most conceptually straightforward of the superconducting qubits uses the lowest two energy levels of the system, as represented in figure That approach, called a phase qubit, was pioneered in 2002 by Martinis and others.<\/p>\n 7<\/a><\/p>\n But it was pre-dated by a different scheme, called a charge qubit, in which Cooper pairs are made to tunnel one by one across a Josephson junction to an isolated superconducting island.<\/p>\n 8<\/a><\/p>\n The states with some number n and n\u202f+\u202f1 Cooper pairs on the island are designated as the qubit\u2019s 0 and 1 states.<\/p>\n A refined version of the charge qubit, called a transmon,<\/p>\n 9<\/a><\/p>\n is currently favored by many quantum computing research groups. Transmons are the basis, for example, of the Google Quantum AI team\u2019s Willow chip, which recently achieved a long-sought milestone in quantum error correction. To counter the inherent delicacy of quantum states, researchers have hoped to build redundancy into a quantum computer by combining the states of many physical qubits to make one logical qubit. But that strategy works only if the physical qubits have a low enough error rate that adding more of them makes things better, not worse. And the Willow chip has done just that.<\/p>\n 10<\/a><\/p>\n But Google researchers aren\u2019t the only ones to be making great strides in quantum error correction and other prerequisites to practical quantum computation. Other teams are right on their heels, with implementations that use neutral atoms or trapped ions rather than superconducting circuits. It remains to be seen which qubits, if any, will be the building blocks of the quantum computers of the future.<\/p>\n Of the leading qubit contenders, superconducting qubits stand out in several ways. All qubits are quantum systems with discrete states, much like those of the atoms that occur in nature. And most qubits are either actual atoms or something similarly small. Superconducting qubits, however, are orders of magnitude larger\u2014large enough to be connected with wires in much the same way as the components of conventional computing hardware are. And because they\u2019re engineered structures, their properties can be fine-tuned: Their interactions can be made far stronger and faster than those of natural-atom qubits, so they could potentially lead to faster computing speeds.<\/p>\n Understanding the answer<\/p>\n Despite recent advances, quantum computers are not yet a mature technology. In that respect, they stand in stark contrast to the neural networks\u2014highlighted by the 2024 physics Nobel, covered in a December 2024 <\/a> Physics Today news story\u2014which are already having disruptive, world-changing effects throughout society, for good or for ill.<\/p>\n Of course, not every Nobel Prize in Physics is connected to a practical technology. The 2015 prize, for example, honored the discovery that neutrinos spontaneously change flavor as they travel (covered by PT in December 2015 <\/a>). Neutrino oscillations aren\u2019t the basis for any consumer products, and they probably won\u2019t ever be\u2014although one never knows for sure.<\/p>\n But neutrino oscillations were an unexpected answer to a fundamental question about the universe. They\u2019re evidence that there\u2019s something going on in the subatomic world that\u2019s not well described by the standard model of particle physics, and they pointed toward places to look for answers to even deeper questions.<\/p>\n And that\u2019s not quite the story of the 2025 prize either. The fact that macroscopic collective variables obey the Schr\u00f6dinger equation was, strictly speaking, not known for sure until it was observed. The observation did rule out some alternative theories that had been floated, such as the idea that above some suitably defined size scale, quantum mechanics just doesn\u2019t apply. But the results themselves weren\u2019t as revelatory as some years\u2019 prizes are.<\/p>\n No one who\u2019s not on the Nobel Committee can be sure of the reasoning for awarding any particular prize. But the value of the work by Clarke, Devoret, and Martinis seems to be in its effects on how physicists do physics. Their experiments expanded the range of parameter space that can be brought under experimental control (and as such, their work is reminiscent of the 2023 prize <\/a>, for the creation of attosecond laser pulses, or maybe even the 2017 prize <\/a>, for the development of gravitational-wave observatories). Beyond qubits, their work has ramifications for basic research, including the field of circuit quantum electrodynamics.<\/p>\n
\n
\n 1(b)
\n <\/a>
\n, has equally spaced quantum states. As the laureates and colleagues have noted, with a temperature of 10 mK, an inductance of 350 pH, and a capacitance of 15 pF\u2014all experimentally realizable values\u2014the energy-level spacing would dwarf the system\u2019s thermal energy, and quantum effects would dominate.<\/p>\n<\/p>\n
![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/12\/miller-feature-fig1.png\")
<\/p>\n
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\n 2(a)
\n <\/a>
\n, with a thin nonsuperconducting layer at the interface. Cooper pairs in the superconductors can tunnel through the interface\u2014but importantly, the tunneling through that physical barrier is distinct from the macroscopic quantum tunneling that the laureates were seeking to demonstrate.<\/p>\n
\n
\n 2(b)
\n <\/a>
\n, is a series of energy levels that aren\u2019t equally spaced and plenty of energy barriers for the system to tunnel through.<\/p>\n<\/p>\n
![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/12\/miller-feature-fig2.png\")
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\n 2(b)
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\n into the tilted one of figure
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\n 3(a)
\n <\/a>
\n. Now the tunneling entity had somewhere to go: If it escaped the metastable state in the energy well it started in, it would go tumbling down the potential-energy hill, which would be observable as the spontaneous appearance of a voltage drop across the Josephson junction.<\/p>\n
\n
\n 3(b)
\n <\/a>
\n shows one way of plotting their results. The horizontal axis is the actual temperature, and the vertical axis is the temperature that would yield the escape rate that they observed, assuming that all the escapes happened classically. In the upper right part of the plot, those temperatures are equal, but in the lower left, the effective escape temperature levels off while the real temperature continues to fall: clear evidence of tunneling.<\/p>\n<\/p>\n
![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/12\/miller-feature-fig3.png\")
<\/p>\n
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\n 2(b)
\n <\/a>
\n, as the qubit\u2019s 0 and 1 states. The laureates had shown that a blast of microwaves at the right frequency can excite the circuit from one state to the other. And just like with other electromagnetically excitable systems, pulses of precise duration can partially transfer the system between the two states and thereby create any desired coherent superposition of 0 and 1.<\/p>\n