John Clauser

John Clauser
Born	John Francis Clauser 1 12, 1942
Birthplace	Pasadena, California, U.S.
Nationality	American
Occupation	Physicist
Known for	Bell test experiments, CHSH inequality
Education	Columbia University (Ph.D., 1970)
Awards	Nobel Prize in Physics (2022) Wolf Prize in Physics (2010)
Website	[http://www.johnclauser.com/ Official site]

John Francis Clauser (born December 1, 1942) is an American theoretical and experimental physicist whose work on the foundations of quantum mechanics helped transform a philosophical debate into an empirical science. Born in Pasadena, California, Clauser is best known for conducting, with the late Stuart Freedman, the first experimental test of Bell's theorem in 1972 at the University of California, Berkeley — an experiment that demonstrated the reality of quantum entanglement and the violation of Bell inequalities. This pioneering work, along with his co-development of the CHSH inequality (Clauser–Horne–Shimony–Holt inequality), laid the groundwork for the field of quantum information science. For these contributions, Clauser was awarded the 2022 Nobel Prize in Physics, shared jointly with Alain Aspect and Anton Zeilinger, "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science." Clauser's career has spanned positions at Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, and the University of California, Berkeley. In later years, Clauser attracted controversy for publicly declaring himself a denier of the scientific consensus on climate change.

Early Life

John Francis Clauser was born on December 1, 1942, in Pasadena, California. He grew up in an academic environment; his father, Francis Clauser, was an aeronautical engineer and professor at the California Institute of Technology (Caltech). Growing up in Pasadena and in proximity to one of the world's leading scientific institutions, Clauser was exposed to physics and engineering from an early age. His upbringing in a household connected to Caltech provided him with early access to scientific thinking and experimentation.

As a young man, Clauser developed a strong interest in experimental physics. In a 2022 interview with Caltech, he described the intellectual curiosity that drove him toward the study of the fundamental nature of reality, and particularly toward the questions raised by quantum mechanics about the behavior of particles at the subatomic level. This early interest would ultimately lead him to confront one of the deepest conceptual questions in twentieth-century physics: whether the strange correlations predicted by quantum mechanics between distant particles could be real, or whether they pointed to a deeper, hidden layer of reality as Albert Einstein had suggested.

Education

Clauser attended the California Institute of Technology as an undergraduate, where he studied physics. He later pursued graduate studies at Columbia University in New York, where he completed his doctoral research under the supervision of Patrick Thaddeus, an astrophysicist. Clauser's doctoral dissertation, titled "Measurement of the Cosmic Microwave Background by Optical Observations of Interstellar Molecules," was completed in 1970. Notably, his thesis topic was in astrophysics rather than in the foundations of quantum mechanics that would later define his career. It was during his time at Columbia that Clauser encountered the work of John Stewart Bell, whose 1964 theorem proposed a way to experimentally test whether quantum mechanics or local hidden-variable theories provided a more accurate description of nature — a discovery that would redirect the course of Clauser's scientific life.

Career

The CHSH Inequality

While still a graduate student at Columbia University, Clauser became deeply interested in the foundational questions raised by Bell's theorem. In 1964, physicist John Stewart Bell had published a landmark paper proposing that the predictions of quantum mechanics and those of local hidden-variable theories could, in principle, be distinguished by experiment. However, Bell's original formulation was not directly amenable to practical experimental testing. In 1969, Clauser, along with Michael Horne, Abner Shimony, and Richard Holt, published a paper that reformulated Bell's inequality into a form that could be tested in the laboratory. This reformulation became known as the CHSH inequality (Clauser–Horne–Shimony–Holt inequality).

The CHSH inequality provided a concrete, measurable criterion: if experimental results violated the inequality, local hidden-variable theories would be ruled out, and the quantum mechanical prediction of entanglement correlations would be confirmed. This was a critical theoretical contribution because it translated a philosophical debate — one that had been ongoing since the famous 1935 Einstein–Podolsky–Rosen (EPR) paper — into the language of laboratory measurement. The CHSH paper became one of the most cited works in the history of quantum physics and remains foundational to the field of quantum information science.

The Freedman–Clauser Experiment (1972)

After completing his doctorate at Columbia, Clauser moved to the University of California, Berkeley, where he began working as a postdoctoral researcher at the Lawrence Berkeley National Laboratory (then known as Lawrence Berkeley Laboratory). It was at Berkeley that Clauser, together with graduate student Stuart Freedman, carried out the first experimental test of the CHSH inequality in 1972. This experiment, known as the Freedman–Clauser experiment, is considered a landmark in the history of physics.

In the experiment, Clauser and Freedman generated pairs of entangled photons using calcium atoms excited by ultraviolet light. The entangled photons were sent to two separate detectors, where their polarization was measured along various angles. The results clearly demonstrated a violation of Bell's inequality, consistent with the predictions of quantum mechanics and inconsistent with any local hidden-variable theory.

The experiment was conducted under conditions that were, at the time, considered extremely challenging. Clauser later described the technical difficulties involved in building the apparatus, which required precise optical alignment and the ability to detect individual photons. The result was a decisive demonstration that quantum entanglement — what Einstein had famously dismissed as "spooky action at a distance" — was a real physical phenomenon.

Despite the significance of the result, the experiment was initially met with limited enthusiasm from the broader physics community. At the time, most physicists considered the foundations of quantum mechanics to be a settled matter and viewed experimental tests of Bell's theorem as philosophically interesting but not central to the progress of physics. Clauser himself noted in later interviews that pursuing this line of research was considered career-threatening, as the physics establishment of the era was skeptical of work on the interpretive questions of quantum mechanics.

Stuart Freedman, Clauser's collaborator on the experiment, passed away in 2012 and was therefore ineligible for the Nobel Prize when it was awarded in 2022. The Nobel Committee and the University of California, Berkeley both acknowledged Freedman's essential contribution to the work.

Later Research and Career

Following the 1972 experiment, Clauser continued to refine and extend his experimental tests of quantum mechanics. He conducted additional Bell test experiments with improved apparatus, seeking to close potential loopholes that might allow a local hidden-variable interpretation of the results. His subsequent work helped to strengthen the case that the violations of Bell inequalities were genuine features of nature rather than experimental artifacts.

Clauser worked at the Lawrence Berkeley National Laboratory and later at the Lawrence Livermore National Laboratory. He also held a position at the University of California, Berkeley. Over the course of his career, Clauser made contributions not only to the experimental testing of quantum foundations but also to the broader development of quantum optics and precision measurement techniques.

In later decades, as the field of quantum information science emerged and grew, the significance of Clauser's early work became increasingly appreciated. The CHSH inequality and the Freedman–Clauser experiment are now recognized as foundational contributions that helped make possible subsequent developments in quantum computing, quantum cryptography, and quantum teleportation.

Nobel Prize in Physics (2022)

On October 4, 2022, the Royal Swedish Academy of Sciences announced that Clauser, along with French physicist Alain Aspect and Austrian physicist Anton Zeilinger, had been awarded the Nobel Prize in Physics "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science."

The Nobel Committee described how the three laureates' work had built upon one another in a chain of increasingly sophisticated experiments. Clauser's 1972 experiment provided the first empirical evidence that Bell inequalities could be violated, confirming the quantum mechanical prediction. Aspect's experiments in the early 1980s closed an important loophole in Clauser's original experiment by rapidly switching the measurement settings during the experiment. Zeilinger's group subsequently demonstrated a range of quantum information phenomena, including quantum teleportation.

The University of California, Berkeley and the Lawrence Berkeley National Laboratory both issued statements recognizing Clauser's achievement and noting that the foundational experiment had been performed in their facilities. Caltech also recognized Clauser as an alumnus who had made a defining contribution to modern physics.

The Nobel Prize citation highlighted the broader implications of the laureates' work, noting that their experiments had established the reality of quantum entanglement and had opened new avenues for quantum technology. The Nobel Committee stated that the development of experimental tools and methods by the three laureates had "cleared the way for new technology based upon quantum information."

In 2011, more than a decade before receiving the Nobel Prize, Clauser, Aspect, and Zeilinger had been identified as potential Nobel laureates when Thomson Reuters (now Clarivate Analytics) admitted them to their Hall of Citation Laureates based on their citation impact in the scientific literature.

Personal Life

Clauser has maintained his residence in California and operates through J.F. Clauser & Associates, a research consulting firm.

Climate Change Views

In 2023, following his Nobel Prize award, Clauser attracted significant public attention and controversy for his statements on climate change. He publicly declared himself a "denier" of the scientific consensus on climate change, asserting that climate models are unreliable and that the role of clouds in regulating Earth's temperature is not adequately accounted for in mainstream climate science. His views were reported in The Washington Post and other major media outlets, prompting discussion about the phenomenon of Nobel laureates making public pronouncements on topics outside their fields of expertise.

Climate scientists responded to Clauser's claims. The climate science blog RealClimate published a detailed critique of Clauser's arguments, analyzing his assertions about cloud feedback and concluding that his understanding of climate science contained fundamental errors. Clauser's climate views remain a subject of ongoing discussion in both the scientific community and the media.

Recognition

Clauser's contributions to the foundations of quantum mechanics have been recognized with numerous awards and honors over the course of his career. His most significant recognitions include:

• Nobel Prize in Physics (2022) — shared with Alain Aspect and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science."
• Wolf Prize in Physics (2010) — shared with Alain Aspect and Anton Zeilinger for their fundamental conceptual and experimental contributions to the foundations of quantum physics, specifically for an increasing series of tests of Bell's inequalities using entangled quantum states.
• Thomson Reuters Citation Laureate (2011) — Clauser, Aspect, and Zeilinger were admitted to the Thomson Reuters Hall of Citation Laureates, which identified them as likely future Nobel Prize recipients based on their citation impact.

In addition, Clauser's 1969 CHSH paper and the 1972 Freedman–Clauser paper are among the most cited works in the history of physics. His contributions are recognized across multiple academic databases, including Google Scholar, Scopus, and zbMATH.

Legacy

John Clauser's work occupies a central place in the history of quantum physics. The CHSH inequality, which he co-authored in 1969, transformed the abstract philosophical debate about quantum mechanics — the question of whether entangled particles truly influence each other instantaneously across distance, or whether hidden variables could explain the observed correlations — into a question that could be answered by experiment. The 1972 Freedman–Clauser experiment then provided the first empirical answer: the correlations predicted by quantum mechanics are real and cannot be explained by local hidden-variable theories.

These contributions are now recognized as having opened an entirely new field of science. The experimental confirmation that Bell inequalities can be violated was an essential precondition for the development of quantum information science, which includes quantum computing, quantum cryptography, and quantum communication networks. As the Nobel Committee stated in its 2022 citation, the work of Clauser, Aspect, and Zeilinger "cleared the way for new technology based upon quantum information." The Nobel Prize organization has described how quantum technologies, though still in early stages of development, are advancing rapidly and hold the potential to transform computing, communications, and sensing.

Clauser's willingness to pursue experimental tests of Bell's theorem at a time when the physics community considered such work marginal is itself a notable aspect of his legacy. The prevailing attitude in physics during the late 1960s and early 1970s was that the interpretation of quantum mechanics was a philosophical rather than a scientific question, and that experimental tests of Bell's inequality were unnecessary. Clauser's decision to undertake this work despite professional risk contributed to a shift in the culture of physics, opening space for serious experimental and theoretical investigation of quantum foundations.

The PNAS profile of the 2022 Nobel laureates noted that the path from the CHSH inequality and the Freedman–Clauser experiment to the modern field of quantum information science represents one of the most consequential developments in late twentieth-century and early twenty-first-century physics.