Quantum Strangeness

A typical tabletop quantum experiment. These experiments are not particularly spectacular to look at—but their results can be earthshaking. (This one is an experiment by David Hall of Amherst College, on a quantum phenomenon known as a Bose–Einstein condensate.) Photo by George Greenstein.

Figure 3.1

Albert Einstein. Although he was one of the creators of quantum mechanics, he never accepted it. Over and over again, Einstein argued that the theory was incomplete because it failed to describe subatomic reality. The arguments that Einstein advanced are the fertile soil from which grew the discoveries described in this book. Photo courtesy of the American Institute of Physics Emilio Segre Visual Archives.

Figure 4.1

Niels Bohr. Also one of the creators of quantum mechanics, Bohr argued that Einstein’s search for a more complete theory—one that would describe microscopic reality—was misguided. Indeed, Bohr argued, the refusal of quantum theory to do so was not a problem but a discovery—a profound philosophical insight. Photo courtesy of the American Institute of Physics Emilio Segre Visual Archives.

Figure 4.2

Bohr and Einstein … in the midst of a furious battle? Although they disagreed profoundly, their disagreements were never personal. In fact, they had deep affection and respect for one another. Photo courtesy of the American Institute of Physics Emilio Segre Visual Archives.

Figure 5.1

A spinning object and its axis of spin.

Figure 6.1

Figure 9.1

Naive view of the entangled state. It makes intuitive sense, but it will turn out to be wrong.

Figure 9.2

The spin axis of Alice’s electron points along one of the indicated directions.

Figure 9.3

The spin axis of Bob’s electron points along one of the indicated directions.

Figure 9.4

Bob’s detector points in a different direction than Alice’s.

Figure 9.5

The electron that reaches Bob has one of the indicated spin axes.

Figure 9.6

When do Bob’s and Alice’s measurements agree?

Figure 10.1

John Bell lectures. His famous theorem showed that no local description of submicroscopic reality could make the same predictions as quantum mechanics. On the blackboard behind him can be seen a segment of this famous theorem (at the top). Photo courtesy of CERN.

Figure 10.2

John Bell and his wife Mary at dinner with friends. Mary Bell is also a physicist: indeed, the two Bells often collaborated. Photo © Renate Bertlmann.

Figure 12.1

Clauser later in life. John Clauser performed the first pioneering experiments on Bell’s Theorem. Bell had shown that we have not one but two theories: one is quantum mechanics, and the other is some theory that would fully describe the reality underlying quantum phenomena. Clauser realized that an experiment could be done that would tell us which was correct. His result favored quantum mechanics. Photo courtesy of John Clauser.

Figure 12.2

Alain Aspect. Clauser’s experiment had a potential loophole: that somehow the two entangled particles could communicate with one another. Aspect closed that loophole by randomly changing the “questions” asked of them.

Figure 12.3

Anton Zeilinger. Clauser’s experiment had another potential loophole: that the “questions” asked of the two particles only seemed random, but were in fact being dictated by some unknown mechanism. Zeilinger’s experiment showed that this mechanism, if it existed at all, lay far off in the universe and operated far back in the past. Photo courtesy of the Mind & Life Institute, © The Mind & Life Institute.

Figure 12.4

Carlos Abellán (left) and Morgan Mitchell (right). Photo: ICFO.

Are the “questions” asked of the two particles really random? All previous experiments had relied on some physical mechanism to achieve randomness—but mechanisms obey the laws of classical physics, and so are not truly random. In the “the BIG Bell Test experiment” vast numbers of people were enlisted to use their free will to create randomness.

Figure 12.5

The app they created. Image: Maria Pascual (Kaitos Games).

Figure 15.1

Jian-Wei Pan at an experiment. Photo courtesy of the Micius Group.

Figure 15.2

Launch of the Micius Satellite. Photo courtesy of the Micius Group.

Figure 15.3

The Micius Satellite.

Jian-Wei Pan is the leader of a group that launched a “quantum machine” into orbit about the earth. This machine, the Micius satellite, has been used to securely transmit a cryptographic key from one place to another, and to teleport a quantum state from one place to another. Photo courtesy of the Micius Group.