FAQ: How Are Entangled Particles Created? [Video]
The number-one question that people ask me when I talk about nonlocality is: how are entangled particles created? I didn’t say much about this in the first edition of my book because the details don’t matter for my overall argument, but since everyone wants to know, I figure I should elaborate. (I've also added an appendix to the paperback edition.) I’ve created an animation (below) to convey the essential features of the process.
Quantum optics experiments typically create entangled photons using a crystal of barium borate. This material takes on two different crystalline forms; the one of greater interest is the beta phase. The photo at right shows a sample in a laboratory mount. It looks like a little prism. But unlike the glass in ordinary prisms, the crystalline material is nonlinear: it is not an inert substrate, but its refractive index can be modified by the light that passes through it, enabling all sorts of novel optical effects.
Courtesy of Enrique Galvez, Colgate University
In particular, the crystal allows two light beams to interact, which they wouldn’t otherwise do. The interaction can amplify one of the beams at the expense of the other, as well as create a third beam. In fact, the beam that gets amplified need not be a "beam": it might simply be random quantum noise in the electromagnetic field. If you set up the crystal properly, the amplification is so powerful that it turns the noise into a proper light beam. A single incoming beam (typically blue or ultraviolet) can thus conjure up two beams (typically red). This process occurs particle by particle: each blue photon splits into two red ones.
Courtesy of Centre for Quantum Technologies, National University of Singapore
The splitting, known as spontaneous downconversion, is a low-probability event. Only about one in a billion photons in the incident beam interacts with quantum noise and divides; the rest continue straight through the crystal unaffected. For this reason, you'd never see the beams with the unaided eye. The above image from the Centre for Quantum Technologies in Singapore is a long exposure. The photographer dragged a piece of tissue along the beams to reveal their paths.
By virtue of their common origin inside the crystal, the outgoing red photons can be entangled in any of their properties: energy, momentum, polarization. For simplicity, experimentalists usually concentrate on polarization. The crystal has an optic axis. If the laser light is polarized in the plane defined by that axis, outgoing photons will be polarized perpendicular to the axis. Vertical polarization in, horizontal out; horizontal in, vertical out. Depending on how you arrange the beam and crystal, the outgoing photons can have the same polarization (known as Type I downconversion) or exactly the opposite polarization (Type II). It doesn’t really matter; either way, the photons are perfectly correlated. This correlation reflects the symmetry of the crystal about its optic axis.
Voilà, two photons. But when you want two entangled photons, you need an extra ingredient: the polarization has to be indeterminate. This is really the essence of entanglement, the reason it’s so mysterious. The photons have the same polarization, but that polarization is not horizontal, not vertical, not circular. It's just plain nothing, a blank that has yet to be filled in, according to the standard interpretation of quantum mechanics.
After all, if the photons did have a specific polarization, there’d be no mystery. You’d create two identical photons and later measure them to be identical, which is no weirder than pairing two socks as soon as they come out of the laundry and later observing that they’re the same color. Entangled photons, in contrast, are like a pair of socks that don’t have any particular color. Each assumes a color only when measured, and both assume the same color. If that boggles your brain, it should. This is what it means to be nonlocal: you can make a statement about the system as a whole—namely, "the parts are the same"—but not about any individual part.
To produce this indeterminacy with the barium borate crystal, you sandwich two thin layers of the material, one oriented vertically, the other horizontally. Then you send in light that is polarized in neither a horizontal nor a vertical direction, but on a diagonal. As long as the layer is thinner than the beam, there’s a quantum uncertainty about which layer the beam will interact with and, therefore, what polarization the outgoing photons will have. Their ambiguity is resolved only when they strike the polarizers and are measured.
Just upstream of the crystal, you can place an optical element known as a waveplate, which can be oriented to ensure the laser photons striking the crystal are diagonally polarized or not. This waveplate therefore decides whether the photons emerging from the crystal are entangled.
What’s nice about the crystal is that it gives you a very controlled way to create entangled photons. I want to emphasize, though, that there’s nothing really special about this crystal. The fact it’s a crystal makes it sound mysterious, but entanglement can be produced in any number of ways. I did a version of the experiment in my basement using a sample of radioactive sodium-22, which gives off entangled gamma rays. Edward Fry of Texas A&M and Richard Holt of the University of Western Ontario used mercury atoms in their pioneering entanglement experiments back in the ’70s. Mercury can emit two photons in rapid succession, and those photons will be entangled. Even the mercury in ordinary fluorescent lights emits entangled photons, Fry says.
The hard part about entanglement isn’t creating it, but creating it in a way that lends itself to measurement. Because the physical implementation isn’t important, it’s perfectly valid to think about it at a higher level of abstraction, such as the coin-flipping metaphor I use in my book. The metaphor is grounded in concrete physics.