For the first time ever scientists from the University of Rochester have directly measured the orbital-angular-momentum of photons in a high-dimensional quantum state. The state consists of 27 dimensions to be exact. In actuality this is probably far lamer than it sounds, right? Nope, it’s just as mind blowing as you would imagine.
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It’s amazing enough that we have the ability to even theorize a 27-dimensional quantum state, but now we can directly measure one. Our 3-dimensional brains use our eyes to see the world in two dimensions. 3-dimensional sight is actually just a trick evolution has taught us. Despite living in a world defined by length, width, and height, the universe itself actually exists in many more spatial dimensions, possibly an infinite number.
High dimensionality is the stuff of quantum physics (physics of the smallest scales of the universe) since these other dimensions are only apparent at incredibly tiny scales. Although scientists have never physically perceived another dimension of space, extra dimensions are a mathematical inevitability when describing the quantum universe.
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Quantum physicists observe a quantum system’s current state and behavior (known as a wavefunction) and attempt to derive the original quantum state as well as the probable future states of the system. The problem is that the act of observation itself alters the state of a quantum system. This is known as the observer effect. The observer is just as much part of the system as what is being observed. Very weird. Additionally, due to the Heisenberg Uncertainty Principle, which states that the more you know about one aspect of a system, the less you will know about another aspect (especially with regards to position vs. momentum), it is impossible to attain accurate data about the entirety of any given system. That is, before the advent of direct measurement.
Before direct measurement, quantum physicists used quantum tomography to derive the various states of a quantum system. Quantum tomography is the process of reconstructing the original quantum state of a wavefunction using bits and pieces of impartial data one bit at a time. It involves taking many measurements of an identical quantum state and comparing the changes made by observation in each one to narrow down the source quantum state. This is like creating a 3d image out of many 2d images. By directly measuring a quantum state, physicists are able to reconstruct the source quantum state without any post-processing. This dramatically speeds up the process and could eventually lead to breakthroughs in the feasibility of quantum computing.
The idea of direct measurement seems to contradict everything we know about quantum physics. If we directly measure something, our act of taking a measurement should alter the system in some way, thus skewing the results and totally collapsing the quantum state. Quantum physicists may not be known for their social prowess, but one thing they are known for is being very clever. Knowing that a direct measurement will collapse a quantum state, physicists take a measurement so gentle that the quantum state is only slightly altered; it doesn’t ever completely collapse.
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This “weak” measurement is then followed by a “strong” measurement of another unmeasured variable in the system. This sequence of weak and strong measurements is repeated on many identical quantum systems until the source wavefunction is precisely known. By being less sure about their intended measurement, physicists can take more precise measurements of peripheral variables in the measured field. Remember, the more you know about A, the less you know about B, and vice versa. They measured B more precisely by knowing less about A. It’s a paradoxical but effective method.
The direct measurement technique was first developed in 2011 by scientists at the National Research Council Canada, who used it to determine the position and momentum of photons. This study is the first major breakthrough in the direct measurement of a quantum state wavefunctions since 2011.
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According to lead author Dr. Mehul Malik, a post-doctoral research fellow at the University of Vienna,
It is sort of like peeking into the box to see if Schrödinger’s cat is alive, without fully opening the box. The weak measurement is essentially a bad measurement, which leaves you mostly uncertain about whether the cat is alive or dead. It does, however, give partial information on the health of the cat, which when repeated many times can lead to near certain information as to whether the cat is alive or dead.