In the famous gedanken experiment posited by famed quantum physicist Erwin Schrodinger in the 1930’s, a hypothetical cat resides in a closed box with a radioactive atom which may exist in a stable or decaying state; if the atom decays, it triggers the release of a poison contained in a flask, which would kill the poor, putative puttycat. But until we open the box, the cat is both alive and dead (note: this previous description has been edited/corrected for accuracy).
The opening of the box is an analogy for performing a measurement on the quantum system. The essential quantum concept here is that, until the measurement is made (i.e., the box opened) the radioactive atom, like the cat (both “quantum objects”), exists in a superposed state — it is both decaying and not decaying (and the cat is both dead and alive) simultaneously. Once the box is opened, the superposed state “collapses” and the cat is either alive or dead.
Such a superposition of quantum states — like another bizarre quantum property known as entanglement in which the states of two quantum particles are linked regardless of the distance between them — apply to exceedingly tiny objects; they do not (according to Schrodinger and many modern physicists) apply to everyday objects in the macro world that we can see and touch.
For many physicist, the idea of applying this quantum effect to larger scale systems is absurd. There is a decided lack of large-scale systems or objects (“cats” if you will) that exhibit this curious property observed in the quantum realm. It would seem that a definitive boundary exists between the micro and macro worlds. Still, some physicists continued to wonder if such effects like superposition and entanglement can be observed on large scales.
Large-Scale Quantum Effect Achieved – More Like a ‘Kitten’ Than A ‘Cat’
Now, in two separate and recent lab experiments (by Lvovsky et al, and Gisen et al), the property of quantum entanglement has been conferred to a large-scale system for the first time.
A team led by Alexander Lvovsky at the Russian Quantum Center (in Moscow), sought to replicate the Schrodinger’s cat experiment on a larger scale (and more faithfully to the original concept) using a semi-transparent mirror to put a single photon (the quantum particle of light) into a superposed state (one in which the photon passes through the mirror, the other in which it is reflected). Once this mixed state was achieved, they were able to “entangle” the two photons such that the behavior of one photo state was linked to the other.
Then, in a clever bit of technological manipulation involving high-speed lasers, one of the quantum states was amplified so that it was imposed on hundreds of millions of photons (a mass of photons easily visible to the unaided eye, in theory, although the team used a non-visible wavelength of light). Following this, they returned the amplified light to its original one-photon (superposed) state.
Upon measurement, they were able to confirm that the entangled state had been preserve throughout the duration of the experiment even while one of the quantum states had been imposed on a macroscopic system (the array of hundreds of millions of photons) for a period of time.
A similar result was achieved at the thousand photon scale by Nicolas Gisin and colleagues at the University of Geneva in Switzerland using a slightly different experimental set-up.
The physicists assert that their experiments represent the first time quantum entanglement has been achieved between a quantum (micro scale) object and a macroscopic object.
“Our breakthrough has been that, so far, people have been able to build these superposition states containing only a few photons, and we’ve been able to do it with 160 million photons,” says Lvovsky.[source: see link, below]
Applications and Meaning of the Experiments
One application of this micro-macro entanglement effect could be to improve the precision of interferometers which use entanglement to measure small differences in micron and nano-scale lengths. Many other uses may also be possible here, perhaps even in quantum Artificial Intelligence.
There have been previous successes in getting larger objects to exhibit quantum properties. In one case, two 3mm-sized diamonds were entangled (Lee, Sprague), and in a second experiment (Purdy et al), a tiny drum the size of a grain of sand was shown to obey Heisenberg’s Uncertainty principle (which prevents us from knowing particle’s position and momentum at the same time).
These more recent experiments extend this work much further and represent a more complex and precise execution and application of the effect. As a proof-of-principle, the experiments convincingly demonstrate that the laws governing the quantum realm may also apply to the classical realm of observable objects.
And while the large-scale object here is not quite at the level of an actual cat (and that would be cruel to the cat, in any event), according to MIT physicist Seth Lloyd (quoted in the recent New Scientist article):
“It’s not the entanglement of something as big as a cat, but it’s at least a kitten.”
Results of the two experiments were presented at the Second International Conference on Quantum Technologies, in Moscow, Russia, on July 23.
Journal references: Nature Physics (Lvovsky et al) ‘Observation of micro–macro entanglement of light’ ; Nature Physics (Gisin et al) ‘Displacement of entanglement back and forth between the micro and macro domains’
Some source material (including quotes) for this post came from the New Scientist article: ‘Schrödinger’s ‘kittens’ made in the lab from photons’ by Katia Moskvitch,