Scott’s Supreme Quantum Supremacy FAQ

It can't really be explained, we kind of accept it works like it has been in both states at once until the box was opened. Similarly, we don't really know how to explain how particles travel by both slits at once in the double-slit experiment.

thank you for that and not trying to explain it in an additional convoluted way

Essentially, they use superconducting electronics to create a quantum circuit. It's not the same logic gates as a traditional computer chip and not based on the same physics. The details are difficult & messy.

There are several reasons it doesn't scale easily to more qubits, but you can imagine that you don't want the chip to be large (must be cooled to 25mK!) but the qubits should be spaced quite far apart so they don't influence each other. Also, it's not a very simple circuit, so the layout of the transmission lines ("wire on a chip") becomes difficult to manage. The last problem also scales badly with the number of qubits (the middle qubit becomes progressively harder to reach).

There is a paragraph in the (leaked) paper that describes their chip:

> In a superconducting circuit, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum mechanically [2, 30]. Our processor uses transmon qubits [6], which can be thought of as nonlinear superconducting resonators at 5 to 7 GHz. The qubit is encoded as the two lowest quantum eigenstates of the resonant circuit. Each transmon has two controls: a microwave drive to excite the qubit, and a magnetic flux control to tune the frequency. Each qubit is connected to a linear resonator used to read out the qubit state [5].

If you want a physical picture, check out [6]: https://arxiv.org/abs/cond-mat/0703002

edit: source [6] is more appropriate and open to access

Google's device is not encoding complex computational problems. Its just being told to arrange its qbits into a random series of gates. Could it do the Fizz Buzz algorithm? Or output the Fibonacci sequence? If not, then in what way is it programmable?

> What exactly is a qubit?

A bit is like a boolean type, has the values of true and false. Or you treat those values as 0 or 1, then gather a bunch of bits to build useful numbers.

A qubit is like a pair such that these numbers MUST satisfy the following constraints:

    pair.left^2 + pair.right^2 = 1
    pair.left and pair.right can be any complex number

Why such a composite type with weird constraints you may ask? Because that's how properties of really small particles behave in the real world. So the hope is, maybe if we can build our software using this weird data type called qubit, we can implement computation on quantum hardware without abstracting every problem using a dump type like a boolean or its aliases/collections.

Remember that classical computers use a clock to flip bits over time.

A similar quantum computer would manipulate qubits instead.

A bit has the storage capacity of 2 distinct bits of information.

A qubit has the storage capacity of 2 complex numbers, which corresponds to 4 floats, which is at least 16*8 bits of information if we are conservative about our assumptions.

> Is it made out of logic gates?

Kind of. Most of the current logic gates are built with semiconductors. It means by applying different voltages/currents/flux etc to different parts of a solid material, we can alter what we measure in some other part of the same material.

A quantum logic gate uses the same principle but in order to achieve the desired speed and storage advantages, it uses an object with a measurable property that at least approximately behaves like a quantum mechanical object. Common semiconductors are too crowded of atoms in terms of their body parts to make good quantum materials. They touch to each other and are almost always exposed to air. Their running temperatures are undesirably high.

A really dark (literally without a single photon) vacuum chamber that holds a really small amount of floating matter in the middle, frozen with lasers up to 0.000...1 Kelvin would make a good example of a stable but expensive qubit. We can measure this qubit by destroying its state, i.e by applying a magnetic field and measuring the emitted photon's location, polarization or frequency. The problem is, copy pasting this device to build a circuit is really hard due to logistics and auxiliary machinery required to keep all the state stable.

> How come our current limit is only around 54 or so?

That's not a fundamental limit but an engineering one, due the issues I mentioned above. The bigger the device gets, the harder to maintain its stable state. The method currently used for reaching this limit really looks like what is used in the 60s. History is repeating itself with a small twist, the running temperature is extremely low this time. Not liquid nitrogen low, but compressing atoms by sniping them from distance via laser on multiple directions low.

There is also one more problem that is unique to quantum computers. You have to measure the same qubit multiple times to be able to read those complex numbers since their values are determined statistically. You either represent a qubit with multiple qubit like devices or you use the same device to try your measurement repeatedly. Each approach comes with its own drawbacks.

> Are qubits, and quantum computers by extension, not even electronic circuits?

Even today, non quantum circuits are sometimes non electronic in some of their parts. Fiber optics, supersonic emitters/receivers and photoelectric sensors are good examples.

To this day, it is not clear whether the first consumer quantum CPU will be entirely electronic or not.