Merging Plasmonics, Nanophotonics- Quantum Computing

Could this quantum computer be the real deal?

Could this quantum computer be the real deal?
Photo illustration by Aurich LawsonI have been writing about quantum computing for a while now. If you look at my recent writing, though, you won’t find much about quantum computing. Why? Well, it all felt a little repetitive. The publications were still coming, but each new one seemed very much like the previous one. I’m not being cynical here; sometimes you just burn out on a subject.

In that light, it takes something special to attract my attention. It turns out that making something that looks and feels like a complete quantum computer—albeit on the smallest of scales—will definitely attract my attention. What we have here, ladies and gentleman, is nothing more or less than the first quantum microprocessor.

Quantum computing has turned out to be a challenge because it relies on encoding information in quantum bits (qubits) that have two fundamental properties. The first is coherence, which allows qubit states to naturally change in a syncronized manner. The second is quantum entanglement, which correlates the states of different qubits with one another. When we perform operations and measurements on a qubit that is entangled with another qubit, we automatically learn about and modify the state of its partner. This provides a sort of quasi-parallelism that allows a quantum system to perform some calculations faster than a classical computer.

But a computer is more than its bits. You need a register to hold qubits and perform operations on them. You need a memory, so that you can store qubits between operations. And you need to be able to initialize and readout the qubit so that you can begin and end a calculation. Now, there are groups of researchers who have done all of these separately. And, using trapped ions, some groups can even claim to have done the whole lot together. But I don’t think anyone seriously thinks that tables full of optics, lasers, and vacuum systems is the way to quantum computing nirvana.

Hydrogen in (3,0,0)-state.

No, quantum computing nirvana is firmly in the realm of solid-state physics. Unfortunately, this is where the problems begin. Qubits don’t last long in the solid state. Entanglement lasts a few hundred nanoseconds and coherence decays away faster than a banking regulation. Yet despite these problems, a group of researchers have managed to make an entire quantum microprocessor out of superconducting qubits.

Admittedly, the computer is rather simple: a two-qubit register made from SQUIDs (superconducting quantum interference devices), two additional SQUIDs that can be used to zero the register (and act as readout), and microwave resonator striplines, which act as memory. The most significant part, however, is a bus that couples the two register qubits together. This bus enables the researchers to program the register to perform different logic operations. That is what makes this something I am willing to call a microprocessor—though it can’t load up a sequential set of instructions into a memory element and execute them.

This all works through the magic of magnetic fields. (What, do we understand magnets now?) The microwave frequency that a SQUID likes to operate at depends on the magnetic field it is exposed to. The resonators have a fixed geometry that will only resonate at one microwave frequency. So a memory can be read or written by changing the magnetic field so that it is the same as that as the resonator. The same is true of the zeroing registers.

As a result, operations are really just a case of ramping magnetic fields up and down. Operations between qubits are performed by applying microwave pulses on the bus between them.

Molecule of alanine used in NMR implementation...

Image via Wikipedia

Conceptually, It is all very simple. It also is likely to scale well, since you just need to be able to choose different operating frequencies for each memory element and qubit.

Of course the qubits still don’t last very long. Their entangled states last 400ns, and the memory holds its value for four times longer. But the length of microwave pulses required to perform a logic operation are on the order of 30ns, so that 400ns is an absolute age.

No doubt there are plenty of steps, pratfalls, and other interesting hiccups along the way, but this bit of work shows how incremental improvements can come together into something that looks quite spectacular.

Science, 2011, DOI: 10.1126/science.1208517

From ARS Technica

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Quantum entanglement   –    Explained

Quantum entanglement is one of the most misused concepts around. Entanglement is delicate, rare, and short-lived. At its heart, quantum entanglement is nothing more or less than a correlation between two apparently separate quantum objects. Having discovered that, you might ask “so what is all the fuss about?” The answer lies deep in quantum mechanics.It is perhaps best to take an example. I shine a laser light through a special crystal. This crystal will occasionally, grab a photon from the light stream and split into two photons with a lower frequency—the sum of the two frequencies adds to that of the original photon—one of the photons is polarized vertically and the other horizontally.

Because we don’t know the polarization state and frequency of each photon, we say that they are in a quantum superposition of the two polarization states and all possible frequency combinations. If we were to measure the frequency and polarization of one photon, we would know immediately what the frequency and polarization of the other is. This is because they are linked by the single physical process that generated them. Classically, we could say “aha, these two photons were always in these states, so there is no need to complicate things further.”

But, nothing could be further from the truth—well, actually, many things could be further from the truth, but this still ain’t true. If I am careful, I can set up my generation process so that it always generates vertically and horizontally polarized photons. And I can pass one of the photons through a device that modifies its polarization. If I then perform measurements on both photons, I will find that the only way to understand the results is that my modification of one photon’s polarization state must have also modified the other photon’s polarization state.

Magnetic field lines around a magnetostatic di...

We know that these photons are not behaving classically but it is stranger than you might think. If these two photons were separated by millions and millions of kilometers, the modification of one photon’s polarization state is still balanced by the modification of the other photon’s polarization state. This happens because the two photons are a single quantum object—that is, they are described by a single mathematical function that cannot be broken up into separate descriptions for each photon. Furthermore, there is no information exchange involved; the changes do not have to move from one photon to the other, they simply are.

Since I have described something that looks like it can enable long distance communication, lets deal with that as well. It can’t. Quantum mechanical measurements are often extremely limited. In this case, we can’t ask a photon “what polarization are you?” We can only ask “are you vertically polarized?” The photon’s answer will always be yes, or no. But, you cannot know if that is because the other photon has been measured—measuring one photon instantly sets the other photon’s polarization in stone—or because it was in a superposition state and you measured first.

The only way to resolve that conundrum is to have a speed-of-light communication channel that says something like “measure now.” In which case, you might as well just send the all the information over the speed-of-light channel.

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Coherence   –    Explained

Coherence is superficially rather simple, but it can be a difficult concept to understand. Coherence is the predictability of an oscillator; how long, in time and space, we can accurately calculate the state the oscillator will be in.That’s very simple, but oscillators turn up in the oddest of places. Let’s start with something simple: a laser oscillator.

A laser can have three different coherences associated with it: time, length, and (confusingly) space. A laser emits light, which consists of an oscillating electric field. Ideally, once we have measured the field, it is always predictable but, in reality, this is not the case. Over time, the laser oscillator will drift, or even stop and restart. As a result, there is a limit to how far into the future the oscillator is predictable, called the coherence time. The coherence length is simply the distance that the laser light can travel in the coherence time. The spatial coherence of the laser beam is the predictability of the oscillator over the width of the laser beam at a frozen moment in time.

At heart, all coherence is this simple, but it turns up in unexpected places. A classic example is atomic physics. Imagine you have a cloud of atoms,  all of which are sitting in the same ground state.  We can shine a light that is resonant with an excited state of the atom on the cloud. As a result, some atoms absorb light and enter the excited state.

Stylized lithium-7 atom: 3 protons, 4 neutrons...

The combination of the light field and the atomic cloud form something very unexpected: an oscillator. The atomic cloud can store energy by absorbing photons and entering the excited state. The light field can also store energy in its electromagnetic field. So we have two energy storage systems that are coupled—energy can oscillate from one storage place to the other.

All the atoms start in the ground state. The illuminating light field drives them all into the excited state and then stimulates emission from the excited state, causing the atoms to decay to the ground state. If you measure the fraction of atoms in the excited state as a function of time, you get a nice oscillator. And, since it is an oscillator, you can define a coherence—this time the coherence is between the ground state and the excited state.

You might think that this system will remain coherent: in a fixed population of atoms, an atom is either in the ground state of the excited state. But an atom in the excited state might decay into some other, non-ground state, or interatomic collisions might cause a continuous rain of atoms out of the excited state. These sorts of processes cause the oscillations to decay over time, leaving an apparently constant fraction of atoms in the excited state and a constant fraction in the ground state—even though, individually, the atoms are still being excited into and decaying out of the excited state.

So, the coherence between the ground state and the excited state has a limited time, which we measure by looking at how fast these oscillations, called Rabi oscillations, decay away.

Coherence: a simple concept that, thanks to the madness that is quantum mechanics, turns up in some very unexpected places.

From ARS Technopaedia

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2 thoughts on “Merging Plasmonics, Nanophotonics- Quantum Computing

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  2. Pingback: PopSci Q&A: Seth Lloyd Talks Quantum Computing and Quoogling The director of the Center for Extreme Quantum Information Theory at MIT answers our biggest questions By Flora Lichtman « New Age

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