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A Photonic C-NOT Gate Breakthrough for Quantum Computing

Physicists have built a quantum logic gate that combines a quantum dot that fires photons with a photonic circuit that processes them

In the race to build powerful quantum computers, many groups are competing to build logic gates that can process quantum information and still be connected together on a large scale. 

One important question remains unanswered, however: what should the devices use to carry quantum information? 

Schemes involving charged particles such as Ion traps, electron circuits and superconductors have long looked promising because the qubits they hold can be easily manipulated with electric and magnetic fields. Charged particles also interact easily with each other in a way that can be made to process data.

The problem, of course, is that stray fields also interact with charged particles, causing the quantum information they carry to leak away. Stray fields litter the universe like the plague and this severely reduces the utility of these types of devices.

One alternative is the humble photon, which is unaffected by stray fields and can travel many kilometres through a waveguide without interacting with the environment. 

The trouble with photons is twofold. First, they are hard to produce individually, tending instead to come in bursts of several. That makes it hard to create them. 

Second, they tend to pass through one another unnoticed, like ships in the night. That makes it hard to process  the information they carry.

However, various groups have made significant process in solving these  problems. Lots of labs have developed photon guns that can be made to emit single photons, one at a time. 

At the same time, other labs have created photon circuits in the form of interconnecting waveguides that force photons to interact and thereby process the information they carry. These circuits are like tiny Scalectrix sets in which the cars collide where the track narrows.

In this circuit, the qubits are path-encoded meaning that the presence of a photon in one track is a 1 and its absence is a 0, for example.  When they come together they interfere, thereby processing the information they carry. 

But the efficiency of this interaction depends crucially on both photons being identical. Small differences in wavelength, for example, can dramatically reduce the performance.  But making identical photons is hard.

Today, Andrew Shields at Toshiba Research Europe Limited in Cambridge, UK, and a few buddies say they’ve solved this problem by integrating both these developments into a single device that acts like a C-NOT logic gate.

“The Controlled-NOT (CNOT) gate we demonstrate is the basic building block of quantum logic, since in combination with one qubit gates it can be used to perform any quantum operation,” say Shields and co.

This logic gate consists of a pillar of indium arsenic that acts as a quantum dot–it emits a single photon when zapped with laser light of a specific frequency. This is coupled to a photon racetrack carved out of silicon. 

A C-NOT logic gate requires two input photons. So the circuit works by zapping  the quantum dot twice, generating two photons. These are identical because they’ve come from the same dot. 

The photons then travel to a beam splitter that sends them down the appropriate paths (one of which has a built in delay that means determines when the photons enter the circuit relative to each other).

Shields and co have measured the truth table of their logic gate. They say it matches their theoretical predictions and can be made better with a few tweaks.

What’s significant about this approach is its scalability. Shields and co say it ought to be possible to build many quantum dots and circuits onto a single integrated chip. And the differences between photons from different quantum dots can be minimised by triggering them all with the same laser pulse. 

That’s handy but it’s not all plain sailing. The device must be cooled to 4.5 Kelvin, the operating temperature of the quantum dot, and the results of a single logic operation take some 30 minutes to collect.

Clearly that will have to change to make these devices viable. But that’s an engineering challenge these guys will surely relish.

Ref: Controlled-NOT Gate Operating With Single Photons

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