Laser technology illuminates the path to quantum computing

With the exception of superconductivity, almost all other mainstream quantum computing schemes can be implemented without lasers. Advances in high-purity, low-noise laser sources are facilitating the development of practical quantum computer hardware, but the challenges of frequency control and phase noise management remain. This paper will describe the important role of lasers in quantum computing and the challenges they face.

01Laser systems will play a pivotal role

Although the idea of using quantum mechanical principles such as superposition and entanglement for computing has been around for at least a few decades, the technology to implement practical quantum computers has only started to emerge in the last few years. High-purity, low-noise laser sources are one of the key enabling technologies for emerging quantum computing architectures. Advanced laser systems will play a pivotal role in the disruption of the field of computer science by quantum hardware.

The interest in building laser systems suitable for quantum computing is related to the inherent advantages of quantum computing itself. Just as bits are the basic building blocks of modern digital computers, two-energy systems called quantum bits form the basis of quantum computers. A quantum bit can exist in a coherent superposition of two binary states (zero and one), so it can be used to perform certain calculations much faster than conventional computers.

Many practical problems in physics, chemistry and biology, as well as in fields as diverse as accounting and automotive design, could benefit from quantum computing techniques. The problems currently faced in these and other fields require exponential execution times that would take decades to solve on even the fastest conventional supercomputers. These calculations can be performed much faster (in polynomial time) on a suitable quantum computer. These range from modelling the world around us to optimisation, sorting problems, large prime factors (the basis of modern cryptography supporting trillion dollar global e-commerce systems), and improving existing algorithms such as artificial intelligence and machine learning. While quantum computers will not replace conventional systems any time soon, there are enough high-value problems in this category to drive millions of dollars of investment in the development of practical quantum computers.

It is useful to think of quantum computing as the initial stage of its development, just as it is for conventional computers. In fact, before Charles Babbage and Ada Lovelace developed the first modern computers or difference engines for computing logarithms, performing such calculations was done by skilled humans, whose job title was 'computer' (just as people who work with paint are called a painter).

There are many historical examples of such 'computers', including Katherine Johnson, Dorothy Vaughan and Mary Jackson, who played an important role in the early years of the US space programme. babbage's differential engine The use of mechanical gears to execute a program developed by Lovelace using paper tape punching was arguably the first implementation of bits in a mechanical computer. Subsequent computer architectures found new ways to represent bits, including voltage and current fluctuations in vacuum tubes, polarised photons and the state of electrons in silicon chips.

02Implementing quantum bits

There are also many ways to achieve quantum bits, and it is likely that we have not yet found the best way to implement them in a quantum computer. For example, commercially available systems, such as IBM's Q system and similar hardware from Google and Intel, rely on superconducting wires cooled to near absolute zero to achieve quantum bits. The success of these systems has led researchers to investigate alternative quantum computing architectures that can operate under less extreme environmental conditions. Laser systems with special properties have been crucial to many recent efforts because of the role they play in multi-energy level physical systems.

Two-state energy gap under laser excitation.

A quantum bit is a two-energy system in which two states can exist in a stable superposition. For example, consider two internal energy states of an atom: the ground state and the excited state. There is a discrete energy gap between these states, and they can be coupled together with laser radiation of a specific frequency (where the laser energy and frequency are determined by Planck's constant h). The states of the atoms have a well-defined phase relationship with the laser radiation (in other words, the coupling between the laser and the atoms is coherent). By applying laser pulses of controlled frequency and duration, it is possible to produce superposition states between two atomic levels, effectively controlling the probability of finding an atom in an excited or ground state.

This process shows whether an atom is in the ground state, the excited state or both at the same time (similar to the famous Schrödinger's cat experiment). When a quantum bit is measured, it collapses to the excited or ground state with a certain probability and can revert to the conventional |0⟩ or |1⟩ values. By combining multiple two-energy systems, it is possible to create entangled states in which the value of one quantum bit affects the value of another quantum bit. Precisely tuned laser pulses that control coherent interactions are used to construct quantum logic gates, just as conventional computers use Boolean logic gates for bits.

In a quantum computer, a sequence of quantum gates operating on one or more quantum bits is used to implement a quantum algorithm. We can measure the probability of a quantum gate working as designed (or more generally, the probability of achieving a target quantum state) as the fidelity of a quantum logic gate. The fidelity is essentially the probability of successful entanglement between two states. If the fidelity drops below a threshold level, an error in the gate occurs and quantum computation is corrupted. The fidelity of a quantum gate is limited by the degree of control we have over the parameters of the laser pulse used to interact with the quantum bits. This means that very high purity, low noise light sources are the basis for building practical, scalable quantum computers.

A type of quantum bit based on a single trapped atom is known as an optical quantum bit. In an ion trap, the energy levels of the atoms are chosen so that the excited state lasts for as long as possible (one to two seconds using the current system). There are significant research challenges to overcome in achieving practical optical quantum bits. The most important of these is the laser linewidth, which sets an upper limit on the coherence time for the interaction between the laser source and the quantum bits. In order to take advantage of properties such as superposition, a quantum computer must complete all calculations before the system loses coherence. This is akin to using the calculator function when your phone is out of battery; you only have a limited amount of time to complete your work before the phone runs out of battery and calculations are forced to stop.

It is desirable to maximise the coherence time in a quantum system to enable longer calculations to be performed. Achieving a long excited state lifetime requires a laser source with an extremely narrow emission linewidth, roughly in the 1 Hz range. For highly fine laser cavities, substantial linewidth reduction and stabilisation is required, as it is insensitive to small vibrations and other noise sources.

Block diagram of a frequency-doubled phase-locked laser system for high-fidelity entanglement gates in an ion-trap quantum computer (red line: laser beam; black line: electronics).

An advanced linewidth reduction system was developed using a Ti:sapphire laser with a central wavelength of 729 nm and a linewidth of 1 Hz, stabilised by feedback from a highly detailed external optical cavity. This makes it possible to create high-fidelity entangled four-bit logic gates. More specifically, an ion trap can be constructed using a steel vacuum chamber cooled to near -450°F. A dozen lasers of slightly different frequencies are fired into the chamber, ionising a combination of calcium and strontium atoms which are held and clustered together in an electric field to form a crystal.

The laser cools the calcium and strontium ions at the frequencies required to entangle them and reads out the results, all belonging to the same part of the optical spectrum, which simplifies the requirements of the laser system. Such material systems can also be manipulated by infrared light, for which there are a variety of available laser sources, while other materials require ultraviolet frequencies to excite and trap the ions. Although equipment specifications vary, output powers in the order of a few hundred milliwatts to a watt or more should be possible in the short term using these devices. The energy states of strontium and calcium can be entangled, so that reading the state of a quantum bit (for example, by interrogating the crystal with a laser wavelength that only interacts with the calcium ion) will also yield the state of a strontium quantum bit.

The 94% fidelity measured in calcium/strontium crystals is sufficient to demonstrate that this concept is feasible for quantum computing; this variation in structure has achieved a fidelity close to 99%, one of the highest gate fidelities reported to date.

Another approach, called hyperfine quantum bits, encodes information in two sublayers of the ground state of alkaline earth materials. Two laser frequencies separated by a few gigahertz (the energy gap of a quantum bit) provide coherent coupling via excited Raman jumps. In this case, the ability to control coherence is not determined by the linewidth of each laser, but by the relative phase noise between the two sources. Typically, these two sources can be achieved by phase-locking the two lasers to maintain a precise frequency offset. Electro-optical modulation can also be used to generate two frequencies via sideband generation. The Raman leap is non-resonant, which means that relatively high optical power is required to achieve a high fidelity logic gate. Such systems require low phase noise and high power, typically at UV wavelengths.

One recent implementation of calcium ion hyperfine quantum bits uses two phase-locked frequency doubled titanium gem lasers operating at 397 nm with a frequency offset of 3.2 GHz. the combined system can deliver a total power of over 3 W. This approach is suitable for high-fidelity quantum gates because of its low phase noise (7 mrad rms between 10 kHz and 1 MHz).

03Technical challenges remain

Many technical challenges remain, including scaling these systems to a sufficient number of quantum bits to perform practical calculations. Systems like those described earlier demonstrate only a few quantum bits at a time; hundreds or thousands of quantum bits are expected for many applications. This will require a significant increase in laser output power by several watts to tens of watts or higher, while maintaining low noise and fine linewidths. Precise alignment of optical components in future systems may also benefit from advances in integrated optics and modulators.

In addition, implementations of quantum bits that do not use ion traps, but rather encode quantum bits as individual photons in two optical modes (e.g. optical polarisation), are proposed. In principle, photonic quantum bit systems do not need to be cooled near absolute zero to operate and can use fibre-optic cables to facilitate the long-distance transmission of quantum bits, provided that the cables maintain their optical mode properties during the propagation of the quantum bits. This is a significant challenge, as the photon attenuation and loss rates of many fibre-optic systems are much greater than the coherence time of the quantum bits in question.

Experimental devices using this principle have been demonstrated to operate at wavelengths near 780nm, and many researchers and institutions are still working on modified photonic quantum logic gates. For example, recently developed 4 x 10 mm photonic quantum bit devices implementing 8 quantum bit quantum computers are already accessible via cloud-based interfaces. This includes efforts supported by Cisco's new quantum research team, which is demonstrating optical quantum bits integrated into photonic circuits using an aluminium gallium arsenide (AlGaAs) laser source.

It is too early to say which of these approaches will be the equivalent of solid-state semiconductor processing for traditional computer chips, but we can expect high-quality laser sources to play a key role in any quantum computing architectures that emerge in the future.

Reference link：

https://www.laserfocusworld.com/lasers-sources/article/14235087/lasers-light-the-way-toward-quantum-computing