The power is in the qubit
anticipated advantage of quantum computers over regular computers lies in the
basic building blocks. In regular computers, the smallest information carrier
is the bit which can take either the value 0 or 1. Generally, 1 is
represented by an electrical voltage (typically 5 volts) being on, and 0 by no
contrast, a quantum computer uses quantum bits – qubits for short –
which can be both 0 and 1 at once, due to the quantum property known as superposition. The qubits are
typically made of subatomic particles such as electrons or photons.
each qubit can represent two values at once, the total number of possible simultaneous states doubles with each added qubit. Two qubits can represent four values at
once, three qubits gives eight possible values, and so on. It starts slow, but
grows faster and faster. Already 300 qubits could represent more values than
there are particles in the entire universe. And it only takes 50–60
well-functioning qubits to exceed the computing power in today’s
The story begins
famous physicist Richard Feynman came up with the idea of using quantum systems
for calculations in the 1980’s. More precisely, he saw that it could be useful
for simulations in physics research.
that time, no serious attempts were made to realise the idea: The practical
significance of a quantum computer seemed small compared to the fact that
nobody knew to how to correct the errors that would inevitably arise in a
quantum computer and the lack of useful algorithms (a quantum computer cannot
be programmed in the regular way).
situation changed drastically in 1994 when mathematician Peter Shor published a
quantum algorithm which rapidly finds the prime factors of a given large number
– the key to breaking today’s encryption codes. A year later, he showed how a
special error correction code can deal with the errors arising in a quantum
computer. This sparkled a strong interest in realising a quantum computer.
Today, great efforts are made all over the world, at universities as well as
How to build a quantum
there is no simple guide on how to build a quantum computer as it is a very
difficult and complex task. But at least, this guide provides a rough overview
of the very basics:
1. Select hardware
A quantum computer could be based on basically any quantum mechanical object, as long as it has two quantum states that can be designated as values 0 and 1. For example, an ion having two different energy levels, a superconducting circuit with or without oscillating energy, or a tiny semiconductor particle – a quantum dot – with different charge or spin states. Other alternatives are squeezed microwaves, Majorana particles, implanted ions, and photons.
The most promising and developed techniques this far are superconducting circuits and ions. Superconducting circuits are fabricated on a microchip, whereas ions are suspended by electromagnetic fields in a so-called ion trap.
2. Isolate from surroundings
Quantum states are extremely sensitive, and collapse if they are exposed to disturbances. Therefore, the qubits need to be thoroughly isolated from the surroundings, if not to “forget” their value immediately. For many types of hardware, this means placing the qubits in an isolated vacuum chamber cooled to just above absolute zero temperature, colder than outer space. Also, take other actions you can think of to reduce disturbances.
3. Control the qubits
To make the qubits work for you, you need a way to manipulate them to put them in the desired input states, put pairs of qubits in the spooky condition, called entanglement, perform logic gate operations, and read out the results. In superconducting quantum computers, this is done using microwave pulses that are guided into the superconducting circuits that constitute the qubits. In ion trap computers, laser light of specific wavelengths is used to address and manipulate the qubits.
4. Improve and practice
Try running a quantum algorithm. Probably, you will find that the qubits forget their values long before the end of the algorithm. Go back to step 1 and try to remove any imperfections in your quantum hardware. Repeat step 2 and take every action you can think of to eliminate disturbances. Hopefully, this will increase the qubit lifetimes, that is the time that they properly keep their quantum state. Then go back to step 3 and remove all possible imperfections in the control equipment. Also, work on increasing the speed in all operations on the qubits. With longer qubit lifetimes and faster operations, you should now be in a better position to run your quantum algorithm.
At the front
mentioned before, superconducting circuits and ion traps are the most developed
techniques for building a quantum computer.
October 2019, Google was first to demonstrate a quantum computer solving a
problem that is beyond the reach of a regular computer. This is a major
milestone in quantum computing, generally referred to as quantum supremacy.
The quantum computer in question is a 53-qubit superconducting quantum computer
named Sycamore. Read more in our news article Big breakthrough for quantum
computers and in Nature.
also built a 53-qubit superconducting quantum computer. It has not been
reported to have outperformed a classical computer, but has been announced to
soon be available for commercial and research activity via the cloud. Another
front player in building superconducting quantum computers is Rigetti
Computing, a venture-backed startup company in California. Their latest
32-qubit quantum processors are also available in the cloud, via Amazon Web
spin-off company from Maryland and Duke universities, is the current leader in
building ion trap quantum computers. They have managed to put 160 ions in their
ion trap, but can this far only connect eleven of them (which is still
impressive – read more in Benchmarking an 11-qubit
quantum computer). IonQ’s quantum computers are
also available via Amazon Web Services.
WACQT’s point of view, it’s not yet time to go for large-scale systems. First,
one has to construct a small-scale system that works extremely well. Going
directly for large systems, without having good-enough qubits and
inter-connections, will unavoidably result in large error rates.
Comparing different quantum
articles often focus on the number of qubits in a quantum computer. However,
this number alone tells very little about its performance. There are several
other useful metrics to be taken into account:
- the connectivity between qubits – the number of other qubits that couple to each qubit,
- the types of quantum-logic gates that can be implemented,
- the reliability, often referred to as fidelity, of the gate operations, and
- the number of parallel operations that can be implemented.
always having to compare large specification sheets, IBM has introduced a
pragmatic single-value metric: quantum volume. It takes into account
both the number of qubits (the “width”) and how long a program that can be
run before errors have accumulated to render the computation useless (the
quantum computer Sycamore has certainly outperformed a conventional
supercomputer in solving a specific problem. However, the solved task is
completely useless, it was chosen solely because it was judged to be easy to
solve for a quantum computer but very difficult for a conventional one.
big milestone in quantum computing is to find a useful problem that is beyond
the reach of regular computers, but which a quantum computer with fifty to a
hundred qubits can solve. We work intensively on this in collaboration with our
industry partners. Probably, it will be within logistics or simulation of large
molecules,” says WACQT researcher Göran Johansson.
computers are predicted to be particularly suitable for solving problems that
involve a large number of possibilities, such as optimisation problems in
logistics or machine learning, and calculation of properties of large
molecules. Breaking today’s encryption codes is however further away, since
running Shor’s famous algorithm requires about one thousand well-functioning
initially, quantum computers will most likely be part of hybrid computing
systems, where a quantum computer operates as a co-processor to a conventional
supercomputer. The conventional processors will do most of the work, whereas
the quantum processor performs the specific calculations that a quantum
computer is significantly better at.