When you expect one thing, but see something else – The Stern-Gerlach Experiment

Susskind and Freidman[1] tell us two aspects of the quantum world stand out as fundamental:

1. harmonic oscillators, and
2. spin.

Harmonic oscillators, well-known in classical physics, are readily expressed in quantum terms. Whereas spin – quantised intrinsic angular momentum of particles – is non-obvious and “strange” to classical physics. Yet harnessing spin is the key to many quantum technologies.

The Stern-Gerlach experiment (SGE) demonstrated spatial quantisation – subsequently identified as electron spin – exists.  In doing so, it “made physicists believe in quantum mechanics”[2].

This post will explore the applications of the Stern-Gerlach experiment in quantum technologies, as well as the challenges and opportunities of harnessing spin.

The Stern-Gerlach Experiment

In 1922, Stern and Gerlach fired a beam of silver atoms through a non-uniform magnetic field towards a glass plate. They intended to show that Bohr’s new theory of the atom – a nucleus surrounded by electrons in fixed orbits[3] – was incorrect.

The experiment relies on each silver atom having a magnetic moment caused by a single unpaired electron, and so their path is altered when passing through the magnetic field.

According to classical physics, these atoms should have a random distribution of magnetic momentum. Therefore, the path alterations should be random – and the pattern expected on the plate should be a diffused beam.

The experimenters believed the classic view would prevail – Stern is said to have said “If this nonsense of Bohr should, in the end, prove to be right, we will quit physics”[4]. However, the result seen was this[5]:

The deflection of atoms into two groups proved the magnetic momentum of the silver atoms is, in fact, quantised and not continuous. A shocking result at the time (and perhaps even still today).

Gerlach sent a postcard to Bohr commenting: “Attached [is] the experimental proof of directional quantization. We congratulate [you] on the confirmation of your theory”.

Strengths

The SGE aimed to give a “clear-cut decision between quantum theoretic model and classical view”[6].

SGE’s great strengths were to “make physicist believe…in quantum”[7], as well as demonstrate several of the “unexpected basic features of quantum physics”[8]. As such, it “spurred the development of quantum theory”[9].

It took a few more years to understand SGE demonstrated quantisation of electron spin[10]; that spin was Pauli’s “additional two-valued property”[11] of electrons; and that spin is intrinsic – an electron has spin but “does not spin”[12]. Indeed:

“…the spin of the electron…is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog”[13]

Despite this, spin is fundamental to modern quantum technologies.

Applications in Quantum Technologies

Spin has subsequently been used in “deep-level” investigations of the atomic world’s fundamental aspects (see[14]). We also harness spin’s quantisation in a number of “high-level” quantum technologies.

Quantum Computing

We use up/down spin states of particles as qubits (a quantum interpretation of digital bits[15]). Superposition of qubits allows for more efficient calculations than classical computers (in certain domains, eg Linear Algebra, Algebraic, Optimisation and Simulation[16]).  Companies such as [17] Research, Quantum computing (no date). [Online] available at: https://research.google/research-areas/quantum-computing/ (Accessed: 10 October 2024).” class=”js–wpm-format-cite”>Google[17], [18], IBM[18] Quantum Computing (no date). [Online] available at: https://www.ibm.com/quantum (Accessed: 10 October 2024). ” class=”js–wpm-format-cite”>IBM[18] and D-wave[19] are all attempting to build quantum computers.

Spintronics

Electronic components themselves are being built to exploit spin of electrons (together with charge) in a field named Spintronics. Examples are Magnetoresponsive RAM (MRAM)[20].

Quantum Communication

Entangled spin states are proposed for secure communication protocols, such as the ping-pong protocol[21], where interception can be detected. Whilst polarisation (spin states) of photons are used to securely share cryptographic keys (eg ID Quantique’s solution[22]) and quantum cryptography itself[23].

Quantum Sensing

We can leverage spin to measure weak magnetic fields with high sensitivity. Similarly spin states are used to improve resolution on MRI machines, as well as introducing Nuclear Magnetic Resonance devices.

Measuring beyond classical precision by using spin states (see[24] and[25])

However, there are challenges and limitation the SGE highlights.

Challenges/Limitations

The SGE also highlights several challenges for our quantum applications.

Noise / Decoherence

External inputs to the system, such as equipment vibrations or fluctuations in temperature, can affect the results. When we are experimenting with such small entities, there is heightened sensitivity. 

Additionally, we need to be concerned about decoherence (wave function collapse) through unwanted interactions. This is one reason the silver atoms in SGE pass through a vacuum in the experiment, to avoid interactions with other atoms.

Measurement

We can only measure spin in one direction at a time. Further, measuring spin in one direction after another, destroys that first measurement. This will limit applications that need to determine multiple spin directions in parallel (or attempting to do so synchronously if relying on previous results).

Scalability

Protecting from unwanted decoherence and noise leads to challenges in scaling up technology. We have to operate at very low temperatures or pressures; and over engineer noise protection.

A particular challenge for quantum computing is implementing error correction for one qubit requires many more qubits (see[26])

Future advances

Eisenhower reminded us to have faith in the “miraculous inventiveness of man”[27] to solve challenges. For inspiration, we can look to history. Early digital computers were the size of rooms with low computational power; now consider the computing power you carry daily in your pocket. We can also look to nature to see the achievable – birds, for example, appear to harness spin under everyday conditions to navigate[28].

Quantum fields may lead to deeper insights (where whilst electrons don’t spin, fields that create electrons do[29].

Conclusion

The SGE was a landmark point in us understanding how our world works – that quantisisation of particle spin is a fundamental reality, even if that is weird from a classical perspective. Its implications extend far beyond the theoretical physics it encouraged to be further developed, influencing a range of technologies that harness spin for practical applications, from quantum computing and spintronics to quantum communication and sensing.

However, there are challenges to practical applications. Quantum rules on measurement may limit what is possible. Protecting against noise and decoherence to get reliable results; and that driving costly protection systems (cooling, isolation etc) which ultimately lead to scalability challenges.

Our innovativeness will overcome those challenges. In the future we’ll use location services without need for GPS satellites, have a quantum engine chips in our iPhones (for more than just secure communications), use hand-portable MRI machines at accident sites.

References