Every day we send almost 300 billion emails, and create 2.5 quintillion (2.5×1018) bytes of data. Much of this information we would readily share with others; still more is garbage, but what about the rest? Digital communications are prevalent in a many aspects of modern life where security is paramount: banking, government, commerce, national defense….
Securely sharing information is a critical challenge for modern information systems. Random numbers, random bit strings of 0’s and 1’s, are at the core of most cryptography protocols. For example, in public—private key exchange, random numbers are used to generate encryption keys. Unfortunately, random numbers are notoriously difficult to generate. In fact, most of the encryption protocols in use today rely on numbers generated using computer algorithms and are therefore pseudo-random, making them potentially vulnerable to hacking.
Quantum mechanics offers us a route out, allowing generation of fundamentally unhackable random numbers: measurement of a suitably-prepared quantum system can generate a random outcome, and thereby a quantum random number. In the Quantum Technologies group at the National Research Council, we have developed a quantum random number generator (QRNG) based on the nonlinear optical process of stimulated Raman scattering . When a ‘pump’ laser pulse is focussed into a potassium titanyl phosphate (KTP) waveguide, some of the photons scatter inelastically, leaving vibrational energy in the crystal, and producing a red-shifted ‘Stokes’ pulse .
The Stokes pulse grows as it propagates through the waveguide, with Stokes photons stimulating further scattering events. Critically, this process is initiated by the purely quantum mechanical vacuum fluctuations of the electromagnetic field; these fluctuations are an exceptionally broadband noise source. Raman scattering provides an ultrafast way of converting these fluctuations into an easily measurable pulse of light, while maintaining the quantum properties of the light field. We generate random numbers by measuring the optical phase [1,2] or energy  of each Stokes pulse, which is random on every laser shot because of its quantum origin. Pulses only picoseconds (10-12s) or femtoseconds (10-15s) in duration can be used to amplify the fluctuations using Raman scattering, before the crystal lattice returns to its initial state and the experiment can be repeated. Due to the very rapid “reset time” of the system , the physical limit on the bit generation rate is a massive 1012bps (bits per second), making this an attractive technique for applications requiring large random data volumes.