Electronics For Quantum Communications

Our secure digital communications so far have functioned on the principle of key-based encryption. This involves generating a key of appropriate length, which is then used to encrypt the data. Because distributing the keys is difficult, the keys are reused rather than regularly generating new ones.

The regular use of the keys opens up the encryption process to attacks by mathematical methods. Protection against such attacks currently is afforded by appropriate key lengths, since the compute time required by the mathematical methods for key recovery increases exponentially with the key length. This means the key lengths must be adapted already today to the growing potential of computing technology.

However, the greatest danger of the keys used in current encryption methods being recovered comes from the use of quantum computers. Because developments in this area are proceeding rapidly, quantum computers that are capable of recovering current and future key lengths in fractions of a second soon could be available. This is possible because with quantum computers, key recovery time scales linearly with the key length rather than exponentially. Classic encryption algorithms would then no longer be secure, because lengthening the key would not offer additional security.

In anticipation of this situation, research has been under way for a number of years in the area of quantum communications. The focus here is on secure communication by means of entangled quanta (in the form of photons). This requires generating entangled quanta and sending one to the recipient while the other remains with the sender. The entangled quanta have special properties that are identical for both quanta. If a quantum is intercepted on the way to the recipient, and then fed back into the stream after manipulation, it loses the typical properties of the encrypted pair. Upon arrival at the recipient, the manipulation can be discovered by comparison with the quantum held by the sender.

The system designs for quantum communication are complex electrical-optical systems. A complex optical setup with (semi-) transparent mirrors is required to generate entangled photons. Various electronic components are also required to control the photon source that must frequently operate at extremely short time scales.

The photons are often detected using single photon detectors. The achievable energy levels are very low, and electronic components are required for analyzing such low energy levels. Furthermore, the analysis electronics must operate with extreme speed – analysis rates in the GHz range are often required.

High-precision instruments are also required for measuring the arrival time of the voltage pulse from the single photon detector. Various mathematical methods are needed to recover the individual photon states in order to ensure that the received photon retains the same state as its counterpart held by the sender. Complex signal processors are used here, which are frequently designed as a combination of an FPGA and a DSP.

The required electronics are currently built from individual components. If quantum communication is to become standard, however, the electronic components must be implemented in just a few circuits. Work is currently beginning on the first subcomponents, such as fast analog-digital converters (ADCs) together with the digital analysis electronics consisting of FPGA and DSP.