Do I Invest in Quantum Communications Links For My Company?

By Matias Castro


  • Introduction
  • How Quantum Communication Works
  • The Problem with Quantum Communication
  • Public Keys: the Alternative To Quantum Communication
  • Verifying That a Sender Sent a Message Via Quantum Effects
  • Using Quantum Communications Instead of Key Encryption
  • How Far Quantum Communication Has Progressed Towards Implementation
  • Conclusion


    Transmitting confidential information from one person to another has been a goal of many a general or chief executive. In recent times a standard encryption technique has emerged which involves the use of a key to change information into an unreadable form unless you are the possessor of the key. This uses the fact that large numbers are very hard to factor. With this system the sender and receiver both must have a key so the weak link in this system is transmitting the key to the receiver, this is where quantum effects can help.

    If the person transmitting the key knows that someone has eavesdropped the transmission then they can simply transmit a new key till there is no eavesdropper. Thus giving a key that only the sender and receiver know. The problem with conventional sending methods is that many eavesdroppers can do so without being detected and hence compromising the secrecy of the key. However by using quantum effects we can make it so that an eavesdropper cannot cover up his tracks.

    How Quantum Communication Works

    This sort of communications works by the fact that once quantum data is intercepted it cannot be retransmitted 100 percent accurately. A typical quantum comms systems uses photons which can have two different polarisation's. Data can be transmitted in either linear or horizontal polarisation's. So if say a person A wanted to send to a person B, they would send bits with random polarisation's, then if they were intercepted by C, C has to know what receiving polarisation technique to use. If the wrong one is used then the information is destroyed and cannot be retransmitted to B to disguise the fact that C eavesdropped. From the heisenberg uncertainty principle (e.g. observing the information changes it) C will only be able to pick the correct phase to transmit 75 percent of the time. So for the communication system to be able to detect an eavesdropper it has to have a lower error rate than this. BT recently tested this out over a 10 kilometre optical cable and managed to identify 91 percent of the photons correctly.

    The Problem with Quantum Communication

    So now we have a system for transmitting these keys so that no one except the sender and receiver have knowledge of it and hence can send messages safely. However now the weak link may be the key itself, given enough time it is possible to break the key (this can be combated by adding more digits to the key). Also an algorithm has been invented by Shor which allows these keys to be factored in a few seconds, but his algorithm requires the invention of a quantum computer. A prospective buyer of this system would have to weigh up the probabilities that this will happen in the future.

    Public keys: the Alternative to Quantum Communication

    Another reason that it may not be worth while for the prospective manager to invest in this is that there has already been invented a system for distributing keys safely so that only the receiver can decode them. This system is the public key encryption, where there are two keys a public one and a private one. Someone who wants to send encrypted data to a friend asks for there public key, encrypts the data sends it then the receiver decrypts it with his private key (this works on the premise that it is easy to work out the public key from the private but not visa versa). A fault in this system is that eventually computers may become fast enough that these keys can be worked out in hours instead of millennia however all that needs to be done to solve this problem is producing longer keys, rather than having to by a whole new communications system. Also a mathematician may invent a way of working out the private key from the public key. Public keys could also be cracked by a quantum computer but would probably cost you less to implement.

    Verifying That the Sender Sent a Message Via Quantum Effects

    So it would seem that on this use alone quantum communication is not very useful to a prospective company, however there is another usage for this system that could be highly useful for a certain type of company that needs to know that the message being sent is from the intended sender (and that it is what the sender intended to write) and not a third party that tapped into the communications link and altered the message or just sent a message of his own. This would be helpful in transfers between banks were it is necessary to know who is sending information to you e.g. you do not want to start sending money to a Swiss bank account when you meant to be sending it to a fellow bank. This method still does not stop someone from getting to the senders terminal and impersonating him there.

    Using Quantum Communication Instead of Key Encryption

    Another possibility for a communications systems which stops eavesdroppers from reading the contents of a message even if the data is sent unencrypted is to send down the line messages constantly where there sole purpose is to check whether the link is being tapped or not. If it isn't then at a randomly selected time to send the data down the line. This system relies on being able to track down an eavesdropper who is constantly on the line. There will be a small probability that the eavesdropper will suddenly start to check the line but as long as the message doesn't last for very long this should be small enough. This would only become viable if key encryption was able to be cracked easily, so again a manager would have to weigh up the possibilities of how likely this will become so. Although this may seem a risky way of sending data if public key encryption is broken, it may be the only alternative other than by carrier pigeon.

    How far Quantum Communication Has Progressed to Implementation

    Quantum communication is not some subject which is still in the theoretical stage it has been tested out and works. In 1989 Bennet, Brassard and colleagues transmitted a secret key over 30 cm, then BT transmitted 60,000 bits of information in one second over a distance of 10 kilometres, and finally communication was achieved under lake Geneva over 23 kilometres showing that this sort of communication is not off in the distant future but can be achieved now.


    If you are director and are trying to decide whether to upgrade to quantum communication links. There are various points you need to consider before investing. If the data you are transmitting has a long shelf life then consider getting these links, as data may be compromised by future gains in computer power or someone solving the public key private key encryption equation. Also if it is paramount that you know that the data is from the sender, and not a third party trying to take advantage of you by feeding you incorrect data, then think seriously about acquiring this new technology. Ultimately whether this is implemented on a large scale depends very much on the cost of producing these links compared to traditional methods.


  • Experimental Quantum Cryptography Nov. 1991 C. Bennet, F. bessette, G. Brassard L. Salvail, J Smolin.
  • New Scientist Quantum Communications thwarts eaves droppers 9 dec 1989 David Deutsch.
  • The Fabric of reality Jan 1997, David Deutsch.
  • New Scientist A quantum leap in secret technology 30 jan 1993 by William Bown.
  • Nature vol.378 p449 Quantum link under Lake Geneva.